Promotion of ausferrite formation in as-sintered Fe–Mo–Si–C-(Cu) composites due to Cu addition

Abstract

A sintered composite with ductile iron microstructure is novel type of materials. Design of these sintered composites can be done based on the interaction between iron or iron-based powder and silicon carbide powder. Previous result revealed that the matrix of the reference sintered composite, produced from pre-alloyed Fe-0.85Mo + 4.0 wt % silicon carbide powder mixture, had a complex microstructural feature consisting of a black particle (solid silicon carbide residue core/graphite shell) enveloped with ferrite halo, pearlite and ausferrite. The matrix of the reference sintered composite could be modified by using proper composite composition. It was hypothesized that the addition of copper powder, the austenite stabilizer, to the reference sintered composite would alter the microstructure due to change of austenite stability. It was found that when copper powder contents of ≥2.0 wt % were added to the reference sintered composite, pearlite was completely replaced by ausferrite. The ausferrite scales decreased with increasing copper content. When 6.0 wt % copper was added, the large free copper particles and fine spherical Cu particles were observed in ferrite halos and bainitic ferrite plates of ausferrite. Ultimate tensile strengths of >650 MPa and yield strength of >350 MPa were obtained in experimental sintered composites. The typical strength-ductility trade off was not observed in experimental sintered composites. Low elongation values were observed in sintered Cu-added composites although they had high austenite fraction. It is suggested that the transformation-induced plasticity (TRIP) effect, which occurs by the transformation from retained austenite to martensite during straining, is not fully obtained from high austenite fraction.