The effects of tungsten powder size, matrix phase content, copper addition, and sintering temperature on density and mechanical properties of sintered W-Ni-Fe heavy alloy have been studied. The W particle sizes were 0.95μ, 2.91μ, 6.0μ, and (Ni + Fe) contents were 10% and 2%. The Ni to Fe ratio were always maintained at 1 to 1. For 90W-5Ni-5Fe specimens part of Ni and Fe were replaced by 0.5, 1, and 2% Cu.
The tensile test specimens were prepared by usual powder metallurgy techniques. The compacts were sintered at various temperatures between 1000℃ and 1460℃, for 30 minutes in hydrogen atmosphere. The density, ultimate tensile strength, elongation, and hardness of the sintered specimens were measured.
For all 90W-5Ni-5Fe specimens with different W powder size densities around 99% of the theoretical density were obtained after sintering in the liquid phase sintering temperature. In solid state sintering, the specimens with smaller W powder size showed larger sintered density.
Both the ultimate tensile strength and elongation of these specimens show rapid increase when the sintering temperature approaches the expected solidus range. Although for specimens sintered at around 1400℃ it is possible to obtain high densities, the mechanical properties were found to be poor.
In solid state sintering between 1100-1350℃ the specimens with 2% (Ni+Fe) show higher relative density than those with 10% (Ni+Fe). This result appears to be consistent with Brophy's observation that in activated sintering of W-Ni compacts the highest densification occurs with 0.25% Ni. The specimens with 2% (Ni+Fe) showed low tensile strength and quite brittle at all sintering temperatures.
For 90W-5Ni-5Fe specimens, the size of W grains after 30 minutes of sintering at 1460℃ appears to be independent of the original W powder size. And the W grain size of 90W-5Ni-5Fe specimens are smaller than that of 98W-1Ni-1Fe after liquid phase sintering.
For 90W-5Ni-5Fe specimens small Cu addition has little effect on densification during sintering. Substituting 1 or 2% of copper for (Ni+Fe) in the matrix phase produced a typical liquid phase sintered microstructure, in the temperature range of 1350 to 1400℃, which was not found in the specimens with 0.5% copper added or without copper. The tensile and elongation properties of the specimens with 1 or 2% of copper were at a maximum at the lowest temperature which gives typical liquid phase sintered microstructure. By increasing the temperature above the lowest liquid phase sintering temperature, the tensile strengths and elongations of the specimens were decreased.
Some attempts were made to explain the observed results by the present theories of liquid phase sintering and activated sintering. The results on W grain growth are difficult to explain unambiguously by present theories.
