Interphase boundary precipitation in liquid phase sintered W-Ni-Fe and W-Ni-Cu alloys
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INTRODUCTION
SINTERING of tungsten-rich W-Ni-X powder compacts (X = Fe, Cu, Cr, or Co) at a temperature at which the minor constituents form a liquid phase allows rapid densification to high density, particulate composites. The resulting microstructure typically comprises a continuous network of approximately spheroidal tungsten single crystals embedded in a ductile, face-centered cubic Ni-X-W matrix phase. The resultant mechanical properties combine useful strength, ductility, and toughness, ~ and appear to be determined primarily by (i) microstructural parameters such as matrix volume fraction and tungsten particle size, size distribution, and contiguity;~,2,3 (ii) the relative strengths of the phases present; 4 and (iii) the integrity of the interfaces that develop between these phases. In those alloys exhibiting both low strength and poor impact toughness at ambient temperatures, fracture occurs primarily by a process of interfacial decohesion, with little or no plastic deformation of either of the constituent phases. 3'5'6 In some instances evidence of a brittle third phase is observed7's to surround tungsten particles on an impact fracture surface. The impact resistance of the alloys is influenced strongly by the matrix composition, the cooling rate from the sintering temperature, and by post-sintering heat treatments. B.C. MUDDLE, formerly with Department of Mechanical and Industrial Engineering and Materials Research Laboratory, University of Illinois at Urbana-Champaign, is now with the Department of Materials Engineering, Monash University, Clayton, Victoria, 3168, Australia. This paper is based on a presentation delivered at the symposium "Activated and Liquid Phase Sintering of Refractory Metals and Their Compounds" held at the annual meeting of the AIME in Atlanta, Georgia on March 9, 1983, under the sponsorship of the TMS Refractory Metals Committee of AIME. METALLURGICALTRANSACTIONS A
Ductility and impact toughness of W-Ni-Fe alloys vary with the weight fraction Ni:Fe, the optimum properties being found8'9'1~within the range 1:1 to 3:1. The slow furnace cooling from the sintering temperature that is common in commercial practice is detrimental to impact toughness, 1'7 while it is now well established7 that post-sintering heat treatments at temperatures in the range 1200 to 1350 ~ followed by water quenching can produce significant improvements in the impact properties of as-sintered, furnacecooled alloys in the above range of compositions. These effects of composition and processing conditions on subsequent mechanical behavior are not well understood, but it has been suggested7'1~ that their influence is, at least in part, connected with the precipitation of a brittle intermetallic phase at the tungsten-matrix interphase boundaries and the subsequent brittle failure of the materials by fracture along these interfaces or by cleavage through the intermetallic. Such proposals would imply that the stability of the precipitate phase is sensitive to matrix composition, that it forms during sintering
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