Shock-induced martensitic transformations in near-equiatomic NiTi alloys

Shock-impact generated tensile-stress pulses were used to induce B2-to-monoclinic martensitic transformations in two near-equiatomic NiTi alloys having different martensite transformation start (Ms) temperatures. The NiTi-I alloy (Ms ≈ +27 °C) impacted at room temperature at 2.0 and 2.7 GPa tensile stress-pulse magnitude, showed acicular martensite morphology. These martensite needles had a substructure containing microtwins, typical of "stress-assisted" martensite. The NiTi-II alloy (Ms ≈ -45 °C) showed no martensite formation when shocked with tensile-stress pulses of 2 GPa. For tensile stresses of 4.1 GPa, the alloy showed spall initiation near the region of maximum tensilestress duration. In addition, monoclinic martensite needles, with a well-defined dislocation substructure, typical of "strain-induced" martensite, were seen clustering around the spall region. No stress-assisted martensite was formed in this alloy due to its very low Ms temperature. The present article documents results of the use of a metallurgical technique for generating large-amplitude tensile stress pulses of finite duration for studies of phase transformations involving changes from a high density to a low density state.