Plastic straining effects on the microstructure of a Ti-rich NiTi shape memory alloy

A Ni-52 at, pet Ti shape memory alloy, cold drawn to 30 pet, was annealed a t 1173 K for 1 hour, water quenched, and then subjected to differential sca nning calorimetry (DSC). No evidence of the premartensitic R transformation was found during either the forward or the reverse transformation. Microst ructurally, it was found that the alloy possessed a relatively large volume fraction (similar to0.05) of coarse second-phase brittle particles. These precipitates acted as preferential sites for martensite plate nucleation an d gave rise to a "starlike" morphology. The tensile and compressive propert ies of the alloy in the as-received condition were also investigated. The a lloy exhibited relatively good ductility (fracture strain = 0.28), which wa s attributed to its inherent ability to relieve or delay the development of plastic instabilities through rapid strain hardening. In addition, X-ray d iffraction (XRD) of deformed specimens indicated the presence of an extrain tensity peak corresponding to the B2 phase (110)(B2) when the alloy was pla stically deformed in compression. Accordingly, it is suggested that plastic deformation induces the reverse transformation to the B2 phase in highly s tressed local regions. Transmission electron microscopy (TEM) of deformed m artensite structures showed slip lines probably due to dislocation slip, as well as variant interpenetration. Besides, optical and scanning microscopy of regions adjacent to the fractured surfaces indicated that fine martensi te plates and/or "apparent" new grains develop at regions of prior stress i ntensification (former crack-tip regions) during crack propagation.