Interfacial segregation and embrittlement in liquid-phase sintered tungsten alloys

Abstract

When tungsten-rich W-Ni-Cu or W-Ni-Fe alloys are liquid-phase sintered the resulting microstructure consists of a continuous network of spheroidal tungsten single crystals embedded in a ductile f.c.c. Ni—Cu—W or Ni—Fe—W matrix. The mechanical properties of these particulate composites are controlled primarily by the relative strengths of the phases present and by the strengths of the interfaces developed between them. The impact strength of conventionally prepared material furnace-cooled from the sintering temperature (1430-1470 °C) may be approximately doubled by reheating to temperatures in the range 1150—1350 °C for 1—2 h and water quenching. Both optical and transmission electron microscopies have failed to reveal any significant change in microstructure accompanying this solution treatment and it seems likely that the inherent strengths of the tungsten—matrix and tungsten-tungsten interfaces are of major importance in controlling the impact properties. In brittle, furnace-cooled specimens fractured at ambient temperatures, failure occurs primarily along these interfaces and the fracture surfaces, particularly the areas of tungsten-tungsten impingement, have the smooth featureless appearance often associated with intergranular failure and commonly attributed to grain boundary segregation of impurity elements. The present study was thus initiated to investigate the possible contribution of interfacial segregation to the embrittlement of these alloys.