Transformation pattern of shape-memory NiTi alloy during stress-biased thermal cycling.

Despite the superelastic deformation of NiTi has been documented and analyzed elaborately, its shape-memory behavior during stress-biased thermal cycling has not been thoroughly unveiled. This paper examines the evolution of transformation pattern in NiTi upon thermal cycling over a wide range of biasing stress. For the present shape-memory NiTi, both forward and reverse transformations proceed via the growth of localized deformation band (LDB) under low biasing stress (σ bias ≤ 150 MPa), while LDB only appears during forward transformation and reverse transformation is uniform under high biasing stress (σ bias ≥ 200 MPa). This is the first time that delocalization (conversion of deformation mode from localization to homogeneity) is observed during stress-biased thermal cycling. We clarify that the intrinsic undercooling/overheating of martensitic transformation results in unstable thermomechanical response, and it is the origin of localization in shape-memory NiTi. It is found that transformation-induced plasticity (TRIP), which is dominated by dislocation slip and deformation twining, not only causes irreversibility in the present shape-memory NiTi but also leads to delocalization through stabilization of intrinsic thermomechanical response of reverse transformation. • The first time that delocalization is observed under stress-biased thermal cycling of shape-memory NiTi. • TRIP dominated by dislocations and deformation twins causes irreversibility in the present shape-memory NiTi. • The intrinsic undercooling/overheating of martensitic transformation is the origin of localization in shape-memory NiTi. • Stabilization of intrinsic thermomechanical response by TRIP leads to delocalization during stress-biased thermal cycling. [ABSTRACT FROM AUTHOR]