Pseudoelasticity in Co-Ni-Al single and polycrystals

The pseudoelastic responses of heat-treated CoNiAl single crystals with [0 0 1] and [1 1 5] orientations, multicrystals of nominally [1 2 3] orientation, and polycrystals are investigated under tension and compression stress states. The highest transformation strains are found under pseudoelasticity for the [0 0 1] orientation in tension and compression as 6.2% and 4.1% respectively. Experiments reveal tension-compression asymmetry of the critical transformation stress and of the stress hysteresis. The pseudoelastic stress-strain response is limited in tension to a much narrower temperature range than that in compression. The levels of strain recovery and the size of the stress hysteresis reveal the influence of dissipative mechanisms. A thermodynamics framework is proposed for describing the role of plastic flow and the internal stress fields on the stress hysteresis behavior. Single crystals exhibit considerable recovery compared to polycrystals and multicrystals. Extensive transmission electron microscopy results confirm the increased plastic flow at the austenite-martensite interfaces and in the secondary phase.