Dynamic deformation and fracture behavior of tungsten heavy alloys have been investigated. The sintered 90W-7Ni-3Fe (wt%), 93W-5.6Ni-1.4Fe and 93W-4.9Ni-2.1Fe heavy alloy samples were packed with carbon black powder and annealed at various temperatures and for various times in $H_2$. With the carburization treatment, WC formed on the surface of the samples. In all of the samples with a WC layer, a η-phase region and a carbon-segregated region formed consecutively below the WC layer. With increased carburization temperature and time, the surface hardness increased, but the impact energy decreased. The dynamic torsional test results indicated that for the carburized tungsten samples, cleavage fracture occurred in the center of the gage section with little shear deformation, whereas shear deformation was concentrated at the center of the gage section for the conventionally processed sample without carburization. The deformation and fracture behavior of the carburized samples correlated well with the observation of the impacted penetrator specimens, i.e., microcrack initiation at tungsten particles and cleavage crack propagation. Since the cleavage fracture mode is thought to be beneficial for self-sharpening, these findings suggest the beneficial effect of the surface carburization on the penetration performance.
A carburization technique using a Cr powder layer has been developed to control the diffusion depth of carbon in W-Ni-Fe heavy alloys. The sintered 93W-4.9Ni-2.1Fe heavy alloy samples were covered with a Cr powder layer of about 1 mm thickness, and then packed with carbon black powder. The packed samples were heat-treated at 1150℃ for 10 min in $H_2$, and then for 50 min in $N_2$. The carburization treatment resulted in the formation of $Cr_7C_3$ and $Fe_3W_3C$ around the tungsten grains from the sample surface with a thickness of 40 - 50㎛. This carburized layer was much thinner than that formed without a Cr powder layer under the same experimental conditions. With the surface carburization, the surface hardness increased by ~ 75%, from 508 VHN to 888 VHN, and the impact energy decreased by ~ 31%, from 123 J to 85 J. After the carburization treatment, the main fracture behavior changed from smearing of matrix to cleavage of tungsten grains in a dynamic torsional test. A high-speed impact test showed that the surface carburization of penetrators induced the formation of many cracks around the penetrator surface, enhancing the self-sharpening, and improved the penetration performance. It appears that the developed technique provides an easy method of carburization without serious deterioration of the toughness of the material.
