On the volume change in Co–Ni–Al during pseudoelasticity

CoNiAl alloys are a new class of shape memory alloys, which exhibit pseudoelastic strains as high 6% over a broad range of temperatures. Based on the crystallographic lattice constants, a substantial volume change upon transformation is expected at the mesoscopic level, yet it has not been measured previously. Transformation strains are established in three mutually orthogonal directions in the [0 0 1]-oriented CoNiAl single crystals under compression. Experiments reveal that the transformation volume change is approximately 2% based on determination of strains on transformed and untransformed locations. Despite the high volumetric strain, the pseudoelastic stress–strain response represents full recoverability with small stress hysteresis. Additional factors that influence pseudoelasticity behavior are discussed particularly the Md − Af interval and the flow resistance, which are both higher for CoNiAl compared to other shape memory alloys.