Comparison of Highly Mobile Twin Boundaries in Cu–Ni–Al and Ni–Mn–Ga Shape Memory Single Crystals

Abstract

We compared twinning systems in Cu–Ni–Al and Ni–Mn–Ga single crystals from macroscale down to atomic scale. Using a newly developed formalism, we studied the twinning stress or mobility and microstructure of the equivalent twin boundaries. In Cu–Ni–Al, compound twinning exhibits the twinning stress of 1–2 MPa, and Type II twinning stress is approximately 20 MPa, which is much higher than the twinning stress for Type II in Ni–Mn–Ga (0.1–0.3 MPa). No temperature dependence was found for twinning stress of Type II in either alloy. Transmission electron microscopy revealed that in contrast to Ni–Mn–Ga, there was no internal twinning in Cu–Ni–Al, only stacking faults. The highest density of stacking faults was observed in the presence of Type I twin boundaries. The extremely low twinning stress may be associated with the deep hierarchy of twinning in Ni–Mn–Ga.