© 2019 Elsevier LtdIn the present research, the microstructure and oxidation behavior of an Fe-8.26Mn-5.25Si-12.80Cr-5.81Ni-11.84Co shape memory stainless steel (SMSS) was studied at 800 °C in air for up to 120 h. Phase changes and oxidation mechanism were discussed based on microscopy analyses, thermogravimetric measurements and thermodynamic simulations. The results show that oxidation exposure promotes the formation of the σ, χ and ferrite phases in the metallic substrate. The oxidation behavior follows a parabolic law, with the kinetics of oxidation being controlled by the Mn2O3 oxide growth in the first hours, and by Mn3O4 and MnCr2O4 spinel growth after 24 h of exposure.

© 2018 Elsevier B.V.TWIP steel weld spots were produced by varying the following welding parameters: welding current, welding time and electrode clamping force. Hardness distributions along the weld spots cross section were obtained by Vickers microindentations. The mechanical strength and failure modes of the weld spots were assessed by tensile-shear tests. Weibull statistics was applied to statistically analyse the data from tensile-shear loading. The results highlighted the influence of welding parameters variations on the Vickers hardness and residual stress state. These properties have a positive correlation to the failure modes and so the mechanical reliability of the weld spots. Welds that failed in pullout mode and partial-interfacial failure mode have statistically higher load bearing capacity than those in interfacial failure mode. The combination of a welding current of 8 kA with a welding time of 16 cycles and an electrode clamping force of 2 kN showed to be the optimum parameters for the investigated TWIP steel weld spots. By using this optimum setup, it was possible to supress interfacial failure mode and to obtain pullout as the predominant failure mode, considerably increasing the mechanical reliability of the weld spots.

© 2019, Latin American Applied Research.In this work, samples of the alloy (composed of 50.9% at Ni) were machined and solubilized so all had the same condition for the hot tensile test between the temperatures 350-650 °C. The results showed that yield stress was between 800 and 597.1 MPa and area reduction between 17 and 75% during the hot tensile test in the respective temperatures. The test at 350 °C shows a fragile fracture characteristic verified by the presence of "river marks", which are characteristic of this type of fracture, because this condition does not promote much plastic deformation. At the other temperatures ductile fracture was observed, with "dimples" that also characterize this type of fracture. This work shows the temperature relationship in the mechanical properties, more specifically the hot tensile test, in NiTi alloy.

© 2018, ASM International.NiTi-based shape memory alloys are successful owing to its capacity to cover specific applications unreachable by binary NiTi. The additions of ternary, and even quaternary, elements are intended to change specific properties. Known for its antibacterial activity, Ag became an alloying element in a search for a functional biomaterial; however, the melting appears to hampering the system exploration. A special melting procedure by vacuum arc remelting was developed based on chemical and thermal analysis, via EDS, XRF, and DSC, assessing the element loss and ingot homogeneity, respectively. By alloy design, different Ag content NiTiAg SMA were produced and analyzed on as-cast condition. The melting procedure developed involves specific feedstock cares and preparation, melting, and some remelting steps. The measured chemical composition slightly differs from the nominal due to alloying element loss and the melting reaction thermodynamics. Being the lower the possible, the remelting steps were optimized to maintain the compromise between chemical composition and compositional homogeneity through the ingot, since the Ag content stabilizes along them, also indicating a limited content possible to be alloyed. Ag-yields are content-dependent, while the Ni:Ti relation is stable, being therefore the melting of NiTiAg SMA better performed by VAR than other melting routes under high vacuum conditions.

© 2018, Universidade Federal de Uberlandia. All rights reserved.This study discusses the microstructure, quasi-static mechanical strength and failure modes of TWIP steel weld spots. Weld spots were produced by varying the main resistance spot welding parameters: welding current, welding time and electrode compression force. All the samples showed a remarkable material hardness mismatch between the fusion zone, the heat affected zone and the base material, as evidenced by microindentation maps. Hardness at the fusion zone is lower than that of heat-affected zone and base metal, which facilitates interfacial failure mode during tensile-shear tests. However, high heat inputs promoted the failure mode changes to partial interfacial mode and then to pullout mode during tensile-shear tests as confirmed by Scanning Electron Micrographs. These changes in failure mode were accompanied by a notable increase in tensile-shear strength and energy absorption capability.

© 2018 Universidade Federal de Sao Carlos. All rights reserved.In the current work, XRD, SEM, EBSD and TEM techniques were used to evaluate the microstructure of stainless Fe-Mn-Si-Cr-Ni-Co shape memory steel processed by ECAE and annealed for one hour at temperatures ranging from 650ºC to 950ºC. The results were then correlated with the mechanical and shape-memory properties of the steel. It was observed that the samples containing large grains and a microstructure free of defects or precipitates presented a high volume fraction of multi-variant thermal martensite and stress-induced martensite, resulting in good shape recovery, owing to the memory effect. The grain refinement and precipitation of second-phase particles decreased the volume fraction and number of martensite variants and considerably increased the mechanical resistance, enhancing the elastic shape recovery. It was shown that shape memory properties were essentially related to the mechanical resistance of the matrix, which in turn was related to the microstructure.

© 2018 Universidade Federal de Sao Carlos. All rights reserved.The present work studied the influence of annealing on a Ni-Ti alloy submitted to uniaxial compression. The stress-strain curve under compression showed the absence of the Stress-Induced-Martensite (SIM) plateau and 42% of remaining austenitic phase (B2). identified by the Electron Backscattered Diffraction (EBSD) technique on a Scanning Electronic Microscope (SEM). Annealing at 150 ºC increased the volume fraction of austenite to 70%. Annealing at 200 ºC reduced the volume fraction of austenite to 33% and increased the classical Vickers hardness and the dynamic elastoplastic hardness. This increase suggests the presence of a phase with higher indentation resistance, which can be the R phase reported in the literature. Annealing at 250 ºC decreased the classical Vickers hardness and the dynamic elastoplastic hardness, probably due to recovery. Annealing at 300 ºC advanced the recovery process and promoted some recrystallization, as suggested by the large standard deviation observed in the hardness results.

