Powder Processing and Characterization of W-3Ni-1Fe Tungsten Heavy Alloy
Mechanical alloying, compaction by cold isostatic pressing, and pressureless sintering were used to study the potential for W â 3 wt% Ni â 1 wt% Fe to be processed into the bulk nanocrystalline form as a replacement material for depleted uranium in kinetic energy penetrators. Milling time and sintering temperature were varied from 15 to 100 hours and 1000 to 1300°C respectively. Particle size analysis and SEM showed a bimodal particle size distribution with most of the particles below 10 µm in size. XRD peak broadening analysis showed crystallite size to be reduced to below 50 nm, while peak shifting indicated a reduction in W lattice parameter due to dissolution of Ni and Fe atoms into the W BCC lattice.
Post-sintering bulk characterization showed density increasing strongly with increasing sintering temperature to above 90% of theoretical density at 1200°C. Apparent activation energy for sintering decreased strongly with increasing milling time. SEM micrographs showed a bimodal grain size distribution with some areas of smaller submicron grains and others with larger grains on the order of 1 – 4 µm, likely connected to the bimodal particle size distribution from milling. XRD and SEM also showed the precipitation of two secondary phases during sintering: (Fe, Ni)6W6C incorporating carbon from the grinding media and an FCC solid solution of Ni, Fe, and W. The intermetallic carbide phase will increase strength but reduce ductility of the bulk material, which is not desirable. Micro and macrohardness testing show similar trends as density with a strong correlation with sintering temperature.