High-temperature creep of AlTiB 2 particulate composites

Abstract

Two Al20vol.%TiB 2 composites produced by different processing routes, namely conventional powder metallurgy (PM) and XD TM (XD TM is a proprietary process of Martin Marietta Laboratories), were creep tested under compression in the temperature range 573–673 K to evaluate the steady state creep mechanisms. The steady state creep rate data of the conventional composite, covering alomost five orders of magnitude in creep rate, were rationalized in terms of the substructure invariant model, which predicts a stress exponent of eight and lattice diffusion control, together with the existence of a threshold stress. The higher values of the apparent stress exponent (8–14) and the apparent activation energy (339 kJ mol −1), observed for the XD composite, do not agree with the existing models for dislocation creep even after considering the threshold stress. This is possibly because of the anomalous creep behaviour of the XD composite observed at higher temperatures. A comparison of the steady sreep data for PM AlTiB 2 and AlSiC composites shows that the SiC particulate provides better creep resistance than the TiB 2 particulate. The XD AlTiB 2 composite has better creep strength than the conventional AlTiB 2 composite; however, its creep strength is inferior to the AlSiC composite. It has been suggested that an applied stress-independent load transfer mechanism is required to explain the origin of the threshold stress for steady state creep in discontinuously reinforced Al matrix composites.