MECHANICAL BEHAVIOR OF POLYCRYSTALLINE TUNGSTEN AT ELEVATED TEMPERATURE

The influence of stress, temperature, grain size and subgrain size on the steady state creep rate of polycrystalline unalloyed tungsten has been examined based on the extensive published data available. It was shown that the steady state creep rate of polycrystalline tungsten, and other metals, is given by ε{lunate} ̇ = Sλ2 Deff( σ E)7 where λ = subgrain size, σ = creep stress, E = average unrelaxed elastic modulus, and S is a universal creep constant equal to about 3 × 1040 cm-4. The effective diffusion coefficient, Deff, equals (DLf{hook}L + DDf{hook}D) where DL is the lattice diffusion coefficient, DD is the dislocation diffusion coefficient, and f{hook}L and D are the fraction of atoms participating in lattice and dislocation diffusion respectively. When subgrains form during creep, as in normal behaving materials, λ = kσ-1 (k is a material constant) and the above equation yields the generally observed five power stress law for creep. When subgrains do not form during creep (as in many refractory metals produced by powder metallurgy techniques), λ in the above equation is replaced by the grain size. In this case ε{lunate} ̇αL7 and ε{lunate} ̇;αL2, predictions in agreement with experimental evidence. The power law breakdown for creep at high stresses can be explained by one of two possibilities 1. (i) the creation of excess vacancies by plastic deformation which increases DL in the Deff term 2. (ii) the increase in dislocation density with increasing stress, which influences the dislocation diffusion term, DDf{hook}D, in Deff. © 1969.