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 [001]-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 M_d-A_f interval and the flow resistance, which are both higher for CoNiAl compared to other shape memory alloys.