© 2018 Universidade Federal de Sao Carlos. All rights reserved.The wire drawing mechanic of Ti-49.82Ni (at. %) Shape Memory Alloy (SMA) was investigated through the true stress-strain curves and drawing stresses. The tensile tested solution treated wire presented a four steps elongation at temperatures below the austenite finish temperature (AF), and a conventional one-step behavior above the martensite deformation temperature (MD). The tensile yield stress for the formation of detwinned martensite (DTM) or stress-induced martensite (SIM) increased as the testing temperature increased; however, for larger deformation, the behavior reversed. The efficiency of drawing work, which is the ratio of uniform work to total work, increased from 10% for 0.07 mm2.mm-2 area reduction at 25 °C to 50% for 0.21 mm2.mm-2 at 110 °C. Therefore, wire drawing temperature and area reduction should be combined to increase the efficiency, taking into account the desired properties with reasonable workability. Furthermore, transformation work should be considered on wire drawing shape memory alloys as phase transformation occurs in temperatures below MD.

© 2017 Universidade Federal de Sao Carlos. All rights reserved.Plasma nitriding of a solution annealed and aged 300 grade maraging steel was studied aiming to increase the creep resistance. The surface microhardness reached 1,140 HV, producing 50 μm layer composed of ε-Fe3N and γ'-Fe4N nitrides at the uppermost sample layer. The inner core remained unaltered presenting typical plate-like martensite microstructure of maraging steels with average microhardness of 604 HV. Surface RMS roughness in the nanometric scale increase from 52 nm to 71 nm. The continuous layer of iron nitrides seems to behave as a barrier for oxidation and for inward oxygen diffusion improving the creep resistance by reducing the steady-state creep rate (εs) in 52-65% when compared with the literature results. Dominant creep mechanism is controlled by dislocations climb. Fracture surfaces of specimens presented ductile failure consisting of equiaxed and bi-modal dimples in the fibrous zone surrounded by 45ºshear lip. Nitrided sample presented a reduced ductility, associated to the hard surface layer.

© 2017 Trans Tech Publications, Switzerland.The influence of plasma nitriding of a maraging 300 steel on mechanical properties at high temperature has been studied. Samples were tensile tested at 600°C in four conditions: solution treated (MAR-S), solution treated and aged (MAR-SA), solution treated and plasma nitrited (MAR-SP) and solution treated, aged and plasma nitrited (MAR-SAP). In the same sequence, the yield strength and ultimate tensile strength increased slightly respectively from 1073 to 1189 MPa and 1174 to 1301 MPa, an increase of about 10% due to plasma nitriding. All the samples presented similar values of elongation, around 18%, but the cross section area reduction decreased significantly by plasma nitriding from ~70% for MAR-S and MAR-SA to ~45% for MAR-SP and MAR-SAP, that is an decrease of 36% in average. This decrease is attributed to brittle fracture nucleated at 50 µm thick iron nitride layer. The inner fracture surface of the tensile tested specimens was predominantly ductile presenting characteristic microcavities.

© 2016, Springer Science+Business Media New York.The possibility of a reverse strain-induced martensitic transformation (RSIMT) of ferrite into austenite was evaluated in two duplex stainless steels (DSS) solution treated at 1000, 1100, and 1200 °C. For each temperature, we evaluated the effect of stress–strain state (tensile tests—triaxial; compression tests—uniaxial, and plate rolling—biaxial) on the reverse martensitic transformation behavior. For each strain level, the ferrite volume fraction was measured with a ferritscope, and finite element analyses allowed the correlation of the stress–strain states to the total transformed ferrite. X-ray diffraction and metallographic analyses showed that microstructures of all samples are composed only by ferrite and austenite, and the general trend of lower ferrite contents associated to higher strains corroborates the hypothesis of RSIMT of the ferrite in austenite. The amount of martensitic transformation is proportional to equivalent strain or to the respective strain energy, but different stress–strain states lead to different transformation behaviors. In the triaxial stress state during necking, the amount of transformed ferrite by RSIMT in a specific strain energy level is higher than that obtained in the biaxial stress state of the plate rolling, which in turn is higher than that observed for uniaxial stress state generated by compression tests. It can be then concluded that different strain states, and not only the amount of plastic deformation, affect the behavior of the RSIMT of ferrite in austenite of DSS.

Ti-50.13Ni and Ti-49.62Ni (at.%) shape memory alloy ribbons were fabricated by melt-spinning method at different circumferential wheel velocities. The effects of wheel velocity, chemical composition and heat treatments on microstructure and Transformation temperature were investigated. Differences in wheel velocity led to differences in cooling rate and sample dimension, as well as in phase transformation temperatures. Two heat treatment conditions were studied, 350°C for 1h and 350°C for 5h. In the samples produced at high wheel velocity and heat-Treated at 350°C for 5h, nanosized Ti-rich precipitates were observed in both chemical compositions. Cross-sectional microstructure was studied by optical microscopy; SEM was used to study the nanometric grains and nano precipitation. The transformation temperatures were analyzed by DSC.

This work deals with structural, electrical and mechanical characterization of Ti-50.13Ni and Ti-49.62Ni (at.%) shape memory alloys (SMAs) fabricated at different circumferential wheel velocities. The effect of wheel velocity, chemical composition and heat treatments are investigated. The characterization of crystallographic phases of the Ti-Ni ribbons was carried out using X-ray diffraction. Electrical resistance variations as function of temperature (δR/R %) were analyzed using a non-commercial technique, which consists in a thermal-adjustable bath apparatus revealing the temperatures of B2→R→B19′ two stage transformation, whereby the presence of R-phase can be definitively confirmed. The Stress-Assisted Two-Way Memory Effect was measured by an own designed apparatus with an Linear Variable Differential Transformer captor and a current controlled heating, and results indicate that the as-spun condition, promotes the Stress-Assisted Two-Way Memory Effect. On the other hand, increments in Ni content tend to decrease transformation temperatures and high wheel velocities help to the R-phase formation.

Copyright © 2016 The Authors.NiTi SMA wire with Ni-free surface is desirable as construction materials for a range of biomaterials to actuators associated to shape memory and superelastic properties. In biomaterials a potential problem with NiTi implant devices is the release of Ni in the human body. This work analyzes the effects of nitrogen plasma based ion implantation (PBII) technique in a wire of NiTi with shape memory effect associated to a Ni-free surface. The samples were treated for 60 min at 741 °C, with 16 kV high voltage pulses. Results of the thermo-mechanical properties of the PBII treated samples showed that there is no effect of the PBII treatment on the shape memory effect when compared to NiTi samples with no PBII treatment, which is desirable for applications addressing Ni-free surface and shape memory effect. Although it was noted that the stress-strain test causes cracks perpendicular to the stress direction on the treated surface.

