Influence of Annealing Temperature on the Properties of Co-Ni-Ga Ferromagnetic Shape Memory Alloy

Polycrystalline ingot of Co47Ni23Ga30 alloy was prepared by arc melting constituent elemental powders under argon atmosphere. The alloy ingot was then vacuum sealed in a fused silica ampoule, homogenized at 1230 °C for 24 hours and quenched in liquid nitrogen. X-Ray diffraction patterns of the as-quenched samples revealed single-phase tetragonal structure. The quenched alloy was then separately annealed at 900 °C, 1000 °C and 1150 °C for 6 hours and subsequently quenched in ice water. The alloys annealed at 1150 °C and 1230 °C exhibited a singlephase martensite structure (β′-phase) at room temperature, whereas, presence of a face centred cubic (γ) phase along with the martensite phase was observed in alloy pieces annealed at 900 °C and 1000 0C, respectively. The martensite-austenite structural phase change in this alloy was observed using a Differential Scanning Calorimeter. It was found that the martensite-austenite and austenitemartensite transition temperatures (As, Af, Ms and Mf) shifted to higher temperatures when the annealing temperature was increased. The Curie temperature shifted towards lower temperatures as the percentage of γ-phase increased in the alloy. The saturation magnetization did not show any appreciable change when the annealing temperature was changed. Presence of the additional γ- phase in the alloy annealed below 1150 °C was confirmed by Optical Microscopy and Scanning Electron Microscopy analysis. The influence of the annealing temperature on the properties of this ferromagnetic shape memory alloy composition is discussed in the paper.