Microstructural development and thermal stability of aluminium-based composites processed by severe plastic deformation.

摘要

Equal channel angular pressing ECAP is a process whereby simple shear is applied to a billet during multiple passages through an angled channel of constant cross section. The process is capable of generating very large plastic strains that significantly refines the microstructure without altering the external dimensions of the billet. A number of properties are influenced by grain refinement with the generation of a submicron grain structure SMG by ECAP resulting in improved strength and hardness and enhanced superplasticity.In this thesis, both an AA7075 alloy and AA7075 Al-base metal matrix compositeMMC reinforced with 5 wt. percent of 50 nm diameter SiC particles was produced by apowder metallurgy route followed by hot extrusion. The materials were subsequentlydeformed by ECAP at 350 C to a true effective strain of 4.6 in an attempt to refine themicrostructure and further distribute the SiC reinforcement phase in the composite. Thehigh temperature microstructural stability of both the as-deformed alloy and compositewas investigated to elucidate the effect of the reinforcement phase on continuous and discontinuous grain coarsening. It was found that ECAP generated a fine equiaxed grain size of ~ 2.3 !m and ~1.8 !m in the alloy and composite, respectively. Thecomposite was more refined after ECAP since the SiC particles allow the matrix toundergo more grain refinement during deformation. ECAP was found to be a reasonablemethod for further distributing SiC clusters in this composite which is important foroptimizing the reinforcement phase in terms of ambient temperature strengthening andenhanced grain stability at elevated temperature.Both the alloy and composite were annealed at times up to 5h at 500 C to assess grainstability. During annealing, the grain structure of both materials evolved in acontinuous manner unlike the discontinuous process of recrystallization. Such aprocess is similar to continuous recrystallization observed in a range of heavilydeformed Al alloys. Substantial grain boundary interactions with MgZn2 precipitatesand oxide particles were found in the alloy, with precipitate, oxide and SiC particlesfound in the composite. The strong pinning force exerted by these particles minimisedgrain growth in both materials with the composite exhibiting a finer less than 2.5 !m grainsize than the alloy less than 3.5 !m after extended annealing. This enhanced grain stabilitywas attributed to the high volume fraction SiC particles which resulted in a large valueof the dispersion parameter f/d which results in significant boundary pinning duringannealing. Grain stability was also analysed in terms of a recently-proposed mean fieldmodel of annealing where it was predicted that the composite should not undergodiscontinuous coarsening, as observed experimentally.