Prediction of activation energies for creep and self-diffusion from hot hardness data

Abstract

It is shown that the activation energy for creep or self-diffusion for pure metals can be determined from hot hardness data above 0.75T _m by means of the expressionH/E=G expQ _L/nRT· HereH is the hot hardness,E is the elastic modulus,G is a material constant,Q _L is the lattice self-diffusion activation energy,R is the gas constant,T is the absolute temperature, andn is the stress exponent for creep assumed equal to five. Hot hardness data above 0.5T _m plotted as logarithmH/E against reciprocal absolute temperature reveal two straight lines with a break observed at about 0.75T _m. It is shown that the break occurs at a value of strain rate, ∈, over lattice self-diffusivity,D _L, of about 10⁹, a value associated with power-law breakdown for creep. These observations suggest two conclusions regarding the rate-controlling process during hot indentation testing of pure metals. Between 0.75 and 1.0T _m, the deformation process is associated with lattice self-diffusion and creep flow in the power. law region. Between 0.5 and 0.75T _m the rate-determining process is associated with dislocation pipe diffusion and creep flow in the power-law breakdown region.