Formation mechanism of in-situ V8C7 and V3B4 particles in iron matrix composites by vacuum sintering

The in-situ V₈C₇-V₃B₄/Fe45 composites were fabricated by vacuum sintering using ferrovanadium, carbon black and Fe45 alloy powders as raw materials. The microstructure, phase structure and hardness of sintered Fe45+x wt.% (FeV+C) composite specimens (x = 0, x = 5, x = 10, x = 15, x = 20) were investigated. The results show that the type and shape of in-situ borides and carbides depend on the content of (FeV+C) mixed powders. When x = 5, the composite consists of α-(Fe, Cr, V) solid solution, strip M₇C₃ carbides and block M₂B borides. When x = 10, the V₃B₄ particles initiate inside the M₂B borides by in-situ precipitation, and the excess vanadium reacts with carbon to in-situ form V₈C₇ particles. When x≥15, the M₂B completely transform into V₃B₄. Furthermore, the V₈C₇ and V₃B₄ particles coarsened and tended to be spheroidal and rectangle shape respectively. The V₃B₄ particles coarsen possibly by contacting and merging of the small V₃B₄ particles. However, the V₈C₇ particles coarsen possibly by the Ostwald ripening. The hardness of the composites increases firstly and then decreases with the increase of (FeV+C) mixed powders. The Fe45+10 wt.% (FeV+C) composite has the highest hardness value (54.2 HRC), which is due to the fine boride and carbide reinforcement and little pore defect.