Variation in Strength, Hardness, and Fracture Toughness in Transition from Medium-Grained to Ultrafine Hard Alloy

Resumo

The microstructure and mechanical characteristics of the samples of medium-grained (WC‒8Co), submicron (WC–8Co–1Cr₃C₂), and ultrafine (WC–8Co–0.4VC–0.4Cr₃C₂) hard alloys formed by liquid-phase sintering of powders of corresponding dispersity are investigated. It is shown that the alloy hardness increases from 1356 to 1941 HV with a decrease in the average grain size from 1.65 to 0.37 μm. The comparison with the published data shows that alloys considered in this study are no worse than analogs formed by sintering under a pressure, hot pressing, and induction and spark plasma sintering by hardness and fracture toughness. Herewith, the flexural strength of alloys prepared by liquid-phase sintering is lower by a factor of 1.5–2.5 than that of alloys formed by sintering under a pressure or pressing because of the presence of pores, the maximal diameter of which is evaluated as 40 μm. An analysis of the results and published data for the correspondence of theoretical regularities is performed. It is shown that the dependences of hardness, fracture toughness, and strength on the average grain size of formed alloys and their analogs in general correspond to traditional regularities based on the Hall–Petch and Orowan–Griffiths laws, despite the presence of theoretical prerequisites for the deviation from them.