Abstract
This study investigated phase transformation of Ni50Mn25Ga17Cu8-xZrx (x = 0, 4, 8) alloys after aging and ball milling. For the Cu4Zr4 and Zr8 dual phase alloys, aging has enhanced magnetic susceptibility and magnetic exchange of the matrix, resulting in an increase of the Curie temperature of austenitic matrix. The decrease of martensitic transformation temperatures for the Cu4Zr4 alloy and the increase of martensitic transformation temperatures for the Zr8 alloy after aging should be related to the dissolution and precipitation of the second phase in the matrix, respectively. Ball milling is effective to smash the Cu4Zr4 and Zr8 alloys to fine particles, but cannot fracture the Cu8 alloy to particles, indicating an inherent high ductility and strength of the Cu8 alloy. Therefore, the macroscopic brittleness of the polycrystalline Cu8 alloy was mainly caused by the weak grain boundaries. For the Cu4Zr4 and Zr8 particles, the martensitic transformation and Curie transition of austenitic matrix disappeared and the Curie transition of second phase remained after ball milling. After post-annealing at 800 °C, the Curie transition of austenite was recovered due to the restoration of atomic order, but the martensitic transformation cannot be retrieved which might be caused by the grain refinement of the austenitic matrix after ball milling.
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