Effect of Ni and Ni3Al additions on sintering of molybdenum powders

Sintering of molybdenum powders normally requires very high temperatures to obtain high achieved densities. A small addition of pure nickel powders, however, not only lowers the sintering temperature of Mo but also remarkably increases its sintered density. For example, a 2 mass% addition produces a 94% achieved density even at one-half of the homologous temperature for Mo, the achieved density strongly depending on the sintering temperature. The sintering proceeds quickly with a lower activation energy (143 kJ/mol) compared to the Mo bulk diffusion energy, and the entire sintering is accomplished in a rather short period of time. On the other hand, when Ni-10 mass% Al powders mostly comprised of an f.c.c. Ni3Al phase are added, the sintering temperature is further lowered compared to the case of Ni addition, and the achieved density is greatly increased at a much higher rate. The activation energy for densification (104.8 kJ/mol) is less than that obtained by adding Ni. A Mo-5 mass% (Ni-10 mass% Al) powder mixture sintered at 1573 K exhibits excellent flexial and tensile strengths at ambient temperature. The tensile strength of 880 MPa at room temperature is reduced by only 9% even at 773 K, suggesting a possible application of this material at elevated temperature.