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, Effect of Sintering-Cooling Rate on V Segregation Amount at WC/Co Interface in VC-doped WC-Co Fine-Grained•c

Segregation of Dopants at WC/Co and WC/WC Interfaces in Solid-State-Sintered WC-VC-Cr₃C₂-Co Cemented Carbides

“…6) This was based on the premise that the solubility limit of dopants in the WC phase is fairly_low, and suggests that there is no segregation layer formed during liquid state sintering. It has been verified that the extent of segregation concentration at the WC/Co and WC/WC interfaces is reduced by an increase in the cooling rate following sintering, 7) and although the grain growth rate is independent of the WC crystal orientation during liquidstate sintering, the degree of segregation at the interface is nevertheless dependent on the WC crystal orientation. 7) Furthermore, the inhibition of grain growth by TaC addition in TaC-doped WC-Co cemented carbide is greater than that seen with TiC addition, despite their being no visible segregation layer on the WC(0001) surface.…”

“…It has been verified that the extent of segregation concentration at the WC/Co and WC/WC interfaces is reduced by an increase in the cooling rate following sintering, 7) and although the grain growth rate is independent of the WC crystal orientation during liquidstate sintering, the degree of segregation at the interface is nevertheless dependent on the WC crystal orientation. 7) Furthermore, the inhibition of grain growth by TaC addition in TaC-doped WC-Co cemented carbide is greater than that seen with TiC addition, despite their being no visible segregation layer on the WC(0001) surface. 8) Based on these results, it would seem that the segregation layer of graingrowth inhibitors at the WC/Co interface cannot be present at the liquid-state-sintering temperature, but rather is formed during the cooling stage that follows sintering.…”

“…If we consider the solubility of V in the Co-W-V-C phase (i.e., the liquid and solid Co phases) of a WC-VC-Co cemented carbide based on a pseudo-binary VC-Co phase diagram, 9) then it is apparent that the amount of V added in a previous work 7) was less than the solubility limit of V in the liquid Co phase, but greater than that in the solid Co phase. Given this difference in solubility limits, it is only natural to assume that (V,W)C x segregates on the WC surface during cooling.…”

High Resolution Electron Microscopy Study in ZrC-Doped WC-12 mass%Co Alloys

“…In that case, the growth rate is largely reduced and is strongly affected by adsorption/desorption of dopants on the surface. 14) A rate and attaching period of dopants are closely related to a change in the interface energy when a dopant adsorbs at the step edge. In the case of V, the segregation energy is very preferable for adsorbing, which is suggested by ab-initio calculations performed by Christensen et al 12,13) The dopants whose segregation energy is very small stays on the surfaces for longer time comparing with the dopants with higher energy.…”

“…On the contrary, VC has some solubility in Co-phase at sintering temperature so that oversaturated VC possibly preticipates at WC/Co interfaces during cooling after soaking. Kawakami et al have examined a cooling rate dependency of the segregation behavior of V. 14) According to their report, the amount of V at WC/Co interfaces becomes smaller in the alloys rapidly cooled. In this sense, it is very difficult to determine a precise amount of segregation during sintering.…”