Understanding modulated twin transition at the atomic level

The modulated martensitic variants in a Ni–Mn–Ga ferromagnetic shape memory alloy have been characterized in detail by transmission electron microscopy (TEM). Electron diffraction confirms that at room temperature most of the martensite in this alloy exhibits the 5-layer modulated structure. Trace analysis indicates that the self-accommodated variants have a coherent twinning plane of {12¯5¯} relative to the martensitic phase. One of the twin relations was determined to be (12¯5¯)A//(12¯5¯)C with [2¯1¯0]A//[210]C, revealing the presence of a reflection twin. Based on TEM experimental observations, a model describing the reorientation of the twinned variants on an atomic scale was developed to explain the reversible motion of the boundary. The twin transition between the variants with modulated structure is shown to involve deshuffle on the basal plane of the original variant, atomic displacement parallel to the twin boundary and shuffle again on the basal plane of the product variant.