Abstract
Diffusionless phase transitions are at the core of the multifunctionality of (magnetic) shape memory alloys, ferroelectrics and multiferroics. Giant strain effects under external fields are obtained in low symmetric modulated martensitic phases. We outline 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 act down to the atomic scale. Hence, a martensitic microstructure of nanotwinned tetragonal martensite can form. Coarsening of twin variants can reduce twin boundary energy, a process we could observe from the atomic to the millimetre scale. Coarsening is 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 is released by the formation of a ‘twins within twins’ microstructure that can be observed from the nanometre to the millimetre scale. This hierarchical twinning results in mesoscopic twin boundaries. Our analysis indicates that mesoscopic boundaries are broad and diffuse, in contrast to the common atomically sharp twin boundaries of tetragonal martensite. We suggest that the observed extraordinarily high mobility of such mesoscopic twin boundaries originates from their diffuse nature that renders pinning by atomistic point defects ineffective.
Highlights
Often martensitic microstructures appear more like modern art than physics
This hampers the understanding of martensitic materials like magnetic shape memory alloys [1], ferroelectrics [2] and other multiferroics [3], since their multiscale microstructure is crucial for their functional properties
When considering all possible combinations of martensitic lattice constants in the sample measured by X-ray diffraction (XRD), the c/a of 0.909 obtained from surface topography can only be caused by a c14M a14M twin boundary (c14M/a14M = 0.91)
Summary
While the rigorous mathematical description of these complex microstructures is art on its own, there are only few cases where a nontrivial martensitic microstructure can be illustrated in an intuitive way This hampers the understanding of martensitic materials like magnetic shape memory alloys [1], ferroelectrics [2] and other multiferroics [3], since their multiscale microstructure is crucial for their functional properties. To understand different types of twinned martensitic microstructures we use epitaxial films made from Ni-Mn-Ga magnetic shape memory alloy as a model system. This archetypical ferromagnetic Heusler alloy undergoes a martensitic phase transition which produces a microstructure with twin variants of different crystallographic orientation. The similarities to other modulated phases (in particular in ferroelectrics) are discussed to illustrate the universality of our conclusions
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