Liquid-Phase Sintering (LPS) of Tungsten-Based Heavy Alloys: Hafnium and CopperAddition

摘要

Tungsten heavy alloys of composition 80W-20(Hf-Cu) weight-percent have beenfabricated by liquid phase sintering (LPS). Two hafnium-rich, near eutectic compositions of the Hf-Cu system were utilized to lower the sintering temperature in the W-Hf system. Intermetallic compounds can be harmful to the mechanical properties of tungsten-based heavy alloys and avoiding them was deemed to be critical in the development of these new alloys. Therefore, an understanding of the processing conditions by which these compounds are formed was a vital component of this work. The threshold temperature for the formation of the intermetallic HfW2 was determined by powder x-ray diffraction (XRD) to be approx. 1,400 deg C. This threshold temperature was well above the predicted LPS temperature of 1,310 deg C for hafnium-rich compositions in the Hf-Cu binary. To study the effects of cooling rates on the formation and stability of another possible intermetallic phase CuHf2, uniaxially pressed 90Hf-10Cu elemental powder blends were heated to 1,310 deg C for 30 min and subjected to four different cooling rates. The CuHf2 phase was, however, observed at room temperature for all cooling rates examined. Two ternary W-Hf-Cu compositions were selected by fixing the tungsten content at 80 weight-percent and utilizing two near eutectic Hf-Cu compositions. Copper additions in the W-Hf alloy system, however, resulted in the formation of the HfW2 intermetallic phase at temperatures as low as 995 deg C. The microstructure was comprised of interpenetrating, semicontinuous networks of nearly pure W with HfW2.