Recent research showed that the microstructure of tungsten heavy alloy could be modified through additions of molybdenum or rhenium. The peculiar microstructure like bimodal grain size distribution and irregular grain shape of this alloy, however, has not been understood. In part I, the microstructural evolutions during liquid phase sintering of 80W-10Mo-7Ni-3Fe (weight percent) compacts have been studied by using W powders of different size. When using fine W powder of 1.3 μm size, the grains of irregular shape which had been observed previously disappeared in short sintering time, and a typical heavy alloy structure resulted. In case of coarse W powder, however, irregular shape of grains and bimodal distribution of grain size were remained during prolonged sintering. The grains of a duplex structure were found to consist of the equilibrium W-Mo(Ni,Fe) grains and pure W particles in the matrix. The presence of pure W grains has been explained by the formation of a molybdenum-rich liquid at early stage of sintering because of lower liquidus temperature of molybdenum with Ni and Fe than that of tungsten. Because the liquid is believed to be saturated mostly with Mo, W particles can dissolve very slowly and remain for longer time. The early recovery of rounded grain shape for the specimen prepared from fine W powder is therefore attributed to fast dissolution of pure W particles at early stage sintering because of smaller particle size. The specimen prepared from 5.4 μm W powder shows a very low impact energy due to the segregation of C and O at the W particles interface, in contrast to slight improvement of ultimate tensile strength.
Depleted uranium is known to be a more effective penetration material than an equivalent density tungsten heavy alloy because of the ability of DU to self-sharpen during penetration. Recent experiments have provided evidence that the superior performance of DU is rooted in the ability of this material to fail through the formation of adiabatic shear. It was found that W-Ni-Mn alloy could fail via localized shear instability. However, a hydrogen atmosphere, usually used in the sintering of tungsten heavy alloys does not fully prevent the oxidation of manganese. The oxide results in pore retention that inhibits the full densification of this alloy. In part II, the liquid phase sintering of W-Ni-Mn alloy have been studied by using reduced or normal W powder. By using reduced W powder, W-Ni-Mn alloy resulted in sintered higher density of about 97~98%, compared to 93~94% of the specimen prepared from normal W powder. Because the pre-sintering of tungsten heavy alloy was carried out about at 800℃ in $H_2$ for the reduction of oxide, the oxide reduction of W powder by Mn powder seems to occur at the lower temperature than 800℃. The incomplete oxide reduction of Mn powder caused the change of matrix composition, which resulted in the change of grain size. Because W content in liquid decreased in the specimen prepared from reduced W powder, the grain size was finer than that from normal W powder. The infiltration of Mn-containing melt into W compact was studied for achieving full density. Although most of the open pore in W skeleton were filled with melt without pore formation, some closed pores were observed in the outside of W skeleton.