© 2016 Trans Tech Publications, Switzerland.An in situ high-temperature X-ray diffraction (HTXRD) study in maraging 300 steel was carried out to study the martensite to austenite transformation and effect of time of exposure in the austenite reversion below austenite start temperature. Solution annealed materials were subjected to controlled heating-holding cycles. The first sample was heated at a rate of 10°C/min from room temperature to 800°C, showing that the microstructure is completely martensitic (α110) until 600°C. From 650°C until 800°C, the microstructure is gradually changing from martensitic to austenitic, showed by the increasing peaks of λ111 and reducing peaks of α110. At 800°C the microstructure is completely austenitic (λ111). Another sample was heated at 10°C/min from room temperature to 600°C and held for 4 hours. At 600°C, at 0 h time of exposure, only a martensitic peak was observed. An austenite peak can be observed after some time of exposure at this temperature. The volume fraction of austenite increased with increasing time of exposure at 600°C, reaching 50/50 volume fraction after 4 hours of exposure. XRD diffraction patterns for the same sample that was held for 4 hours at 600°C and then cooled down in air to room temperature showed the same intensity of austenite and martensitic peaks found in situ at 600°C for 4 hours (retained austenite), with the volume fraction of 50/50 of austenite and martensite phases. The HTXRD technique can be used to identify and quantify martensite to austenite transformation and austenite retention.

© 2015 Material Reserch Society.Shape memory alloys (SMA) are metallic attractive engineering materials due to their capacity to store pre-defined shapes through a thermally induced phase transition from a solid state. This paper aims to evaluate the influence of solubilization thermal treatments on a NiTi shape memory alloy originally fabricated by vacuum induction melting and then reprocessed by plasma melting followed by injection molding (Plasma Skull Push Pull process) into different metal molds (steel, aluminum, brass and copper) in order to compare the thermal properties regarding to its raw state. The thermal treatments of solubilization were carried out at 850°C in different times (2n function, n = 0, 1,2 and 3, in hours). The influence of solubilizing treatments in the NiTi shape memory alloy was analyzed using the following characterization techniques: Differential Scanning Calorimetry (DSC) and Electrical Resistance as a function of Temperature (ERT). The results demonstrate that the solubilization heat treatments applied on the reprocessed NiTi shape memory alloy through the plasma skull push pull process, provides important changes in the phase transformation of the material. Therefore, it was demonstrated that it is necessary to solubilize the material after melting or remelting the NiTi shape memory alloy via this process to obtain mini-Actuators products with homogeneous properties.

© 2016 Trans Tech Publications, Switzerland.A study was conducted on the corrosion behavior and characteristics of the passive oxide film of Fe-Mn-Si-Cr-Ni-(Co) shape memory stainless steels (SMSS) in a concentrated nitric acid (HNO3) solution, based on potentiodynamic polarization, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) analyses. The results indicated that Fe-Mn-Si-Cr-Ni-(Co) SMSSs exhibit a passive behavior similar to that of 304L austenitic stainless steel (304L SS). However, unlike 304L SS, their high silicon (Si) content renders them insensitive to intergranular attack in highly oxidizing environments. The XPS analysis also indicated that Si appears to be the main element responsible for the high protectiveness afforded by the passive film formed on Fe– Mn–Si–Cr–Ni–Co SMSS.

© 2016, Journal of Aerospace Technology and Management. All Rights Reserved.In order to quantify ferrite content, three techniques, XRD, ferritoscope and optical metallography, were applied to a duplex stainless steel UNS S31803 solution-treated for 30 min at 1,000, 1,100 and 1,200 °C, and then compared to equilibrium of phases predicted by ThermoCalc® simulation. As expected, the microstructure is composed only by austenite and ferrite phases, and ferrite content increases as the solution treatment temperature increases. The microstructure presents preferred grains orientation along the rolling directions even for a sample solution treated for 30 min at 1,200 °C. For all solution treatment temperatures, the ferrite volume fractions obtained by XRD measurements were higher than those achieved by the other two techniques and ThermoCalc® simulation, probably due to texturing effect of previous rolling process. Values obtained by quantitative metallography look more assertive as it is a direct measurement method but the ferritoscope technique should be considered mainly for in loco measurement.

© Owned by the authors, published by EDP Sciences, 2015.NiTi Shape Memory Alloys (SMA) became an interesting research field in metallurgy and physics, among many others, since its advent, back in 1960's. Up to date, this interest remains due to the great range of unexploited research possibilities, in addition to the large number of application accessed by these alloys. Within those research possibilities, NiTi SMA has been used as the basis for new alloys by the introduction of a third, and even a forty, chemical element, what is called NiTi-based SMA. Silver addition was made via arc remelting to produce Ni54.99Ti43,01Ag2,00 and Ni52,99Ti45,01Ag2,00 wt.%, so that silver acts as a substitute for Ni in Ti-rich, as well as a substitute for Ti in Nirich alloys, respectively; for comparison reasons, an alloy Ni53,99Ti44,01Ag2,00, with near-equiatomic Ni:Ti relationship, was melted. Remarks on metallurgy of ingot melting, microstructure developed and thermal properties of these alloys was accessed.

© Published under licence by IOP Publishing Ltd.Dilatometric study in maraging 300 steel was carried out to study the effect of heating rate on precipitation of intermetallic phases and martensite to austenite transformation. Solution annealed material were subjected to controlled heating-holding-cooling cycles. The martensite to austenite transformation splits into two steps at lower heating rates. The first step enhanced by slow heating rate, occurs through a diffusion process, while the second step, enhanced by a fast heating rate, occurs though a shear process. The extent of precipitation decreases with heating rate, suggesting that precipitation occurs primarily by a diffusional process.

© 2015, Journal of Aerospace Technology and Management. All Rights Reserved.The main goal of this paper is to analyze if it is feasible to employ a trained shape memory alloy wire as a linear actuator to modify the camber of a morphing wing rib. In order to achieve this purpose, a morphing rib with a compliant trailing edge was proposed, developed, and subjected to structural analyses to ensure its flexibility. After the rib configuration was set, it was manufactured by a 3-D printer. The NiTi wire used as actuator was trained by a thermomechanical procedure based on a cycling process with a constant load application to present the two-way shape memory effect. In that way, the wire presents a determined length at its low-temperature phase and a shorter one at its high-temperature phase. Since the wire contraction and the torque applied by it are two crucial factors to define the camber curvature, it was decided to study two different wire lengths: 103.5 and 152.1 mm. The aerodynamic performance of the morphing cambered airfoils was studied using XFOIL software and compared to that of conventional airfoils with single hinged flap. The results show that both morphing airfoils present better aerodynamic performance for small angles of attack.

