Stress-induced martensitic transformation behavior of a Ti-Ni-Hf high temperature shape memory alloy

The stress-induced martensitic transformation behavior and the microstructure of stress-induced martensite (SIM) in a Ti36Ni49Hf15high temperature shape memory alloy (SMA) have been investigated using tensile tests and transition electronic microscopy (TEM) observations. It shows that, compared with the TiNi SMAs, there is no stress plateau in the stress-strain curve as the TiNiHf alloy deformed in an austenite. The martensitic transformation enthalpy is calculated to be - 106.84 cal/mol. The martensite variants mainly show preferential oriented morphologies for the TiNiHf alloy deformed to 8% at 523 K. The substructure of SIM and deformed SIM in the present alloy are (001) compound twin. Martensite variants are (011) type I twin related. Further, increasing the deformation temperature and deformation strain, the preferential oriented SIM variants gradually develop to martensite variants with variant-crack/variant-intersect morphologies. The inexistence of stress plateau in the stress-strain curve of TiNiHf may mainly result from the dislocation slip during the stress-induced martensitic transformation.