Observation on rate-dependent cyclic transformation domain of super-elastic NiTi shape memory alloy

Formation and evolution of transformation domains in the super-elastic NiTi shape memory alloy (SMA) subjected to the strain- and stress-controlled cyclic tension-unloading tests were observed by using a non-contact digital image correlation method. The effect of loading rate on the transformation domains of NiTi SMA sheet was investigated. It was found that the number of transformation domains increases monotonically with the increasing loading rate. At relatively low loading rate, the deformation mode changes from a localized one to a homogeneous one with the increasing number of cycles. The residual martensite domains occur in the cyclic tests of the NiTi sheet at a high loading rate due to the hindrance of an increased dislocation density to the reverse transformation from the martensite to austenite phases. Moreover, the strain and temperature distributions obtained in the stable stages of stress-controlled cyclic tests are more uniform than that in the strain-controlled ones. The competition between transformation heat production and heat exchange results in the rate-dependent transformation domain, which reaches an equilibrium state in the stable cycle.