HIGH-TEMPERATURE CREEP OF AL-TIB2 PARTICULATE COMPOSITES

Two Al-20vol.%TiB2 composites produced by different processing routes, namely conventional powder metallurgy (PM) and XD(TM) (XD(TM) is a pr oprietary process of Martin Marietta Laboratories), were creep tested under compression in the temperature range 573-673 K to evaluate the s teady state creep mechanisms. The steady state creep rate data of the conventional composite, covering almost five orders of magnitude in cr eep rate, were rationalized in terms of the substructure invariant mod el, which predicts a stress exponent of eight and lattice diffusion co ntrol, together with the existence of a threshold stress. The higher v alues of the apparent stress exponent (8-14) and the apparent activati on energy (339 kJ mol(-1)), observed for the XD composite, do not agre e with the existing models for dislocation creep even after considerin g the threshold stress. This is possibly because of the anomalous cree p behaviour of the XD composite observed at higher temperatures. A com parison of the steady state creep data for PM Al-TiB2 and Al-SiC compo sites shows that the SiC particulate provides better creep resistance than the TiB2 particulate. The XD Al-TiB2 composite has better creep s trength than the conventional Al-TiB2 composite; however, its creep st rength 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 d iscontinuously reinforced Al matrix composites.