Cyclic deformation and austenite stabilization in Co35Ni35Al30 single crystalline high-temperature shape memory alloys

This study reports on the role of repeated stress-induced martensite (SIM) transformations on the pseudoelastic (PE) behavior of solutionized Co35Ni35Al30 [0 0 1]-oriented shape memory single crystals under both isothermal and non-isothermal conditions (referred to as “training”). It is demonstrated that training results in austenite stabilization and strengthening, consequently increasing the critical transformation stress levels for the SIM (σcritSIM), and promoting excellent cyclic stability and reproducibility of the PE response. This is attributed to the formation of dense dislocation arrangements and fine coherent sub-nanometer hexagonal close-packed Co and γ′ (Ni3Al:L12) precipitates during training. The training that involved cyclic loading conditions was more effective than the monotonic stress–strain tests at different temperatures in modifying SIM characteristics bringing about (i) a large pseudoelastic temperature range of 350 °C with σcritSIM levels reaching 1 GPa with complete recoverable strains of 2%, and (ii) excellent stable cyclic PE response at temperatures as high as 250 °C.