Cyclic deformation and austenite stabilization in Co 35Ni 35Al 30 single crystalline high-temperature shape memory alloys

Abstract

This study reports on the role of repeated stress-induced martensite (SIM) transformations on the pseudoelastic (PE) behavior of solutionized Co 35Ni 35Al 30 [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 ( σ crit SIM ), 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′ (Ni 3Al:L1 2) 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 σ crit SIM levels reaching 1 GPa with complete recoverable strains of 2%, and (ii) excellent stable cyclic PE response at temperatures as high as 250 °C.