Microstructure, magnetic and mechanical properties of Fe–28Cr–15Co–1Si cast magnets containing Mo and Ti additives

Abstract

In this study, the development of Fe–28Cr–15Co–1Si–xM, M = Ti and Mo, x = 1, 2, 3 (wt.%) cast permanent magnets by a thermomagnetic treatment was investigated. The alloys were melted in a vacuum induction furnace (VIM) using pure raw materials and then cast in alumina tubes. Structural, mechanical, and magnetic properties of the alloys were evaluated at different stages of heat treatment. Specimens in the solution annealed condition, showed a coarse grain with a body-centered cubic (BCC) structure (α phase), which decomposes into two α₁ and α₂ phases by a spinodal transformation during the thermomagnetic treatment. By addition of Mo to Fe–28Cr–15Co–1Si alloy, the strain energy of decomposition increases by concentrating in the α₂ phase. This results in a difference in the lattice parameter between α₁ and α₂ phases and an improvement in the magnetic properties of the alloy. The presence of Ti in the alloys raised the amount of α₁ phase, which eventuates in the suppression of the unwanted σ or γ phases in the annealed state. Ti and Mo additions also affect the development of shape anisotropy in the ferromagnetic α₁ particles, which in turn improves the magnetic properties of the alloys. The best magnetic properties turn out to be in Fe–28Cr–15Co–1Si–3Mo, and Fe–28Cr–15Co–1Si–2 Ti alloys: B_r = 0.98 T (9.8 kg), H_c = 67.66 kA/m (0.85 kOe) and (BH)_max = 41.27 kJ/m³ (5.2 MGOe), and B_r = 0.93 T (9.3 kg), H_c = 65.27 kA/m (0.82 kOe) and (BH)_max = 38.09 kJ/m³ (4.8 MGOe), respectively. The hardness test results verified that in the Fe–Cr–Co-based alloys with spinodally decomposed structure, the internal stresses are more effectual than composition gradients on the alloy’s strength.