Influence of heat treatments on microstructures in an Fe--Co--V alloy

The microstructural changes in an Fe-Co-V alloy (composition by wt pct: 2.97 V, 48.70 Co, 47.34 Fe and balance impurities, such as C, P and Ni) resulting from different heat treatments have been evaluated by optical metallography and transmission electron microscopy. Results indicate that, on air cooling or quenching into iced-brine from the high temperature single-phase γ (fcc) field, vanadium can be retained in a supersaturated solid solution (α2) which has bcc structure. For the range of cooling rates employed, a portion of the material appears to undergo the γ-α2 transformation massively and the remainder martensitically. Also antiphase boundaries are observed in the air-cooled samples. On annealing in the two-phase α1 + γ field, α2 decomposes into vanadium-rich subgrains (γ) and vanadium-poor subgrains (γ1), and only the former undergo the γ → α2 transformation during air cooling or iced-brine quenching. The α1t subgrains in a sample, slowly quenched in quartz, show superlattice dislocations and antiphase boundaries, whereas both the transformed and untransformed areas exhibit (100) superlattice reflections. There is, however, no evidence of long-range order in the specimens quenched into iced-brine. The two-phase annealing sequence followed by a 2 h anneal at 600°C and air cooling results in precipitation within the vanadium-rich, transformed subgrains. Also there is evidence of long-range order in both types of subgrains.