Coarsening of tungsten grains in liquid nickel-tungsten matrix

Abstract

In sintered W-Ni alloys with 1,7, and 30 wt pct Ni the tungsten grain growth in liquid matrix at 1540°C was investigated. The observed grain size distributions and growth rate are compared with the theoretical predictions of Wagner, Lifshitz and Slyozov, Lay, and Ardell. In the 70 pct W-30 pct Ni alloy the tungsten particles settled to the bottom of the specimens immediately upon melting of the matrix, but the spherical grain shape is maintained during the initial stage of annealing. In these specimens the linear intercept distribution of the grains agrees with the prediction of the LSW (Lifshitz, Slyozov, and Wagner) theory for the reaction controlled growth mechanism. On the other hand the growth rate appears to follow the t^1/3 law predicted for the diffusion controlled mechanism. These results are consistent with Lay and Ardell's theory in which the concentration gradient around grains is inversely proportional to the average grain size in the limit of small matrix fraction. In the alloys with 1 and 7 pct Ni a meaningful comparison of the observed linear intercept distribution of the grains with theoretical predictions is difficult because of grain contact flattening due to densification. The grain growth is larger with less matrix fraction in the specimens and this result provides an evidence for the diffusion controlled grain growth during the liquid phase sintering of this alloy.