Stress-induced martensitic transformation and microstructure of a Ti36Ni49Hf15 high-temperature shape-memory alloy

The stress-induced martensitic transformation behavior and the microstructure of stress-induced martensite (SIM) in a Ti36Ni49Hf15 high temperature shape memory alloy (SMA) have been investigated using tensile tests and transmission electron microscopy (TEM) in this paper. It shows that, compared with TiNi SMAs, there is no stress plateau in the stress-strain (S-S) curve as the TiNiHf alloy deformed in austenite. The martensitic transformation enthalpy is calculated to be 106.84cal/mol. The martensite variants mainly show preferential oriented morphologies for the TiNiHf alloy deformed to 8% at 523k. The substructure of SIM and deformed SIM in the present alloy is (001) compound twin. Martensite variants are (011) type I twin related, Further increasing the deformation temperature and strain, the preferential oriented SIMs gradually develop to martensite variants with variant-crashed / variant-intersected morphologies. The inexistence of stress plateau in the S-S curve of the TiNiHf alloy may mainly result from the dislocation slip during stress-induced martensitic transformation.