A study on the improvement of the sintered density of W-Ni-Mn heavy alloy
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I.
INTRODUCTION
BASED on a recent investigation by Magness, it is believed that the penetration performance of a kinetic energy penetrator is markedly dependent upon the self-sharpening ability that stems from the adiabatic shear band of the penetrator. Although the conventional tungsten heavy alloy, which consists of W, Ni, and Fe, has good mechanical properties at lower strain rates, its penetration performance is inferior compared to that of depleted uranium. This inferiority might be associated with the low tendency to form adiabatic shear bands in the tungsten heavy alloy.[1,2] The influencing material parameters on the formation of adiabatic shear bands are material density, melting point, specific heat capacity, strain-hardening exponent, and thermal conductivity.[3] Among these influencing factors, the thermal conductivity may contribute dominantly to the adiabatic shear band, because the adiabatic condition is directly related with thermal conduction through the material. The thermal conductivity of Mn is as low as one-tenth that of Fe; therefore, the addition of Mn, instead of Fe, into the conventional heavy alloy system may give rise to increased adiabatic shear bands. Compared to those of the traditional tungsten heavy alloy (W-Ni-Fe), the W-Ni-Mn alloy also has benefits due to decreased sintering temperature and in the refinement of W grain size for the liquid phase sintering process.[4,5] However, there is a sintering problem revealed by lower sintered density, because the Mn powder is easily oxidized, and its oxide is not reduced even under a high purity hydrogen or vacuum environment. This problem limits its application to the kinetic energy penetrator used for defeating modern armor materials. In the present study, liquid phase sintering has been car[1]
ried out under controlled atmosphere, considering that the oxidation of Mn is mainly due to the water vapor released from the as-received powders of W and Ni during their reducing reaction. The objective of this work is to obtain the W-Ni-Mn heavy alloy with a full sintered density.
II.
Average grain sizes of the tungsten, nickel, and manganese powders used in this study are 2.5, 3.0, and 3.5 mm, respectively. The powders are weighed and blended for a composition of 90W-6Ni-4Mn (wt pct) without a binder in a tubular mixer for 8 hours. The mixed powder is compacted under a pressure of 100 MPa into a cylindrical bar (10-mm diameter 3 10-mm height). Liquid phase sintering is performed in a horizontal furnace under controlled atmosphere with high purity nitrogen and/or hydrogen gases. The hydrogen gas used in the present work shows a dew point of 250 7C. The sintering cycle consists of heating up to reduction temperatures under a nitrogen or hydrogen gas, holding at the reduction temperatures for 30 minutes to 4 hours after the atmosphere is changed to dry hydrogen, and sintering at 1260 7C for 1 hour. The heating rate and reduction temperature used are varied from 5 7C/min to 25 7C/min and 1050 7C to 1200 7C, respectively. The effects of sintering
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