Abstract
Metamagnetic shape memory alloys are a unique class of materials capable of large magnetic field-induced strain due to reverse martensitic phase transformation. A precondition for large shape change is martensite deformation, which heavily depends on microstructure. Elucidation of microstructure is therefore indispensable for strain control and deformation mechanics in such systems. The current paper reports on a self-accommodated martensitic microstructure in metamagnetic Ni50Mn37.5Sn12.5 single crystal. The microstructure here is hierarchically organised at three distinct levels. On a large scale, martensite plate colonies, distinguished by intercolony boundaries, group individual martensitic plates. Plates are separated by interplate boundaries and deviate by 2.2° from an ideal twin relation. On the lower scale, plates are composed of subplate twins. Conjugation boundaries separating two pairs of twins arise in relation to a subplate microstructure. Modulation boundaries separating two variants with perpendicular modulation directions and with parallel c-axes also appear. Mechanical training frees larger plates from fine subplate microtwins bringing macro-lamellae into twin relation, what then permits further detwinning until a single variant state.
Highlights
Research into Ni–Mn–(Sn, In, Sb) metamagnetic shape memory alloys has flourished since the discovery of the magnetic field-induced shape recovery by reverse martensitic phase transition in Ni–Co–Mn–In alloy [1]
Straight lamellae run across the entire colony, while bent and bifurcated ones bearing signs of pronounced branching terminate at a shorter distance from intercolony boundaries (ICBs)
Ni50Mn37.5 Sn12.5 single crystal transforms into a martensite state with three distinct levels of hierarchically organised microstructure
Summary
Research into Ni–Mn–(Sn, In, Sb) metamagnetic shape memory alloys has flourished since the discovery of the magnetic field-induced shape recovery by reverse martensitic phase transition in Ni–Co–Mn–In alloy [1]. On a mesoscopic scale the resultant martensite phase shows a hierarchical, self-accommodated microstructure composed of a mixture of different symmetry-related martensite variants organised at various length measures in order to reduce the overall transformation strain [9,10,11].
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