Segregation to interphase boundaries in liquid-phase sintered tungsten alloys
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I.
INTRODUCTION
THE microstructure
of liquid-phase sintered tungstenbased alloys comprises a continuous network of approximately spheroidal tungsten single crystals ( - 2 0 to 30 ~m in diameter) embedded in a ductile, nickel-based matrix phase. This composite structure results in high density materials (16 to 18 Mg m -3) with a useful combination of mechanical properties; the ultimate tensile strength is typically 800 to 1000 N mm -z, tensile elongation 2 to 20 pct, and impact strength 10 to 200 J (unnotched Charpy impact energy). Their properties are, however, observed to vary markedly with variations in composition and sintering conditions, and there is an increasing effort to understand and control those factors influencing, in particular, the resistance of the materials to impact loading. Following a study of the impact properties of liquid-phase sintered W-Ni-Cu and W-Ni-Fe alloys, it was recently suggested 1'2 that the strengths of the interfaces within the structure, both the tungsten-matrix interphase boundaries and the tungsten-tungsten grain boundaries, were of major importance in determining the impact strength of a given alloy. In specimens of both alloy systems conventionally furnace-cooled from the sintering temperature, brittle failure was observed to occur predominantly by fracture along these interfaces and there was little accompanying plastic deformation of either of the phases present. Analysis of the average composition of the fresh fracture surfaces by Auger electron spectroscopy (A.e.s.) indicated the presence of an enhanced concentration of impurity elements phosphorus and sulfur, implying interfacial segregation of these ele-
C. LEA is on the research staff of the Division of Materials Applications at the National Physical Laboratory, Teddington, United Kingdom; B. C. MUDDLE, formerly in the Department of Metallurgy and Materials, University of Cambridge, United Kingdom, is now a Member of the Staff of the Department of Mechanical and Industrial Engineering, University of Illinois at Urbana, IL 61801; and D.V. EDMONDS, formerly in the Department of Metallurgy and Materials, University of Cambridge, United Kingdom, is now in the Department of Metallurgy and Science of Materials, University of Oxford, United Kingdom. Manuscript submitted June 28, 1982. METALLURGICALTRANSACTIONSA
ments during sintering or during the slow furnace cool from the sintering temperature. High temperature heat treatment (typically one to two hours at 1350 ~ of the as-sintered material followed by water quenching was found to produce a significant improvement in impact properties. This improvement arose from a marked increase in the level of interphase cohesion between the matrix phase and the tungsten particles. The improved strength of the interphase boundaries was in turn associated with a decrease in the level of interfacial contamination by impurity elements. In the absence of other detectable changes in microstructure, it was concluded2 that the improvement in the impact strength with heat treatment was ass
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