Twin boundary structure of the modulated variants in a Ni–Mn–Ga alloy

Han, Ming; Kong, Fanping

doi:10.1016/j.jallcom.2007.04.025

Cited by 12 publications

(6 citation statements)

References 29 publications

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“…They suggested that some rearrangements can reduce the misfit, but still the incompatible twin boundary is expected to have an unfavourable high energy -in agreement with the consideration in Chapter 8. Han et al [51,52] examined mesoscopic twin boundaries by TEM. Though they describe these boundaries as rather flat on the nanometer scale, the micrographs reveal fringes with a thickness of up to several nanometres.…”

Section: Mobility Of Mesoscopic Of Twin Boundariesmentioning

confidence: 99%

Modulated martensite: why it forms and why it deforms easily

et al. 2011

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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 for their functional properties by analysing the martensitic microstructure of epitaxial Ni-Mn-Ga films from the atomic to 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 retaining 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 which can be observed from the nanometre to 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 observed extraordinarily high mobility of such mesoscopic twin boundaries originates from their diffuse nature which renders pinning by atomistic point defects ineffective.

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Section: Mobility Of Mesoscopic Of Twin Boundariesmentioning

confidence: 99%

Modulated martensite: why it forms and why it deforms easily

et al. 2011

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“…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][15][16][17][18][19][20][21][22][23]. More recently, attempts to study variant organisation and mechanical detwinning have been performed for Ni 50 Mn 38 Sn 12 [24] and Ni 2 Mn 1.44 In 0.56 polycrystalline alloys [25], which were found to contain no nanotwins inside larger, misoriented plates.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2017

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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 Ni 50 Mn 37.5 Sn 12.5 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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“…Though the martensite structure is controlled to a large extent by the composition, there are compositions around 29-30 at.-% Mn and 20-21 at.-% Ga, for which multiple martensite structures were reported [6,7]. Due to the monoclinic lattice, a variety of twinning modes operate in both 10M and 14M martensites [8,9]. The magnetic-field-induced strain (MFIS) strongly depends on the twin microstructure which can be modified through appropriate training (e.g., [10]).…”

Section: Introductionmentioning

confidence: 99%

Characterizing Twin Structure and Magnetic Domain Structure of Ni-Mn-Ga through Atomic Force Microscopy

Reinhold

Knowlton

Müllner

2013

Icomat

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Ni-Mn-Ga is a ferromagnetic shape memory alloy that deforms by twin boundary motion. The magneto-mechanical properties depend strongly on the twin microstructure. A thermomechanical treatment was applied to a Ni-Mn-Ga single crystal with coexisting 10M and 14M martensite structures to create twin boundaries and align the short crystallographic c direction preferentially perpendicular to the surface. The resulting twin structure was characterized using atomic force microscopy (AFM) to obtain the surface relief caused by twinning. Magnetic force microscopy (MFM) was used to find the direction of easy magnetization (which coincides with the crystallographic c direction) of each twin. Among 18 possible twinning histories, ac twinning in 10M martensite was identified as the unique solution. The combination of AFM and MFM after thermomechanical treatment provides a nondestructive characterization of the twin microstructure including the identification of the crystallographic orientation of each twin and of each twin boundary plane.

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Twin boundary structure of the modulated variants in a Ni–Mn–Ga alloy

Cited by 12 publications

References 29 publications

Modulated martensite: why it forms and why it deforms easily

Modulated martensite: why it forms and why it deforms easily

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

Characterizing Twin Structure and Magnetic Domain Structure of Ni-Mn-Ga through Atomic Force Microscopy

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