Effect of Temperature on the Fracture Toughness of a NiTiHf High Temperature Shape Memory Alloy

The fracture toughness of Ni50.3Ti29.7Hf20 high temperature shape memory alloy was systematically investigated as a function of temperature. A set of nominally isothermal fracture toughness tests were conducted on disk-shaped compact tension specimens at five temperatures corresponding to three thermodynamical conditions: (i) below martensite finish temperature to obtain the fracture toughness of martensite (ii) above martensite start temperature in austenite but below the martensite desist temperature (Md, the temperature above which the austenite does not transform), in order to find the fracture toughness when stress induced martensitic (SIM) transformation takes place close to the crack tip, and (iii) above Md, in order to obtain the fracture toughness of austenite. The extent of the inelastic zone near the crack tip was detected using digital image correlation, and the fracture surfaces were examined. The fracture behavior was highly temperature/phase dependent. The fracture toughness of the transforming material was higher than that of austenite and martensite, i.e. SIM transformation acts as a toughening mechanism. This was attributed to the differences in strain hardening behavior in detwinning, martensitic transformation, and plastic deformation regimes of the stress–strain response, where SIM transformation occurs with the lowest strain hardening rate. The fracture toughness values obtained here are lower than those of equiatomic NiTi.