Microstructural evolution during hot deformation of the 6061 aluminium alloy based Al₂O₃ metal matrix composites

The development of conventional direct casting techniques to produce aluminium alloy based metal matrix composites and the successful fabrication of these composites by extrusion and forging processing has contributed to their successful commercialization. The Center for Metallurgical Process Engineering recently initiated a collaborative research study with the Ontario Center for Materials Research to develop a mathematical model describing microstructural evolution of 6061-alumina metal matrix composite during extrusion processing. The research described in this thesis was directed at quantifying the microstructural changes associated with hot axisymmetric testing of a 6061 aluminium alloy reinforced with 10, 15 and 20% alumina particulate. Cylindrical specimens 10 mm diameter x 15 mm height were compressed in the temperature range of 573-823 K (300-550 °C), at a strain rate of 0.01-10 /s. The resulting true stress-true strain data corrected for deformation heating were used by another researcher to develop the constitutive equations for hot deformation. True local strains at different points within the heterogeneously deformed specimen were determined by metallographic measurements of the aspect ratio of the deformed grains. To reveal the aluminum grain boundaries, existing etching techniques were modified. Grain and subgrain size evolution was investigated by optical metallography. Recovery behavior of the 6061 and the composites was very similar while recrystallization behavor differed markedly. Only up to 5% fraction recrystallized was observed in the 6061 material deformed at temperatures above 723 K and annealed at the deformation temperature, while grain growth was more apparent. Deformation at 723-773 K and annealing at 798 K produced up to 8% fraction recrystallized. Five regions with different microstructural behaviour were identified for the composites: i) below 673 K recovery is sluggish and deformation flow lines persist during annealing; ii) at temperatures above 673 K at strain rates less than 0.5 /s, recovery is intensive, the optically visible subgrains form and the original grain boundaries persist during deformation; iii) at strain rates from 0.1 to 1 /s static recrystallization occurs; iv) at strain rates from 1 to 5 /s in the composites there is a region of metadynamicre crystallization; v) at strain rates above 5 /s dynamic recrystallization occurs. The mixed static/metadynamic recrystallization kinetics (static or dynamic nucleation and static growth of recrystallized grains) in the composites in the temperature range 723-788 K for strain rates of 0.5-5 /s and strains of 0.4-1.2, could be described using the Avrami equation with n=1.4 and the time for 50% recrystallisation as: t₅₀% = 3.1‧10⁻¹⁸‧έ⁻ˣ‧exp(293000/RT)‧ε⁻ⁿ ; with x~2 and n=5.3. In the region exibiting dynamic recovery, the temperature-strain rate dependence of the resulting subgrain size was determined to be dsub = -0.686+0.044‧In(Z), in good agreement with literature data. After deformation to a strain of 1, the average angle between the maximum cluster or particle dimension and the matrix flow direction was measured as 21 and 30 degrees respectively. No particle fracture, particle debonding or de-clustering was observed.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat