Cyclic deformation and austenite stabilization in Co_(35)Ni_(35)Al_(30) single crystalline high-temperature shape memory alloys

摘要

This study reports on the role of repeated stress-induced martensite (SIM) transformations on the pseudoelastic f PE) behavior of solu-tionized Co_(35)Ni_(35)Al_(30) 0 0 l-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 (sigma_(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 gamma' (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 deg C with sigma_(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 deg C.