In situ multi-fields investigation on instability and transformation localization of martensitic phase transformation in NiTi alloys

Abstract

When NiTi shape memory alloys thin strips under the uniaxial tensile deformation, the stress-induced martensitic transformation tends to exhibit strain localization and instability, and the sample shows reversible transformation bands evolution on macroscopic. The displacement, strain and temperature fields were investigated with in situ optical method, and the full-field strain and temperature information about martensitic localization were quantitatively obtained during uniaxial loading-unloading conditions. Strain field is calculated by digital image correlation (DIC) method, and the temperature field is captured by infrared thermograph, the thickness field and in-plane rotation angle are also calculated by DIC data. The strain, temperature variation, thickness, in-plane rotation both inside and outside of the transformation bands were studied when it nucleation, expansion, combination, reduction and disappear. The results show that transformation strain of the samples are mainly concentrated inside the transformation bands but small outside, and temperature variation mainly concentrated in the transformation fronts, the thickness field in transformation bands is 2% smaller than out of bands. In-plane rotation angle is not only concentrated in the transformation fronts, but also heterogeneous in the transformation bands. In addition, the maximum in-plane rotation angle during tension is 1.5°. The whole loading-unloading progress is full thermal coupling, transformation localization, martensite and austenitic critical nucleation stress are greatly influenced by temperature variation. © Copyright;.