On the activation energy of high temperature creep in metals

The temperature dependence of the activation volume or ‘stress-concentration factor’ q appearing in the expression relating the equilibrium tensile creep rate dε/dt, the temperature T and the applied tensile strew σ has been studied in the case of several pure metals subject to creep above about 1/2 T m, where T m, is the melting point. On writing q=nb 3, where b is the distance of closest approach of atom in the lattice, the relation was found to be valid in all cases, with Q=αT m, and α=4.9 (cal/g atm)/°K, i.e. Q is equal to about twice the latent heat of melting. The increase of q with temperature is ascribed to the increase in the mean spacing between adjacent logs in screw dislocations, and Q appears to be the activation energy for the conservative motion of jogs along dislocations. In face-centred cubic metals large values of n 0 are found to be associated with low stacking fault energies, and vice versa.