From mechanical behavior and elastocaloric effect to microscopic mechanisms of gradient-structured NiTi alloy: A phase-field study

Abstract

The introduction of gradient structure has been recently reported as a practical way to tailor the mechanical behavior of NiTi alloy while improving its elastocaloric effect simultaneously. In this research, a thermo-mechanically weakly coupled phase-field model considering crystal plasticity is employed to investigate the microscopic mechanisms of the elastocaloric effect related to the gradient structure, especially from the point of view of the two-way interaction between stress and martensite variants. A set of polycrystalline systems are established to accomplish the gradual transition from a fine-grained specimen to a coarse-grained one by the grain-size gradient. The gradually changed mechanical response and phase transformation characteristics during the superelastic simulations embody the effectiveness of the grain structure as a regulator of the material properties. The Brayton cycle simulations conducted next revolve around further validation of the proposed model and variant-related interpretation of the effects of the grain structure. Predictions relevant to histories of stress and temperature agree well with the experimental results, and more remarkably, the linear stress-temperature relationship observed during the stress drop of the first hold in the Brayton cycle conforms perfectly to the classical Clausius–Clapeyron relation. Together with martensite reorientation, the propagating martensite phases, where the increasing non-major variants are necessary complements to the major one preferentially aligned with the external stress, are largely responsible for this stress drop. Regarding the elastocaloric effect, the gradient structures achieve very similar cooling capacity but higher cooling efficiency compared to the coarse-grain one, taking advantage of more martensite, particularly the major variant, produced if provided with the same transformable area (excluding the grain boundaries). Besides, a steeper gradient in structures is prone to alter the composition of martensite variants and more non-major variants are demanded to alleviate the developing deformation mismatch among layers with different grain sizes.