Self-accommodated and pre-strained martensitic microstructure in single-crystalline, metamagnetic Ni–Mn–Sn Heusler alloy

Journal of Materials Science, Feb 2017

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.

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Self-accommodated and pre-strained martensitic microstructure in single-crystalline, metamagnetic Ni–Mn–Sn Heusler alloy

J Mater Sci Self-accommodated and pre-strained martensitic microstructure in single-crystalline, metamagnetic Ni- Mn-Sn Heusler alloy P. Czaja 1 R. Chulist 1 M. Szlezynger 1 W. Skuza 1 Y. I. Chumlyakov 0 M. J. Szczerba 1 0 Siberian Physical-Technical Institute , Tomsk 634050 , Russia 1 Institute of Metallurgy and Materials Science, Polish Academy of Sciences , 25 Reymonta St., 30-059 Krakow , Poland 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. - 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]. It was further stimulated by the observation of giant magnetoresistance, magnetocaloric and more recently a large elastocaloric effect found in i.a. Ni–Mn–Sn system, which make these materials interesting for variety of functional applications [2–4]. In general, this unique behaviour originates in the thermo-elastic martensitic phase transition (MPT) between austenite and martensite phases, and the driving force is the Zeeman energy (DM H) receiving contribution from the saturation magnetisation difference between the parent and product phase [5]. Earlier studies on Ni50Mn50-xSnx alloys reveal that for critical 5 B x B 25 concentration range, the Heusler L21 austenite phase thermally transforms into martensite, depending on Sn content having 10 M, 14 M, 4O and L10 structures [6–8]. In general, 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 [9–11]. Martensite variants according to the Bain transformation matrix for a typical cubic to tetragonal transformation refer to the different crystallographically equivalent orientations of the tetragonal structure with the c-axis parallel to the three main axes of the cubic austenite. Due to symmetry relations, the number of such possible variants depends on the number of rotations in the austenite and martensite point groups leading to 3 variants for cubic (Fm3m) to tetragonal (I m4 mmÞ, 6 for cubic (Fm3m) to orthorhombic (Pmma) and 12 for cubic (Fm3m) to monoclinic (P2/m) transformations [10]. Detailed understanding of the resulting martensite microstructure is paramount for the control of twin-boundary mobility and thus overall mechanical properties of the low-temperature martensite phase, which by analogy to conventional shape memory alloys requires pre-deformation by an external loading in order to realise the shape recovery accompanied by an output stress. The pre-deformation is mediated by the detwinning mechanism, operational during the training process often applied to harvest a single variant martensite state and conducted by a sequence of uniaxial compression tests along the \001[ directions referred to the austenite phase [12, 13]. The initial self-accommodated microstructure as well as the detwinning process has been elucidated in more detail for magnetic Ni–Mn–Ga alloys, while considerably less attention has been called in this regard to Ni–Mn–(Sn, In, Sb) alloys [14–23]. More recently, attempts to study variant organisation and mechanical detwinning have been performed for Ni50Mn38Sn12 [24] and Ni2Mn1.44In0.56 polycrystalline alloys [25], which were found to contain no nanotwins inside larger, misoriented plates. In the current paper, the self-accommodated and pre-strained microstructure is disclosed in Ni50Mn37.5Sn12.5 single crystal with a modulated 4 M martensite structure, which is capable of near the theoretical limit 7.9% longitudinal strain [26]. It is demonstrated that l (...truncated)


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P. Czaja, R. Chulist, M. Szlezynger, W. Skuza, Y. I. Chumlyakov, M. J. Szczerba. Self-accommodated and pre-strained martensitic microstructure in single-crystalline, metamagnetic Ni–Mn–Sn Heusler alloy, Journal of Materials Science, 2017, pp. 5600-5610, Volume 52, Issue 10, DOI: 10.1007/s10853-017-0793-3