The effect of vanadium and vanadium plus titanium on the magnetic and mechanical properties of FeCrCo hard magnets

The microstructure and magnetic properties of the FeCrCo hard permanent magnetic alloys with and without vanadium or vanadium plus titanium were studied using transmission electron microscopy and transmission Lorentz microscopy. Mechanical and magnetic properties were studied in parallel at different stages of the production. The three alloys 57Fe- 15Co-28(Cr, V, Ti) (all compositions are in weight per cent) used for the present investigation had 28Cr, 23Cr-5V and 23Cr-3V-2Ti respectively. It is known that the magnetic properties of these alloys are improved by a heat treatment in a magnetic field. Improvement in the magnetic remanence Br is due to elongation of the FeCo (α1) phase parallel to the magnetic field direction. Subsequent step-aging or continuous-cooling treatments cause further phase separation with increased compositional diffences between the phases without any microstructural changes and thus increase the coercive field Hc. Lorentz microscopy shows that the matrix is weakly magnetic and that the magnetic hardening is due to domain wall pinning. Mechanical testing and fracture analysis show that the alloys are very ductile in the as-quenched state but become very brittle after the aging treatments. The embrittlement is severe for the ternary alloys and is most effectively retarded by the addition of vanadium or vanadium plus titanium. It is attributed to the high chromium content of the chromium-rich (α2) phase and to the large grain size (a result of high homogenization temperature) of the ternary alloys. Since the addition of vanadium or vanadium plus titanium does not change the magnetic properties but improves the mechanical properties and makes the processing more convenient, the alloys with vanadium or vanadium plus titanium seem to be of commercial interest.