Elevating and controlling the martensitic-austenitic transformation temperature above room temperature for Ni-Co-Mn-Al ferromagnetic shape memory alloys

Rapidly quenched Ni50-xCoxMn35Al15 (x = 7 - 13) alloy ribbons with thickness of ~ 20 μm and width of ~ 3 mm were prepared by melt-spinning method. All the ribbons are crystallized with columnar grains having diameter of 1 - 3 µm and orientation perpendicular to the ribbon surface. Two crystalline structure types of the cubic Ni2MnAl and the tetragonal Ni3Al respectively corresponding to the austenitic and martensitic phases are revealed at room temperature and their fraction clearly depends on the Co concentration. The martensitic-austenitic (M-A) structural transformation related to shape memory effect (SME) is observed on all the ribbons except the one with x = 13. The process of the M-A transformation entirely occurs above room temperature and can be controlled in the range of 360 - 430 K by changing Co concentrations from 7 to 12 at%. The Curie temperature of the austenitic-ferromagnetic phase, TCA, increases monotonically with increasing the Co concentration. The external magnetic field greatly affects the M-A structural transformation and the magnetic transition of the alloy ribbons.