Enhancement of microstructure-based magnetic, electronic, and lattice contribution in a CoNiAl ferromagnetic shape memory alloy

Abstract

In this study, a Co-Ni-Al system with nominal compositions Co42Ni31Al27 and Co41Ni32Al27 was synthesized. The structural and microstructure of these confirm the presence of a non-ferromagnetic face-centered cubic (γ) phase interspersed between the grains of a ferromagnetic body-centered cubic (β) phase. Notably, γ phase is increased by 1.5 times in the Co41Ni32Al27 sample due to the 1% substitution of Co by Ni. The microstructural tuning induced a higher thermal hysteresis in the shape memory effect of Co41Ni32Al27 with an increase in enthalpy during the phase transition (Austenitic ↔Martensitic). In addition, the temperature-dependent resistivity, ρ(T) was measured to study the electron-phonon and electron-magnon scattering around the phase transition of the studied samples. The dynamic elastic properties of the studied samples were tracked by the relative change in sound velocity (δv/v) with temperature and elastic recovery was confirmed in both alloys across the 120 K to 300 K range. However, the Co41Ni32Al27 exhibits a high amount of lattice contribution to the shape recovery compared to the Co42Ni31Al27. Moreover, a larger variation in relative resistivity (∆ρ/ρ) for Co41Ni32Al27 compared to Co42Ni31Al27 during the phase transition indicates a larger shape change due to decreased Co content. Furthermore, the Co41Ni32Al27 sample shows higher temperatures of martensitic start (T_Ms≈260K) and Austenitic finish (T_Af≈290K) along with high Curie temperature (T_c=330K). Consequently, the temperature-dependent susceptibility (χ^') confirms the higher magnetoelastic recovery in the Co41Ni32Al27 sample, indicating an enhancement of magnetic field-induced strain (MFIS). Stress-induced Q^(-1) is lower for Co41Ni32Al27 (~2.9×10^(-3)) compared to Co42Ni31Al27 sample (~5.0×10^(-3)) signifying the enhanced mechanical strength.