Microstructural influences on the
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I N T R O D U C T I O N AND B A C K G R O U N D
THE microstructure of a typical liquid-phase sintered W-Ni-Fe alloy (Figure 1) consists of spheroidal bodycentered cubic (bcc) tungsten particles in a face-centered cubic (fcc) W-Ni-Fe matrix. While the tungsten particles contain only trace amounts of Ni and Fe, the matrix alloy contains a significant percentage of tungsten (the tungsten content of the matrix is a strong function of the manufacturing process). For the high tungsten contents (>90 pct) that are of commercial interest, the tungsten particles coalesce to form a microstructure consisting largely of contiguous tungsten grains, although the softer phase is still conventionally termed the "matrix" phase. Variations in the microstructure can be accomplished by varying the alloy composition, consolidation technique, and subsequent thermomechanical processing (such as swaging). A relatively high tungsten content yields high strength and density, while the ductile matrix provides reasonable deformability; this combination of properties makes these materials nearly ideal as kinetic energy penetrators. Since penetrator materials are subjected to high strain rates and large deformations, it is necessary to develop an understanding of the behavior of these alloys under such conditions. There is a considerable body of literature on the quasistatic mechanical properties of W-Ni-Fe heavy alloys. Lt,2J Several studies have also attempted to correlate the observed quasistatic mechanical properties with microstructural information. For instance, Krock and Shepard ltj obtained quasistatic stress-strain curves in tension for a range of tungsten volume fractions; the strength of the alloy appeared to be almost independent K.T. RAMESH, Assistant Professor, is with the Department of Mechanical Engineering, The Johns Hopkins University, Baltimore, MD 21218. R.S. COA~rES, formerly with The Johns Hopkins University, is Mechanical Engineer, Aberdeen Proving Ground, United States Army Ballistic Research Laboratory, Aberdeen, MD. Manuscript submitted May 20, 1991. METALLURGICAL TRANSACTIONS A
of the tungsten content over the range of volume fractions studied (80 to 92 pct W). However, Rabin and German pl observed that the flow stress (in tension) increased with increasing tungsten content; this observation has been corroborated by several other workers, f41 The effect of the degree of swaging on the quasistatic mechanical properties has been studied by several workers, IS1 resulting in the general observation that increased swaging results in an increased flow stress. This increased strength as a result of prior plastic straining is also a function of the tungsten volume fraction, and so several investigators have attempted to distinguish between the behavior of the tungsten grains and that of the matrix. I2'61 The intent is to combine the two observed behaviors for various tungsten volume fractions using a dual-phase composite approach. However, the composition of the matrix is a strong function of the processing route, the grain
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