Sintering atmosphere effects on tensile properties of heavy alloys

The sintering atmosphere has a direct bearing on the residual porosity which in turn has a strong negative influence on the tensile properties of W-Ni-Fe heavy alloys. The present investigation uses various sintering atmospheres to understand pore formation, densification, microstructure, and tensile properties of heavy alloys with tungsten contents ranging from 88 to 97 wt Pct. Pore formation when sintering in a dry hydrogen atmosphere is linked to water vapor generation and its entrapment in pores. A hydrogen dew point effect is associated with solution-reprecipitation of tungsten during liquid phase sintering. The beneficial effect of vacuum sintering has been analyzed in terms of removal of the gases before pore closure. Property degradation during long time vacuum sintering is attributed to preferential matrix vaporization. The negative effect of long sintering times in dry hydrogen is attributed to pore coarsening, which is removed by a three-stage sintering atmosphere treatment.