Mechanical properties of Fe-Cr-Co ductile permanent magnet alloys

The structure and mechanical properties of ternary Fe-Cr-Co magnet alloys containing 9 to 11 wt pct cobalt have been investigated. Fine scale spinodal decomposition of the iron rich bcc α-phase into (α + α₂) structure increased the alloy strength and reduced the ductility. The degree of changes in the mechanical properties depended on the cobalt content and the final aging temperature and time which primarily determines the compositional amplitude. As a result of decomposition, the dislocation movement by slip became more difficult, and the mode of deformation changed from predominantly slip to predominantly twinning. The embrittlement during aging and the fracture behavior of these alloys go through two stages: i) from microvoid nucleation and coalescence type ductile fracture to quasi-cleavage type transgranular fracture (ductile-brittle transition) and ii) from transgranular to intergranular fracture. The cause of the transgranular fracture is attributed to the raised ductile-brittle transition temperature resulting from the increased strength and the tendency for deformation twinning which are likely to make the relief of local stress concentration more difficult. The cause of the intergranular fracture is ascribed to the formation of more or less continuous grain boundary precipitate that forms upon further decomposition at lower temperatures (below ∼540‡C). Both types of embrittlement were found to be reversible upon heat treatment at higher temperatures, either within the (α ₁ +α ₂) range or above the miscibility gap.