Graded phase structure in the surface layer of NiTi alloy processed by surface severe plastic deformation

The graded phase structure and its evolution in the surface of NiTi alloy after surface mechanical attrition treatment are studied using transmission electron microscopy. The initial B2–B19′ phase transformation occurs at low strain level. As the deformation strain increases, the B19′ martensite phase transforms back to B2 phase giving rise to graded phases. Our study reveals that stress induced martensite transformation and deformation heating play important roles in the formation of this graded phase structure.