Ni-Mn-Ga and faceted bicrystals

February 8, 2011

[1] Determination of the orientation relationship between austenite and incommensurate 7M modulated martensite in Ni–Mn–Ga alloys

Z B Li et al

For Ni–Mn–Ga ferromagnetic shape memory alloys, a large magnetic-field-induced strain could be reached through the reorientation of martensitic variants in the martensite state. Owing to the collective and displacive nature of the austenite to martensite transformation, a certain orientation relationship (OR) between the parent and the product phase is required to minimize the transformation strain and the strain energy generated, which brings about self-accommodating groups of martensitic variants with specific orientation correlations. In this work, the microstructural and crystallographic characteristics of martensitic variants in a polycrystalline Ni50Mn30Ga20 alloy were investigated by electron backscatter diffraction analysis. With accurate orientation measurement on inherited martensitic variants, the local orientations of parent austenite grains were predicted using four classical OR for the martensitic transformation. Furthermore, a specific OR, namely the Pitsch relation with (1 0 1)A//(1 View the MathML source View the MathML source)7M and [1 0 View the MathML source]A//[View the MathML source View the MathML source 1]7M, was unambiguously determined by considering the magnitude of discontinuity between the lattices of the product and parent phases and the structural modulation of the incommensurate 7M modulated martensite. The present procedure to determine the OR, without recourse to the presence of retained austenite, is in general applicable to a variety of materials with modulated superstructure for insight into their martensitic transformation processes.

[2] Size and shape evolution of faceted bicrystal nanoparticles of gold on sapphire

O Malyi et al

We produced an array of Au nanoparticles on the basal plane of sapphire via solid-state dewetting of a thin film, and followed the size and shape evolution of individual particles during a 950 °C anneal in air. The particles exhibited {1 0 0} and {1 1 1} facets and shapes that tend to be far from those predicted based upon equilibrium considerations. Most of the single-crystal particles exhibited remarkable size and shape stability up to the longest annealing time, 65 h. The bicrystal particles rapidly transformed into single crystals after the shortest annealing time, 1 h. This transformation was accompanied by an apparent rotation of one of the grains and its alignment along its immobile counterpart. We interpreted both phenomena in terms of very slow mass transport along the singular {1 1 1} and {1 0 0} facets. In the case of bicrystal particles, the fast migration and escape of the boundary from the particle changed the crystallography of the facets in the grain swept by the boundary from a crystallographically singular orientation to a non-special orientation. Such surfaces exhibit high diffusivity and, hence, rapid shape evolution via curvature-driven surface diffusion. A quantitative model of this rotation was proposed and showed to confirm our qualitative description. Based on these findings, we conclude that grain-boundary migration, rather than surface diffusivity, controls the kinetics of mid- and late-term agglomeration of thin polycrystalline films of Au.

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One Response to “Ni-Mn-Ga and faceted bicrystals”

  1. Hylke Koers Says:

    I noted the “View the MathML source” tags in this post, and just wanted to point your attention to MathJax, a piece of open-source software that allows you to render MathML source on your blog and thereby help you make your blog entries more useful for your readers. There is a WordPress plug-in available for easy installation; see http://www.mathjax.org and http://wordpress.org/extend/plugins/mathjax-latex/

    Hope this is useful!


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