© 2015 Elsevier Inc.Abstract In this paper, the high-temperature creep resistance and effects on the austenite reversion and the dynamic evolution of precipitates of maraging 300 steel were investigated. The main strengthening mechanism in a solution treated and aged material is the fine needle shaped Ni<inf>3</inf>(Ti,Mo) precipitates densely dispersed in a single martensitic phase. The specimens were submitted to creep tests at temperatures of 550, 600 and 650 °C and stress conditions of 200, 300 and 500 MPa. Stress exponent (n) varied from 6.0 to 7.2 and activation energy for creep (Q<inf>c</inf>) from 364 to 448 kJ/mol, associated to the tangled and cells arrangements of the dislocations, show that the dominant creep mechanism is controlled by dislocations climb and slip. The experimentally determined threshold stresses are about 25 MPa at 550 °C and close to 4 MPa at 600 and 650 °C. Due to high-temperature creep exposure, part of martensite was reverted to austenite in a range of 17.2% to 48.5%, depending upon the time, temperature and applied stress. At the same time, the Ni<inf>3</inf>(Ti,Mo) precipitates were coarsened and Fe<inf>2</inf>Mo precipitated, leading to undesirable alloy's strength reduction. Volume fraction of reverted austenite showed strong negative correlation with hardness. Fracture surfaces of specimens presented ductile failure consisting of equiaxed and bi-modal dimples in the fibrous zone surrounded by 45° shear lip.

© Springer International Publishing Switzerland 2015.Simultaneous nitriding and aging heat treatment of maraging 300 steel was carried out inside a DC-pulsed plasma nitriding reactor. A single heat treatment cycle was done, as the plasma nitriding and age hardening processes occur at the same ranges of temperatures and times. Samples of maraging 300 steel, in the solution annealed and solution annealed and aged conditions, were tested. Plasma nitriding and aging, carried out at 480 °C for 3 h, increased the surface hardness up to 1140 HV, producing case depths of 50 μm since ε-Fe3N and γ′-Fe4N nitrides were formed in the hardened surface layer. It is observed that the microstructure of the core material remains unaltered as the typical martensite plate-like microstructure of maraging steels. The core hardness of solution annealed samples increased from 331 to 597 HV after the plasma nitriding treatment proving the possibility of nitriding and aging at the same treatment cycle. The pre-aged samples did not show any overaging or martensite reversion to austenite after the simultaneous plasma nitriding and aging treatments, that could be showed by the core hardness of 620 HV and can be related to the time of total aging exposure of 6 h, including pre-aging and plasma nitriding.

The current work evaluated the microstructures and martensitic transformation temperatures of NiTi shape memory alloy (SMA) deformed by equal channel angular extrusion (ECAE). The Ti-55.27wt.%Ni alloy was processed by 1 ECAE pass at 250 °C using a die with an intersection angle of 120°. After processing, samples were annealed at 300 °C, 400 °C and 500 °C for 1h to evaluate the microstructural changes. Microstructural characterization was performed by scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer (EDS) device, and Vickers hardness measurement. Martensitic transformations temperatures were analyzed by differential scanning calorimetry (DSC). Results show that the annealing treatments presented no significant change in the microstructure of the ECAE processed samples. Meanwhile, the DSC curves corresponding to the annealing treatments performed at 300 °C and 400 °C show two step martensitic transformation related to B2→R→B19'. For the annealing at 500 °C, the martensitic transformation temperatures returned to the ST condition, indicating a reduction of the processing defects.

© (2014) Trans Tech Publications, Switzerland.The objective of this work is to evaluate creep behavior of a maraging steel (300 grade) solution annealed before and after superficial treatment of plasma nitriding. Creep tests were conducted on a standard creep machine at stress range of 200 to 500 MPa at 550°C. Samples with a gage length of 18.5 mm and a diameter of 3.0 mm were used for all tests. Creep parameters are determined and a comparative analysis is established with the results gotten from the alloy with and without plasma nitriding. Maraging 300 steel plasma nitrided has showed a similar creep behavior compared with the same alloy without superficial treatment, with creep rate and stress exponent results very close to the material only solution annealed. This result can be associated with the strong impact of reversion of martensite to austenite and overaging at this temperature and time of exposure that minimizes the benefits of a superficial treatment.

© 2014 World Scientific and Engineering Academy and Society. All rights reserved.Maraging steels have low carbon content and are highly alloyed, having as main feature the ability to increase the mechanical strength after thermal aging. Therefore, the objective of this work is to analyze the effect of aging in a Cofree maraging steel, Fe – 0.014% C – 0.3% Mn – 3.9% Mo – 2.1% Cu – 0.19% Si – 11.8% Cr – 9.1% Ni – 1.0% Ti (wt), at different heat treatment times (10 min - 960 min) at constant temperature (550°C), after cold rolling up to 66% and 77% area reduction. Microstructures were analyzed by scanning electron microscopy (SEM - EDS) and mechanical properties by Vickers hardness measurements. The results showed a significant increase in the hardness of the material after aging heat treatment on the solution treated and deformed samples. The aging heat treatment which was harder about 650 HV was 550°C/60 min.

© 2014 Materials Research Society.Shape Memory Alloys (SMA) metallic materials that change their mechanical and physical properties with temperature variation and mechanical loading, surprising engineers and researchers. In this way, one can develop thermomechanical actuators capable, for example, of generating force by blocking the shape recovery or change the natural frequency of a mechanical system by blocking resonance. The processing of these SMA are countless, each one with its specific limitation and particularity. This study aims to evaluate the influence of rapid solidification of a Ni-Ti SMA that is originally manufactured by Vacuum Induction Melting (VIM) and reprocessed by Plasma Melting (PM) followed by injection molding into different metal molds (steel, brass, aluminum and copper). The influence of such a processing is analyzed through Differential Scanning Calorimetry (DSC) and Electrical Resistance as a function of Temperature (ERT) to determine the effects on transformation temperatures. The results demonstrate that by using the copper mold one can provide greater uniformity of the material properties. Thus, there is the possibility of obtaining different kinds of SMA mini-actuators by PM injection in a copper mold and that includes different shapes and sizes that can be studied further.

The effect of the microstructure on the shape recovery in stainless Fe-8Mn-5Si-13Cr-6Ni-12Co shape memory steel (SSMS) was evaluated using tensile tests and reversion temperature of 600°C for a pre-strain of 4%. The tests were performed for a solution treated and annealed conditions at different temperatures after wire drawing of 57% area reduction. The best total shape recovery (TSR) was 83% for a sample deformed and annealed at 850°C. It was concluded that the elastic (or, superelastic) shape recovery (ESR) is high when the austenitic matrix strength is high surpassing the value of shape recovery due to memory effect (SR) and once the austenitic matrix becomes softer, the contribution of SR increases and that of ESR decreases.

