Orientation Dependence of Superelasticity in Ferromagnetic Single Crystals Co49Ni21Ga30
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Dependence of the amount of reversible deformation on the orientation of the crystal axis and testing temperature has been studied using [001] and [ 124] single crystals of the Co49Ni21Ga30 (at %) alloy upon compression. It has been shown that in the [001] crystals with T M s (M s is the temperature of the onset of the forward martensitic transformation upon cooling) the reversible deformation is equal to 5.5-6.5% and consists of the deformation connected with the shape-memory effect (SME) equal to 4-4.2%, and of "ferroelastic" deformation equal to 1.5-2.2%, which is reversible upon unloading. The total reversible deformation exceeds the lattice deformation 0 observed upon the B2-L10 martensitic transformation, which is equal to 4.5%. At T > A f (A f is the finish temperature of the reverse martensitic transformation upon heating), the reversible deformation in [001] crystals is equal to 6.5%. It has been shown electron-microscopically that the reversible deformation equal to 1.5-2.2% in the temperatures range of T = 77-300 K is connected with the development of mechanical twinning in the L10 martensite on (110) L10 planes, which proves to be reversible in the [001] crystals and can be partly irreversible in the [124] crystals. Upon heating, the (110) L10 twins of the stabilized L10 martensite pass into the ( 112) B2 twins of the B2 phase. 2010 Pleiades Publishing, Ltd.
