The effect of vacuum treatment on mechanical properties of W-Ni-Fe heavy alloy has been studied. The specimens of 96 w/o-2.8w/o Ni-1,2w/o Fe composition have been sintered at 1460℃ in hydrogen and then heat treated at 1000℃ in vacuum or hydrogen from 0 to 6 hours.
As-sintered specimens show elongation value of 7.0 ± 0.9% and U.T.S., 750 ± 30 MPa. By vacuum treatment at 1000℃, the elongation and U.T.S. increase markedly to 21.7% and 890MPa, respectively. There is no significant change in mechanical properties with vacuum treating time to 6 hours. But, mechanical properties of the specimens heat treated under hydrogen remain unchanged. When the vacuum treated specimens are exposed again to hydrogen atmosphere at 1000℃, mechanical properties are decreased to the level of as-sintered or hydrogen heat treated specimens.
The fracture surfaces of these specimens have been examined by scanning electron microscopy. As-sintered or hydrogen heat treated specimens fractured mainly along the interface between tungsten grains and matrix. In the ductile vacuum treated specimen, however, ductile tensile fracture of the matrix and cleavage fracture through the tungsten grains have been observed.
It is suggested that after normal sintering process, heat treatment has no effect on mechanical properties of W-Ni-Fe heavy alloy. On the other hand, the vacuum treatment improves the mechanical properties by removing the hydrogen embrittlement effect.