Reducing coercivity by chemical ordering in additively manufactured soft magnetic Fe–Co (Hiperco) alloys

While BCC Fe–Co alloys form an important class of soft magnetic alloys, they are often challenging to process in near net shape, due to the formation of the hard embrittling ordered B2 phase. Additive manufacturing (AM) technologies, such as laser engineered net shaping (LENS), permit processing these alloys in complex near-net shapes while controlling the extent of B2 ordering. The as-deposited samples, consist of elongated BCC grains, and exhibited reasonable values of saturation magnetization (M_s) but rather high coercivity (H_c). After annealing (950 °C/30 min), the Hc values further increased due to the formation of fine recrystallized BCC grains, although these grains were relatively free of residual stresses. A second annealing step (500 °C/50 h) resulted in the M_s increasing by 25% and H_c reducing by 63%. Detailed Transmission Electron Microscopy (TEM) analysis revealed substantially larger ordered B2 domains, separated by anti-phase domain (APD) boundaries in the two-step annealed condition, as compared to the single-step annealed (950 °C/30 min) condition which consisted on nanometer scale weakly ordered B2 domains homogeneously distributed within a disordered BCC matrix. Therefore, the magnetic properties of the LENS processed alloys are significantly affected by the extent of B2 ordering, which can be engineered via appropriate post-processing heat treatments.