Mechanical Behavior of Chemically Vapor Deposited Tungsten at Elevated Temperatures
OAK RIDGE NATIONAL LAB TN
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Many applications of chemically vapor deposited CVD tungsten require that the material be stressed at elevated temperatures. We have run creep-rupture tests at 1650 and 2200 deg 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 deg C the CVD product 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 deg C the minimum creep rate is higher and the rupture life lower for the CVD product. Two microstructural features of importance were noted in the CVD tungsten 1 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 deg 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. We 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.
- Properties of Metals and Alloys