The effect of managanese addition on the microstructure of W-Ni-Fe heavy alloy
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I.
INTRODUCTION
RECENTLY, the addition of Mn into conventional heavy alloy, which consists of W, Ni, and Fe, has received attention to give rise to increased self-sharpening ability that stems from the localized adiabatic shear band of the penetrator,[1,2,3] because it is considered that the increased self-sharpening ability leads to superior penetration performance in kinetic energy ammunition.[4] The enhancement of the adiabatic shear band in the Mn containing alloy is based on the lowered thermal conduction, which promotes thermal softening rather than work hardening, through the material due the extremely low thermal conductivity of Mn compared to those of the other constituents such as W, Ni, and Fe, as shown in Table 1. Traditionally, the addition of Mn has been performed by a liquid phase sintering process, which consists of the mixing of Mn powder with the other elemental powders and sintering under a hydrogen atmosphere.[2,5] However, there is a severe problem of pores forming in the interior of a specimen, which stems from the high oxidation potential of the Mn. In order to overcome this problem, the atmospheric controlling[3] and the resintering treatment methods[6] have been successively proposed. The former method consisted of rapid heating up to reduction temperature under a nitrogen gas, holding after the atmosphere changes to dry hydrogen, and sintering at a temperature of 1260 7C. Heating under nitrogen gas can prevent the formation of Mn oxide by the reduction reactions of Ni and W oxides, and the holding under hydrogen atmosphere contributes to simul-
EUN-PYO KIM and MOON-HEE HONG, Senior Researchers, and WOON HYUNG BAEK, Principal Researcher, are with the Division of Structural Materials Research and Development, Agency for Defense Development, Taejon 305-600, Korea. IN-HYUNG MOON, Professor, is with the Materials Science Division, Hanyang University, Seoul 133-791, Korea. Manuscript submitted February 23, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
taneously reduce all oxides contained in the as-received powders. In the case of resintering treatment method, a compact of Mn powder is placed on the presintered W-Ni-Fe heavy alloy specimen and then resintered at the same sintering condition as the preliminary one. When adding Mn by the method, it is observed that the morphology and growth rate of W grains are altered. The morphological change in W grains was also previously reported when applying the cyclic heat treatment to the specimen[7] or when adding elements such as Mo and Re.[8,9,10] The causes of the former and later morphological changes were found to be nonuniform thermal stress[7] and chemical in-homogeneity in solid grains,[10] respectively. The morphological change in W grains by the addition of Mn may be caused by a different reason from those of previous investigations,[7–10] because Mn is not soluble in the W grains, but in matrix phase, and no cyclic stress is applied. In the present investigation, the effect of Mn addition on the growth kinetics and the cause of morpholo
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