The role of quenched-in disorder in polycrystalline bulk and melt-spun ribbons of Ni50Mn29Ga21

Investigations on the properties of polycrystalline bulk and melt-spun ribbons of composition Ni50Mn29Ga21 are contrasted. Both samples are subjected to quenching after melting and then were homogenized at the same temperature. Though the electron per atom ratio is unchanged, the quenched-in disorder has caused the structural and magnetic transformations to occur in close proximity in the ribbon giving rise to magnetic entropy change of -9.2 J/kg-K at the martensitic transformation. The bulk alloy showed significantly high magnetic field-induced strain of 0.7% at 5 kOe magnetic field and an apparent compressive strain of 5.3% at 40 MPa compression stress, without training the sample. Large grain size and micro-twinning are observed in the bulk, while nano-scale twinning is seen in the ribbon. The variations in the process altered the degree of ordering, which modified the transformation temperatures, the crystal structure, the microstructures of the ribbon and the mechanical properties of the bulk specimen significantly.