High strain rate superplastic flow stress and post-deformation mechanical properties of mechanically alloyed 2024 aluminium alloy reinforced with SiC particles

Abstract

Composites consisting of 2024 aluminium alloys reinforced with volume fractions of 0, 5, 10, and 15 vol.-% of SiC particles were fabricatedfrom the mechanically alloyed powders by an optimised hot compaction and prestraining process. Fine and equiaxed grain structures with grain sizes of <1 μm were observed within the matrix of each alloy. The composite specimens were compressed at temperatures between 733 and 813 K with a wide strain rate range from 10⁻³ to 10 s⁻¹. Two strain rate regions with different slopes from ∼ 5 × 10⁻¹ s⁻¹ were found in log (true stress–log (strain rate) curves. In the lower strain rate region of each alloy, the strain rate sensitivity values m were 0.03–0.16. The threshold stress σ_th for each alloy was estimated using an extrapolation procedure. A linear relationship was found between <disp-formula><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="splitsection10-m1.tif"/></disp-formula> and σ_th where V_f is the volume fraction of SiC particles. In the higher strain rate region of each alloy, m values greater than 0.3 were obtained at 773 K, which is very close to the solidus temperature of 775 K for 2024 aluminium alloy. Moreover, the maximum yield strength and elongation for each alloy at room temperature were also obtained in the specimens compressed at 773 K. Thus, it was found that the optimum temperature for the high strain rate superplastic processing of the composites was just below the solidus temperature of the 2024 aluminium alloy. The grain coarsening resulted in the decrease of post-deformation strength and ductility as well as the m value in hot compression above the solidus temperature.