Ni-Ti-Nb system alloys show wide shape memory hysteresis, suitable for assembly applications. The microstructure is composed by NiTi matrix (with some dissolved Nb) and Nb dispersed particles (with some Ni and Ti content). These particles are to cause the hysteresis widening. This work evaluates the microstructure evolution during wire fabrication process of equiatomic Ni and Ti alloys with increasing Nb content (1.5, 3.0, 6.0 and 9,0%at.). It is shown that as-cast alloys with up to 9% at.Nb and near equiatomic Ni:Ti relation show three main microconstituents: NiTi matrix phase, interdendritic eutectic phase (NiTi + β-Nb) and Ti3(Ni,Nb)2 compound precipitates. It was observed that NiTi matrix phase and eutectic phase (NiTi +β-Nb) have ductile behavior while Ti3(Ni,Nb)2 compound have fragile behavior. There was not much hardness variation during hot swaging (200-300 HV) due to recovery and recrystallization processes. Mechanical hardening prevailed as the mechanism for increase hardness of cold worked samples from 200 to 450 HV. © (2014) Trans Tech Publications, Switzerland.

Shape recovery of a 2 mm in diameter shape memory alloy (SMA) wire produced from EB melted NiTi ingot is presented. The results indicate complete shape recovery as high as 9% and a shape recovery higher than 9% for a pre-strain between 10 and 15%. These excellent results are attributed to the high purity of starting ingot with carbon content of 0.016 wt%. It was observed no change in reverse martensitic transformation temperature for pre-strain up to 9% and for higher pre-strain there was a 50 C increase in reverse martensitic transformation peak temperatures indicating some work hardening of the material. © 2012 Elsevier B.V.

In this work the effect of grain refinement on the shape memory properties of a Fe-Mn-Si-Cr-Ni-Co-Ti alloy was evaluated using compression tests. In order to refine the microstructure, the samples were heavily deformed by Equal Channel Angular Extrusion (ECAE) and then annealed at different temperatures ranging from 723 K to 1323 K. These treatments resulted in the formation of intermetallic precipitates and strengthening of austenitic matrix. The results of compression tests show that the higher degrees of shape recovery (56 % for 4% strain) were achieved by the samples with smaller grain size (12 μm). It was also shown that the contribution of elastic shape recovery increases and the shape recovery due memory effect decreases as the austenitic matrix strengthening increases. © (2013) Trans Tech Publications, Switzerland.

Stainless shape memory steels presents reasonable shape recovery but, lower than the traditional NiTi shape memory alloys (SMA). However, recent results have shown that the shape recovery could be improved by decreasing the austenitic grain size. The present work describes the influence of the austenite grain size on the shape recovery in stainless shape memory steel deformed by equal channel angular extrusion (ECAE) using a die intersection angle of 120°. Two alloys, FeMnSiCrNi and FeMnSiCrNiCo, were deformed by 1 ECAE pass and then they were compared in the deformed state; deformed and annealed at different temperatures for 1 h, resulting different grain sizes. Both alloys were evaluated by compression tests and the results shows an increase in the total shape recovery related to the grain refinement. The best total shape recovery was 73% after a pre-strain of 4% for FeMnSiCrNi alloy. © (2013) Trans Tech Publications, Switzerland.

The substitution of conventional components for NiTi in distinct devices such as actuators, valves, connectors, stents, orthodontic arc-wires, e.g., usually demands some kind of treatment to be performed on the surface of the alloy. A typical case is of biomaterials made of NiTi, in which the main drawback is the Ni out-diffusion, an issue that has been satisfactorily addressed by plasma based ion implantation (PBII). Even though PBII can tailor selective surface properties of diverse materials, usually, only thin modified layers are attained. When NiTi alloys are to be used in the harsh space environment, as is the case of devices designed to remotely release the solar panels and antenna arrays of satellites, e.g., superior mechanical and tribological properties are demanded. For this case the thickness of the modified layer must be larger than the one commonly achieved by conventional PBII. In this paper, new nitrogen PBII set up was used to treat samples of NiTi in moderate temperature of 450 °C, with negative voltage pulses of 7 kV/250 Hz/20 μs, in a process lasting 1 h. A rich nitrogen atomic concentration of 85 at.% was achieved on the near surface and nitrogen diffused at least for 11 μm depth. Tribological properties as well as corrosion resistance were evaluated. © 2012 Elsevier B.V.

The NiTi shape memory alloys (SMA) are employed in various applications, thanks to its specific mechanical properties. However, for many applications, it is interesting to improve its surface mechanical resistance to tribological or chemical attacks. This work aims to analyze the effects of nitrogen plasma based ion implantation (PBII) technique on the surface of NiTi produced by electron beam melting (EBM). The samples were treated for 120min (550°C and 770°C) and 360min (550°C), with 16kV high voltage pulses. The surfaces were analysed by X-ray diffraction (XRD), white light interferometry (WLI) and glow discharge optical emission spectrometry (GDOES). In addition to martensitic and austenitic NiTi, the XRD analysis shows the appearance of titanium nitride TiN and Ni 3Ti phase in the PBII treated samples. Whereas the untreated specimen presents a RMS roughness R RMS around 16nm, the PBII treatments were shown to increase it, from 44nm, at 550°C, to 82nm at 770°C. The nitrogen incorporation ranges from 150nm (at 550°C, whatever the duration), up to more than 500nm (770°C) thanks to temperature activated diffusion. The increase of R RMS may be explained by the sputtering effect but it may be linked to the presence of a buried oxi-nitride phase grown by diffusion at higher temperature. © 2011 Elsevier B.V.

The corrosion properties of three Fe-Mn-Si-Cr-Ni-(Co) shape memory stainless steels were studied based on X-ray photoelectron spectroscopy (XPS) analyses, immersion and polarization tests. The test results were compared with those of a type 304 austenitic stainless steel. The XPS analyses indicated substantial Si content in the anodic passive films formed on shape memory stainless steels in sulfuric acid solution and that the high protectiveness of these films results from a protective film consisting of a (iron, chromium)-mixed silicate. The corrosion rate of the shape memory stainless steels in boiling nitric acid solution was lower than that of austenitic stainless steel. The high silicon content was found to play an important role in the corrosion behavior of these shape memory alloys in highly oxidizing environments. Due to their high manganese content, the shape memory stainless steels showed poor corrosion behavior in 3.5% sodium chloride solution when compared with austenitic stainless steel. © 2012 Elsevier B.V. All rights reserved.

