Steady state creep behaviour of particulate-reinforced titanium matrix composites

The steady state creep behaviour of unreinforced Ti, Ti-Ti2C and Ti-TiB-Ti2C composites has been examined in the temperature range 823-923 K. It is shown that the creep deformation of unreinforced Ti is governed by climb-controlled creep mechanism for which the stress exponent is between 4.1 and 4.3 and the activation energy is 236 kJ mol(-1). For composites, the stress exponents are between 6 and 7 at 823 K but are similar to unreinforced Ti at 923 K. The measured steady state creep rate of composites is found to be 2-3 orders of magnitude lower than unreinforced Ti in the investigated temperature range. It is then established that the origin of creep strengthening at 823-923 K is due to the combined effects of increased modulus of composites and the refined microstructure. It is further shown that the change of stress exponent of composites al 823 K is because of the change in creep mechanism from lattice-diffusion controlled dislocation climb to pipe-diffusion controlled dislocation climb. By analyzing the creep data, a modification in the dimensionless constant, A = 3.2 x 10(5) exp(-24.2 V-r) for lattice-diffusion regime and A = 9.4 x 10(5) exp(-28.1V(r)) for pipe-diffusion regime, where V-r is the volume fraction of reinforcements, is suggested to account for the influence of reinforcements on creep kinetics.