Effect of Cryogenic Heat Treatment on Phase Formation in Co38Ni37Al17Si6Sb2 Ferromagnetic Shape Memory Alloy

In this study, effect of cryogenic heat treatment in polycrystalline Co38Ni37Al17Si6Sb2 ferromagnetic shape memory alloy (FSMA) on phase formation phenomenon was investigated. After the homogenization process, the synthesized Co38Ni37Al17Si6Sb2 alloy was cooled into liquid nitrogen and kept in liquid nitrogen for different time periods. X-ray difraction method was used to determine lattice parameters and crystallite sizes from the samples obtained. The crystallite size of the Co38Ni37Al17Si6Sb2 alloy was calculated using the Debye Scherrer equation. The results show that these alloys were composed of four phases; A1 structure of γ phase, β austenite phase (B2 cubic), body-centered tetragonal L10 martensite and γ′ (ordered fcc L12) phase. Thermal properties of Co38Ni37Al17Si6Sb2 specimens were analyzed by Differential Scanning Calorimetry (DSC). Martensitic transformation temperatures were increased due to the increasing aging time in Liquid Nityrogen. An endothermic peak at ~ 165°C and an exothermic peak at ~ 110°C were observed from DSC results. Scattering Electron Microscope (SEM) and optical microscope experiments were performed to investigate metallographic structures. The findings of the γ, β and L10 phases were confirmed by SEM observations and EDS analysis. As a result of cryogenic heat treatment in the material phase formation phonomenon was discussed. Vickers and Rocwell Hardness methods were determined to find differences of hardness values due to the cryogenic heat treatment time on Co38Ni37Al17Si6Sb2 FSMA. The hardness value of the alloy has been found to rise with increasing cryogenic aging time