Passive films formed on three Fe-Mn-Si-Cr-Ni-(Co) shape memory stainless steels (SMSSs) were studied based on polarization tests, EIS, XPS and Mott-Schottky analyses. The test results were compared with those of a type 304 austenitic stainless steel. The results indicated that silicon plays an important role in the passive film properties of Fe-Mn-Si-Cr-Ni-(Co) SMSSs. Anodic passive films formed on Fe-Mn-Si-Cr-Ni-(Co) SMSSs are highly protective and consist of a chromium oxyhydroxide with incorporation of silicon in the chemical form of a silicate. Mott-Schottky analyses suggested that passive films on Fe-Mn-Si-Cr-Ni-(Co) SMSSs are less defective and thicker than those on austenitic stainless steel. © 2012 Elsevier Ltd.

Nano-hydroxyapatite (n-HA) crystalline films have been developed in this study by electrodeposition method on NiTi shape memory alloy (SMA). The electrodeposition of the n-HA films was carried out using 0.042 mol/L Ca(NO 3) 2 æ 4H 2O + 0.025 mol/L (NH 4) æ 2HPO 4 electrolytes by applying a constant potential of 22.0 V for 120 min and keeping the solution temperature at 70 °C. The characterization of n-HA films is of special importance since bioactive properties related to n-HA have been directly identified with its specific composition and crystalline structure. AFM, XRD, EDX, FEG-SEM and Raman spectroscopy shows a homogeneous film, with high crystallinity, special composition, and bioactivity properties (Ca/P = 1.93) of n-HA on NiTi SMA surfaces. The n-HA coating with special structure would benefit the use of NiTi alloy in orthopedic applications. © ASM International.

This study was developed to understand the influence of chemical composition and austenitic grain size on the wear resistance in stainless shape memory steel. A two-body abrasive wear device was used to understand the wear mechanism involved. They were tested pins with the following chemical composition: Fe-10.3Mn-5.3Si-9.9Cr-4.9Ni-0.006C and Fe-14.2Mn-5.3Si-8.8Cr-4.6Ni- 0.008C after being austenitized at 900 and 1050 °C, followed by water quenching. The surface characterization was performed by optical microscopy and scanning electron microscopy, and the roughness profile evaluation was also conducted. The weight loss was measured after conducting the wear testing, and the wear rates were estimated. The results demonstrated that the alloy with less manganese and higher chromium content has the best wear resistance (between 17.5 and 18.9%). With an increase of the austenitic grain size there was a small reduction on the wear resistance (between 3.0 and 4.1%). The chemical composition demonstrated to have higher influence on the wear behavior than the austenitic grain size. © ASM International.

NiTi SMA is a promising material in the biomedical area due to its mechanical properties and biocompatibility. However, the nickel in the alloy may cause allergic and toxic reactions and thus limiting its applications. It was evaluated the influence of surface modification in NiTi SMA by nitrogen plasma immersion ion implantation (varying temperatures, and exposure time as follows: <250 °C/2 h, 290°C/ 2 h, and 560°C/1 h) in the amount of nickel released using immersion test in simulated body fluid. The depth of the nitrogen implanted layer increased as the implantation temperature increased resulting in the decrease of nickel release. The sample implanted in high implantation temperature presented 35% of nickel release reduction compared to reference sample. © ASM International.

Aim To evaluate two NiTi wires with different carbon and oxygen contents in terms of mechanical resistance to rotary bending fatigue (RBF) under varied parameters of strain amplitude and rotational speed. Methodology The wires produced from two vacuum induction melting (VIM) processed NiTi ingots were tested, Ti-49.81at%Ni and Ti-50.33at%Ni, named VIM 1 and VIM 2. A brief analysis related to wire fabrication is also presented, as well as chemical and microstructural analysis by energy dispersive spectroscopy (EDS) and optical microscope, respectively. A computer controlled RBF machine was specially constructed for the tests. Three radii of curvature were used: 50.0, 62.5 and 75.0mm, respectively, R1, R2 and R3, resulting in three strain amplitudes εa: 1.00%, 0.80% and 0.67%. The selected rotational speeds were 250 and 455rpm. Results The VIM 1 wire had a high carbon content of 0.188 wt% and a low oxygen content of 0.036 wt%. The oxygen and carbon contents of wire VIM 2 did not exceed their maximum, of 0.070 and 0.050 wt%, according to ASTM standard (ASTM F-2063-00 2001). The wire with lower carbon content performed better when compared to the one with higher carbon content, withstanding 29441 and 12895 cycles, respectively, to fracture. Conclusions The surface quality of the wire was associated with resistance to cyclic fatigue. Surface defects acted as stress concentrators points. Overall, the number of cycles to failure was higher for VIM 2 wires with lower carbon content. © 2011 International Endodontic Journal.

The corrosion behavior of three Fe-Mn-Si-Cr-Ni-(Co) shape memory stainless steels (SMSS) in 0.5 M H2SO4 solution was studied through electrochemical and immersion tests. The test results were compared with that of a type 304 (SS 304) austenitic stainless steel. The three SMSSs exhibited a passive behavior in 0.5 M H2SO4 solution; however, their anodic behavior in the active dissolution region was markedly different. The passive current densities of the SMSSs were similar to that of SS 304, although the critical anodic current required for passivation was higher. The corrosion rate of the SMSSs was much higher than that of SS 304. It was observed that the amount of Cr and Mn plays an important role in the corrosion behavior of SMSSs. The best corrosion behavior in acid media was shown by the SMSS that contained the highest amount of Cr and the lowest amount of Mn. © 2011 Elsevier B.V.

The level of impurities, carbon and oxygen, of the NiTi alloy is usually inherent to the melting process or initial raw material. Associated with the formation of precipitates, TiC and Ti4Ni2O, the impurities in the NiTi alloy influence the temperatures of martensitic transformation (MT), i.e, changing the properties of shape memory effect (SME) in the alloy. In this paper aspects of the influence of impurities, carbon and oxygen, in the process of melting by Vacuum Induction Melting (VIM) and Electron Beam Melting (EBM) and remelting by EBM is analyzed by using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and metalografic analysis in order to better understand the influence of those impurities in the SME of the NiTi alloy.

