Research on Novel Multi-step Martensitic Transformation in NiTi Shape Memory Alloys

The martensitic transformation is the intrinsical cause of NiTi shape memory alloy(SMA)with ex- cellent shape memory effect(SME)and superelasticity.In general,NiTi SMA exhibits one step(B2-B19')or two step (B2-R-B19')martensitic transformation behavior.However,NiTi SMA would show some novel multi-step martensitic transformation behaviors under certain condition.This paper reviews all types of multi-step martensitic transformation behaviors,and their forming conditions and mechanisms.An abnormal three-step martensitic transformation behavior (B2-R-B19' and B2-B19')is found in Ni-rich Ti-Ni alloys when aged at intermediate temperatures(400～500℃).This strange behavior is due to preferential precipitation of Ni_4 Ti_3 phase at grain boundary.However,it is found that R-phase transformation occurs in two steps,when aged at low temperatures(200～300℃).The abnormal two-step R-phase transformation is attributed to a large-scale heterogeneity of Ni-content between grain boundary region and grain interi- or,which results from the preferential precipitation in grain boundary regions.