Microstructure evolution during fabrication of NI-TI-NB SMA wires

Abstract

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.

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