A micromechanical view of enhanced reversibility in shape memory alloys

Transformation-induced plasticity (TIP) is the source of poor reversibility and functional fatigue in shape memory alloys (SMA)s undergoing martensitic phase transformation. The TIP is believed to originate from the defect generation at the highly-stressed austenite/martensite transition layer. It is suggested that under satisfaction of compatibility criteria (also known as cofactor conditions) the highly-stressed transition layer is removed and the phase transformation reversibility is highly enhanced. In this study, we employ the inclusion problem of micromechanics along with crystallographic theory of martensite to quantify the generated internal stresses associated with the nucleation of martensite in a wide range of SMAs. The micromechanical calculations show that the nucleation of martensite in SMAs with high degree of compatibility and enhanced reversibility generates much reduced internal stresses. For example, the maximum shear stress generated by nucleation of twinned martensite in Ni _50.3Ti _29.7Zr ₂₀ is ∼ 5 GPa, which is reduced to ∼ 400 MPa (about an order of magnitude reduction) in single-variant martensite of Ni ₃₉Ti ₅₀Pd ₁₁ with excellent reversibility and low functional fatigue. It is also shown that the internal stresses are much lower in a supercompatible twinned martensite microstructure (Cu ₂₅Au ₃₀Zn ₄₅) compared with a conventional twinned microstructure such as those formed in NiTi-based alloys. Analysis of the deformation state of the matrix shows that the underlying mechanism behind the reduced internal stresses is the reduction in the maximum shear strain ((λ ₃ - λ ₁)/2) imposed by the nucleation of martensite. The results present a quantified picture of internal (interfacial) stresses in different SMAs to better understand the origin of reversibility and functional fatigue.