Effect of strain rate on the flow stress of three liquid phase sintered tungsten alloys
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I.
INTRODUCTION
MOST metals
exhibit an increase in flow stress as the strain rate is increased, and extensive examinations of strain rate effects in many individual alloy systems have been undertaken with results summarized in a number of reviews.l-4 Liquid phase sintered tungsten alloys are a recent development and only a small amount of information is available on the effect of strain rate on strength. 5'6'7 The deformation and fracture behavior in these alloys is not fully understood and the contribution of minor phases to intergranular failure is the subject of considerable research effort. 8'9 In this work the flow stresses of three tungsten alloys are examined over a large range of strain rates, 10-3 s -1 to 103 s -~, and the nature of the stress/strain behavior is interpreted in terms of the testing conditions, thermomechanical characteristics, and microstructure.
II.
C H A R A C T E R I Z A T I O N OF M A T E R I A L S
The alloys used in this study are designated as 90WNiCu, 95WNiFe, and 97WNiFeCu, respectively, where the numeral represents the nominal weight percentage of tungsten (W) in the alloy and the other symbols indicate the principal alloying elements. The materials are composed of spheroidal grains of nearly pure tungsten surrounded by a binder phase, which is a solid solution of tungsten with the alloying elements. Typical microstructures (Figure 1) show the differences in grain sizes, grain shapes, and volume fractions of tungsten and binder phase among the three alloys. Table I lists compositions, both nominal and obtained by chemical analysis of the test materials, and also electron probe microanalysis results for the binder phase in each alloy and for a brittle phase which was observed in the 97WNiFeCu alloy. X-ray diffraction was carried out to determine the structure of the binder phase, which was found to be face-centered cubic with a lattice parameter of 0.358 nm for all alloys. Physical property data for the alloys are given in Table II. All the alloys were tested in the as-received condition. Following its sintering treatment the 95WNiFe alloy had
R. L. WOODWARD, N. J. BALDWIN, I. BURCH, and B. J. BAXTER are with the Australian Department of Defence, Materials Research Laboratories, P.O. Box 50, Ascot Vale, 3032, Victoria, Australia. Manuscript submitted December 4, 1984.
METALLURGICAL TRANSACTIONS A
been subjected to a vacuum anneal at 1100 °C followed by rapid cooling. The 97WNiFeCu alloy was in the as-sintered condition, but no information was available on whether the 90WNiCu had received any post sintering heat treatment.
III.
TESTING PROCEDURE
Compression tests were carried out on 4.76 m m nominal diameter cylinders with a height-to-diameter ratio of one. This is a compromise which avoids the problems of buckling at larger height-to-diameter ratios, and friction effects which occur at low height-to-diameter ratios. A thin layer of teflon was used to minimize frictional constraint with the platen ends; however, slight barrelling was observed with some evidence of nonunif
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