Microstructure effects on tensile properties of tungsten-Nickel-Iron composites

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INTRODUCTION

TUNGSTENheavy alloys are W-Ni-Fe or W-Ni-Cu metalmetal composites produced by liquid phase sintering elemental powders. The alloys possess high density, strength, and ductility which makes them useful for several applications, including kinetic energy penetrators. The composite microstructure consists of nearly spherical grains of bodycentered cubic tungsten surrounded by a solidified facecentered cubic matrix phase containing Ni, Fe, and W. Although polycrystalline tungsten normally is brittle at room temperature, the matrix imparts ductility to the composite. Previous research into the mechanical properties of heavy alloys has identified several causes of ductility and toughness variations, including residual porosity, I~l intermetallic precipitation, IS-1~ hydrogen and impurity embrittlement, I'-~71 varying ductile to brittle transition temperature,18'~7'~81incomplete oxide reduction, I31 and tungsten precipitation in the matrix. I~9'2~ The processing routes needed to obtain high strength and ductility are now better understood. The first requirement for good properties is essentially full densification, achieved through control of the liquid phase sintering process. Subsequently, it is necessary to avoid the weakening of the tungsten-matrix interface by hydrogen embrittlement, impurity segregation, or intermetallic phase precipitation. The strength of the tungsten-matrix interface is controlled through appropriate heat treatments. A successful heat treatment involves heating in vacuum or inert gas followed by water quenching. 14'13'171 In past research, processing variations have dominated properties and precluded understanding the underlying microstructure/property relations. For example, Krock f~8~ concluded that the tensile properties of heavy alloys were independent of tungsten volume fraction and matrix mean free path. In contrast, Gurland and Parikh I2q found that yield

B. H. RABIN, formerly Graduate Research Assistant at Rensselaer Polytechnic Institute, is Senior Scientist, P/M Unit, Materials Group, EG & G Idaho, P.O. Box 1625, Idaho Falls, ID 83415. R.M. GERMAN is Professor, Materials Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12180-3590. Manuscript submitted June 1, 1987. METALLURGICAL TRANSACTIONS A

strength and deflection in bending depended on tungsten content. Votava I221 and Ekbom I23j reported that initial strain was localized mainly within the matrix and that at larger strains the deformation of individual tungsten grains resembled that of the composite. Likewise, O'Neil and Salyer ~241found that flow strength depended on tungsten content at low strains, but was independent of tungsten content over a few percent plastic strain. Fracture occurred uniformly through the tungsten grains and matrix, except at the highest tungsten levels when intergranular failure of the tungsten was observed. Churn and German I251found that crack propagation occurred by cleavage of tungsten grains ahead of the crack tip in bend tests. In contrast, several authors have rep

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