Orientation-dependent superelasticity and fatigue of CuAlMn alloy under in situ micromechanical tensile characterization

The in situ micromechanical tensile tests are conducted to characterize the superelastic behaviors for [223], [1214], [325] and [205] oriented Cu67Al24Mn9 micro-slats. The stress-induced martensitic transformations are captured in all textures corresponding to strong crystallographic anisotropy. We propose a one-dimensional constitutive model considering the directional anisotropy of elastic modulus for cubic symmetry and the crystallographic compatibility of twinned martensite. The modeled mechanical behaviors agree with the micromechanical tensile tests well, which suggests that the formation of compatible twins is the primary deformation mechanism. Particularly in the [205] texture, we observed formation of nano-cavities (<100nm) on the lateral surface of the micro-slat. Based on the analysis of compatible martensite twin laminates and fcc slip systems, we theorize that the massive normal elongation and little lateral shear cause the formation, stretching and growth of nano-cavities at nano scales to accommodate the external loads. As a result, the structural and functional fatigue resistance is improved compared to other textures. The experimental and theoretical results in this paper are potentially useful to guide the texture design of Cu-based shape memory alloy for high transformation strain and low functional fatigue.