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.