Composition and phase structure dependence of magnetic properties for Co2FeCr0.5Alx (x=0.9, 1.2) multi principal component alloys

This article investigates the microstructure evolution, phase formation, and magnetic properties of Co2FeCr0.5Alx (x = 0.9, 1.2) complex component alloys, as a function of heat treatment temperatures (at 500, 600, 700, and 1150°C), using XRD, optical microscopy, electron microscopy and vibrating sample magnetometry (VSM). The alloy with 20.4 at% Al (x= 0.9), identified here as C1, consisted of microscale BCC1 phase and BCC nanoscale particles containing mainly Fe and Cr, and B2 matrix with mainly Al and Co. Partial transformation of the BCC1 phase to an FCC phase was observed at 700°C and full transformation at 1150°C, through twinning. For the sample with 25.5 at% Al (x= 1.2), identified as sample C2, there were only nanoscale BCC particles in the B2 matrix with the same element segregation between the phases as C1. This increase in Al (from x= 0.9 to 1.2) content stabilised the B2 matrix phase, reduced the grain size, and increased both saturation magnetisation (Ms) and coercivity (Hc). Moreover, increasing the heat treatment temperature resulted in an increase in grain size of the B2 matrix, volume fraction and average size of the micro BCC 1 and nanoscale BCC phases for both C1 and C2, which also modified the soft magnetic properties, with Ms and Hc increasing up to 600°C followed by a decrease until 1150°C. Using the structural information as inputs for density functional theory calculations of Hc and Ms, it has been found that the Hc is influenced by the grain size of the matrix, and the volume fraction and size of the BCC1 phase at temperature higher than 600°C for C1 and 700°C for C2, but is controlled by nanoscale BCC particles below these temperatures. The Ms is controlled by the elemental diffusion and segregation. Thus, the best combination of Hc and Ms was seen with antiferromagnetic Cr segregated and partitioning in the microscale BCC1 phase, and a B2 matrix with less Cr rich precipitation, formed at 500°C, where the misfit strain between B2 matrix and nanoscale BCC was low.