Abstract
Two bending tests around two perpendicular axes were applied to 10M Ni-Mn-Ga single crystals with five-layered modulated structure. The crystal structure and microstructure evolution were examined using synchrotron radiation and electron backscatter diffraction, respectively. The bend stress results in pseudoelasto-plastic strain due to {101) twins tapering. A close examination of the microstructure reveals an additional pattern indicating microstructural changes in the form of {110) twins. As bending proceeds the {110) twins branch undergoing a significant twin refinement. Additionally, an elastic change of lattice parameters is confirmed yielding a higher total pseudoelastic strain. Unloading restores the initial twin configuration removing a large amount of the {110) twin boundaries, however, this process is followed by incomplete recovery since the samples do not retain its original shape entirely. The paper underlines the differences in mechanism for bending around two perpendicular axes explaining the amount of strain observed under pseudoelastic deformation. Additionally, the results are discussed with respect to minimization of elastic energy due to twin refinement and branching as well as mobility of the {101) and {110) twin boundaries.
Highlights
Low twining stress is an outstanding property of Ni-MnGa magnetic shape memory (MSM) single crystals [1,2,3,4,5,6,7,8]
Easy twin boundary motion leads to reorientation of variants resulting in high permanent strains which can be obtained by a mechanical loading or magnetic field
One of the possible explanations is given by Seiner et al where the high mobility of twin boundaries in 10M Ni-Mn-Ga structure is related with a complex crystal structure of 10M martensite and the order of compatibility between the twinned variants [17]
Summary
Low twining stress is an outstanding property of Ni-MnGa magnetic shape memory (MSM) single crystals [1,2,3,4,5,6,7,8]. The suggested model considered the modulated monoclinic 10M structure with a very small difference in the a and b lattice parameters and the resulting microstructures to be the reasons for high mobility of type II twin boundaries. This correlates very well with other reports given by Faran et al who show some energy barriers, and related with that microstructural elements, the width of which is consistent with distribution of a-b lamination [18,19,20]. It seems that the {110) twins (further referred to as a-b twins) are strongly related with high mobility of {101) twin boundaries [21,22,23]
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