Abstract
Heusler alloy, Ni2MnGa, and its off-stoichiometric solid solutions have great potential applications in refrigeration technology due to their magnetocaloric effect (MCE), which can be strongly influenced by martensitic phase transformations and related crystallography. In this study, five polycrystalline alloys with nominal compositions of Ni2+x Mn1−x Ga (x = 0.14, 0.16, 0.19, 0.22, and 0.24) were prepared by triple arc-melting and characterized by differential scanning calorimetry for measurement of martensitic transformation temperature, optical microscopy for microstructural observation, and X-ray diffraction and transmission electron microscopy (TEM) for detailed microstructural and crystallographic analyses. Martensitic transformation temperature was closest to room temperature for the Ni2.14Mn0.86Ga (x = 0.14) alloy, and the transformation temperature increased with an increase in valence electron concentration (e/a). For the Ni2.14Mn0.86Ga (x = 0.14) alloy, extensive TEM analyses confirmed the presence of modulated 7 M martensites, however, only non-modulated (NM) martensites were observed in all other alloys including Ni2.16Mn0.84Ga (x = 0.16) alloy which exhibited a large MCE. Martensites examined by high resolution TEM were highly twinned in nature, and formed nano-scale twins within twinned microstructures.
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