Mechanical properties of soft magnetic FeCo alloys

The brittle fracture of stoichiometric FeCo has long been a puzzle given its high-symmetry B2 crystal structure, 1/2〈111〉{110} slip, low ordering temperature and relatively low strength. Macroalloying with vanadium (≈2%) improves ductility significantly in the disordered state, but only moderately in the ordered state. Brittle fracture is intergranular in the binary stoichiometric alloy, but is usually transgranular cleavage in the vanadium-containing alloy. Boron and carbon additions are shown not to suppress grain-boundary fracture nor improve the ductility of stoichiometric FeCo, but they produce significant ductility improvements in the ternary FeCo–2V alloy. The mechanism appears to be slip refinement by fine precipitates. It is also shown that for boron-doped (30 wppm) stoichiometric FeCo, the optimum vanadium content for good ductility is 2.1–2.5 wt.%.