Effect of crystal quality on twinning stress in Ni–Mn–Ga magnetic shape memory alloys

Abstract

Low twinning stress (TS) is a prerequisite for magnetic shape memory functionality in ferromagnetic martensites. We compare Ni50Mn28Ga22 (nominal at.%) single crystals from four different producers to reveal the effect of crystal quality on the TS. Near the reverse martensite transformation, the TS is generally low, about 1 MPa, regardless of mosaicity of up to 1.7° and chemical composition deviations of up to 2 at.% of Mn. Pure type I and type II twin boundaries occur in crystals with smooth chemical composition gradients. The corresponding temperature dependences of TS follow universal linear trends with the slopes of −0.045 MPa/K for type I twins and −0.001 MPa/K for type II twins, enabling a very low TS down to 2 K for the latter. An intermediate slope of −0.023 MPa/K appears for mixed type I/type II twin boundaries in a crystal with sharp local chemical deviations. We conclude that the crystal quality affects the magnitude of the TS indirectly, through its impact on the character of the twin boundaries. The effect is weak near the martensite transformation temperature and strong at low temperatures.

Keywords

• Magnetic shape memory
• Twinning stress
• Crystal mosaicity
• Microstructure
• Single crystal
• Ni–Mn-Ga