In-Situ TEM Analysis of the Phase Transformation Mechanism of a Cu-Al-Ni Shape Memory Alloy Fabricated with Different Quenching Speeds

A shape memory alloy (SMA) recovers its original shape after deformation through martensitic phase transformation (PT), which is diffusionless and involves shear deformation. Minimizing PT hysteresis is of crucial importance for reliability of SMA-based devices, where the stress caused by thermal hysteresis leads to functional degradation. As a result, understanding structural factors that control PT, including phase nucleation and growth, is critical for development of materials with high reversibility. In this study, the PT mechanism (from γ1' martensite to β1 austenite phase) in Cu-Al-Ni SMAs was investigated by preparing alloy with different quenching rates, resulting in very different transition temperatures. By characterizing the atomic scale composition and microstructure we show that slow quenching induces nanoprecipitation that change chemistry and strain field of the matrix alloy, while fast quenching avoids the formation of these nanoprecipitates. In-situ monitoring of the PT process in TEM demonstrate a growth-dominant conventional PT mechanism with low hysteresis in the fast quench sample. On the other hand, the alloy with precipitates shows a nucleation-dominant PT mechanism with suppression of phase growth that induces high thermal hysteresis. Our finding provides valuable insights into the fabrication of SMAs with better reliability.