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 al loys having different martensite transformation start (M-3) temperatur es. The NiTi-I alloy (M-s approximate to + 27 degrees C) impacted at r oom temperature at 2.0 and 2.7 GPa tensile stress-pulse magnitude, sho wed acicular martensite morphology. These martensite needles had a sub structure containing microtwins, typical of ''stress-assisted'' marten site. The NiTi-II alloy (M-s approximate to -45 degrees C) showed no m artensite formation when shocked with tensile-stress pulses of 2 GPa. For tensile stresses of 4.1 GPa, the alloy showed spall initiation nea r the region of maximum tensile-stress duration. In addition, monoclin ic martensite needles, with a well-defined dislocation substructure, t ypical of ''strain-induced'' martensite, were seen clustering around t he spall region. No stress-assisted martensite was formed in this allo y due to its very low M-s temperature. The present article documents r esults of the use of a metallurgical technique for generating large-am plitude tensile stress pulses of finite duration for studies of phase transformations involving changes from a high density to a low density state.