Deformation behavior and magnetic properties of equiatomic FeNi single crystals

Abstract

An equiatomic Fe–Ni alloy in its disordered A1 structure is a soft magnetic alloy. On ordering to the L10 ordered phase, it shows significant magneto-crystalline anisotropy and a strong permanent magnet behavior and is of interest as a rare earth-free permanent magnet. However, synthesis of an L10 phase in a bulk form remains a challenge due to its low critical ordering temperature Tc and consequent extremely slow ordering kinetics. This phase is present in asteroids, and how it was formed remains unclear. The likely mechanism is enhanced diffusion kinetics due to extreme dislocation densities and vacancy concentrations produced by deformation during asteroid collisions and the presence of S. Prior to examining extreme deformation in an FeNi alloy comparable to that in asteroid collisions, low strain rate deformation behavior, magnetic properties, and the structure of undoped and S-doped [100]-oriented FeNi single crystals were carried out. Controlled deformation at a strain rate of 1 × 10−5/s showed the yield point to be 89 MPa, and the critical resolved shear stress was 25.7 MPa. The dislocation densities obtained were ∼1017/m2. The saturation magnetization value was ∼147–151 emu/g both before and after deformation, comparable to NdFeB magnets. Coercivity increased slightly from ∼0.04–0.4 to ∼5 Oe after deformation due to an increase in dislocation density. The x-ray diffraction scan of S-doped and deformed single crystals after annealing at 300 °C, just below Tc, showed no evidence of L10 order. These data serve as a baseline for extreme strain rate deformation where much higher dislocation densities and vacancy concentrations can be obtained to facilitate L10 order.