TEM study of pseudo-twin in an off-stoichiometric Ni2MnGa

We have investigated twin-like microstructure of the martensite phase in a Ni55.5Mn20.0Ga24.5 (at.%) alloy by using transmission electron microscope (TEM). The bright field image of the martensite phase showed striped microstructure composed of two crystals. Assuming that both crystals have orthorhombic structures, we calculated the axial ratios c/a and b/a from the 1 1 1 diffraction patterns by using angles between reflections. As a result, we found that the axial ratios of one crystal are slightly different from those of the other crystal. This result implies that the twin-like (striped) microstructure of Ni55.5Mn20.0Ga24.5 is not true twin but is pseudo-twin.

► In Ni–Mn–Ga alloy which has transformation temperature around room temperature, it is difficult to observe in static by using transmission electron microscope.
► In a condition that the magnetic field in TEM hardly influences to the sample transformation, the TEM bright field micrograph of uniform texture in Ni55.5Mn20.0Ga24.5 (TM at 360 K) was captured along [0 1 0] and [1 1 1] direction.
► This texture was revealed that a twin-like banded structure composed of two orthorhombic lattice.
► The precisely measuring of angles subtended by diffraction spots on the [1 1 1] diffraction pattern revealed that two crystals which constructed a twin-like structure have different crystal structure in a microscopic region.