The shape recovery properties of an Fe-Mn-Si-Cr-Ni-Co based alloy were studied using compression tests. Analyzed were the elastic recovery (Er), shape recovery (Sr) and total shape recovery (TSR = Er + Sr) measurements as a function of training cycles. The results indicated that 3.3 was the best ratio between height (ho = 20 mm) and diameter (φo = 6 mm) to obtain a homogeneous deformation and defined loading curves. A major contribution of TSR was attributed to shape recovery. In the last training cycle, a TSR = 90% was obtained where 25% was attributed to Er.

Within the stainless steel family, austenitic steel can be considered as the one with the best weldability and greatest application in the processing industry. However, the properties of this type of steel can be easily degraded during welding operations or plant service. The solidification cracks can be avoided when there is a moderate presence of Δ ferrite in the welded metal, as described in technical literature. On the other hand, Δ ferrite is seldom discussed when applied to base metals, since austenitic stainless steel, annealed and quenched, probably presents only traces of it at this phase. Forged austenitic stainless steel components have presented high amounts of Δ ferrite, sometimes appearing in the intermetallic phases, creating serious concerns for equipment and pipe manufacturers. Herein, a case study is presented where Δ ferrite and intermetallic precipitates, especially the σ phase, hindered the application of ASTM A182 F 317L forged flanges in processing plants.

In this work, three stainless steel alloys (Fe-Mn-Si-Cr-Ni) with shape memory effects were studied by varying austenitizing time and temperature-from 800°C to 1050°C for 2400s and from 600s to 57600s at 1050°C-followed by a water quenching. Optical microscopy results showed no grain size variation up to 900°C with its average dimensions around 30μm. From 950°C to 1050°C, an increase of grain size was noted when compared to 900°C and less. It is possible to note an increase of twice the grain size from 950°C to 1050°C. The results also show that the influence of the treatment temperature on grain growth seems to be more significant than the time elapsed. Also, it was observed that the grain growth depended on the chemical composition, being lower for the alloy with lower manganese and higher chromium content.

This work presents a hardness study and elastic modulus for the ε-martensite and y-austenite phases of an iron based shape memory alloy. Using instrumented indentation, it was possible to determine the hardness and elasticity modulus of these phases separately. The martensitic phase presented a hardness of 7.0 GPa and the austenitic phase presented a hardness of 3.0 GPa. The elastic modulus values were 202 and 137 GPa for the martensitic and austenitic phases, respectively.

Most of the applications of NiTi SMA are as a wire form. In this sense it is important to know the effects of thermo-mechanical processing such as reduction per pass and intermediate annealing on the wire drawing process. For this work they were produced wire by cold drawing using 15 % area reduction per pass with and without intermediate annealing. The starting ingot was produced by VIM process. The influence of thermo-mechanical processing will be related to the martensitic transformation temperatures. © (2010) Trans Tech Publications.

The rocket propellant ignition system uses electro-explosive device actuated by wire electrode. Those wires are usually made by Fe-Ni based alloy with controlled thermal expansion inserted into a ceramic feed-through and are connected to thin resistive wire which is heated through the passage of an electrical current for propellant ignition. The contact between ceramic feed-through and wires should be reliable since sometimes it could fail. A novel alternative process is to use SMA wires taking into account the shape recovery effect constraining the wire inside the feed-through. The recovery stress of 326 MPa for 4% pre-strain should be enough to constrain the wire inside the feed-trough avoiding the gas leakage. © (2010) Trans Tech Publications.

Earlier works showed that NiTi shape memory alloy production by electron beam melting (EBM) is a viable process in which its main characteristic is the low contamination by impurities such as carbon and oxygen. Some difficulties arise when compared to conventional vacuum induction melting (VIM) process such as composition control and complex machine operation. This work focus on the production of ever made large scale 150mm in diameter NiTi ingot produced by EBM showing its viability. The carbon contamination was only 0.016wt% compared to usual 0.05wt% of VIM process. The ingot radial composition homogeneity was proved by small variation presented by direct and reverse peak martensitic transformations temperatures which was around 2°C.© (2010) Trans Tech Publications.

The main inconveniences of vibration on aeronautical structures are discomfort caused to passenger, possibility of noise generation and fatigue (sonic and mechanical). To minimize the vibrations produced by an engine or flight loads, the aeronautical structures shall have to use shockmounts. An aeronautical structure that absorbs vibration by its own can save weight and consequently fuel and may improve its fatigue life. This work analyzes the metallurgical aspect of damping capacity of stainless SMA produced by the group aiming future application as structural aeronautic material. Copyright © 2008 ASM International® All rights reserved.

The martensitic transformation temperatures and enthalpies of NiTi shape memory alloy strongly depend on the content of nickel and on carbon and oxygen impurities. Nickel stabilizes the high-temperature phase while carbon and oxygen enrich the surrounding matrix with nickel. In this work it is shown that, as a consequence, the martensitic transformation temperatures and enthalpy changes are lower than specially prepared low-contamination reference samples. Furthermore, the enthalpy changes increase linearly with increasing peak martensitic transformation temperature. This means that the lower the enthalpies, the lower is the peak martensitic transformation temperature suggesting that at some lower temperature no phase change should occur. © 2007 Elsevier B.V. All rights reserved.

The aim of this work was to study the effect of austenitic grain size (GS) reduction on the structural parameters of the εhcp -martensite in stainless shape memory alloy (SMA). Rietveld refinement data showed an expansion in c-axis and a reduction in a and b-axis with thermo-mechanical cycles for all samples analyzed. Samples with 75 ≤ GS (μ) ≤ 129 were analyzed. It was also observed an increase of the unit cell volume in this phase with GS reduction. The smallest grain size sample (GS = 75 μm) presented a c/a ratio of 1.649, and approximately 90% of total shape memory recovery.

The purpose of this study was to evaluate the fracture resistance and the fracture surface of NiTi SMA engine-driven endodontics files submitted to clockwise torsion. The maximum angular deflection and the maximum torque were analyzed without axial loading. The helical plastic deformations and the fracture surface morphology were evaluated by Scanning Electron Microscopic. The results showed that there was a significant statistical difference in the maximum fracture torque and no statistical difference for the angular deflection for the analyzed files. In relation to the maximum torque at the instant of the fracture, one of the brand presented better performance than the other. According to Scanning Electron Microscopic evaluation all the files showed ductile fracture morphology.

The shape memory effect of stainless shape memory alloy is associated to non-thermolastic γ (CFC) ↔ ε (HCP) martensitic transformation. Recent results generated by our group have demonstrated that the parent austentie grain size is an important parameter for the degree of shape recovery and also in some others properties such as yield stress, hardness and volume fraction of thermal martensite. Using EBSD, this work shows that besides the above-mentioned parameters, the number of martensite orientation variants decreases when the grain size decreases.

