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

Abstract Rapidly quenched Ni 50−x Co x Mn 35 Al 15 (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 Ni 2 MnAl and the tetragonal Ni 3 Al 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, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">C</mml:mi> </mml:mrow> </mml:msub> <mml:mi mathvariant="normal">A</mml:mi> </mml:msup> </mml:math> , 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.