Giant strain effects under external fields were obtained in low-symmetry modulated martensitic phases. An outline was given here of the origin of modulated phases, their connection with tetragonal martensite and consequences owing to their functional properties by analysing the martensitic microstructure of epitaxial Ni–Mn–Ga films from the atomic to the macroscale. Geometrical constraints at an austenite–martensite phase boundary acted down to the atomic scale. Hence, a martensitic microstructure of nanotwinned tetragonal martensite could form. Coarsening of twin variants could reduce twin boundary energy, a process which could be observed from the atomic to the mm scale. Coarsening was a fractal process, proceeding in discrete steps by doubling twin periodicity. The collective defect energy results in a substantial hysteresis, which allows the retention of modulated martensite as a metastable phase at room temperature. In this metastable state, elastic energy was released by the formation of a 'twins within twins' microstructure that could be observed from the nanometre to the millimetre scale. This hierarchical twinning results in mesoscopic twin boundaries. The analysis indicated that mesoscopic boundaries were broad and diffuse, in contrast to the common atomically sharp twin boundaries of tetragonal martensite. It was suggested that the extraordinarily high observed mobility of such mesoscopic twin boundaries originated from their diffuse nature, which rendered pinning by atomistic point defects ineffective.

Modulated Martensite: Why it Forms and Why it Deforms Easily. S.Kaufmann, R.Niemann, T.Thersleff, U.K.Rössler, O.Heczko, J.Buschbeck, B.Holzapfel, L.Schultz, S.Fähler: New Journal of Physics, 2011, 13[5], 053029