Strain-hardening behaviour in sintered aluminium-3.5% alumina composite preforms during axial compression with and without annealing

The present investigation has been undertaken to evaluate the strain-hardening phenomenon experienced in sintered aluminium-3.5% alumina composite preforms during axial compression tests. Powder preforms of three different initial density ratios, namely 0.75, 0.80 and 0.90, with three initial aspect ratios for each density level were prepared using a suitable die-set assembly on a 0.60 MN capacity hydraulic press. Sintering was carried out in an electric muffle furnace for a period of 90 min at 550°C in a nitrogen atmosphere. Each sintered compact was subjected to incremental compressive loading with and without annealing after each step of deformation. The annealing operation was carried out in a furnace at 200°C for 30 min in a nitrogen atmosphere. The strain-hardening exponent n and strength coefficient K were obtained for each initial preform and for each aspect ratio. Analysis of the experimental data shows the existence of empirical relationships between the material parameters namely, n values and K values and the ratio of the initial preform densities to the theoretical density. Further, it was found that n consists of two segments, one representing the work hardening of the matrix material and the other due to densification. It has been also established that a power-law relationship exists between K and the percent fractional theoretical density %(ρd/ρth).