Investigation on Microstructure and Memory Property of NiTi Single Crystal Shape Memory Alloys

Abstract:

Single crystals of NiTi (with 50.0at% Ni) were prepared in a self-designed unidirectional solidification furnace with a graphite mold shell connected with a crystal selector. Both macrostructure and microstructure of the alloy were observed by OM and SEM with EDS, the grain orientation of single crystal was determined by XRD and pole figures, and shape memory effect was evaluated by an instrument developed by ourselves. The results show that dendritic features were presented in both cross section and longitudinal section; the direction of dendrites growth were close to solidified direction; there was no obvious grain boundary observed; the angle between normal direction of cross section and [111] orientation was 15°. Compared with normal NiTi polycrystals, the maximal recoverable strain and memory fatigue property of the alloy were improved because the bad effect of grain boundary on the martensite phase transformation was eliminated, and also [111] orientation was a preferred orientation of NiTi alloy which can provide larger recoverable strain.

Key words:NiTi shape memory alloy; single crystal; maximal recoverable strain; memory fatigue property