Reversible detwinning and texture evolution in a nanocrystalline NiTi alloy during deformation

This study investigated the phenomenon of crystallographic and mechanical reversibility of detwinning of martensite in a nanocrystalline Ni_50.8Ti_49.2 alloy wire by means of in-situ high energy X-ray diffraction and phenomenological theoretical analyses. By studying the formation and evolution of crystallographic textures of the R phase and the B19′ phase during pseudoelastic deformation, it was found that for the R phase the reorientation of lattice correspondence variant pairs (CVPs) and detwinning between the variants within a CVP occur concurrently upon loading, forming a single variant R phase prior to the stress-induced R → B19′ martensitic transformation. This texture evolution of the R phase was reversible upon unloading, indicating retwining of the R phase. The stress-induced B19′ phase formed exhibited two textures, signaling the formation of one internally twined CVP. Detwinning of the B19′ variants within the CVP occurred upon further deformation in conjunction with elastic and plastic deformation. The detwinning of the B19′ martensite was also spontaneously reversible upon unloading, in contrast with the common perception that variant reorientation and detwinning are thermodynamically irreversible. This is explained on the basis of local lattice distortions and internal elastic stresses generated as a result of, or as a penalty for, the violation of habit plane requirement caused by variant detwinning within a CVP. These internal lattice stresses serve as the driving force for self-recovery, or retwinning, of the martensite upon unloading. These findings provide a guidance to the interpretation of the mechanical behavior and design of NiTi alloys of ultra-low elastic moduli.