Highly oriented crystallization, and tunable structural transformation and magnetic transition in Ni-Co-Mn-Al shape memory alloys

Alloy ribbons of Ni50-xCoxMn50-yAly with x = 5-10 and y = 16-17 were prepared by rapidly solidified technique. Highly oriented column-shaped crystalline grains are observed in the alloy ribbons. The formation of martensitic (L10) and austenitic (B2) phases and their structural transformation are strongly dependent on the concentration correlation of Co and Al. Co tends to enhance the fraction and the ferromagnetic interaction of the austenitic phase leading to a greatly increase of the saturation magnetization and Curie temperature of the alloy. The martensitic-austenitic (M-A) structural transformation process can be tuned to occur entirely above room temperature by appropriate concentrations of Co and Al. An irregular variation of the M-A transformation temperature happens in a narrow range, 8-8.5 at%, of Co concentration, while the ferromagnetic-paramagnetic (FM-PM) phase transition temperature monotonically increases. The external magnetic field favors the appearance of the austenitic phase and induces shifts to low and high temperatures for the M-A transformation and the FM-PM transition, respectively.