Many applications of chemically vapor deposited (CVD) tungsten require that the material he stressed at elevated temperatures. We have run creep-rupture tests at 1650 and 2200°C to evaluate the mechanical behavior of this material, and the properties are compared with those of a typical heat of powder metallurgy (PM) tungsten. At 1650°C the CVD prod¬uct has low fracture strains (approx 5%) and a lower minimum creep rate. At high stresses the rupture life is shorter than that of the PM material;at low stresses the rupture lives are about equivalent. At 2200°C the minimum creep rate is higher and the rupture life lower for the CVD product.nTwo microstructural features of importance were noted in the CVD tungsten:(l) the formation and growth of voids and (2) the columnar nature of the grains. Fractographic techniques were used to study void nucleation and growth in the material. Nucleation appears to be spontaneous as the material is heated to elevated temperatures, indicating the presence of an impurity having a high vapor pressure. The growth appears to occur almost entirely by the stress-induced diffusion of vacancies into the void. At 2200°C under stress the voids reach such a large size that they comprise about 10% of the test specimen. The columnar grain structure of the material is important because it is very difficult to get extensive grain-boundary sliding and rotation in this type of structure.nWe have rationalized the creep behavior of this material on the basis of the effects that both the void growth and the columnar grain structure have on the individual deformation processes that sum to give the overall creep behavior.
展开▼
