High-strain deformation processing of NiW alloys

Abstract

This paper discusses the potential of NiW alloys of differing initial microstructures, for high-strain cold deformation processing, and thus their suitability for development as heavily deformed, high strength in situ composites. Varying initial phase morphologies and tungsten volume fractions were produced by variations in initial processing. Liquid-phase sintered structures, containing 48 vol.% W, can be cold rolled to true strains of 2.4. They presumably could be cold worked further, but size limitations imposed by our processing capability prevented that being done in this study. Likewise, low volume fraction (of less than 15 vol.%) tungsten alloys, produced by casting of a hypoeutectic nickel-rich alloy and by transient liquid-phase sintering, can be cold rolled to the same strain as the liquid-phase sintered NiW structure. In contrast, as-cast arc melted alloys cannot be cold drawn to any extent at room temperature. They can be made capable of limited drawability by subjecting them to a coarsening heat treatment, which also spheroidizes eutectic tungsten to some extent, prior to cold working. The limited malleability of this alloy appears to be related to the heterogeneous distribution of nondeforming tungsten particles. Fracture surfaces of material that failed during drawing are populated heavily by such particles. The strengths of cold worked NW alloys of the present study exceed those of comparably deformed directionally solidified NiW eutectic alloys, at least up to deformation strains of 2.4. Since the latter type of alloy manifests extensive work hardening for deformation strains greater than 3.0, we expect this would also hold for the alloys described in this paper. Thus the NiW system, composed of a strong, tough nickel phase and a linear work hardening tungsten phase, is a likely candidate for development as a strong, tough, heavily deformed in situ composite.