Double Twinning in Ni–Mn–Ga–Co

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Magnetic shape-memory alloys tend to deform via magnetic-field-induced and stress-induced twin-boundary motion. The rather low martensite transformation temperature of ternary Ni–Mn–Ga limits the operating temperature for potential applications. By alloying 5 at.% cobalt, the martensite transformation temperature and the Curie temperature was increased from 70 and 110 °C respectively up to 160 °C. In the single crystalline samples two non-modulated structures with tetragonal and orthorhombic lattices were found. The non-modulated orthorhombic structure has similar lattice parameters to the pseudo-orthorhombic 14M Ni–Mn–Ga phase. The single crystal specimen with the non-modulated orthorhombic structure exhibited a cyclic permutation of all three crystallographic axes in response to uniaxial loading. The parallelepiped-shaped sample was compressed repeatedly in all three directions. While maximizing work done by the load during deformation required three different martensite variants to result from deformation in three different directions, only two different martensite variants were found. The analysis of the sample shape revealed two variants mutually related through cyclic permutation of the lattice parameters, which cannot result from a single twinning event. The cyclic permutation is discussed in the light of Crocker’s double twinning mechanism.