On the synthesis, structural transformation and magnetocaloric behavior of Ni37.5Co12.5Mn35Ti15 melt-spun ribbons

We fabricated Ni37.5Co12.5Mn35Ti15 melt-spun ribbons at linear wheel speeds (WS) of 20 and 8 ms-1 (series A and B, respectively). The effect of a short time thermal annealing (30 min.) between 1023 K and 1173 K on the martensitic-like structural transition and the crystal structure, microstructure and magnetic entropy change ΔSM(T) curves and related parameters for as-solidified (AS) samples of series A were studied. Whereas the Curie temperature of austenite (AST) TCA keeps nearly constant, both the reduction of the solidification rate and the increase on the thermal annealing temperature increase the temperature of the structural transformation reducing the magnetization change across the AST to martensite (MST) transition. The martensitic transformation (MT) in AS samples undergoes from a B2-type ferromagnetic (FM) AST with TCA = 328 K to a monoclinic martensite (MST); SEM images evidenced a partially grain-oriented microstructure formed by columnar in shape-elongated grains with their major axis oriented along the thermal gradient during solidification. Magneto-structural transition for AS ribbons of series A occurs in the vicinity of room temperature and is accompanied by a magnetization change of around 63 Am2kg-1. For a magnetic field change of 2 T these samples showed a maximum magnetic entropy change |ΔSM|max of 13.8 (9.5) J kg-1 K-1 for the MST→AST (AST→MST) transformation. This is below the previously reported for this alloy composition (27.2 J kg-1 K-1), and is related to the broader magneto-structural transition.