Eun-Pyo Kim

The effect of yttrium oxide on the sintering behavior and hardness of tungsten

In this study, the relative density and hardness of Y2O3 dispersed tungsten alloy were investigated as functions of the Y2O3 content and sintering temperature. The sintering temperature and the amount of the second phase were varied from 1800 to 2500°C and 0 to 2.0 weight pct, respectively. The relative density of the alloys is higher than that of pure tungsten in the range from 2000 to 2500°C, whereas the density is lower at 1800°C. As the Y2O3 content increases, the sintered density increases at a given temperature. The transition temperature (Ttr), where the relative density of the alloys exceeds that of pure tungsten, is reduced with increased Y2O3 particle content. In order to examine the effect of the second phase on the mechanical property, the hardness of pure tungsten and the alloys are measured. The hardness is mainly dependent upon the relative density of the alloys, rather than the amount of the second phase and tungsten grain size. The relationship between hardness and density is discussed according to the plasticity theory of porous materials.

Matrix penetration of w/w grain

The matrix phase of 93W-5.6Ni-l.4Fe heavy alloy has penetrated into W/W grain boundaries during a newly developed heat treatment which consists of repeated isothermal holdings at 1150 °C and water quenching between them. With the matrix penetration, the impact energy increases from 57 to 170 J, whereas the ultimate tensile strength and elongation remain un- changed. The tensile fracture surface of specimens appears similar regardless of the matrix penetration: predominant cleavage of W grains and dimple fracture of matrix. The fracture sur- face by impact, on the other hand, reveals a change of W/W interface failure toward ductile failure of penetrated matrix with many dimples by increasing the matrix penetration. The in- crease in the fraction of ductile failure and observed crack blunting is attributed to the increase in impact energy with the matrix penetration.