The usual process to produce NiTi shape memory alloys is by vacuum induction melting (VIM) using graphite crucible that contaminates the bath with carbon. The contamination by oxygen comes from residual oxygen inside the melting chamber. A new alternative process to produce NiTi alloys is by electron beam melting (EBM) using water-cooled copper crucible that eliminates the carbon contamination and the oxygen contamination would be minimized due to operation in high vacuum. This work compares the two processes and shows that the carbon contamination is four to ten times lower for EBM compared to VIM products and that the final oxygen content is much more dependent on the starting raw materials. The purity of the final product should be very important mainly in terms of biomedical applications and the contaminations by carbon and oxygen affect the direct and reverse martensitic transformation temperatures. © 2006 Elsevier B.V. All rights reserved.

Based on the fact that the Ti-6A1-4V alloy has good mechanical properties, excellent resistance to corrosion and also excellent biocompatibility, however with low wear resistance, this work aims to test plasma processes or combination of plasma and ion implantation processes to improve these characteristics. Two types of processing were used: two steps PIII (Plasma Immersion Ion Implantation) combined with PN (Plasma Nitriding) and single step PIII treatment. According to Auger Electron Spectroscopy (AES) results, the best solution was obtained by PM for 150 minutes resulting in ∼ 65 nm of nitrogen implanted layer, while the sample treated with PIII (75 minutes) and PN (75 minutes) reached ∼ 35 nm implanted layer. The improvement of surface properties could also be confirmed by the nanoindentation technique, with values of hardness increasing for both processes. AFM (Atomic Force Microscopy) characterization showed that the single step PIII process presented greater efficiency than the duplex process (PIII + PN), probably due to the sputtering occurring during the second step (PN) removing partially the implanted layer of first step (PIII).

Metallurgical grade silicon (MG-Si) is obtained from the reduction of silica (SiO2) in a voltaic arc furnace. The impurities are inherent to the reduction process and they are also dependent on the quality of the initial materials. Among other applications, silicon is used as a substrate for photovoltaic conversion of energy and this conversion is as bigger as greater is the purity of the substrate. Researches are being carried out in some countries with the objective of searching for new processes of silicon purification or new materials that can be used as substrates for energy conversion. In this research, the technique of silicon purification in an electron beam furnace was used, where the melting occurs in a high vacuum and the impurities are extracted by evaporation. MG-Si in bulk form without leaching, with an initial purity of 99.88% in mass and ground, and leached MG-Si, with an initial purity of 99.92%, were used as starting materials. The final purity obtained in both the materials was above 99.999% in mass. These results demonstrate that this process is technically viable, while also eliminating the stages of chemical purification used in other techniques. © 2005 Elsevier B.V. All rights reserved.

Metallurgical grade silicon (MG-Si) is obtained from the reduction of silica (SiO2) in a voltaic arc furnace. The impurities are inherent to the reduction process and they are also dependent on the quality of the initial materials. Among other applications, silicon is used as a substrate for photovoltaic conversion of energy and this conversion is as bigger as greater is the purity of the substrate. Researches are being carried out, in some countries, with the objective of searching for new processes of silicon purification or new materials that can be used as substrates for energy conversion. In this research, the technique of silicon purification in an electron beam furnace was used, where the melting occurs in a high vacuum and the impurities are extracted by evaporation. MG-Si in bulk form without leaching, with an initial purity of 99.88% in mass and ground and leached MG-Si, with an initial purity of 99.92%, were used as starting materials. The final purity obtained, in both materials, was above 99.999% in mass. These results demonstrate that this process is technically viable, while also eliminating the stages of chemical purification used in other techniques. © 2005 Elsevier B.V. All rights reserved.

The usual process to produce NiTi shape memory alloys (SMA) is by vacuum induction melting (VIM) using graphite crucibles which contaminate the bath with carbon. Carbon reacts with titanium precipitating TiC influencing the matrix composition which, in turn, affects the martensitic transformation temperatures. Furthermore, the presence of TiC makes the final product difficult for mechanically process using cold working steps. In this study, we present preliminary results of our attempt to remove the TiC from the melt using ceramic filters during pouring and analyzed the efficiency of the filtering process and some results will be presented. © 2005 Elsevier B.V. All rights reserved.

The usual process to produce NiTi shape memory alloy is by vacuum induction melting (VIM) using a graphite crucible, which causes contamination of the melt with carbon. Contamination with oxygen originates from the residual oxygen inside the melting chamber. An alternative process to produce NiTi alloys is by electron beam melting (EBM) using a water-cooled copper crucible that eliminates carbon contamination, and the oxygen contamination would be minimal due to operation in a vacuum of better than 10-2Pa. In a previous work, it was demonstrated that the technique is feasible for button shaped samples weighing around 30g. The present work presents the results on the scale up program that enables the production of larger samples/ingots. The results are very promising in terms of chemical composition homogeneity as well as in terms of carbon contamination, the latter being four to ten times lower than the commercially-produced VIM products, and in terms of final oxygen content which is shown to depend primarily on the starting raw materials.

This paper presents experimental results relating the initial austenite grain size to bulk hardness, compressive yield stress (σ0.2%) and volume fraction of stress-induced ε martensite. It is shown that the bulk hardness obeys quite closely the Hall-Petch equation while the yield stress, σ0.2%, decreases with decrease of grain size, indicating that the induction of ε martensite mechanically is easier for the materials with finer grains. This fact is corroborated by the observed increase of the volume fraction of stress-induced e martensite with decrease of grain size. Inversely, after shape recovery heating, the volume fraction of residual stress-induced ε martensite increases as the grain size increases, e.g. the increase in grain size hinders the reversible martensitic transformation.

This work presents some preliminary results relating training treatment, training temperature and the formation of α′ martensite to the shape recovery effect of stainless shape memory alloys. For the composition tested, the sample shows some mechanical memory (constant tensile stress at 4% strain and constant yield stress throughout the training cycles) with a very good shape recovery (95% after 4% tensile strain) at a training temperature of 873 K. Its residual strain is related to the generation of perfect dislocations only. For the sample trained at 723 K, the residual strain could be attributed to incomplete reversion of stress-induced ε martensite, in part due to the blocking effect of α′ martensite and also to the generation of perfect dislocations. The influence of α′ martensite on shape recovery is relative and is dependent on training temperature, and the preferential growth of α′ martensite is shown to occur for large grain size. © 1999 Elsevier Science S.A.