Effect of sintering cooling rate on V segregation amount at WC/Co interface in VC-doped WC-Co fine-grained hardmetal

Abstract

In other researchers' studies by HRTEM observation and XMA analysis on VC-doped WC-Co fine-grained hardmetal, the phenomenon that V segregated at total WC(0001)/Co interface was reported and such segregation itself was suggested to be the direct cause for V inhibition effect on WC grain growth. This suggestion, however, seemed to be inadequate, because it is not clear whether this segregation was generated by an equilibrium adsorption mechanism during the temperature-raising-stage and/or temperature keeping-stage of sintering, i.e., during the so-called grain growth-stages, or by a heterogeneous precipitation (hetero-epitaxial nucleation and growth) mechanism during the cooling-stage after the temperature keeping-stage. In this study, the effect of the cooling rate on the amount of V segregation was investigated to solve the above problem. It was found that the amount depended on each WC grain even at a certain cooling rate and the arithmetic average amount became smaller with increasing cooling rate. These findings, a consideration on the basis of the general theory of grain growth in multi-phase system, etc., suggested the following. All of the V segregation at total WC(0001)/Co interface is not necessarily generated by an equilibrium adsorption mechanism, but mainly by the heterogeneous precipitation mechanism. Therefore, such V segregation itself should not be directly correlated to the VC inhibition effect on WC grain growth. The segregation, etc., however, suggested that V atoms adsorb and desorb intermittently at the step and/or kinks, i.e., WC growth site, of WC(0001) crystal surface and thus WC grain growth which occurs according to Ostwald-ripening mechanism is inhibited during the V adsorption and desorption.