Grain-shape parameters for high-temperature creep resistance in powder metallurgy tungsten fine wires

Three grain-shape parameters, f ₁, f ₂, and f ₃, are defined to clarify the morphological effect of grains on the high-temperature creep resistance under the condition that no grain boundary cavitation occurs. The parameter f ₁ is characteristic of complicated grain shapes, suggesting that it can be a measure of the interlocking grain structure. The parameter f ₂ is an important parameter when torsional stresses are imposed on coiled wires, and f ₃ is characteristic of the short-range roughness of grain boundaries only when f ₁ is not greatly changed. The minimum creep rate decreases as the grain aspect ratio, R, increases for R <30, and the creep rate increases as R increases for R >30. The parameter f ₁, as well as Raj and Ashby’s model, gives a satisfactory explanation for the former behavior. On the other hand, it is proven that their model must be modified using f ₃ to explain the latter behavior because of the highly elongated grain configuration that is associated with R >30.