Martensite Transformation in Ni-Mn-Ga Ferromagnetic Shape-Memory Alloys
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Martensite Transformation in Ni-Mn-Ga Ferromagnetic Shape-Memory Alloys ´ ZPITA, M.L. RICHARD, J. FEUCHTWANGER, S.M. ALLEN, R.C. O’HANDLEY, P. LA and J.M. BARANDIARAN The crystal structure of Ni-Mn-Ga ferromagnetic shape-memory alloys is extremely sensitive to composition. Several martensitic structures including tetragonal (five-layered), orthorhombic (seven-layered), and nonmodulated tetragonal have been observed. Temperature-dependent X-ray diffraction measurements and calorimetry have revealed markedly different transformation behavior in the tetragonal and orthorhombic materials. The orthorhombic material shows a much larger difference between the martensite start and finish temperatures as compared to tetragonal martensite. The difference in transformation character can be explained from a thermodynamic standpoint by including the difference in the strain energy contribution for the two different martensite phases. I. INTRODUCTION
NI-MN-GA based Heusler alloys have been shown to exhibit the so-called magnetic shape-memory effect (magnetoplasticity) through magnetic field-induced twin-boundary motion. Unlike the thermoelastic shape-memory alloys, which use the austenite to martensite transformation to achieve a large output strain, the magnetic shape-memory effect occurs entirely within the martensitic phase by fieldinduced twin-boundary motion. The maximum strain achievable in these alloys is extremely sensitive to the martensitic crystal structure, which is a strong function of composition. Up to 10 pct field-induced strain has been observed in the orthorhombic/14M martensite,[1] while up to 6 pct has been reported in the tetragonal/5M martensite.[2,3,4] The work reported here details the characterization of the martensitic transformation behavior of a set of alloys whose martensitic transformation temperatures are at or above room temperature. The character of the martensitic transformation of these alloys is of particular technical interest due to the possibility of achieving magnetic field-induced strain at room temperature. It is important to understand the thermodynamics and kinetics of the transformation in order to ensure that the material remains in the martensitic phase throughout the operational temperature range. II.
of 5.0 mm/h. The furnace was backfilled with purified argon to a positive pressure of 6.8 3 105 Pa to minimize evaporation of manganese during crystal growth. Several crystals grown in this way exhibited composition gradients, and thus, many samples, each of a different composition, could be obtained from each large single crystal. Crystals were heat treated for 24 hours at 900 °C under purified argon atmosphere. Individual samples of approximately 2 to 3 mm in length were cut from each crystal and crushed into powder. Powders were annealed at 700 °C for 3 hours to relieve stresses imparted during crushing. Compositions of the powders were measured to ensure homogeneity. Single-crystal discs of 3 mm in diameter were also cut from several single-crystal boules of different compositions. X-ray
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