HIGH-STRAIN RATE SUPERPLASTIC FLOW-STRESS AND POST-DEFORMATION MECHANICAL-PROPERTIES OF MECHANICALLY ALLOYED 2024-ALUMINUM-ALLOY REINFORCEDWITH SIC PARTICLES

Composites consisting of 2024 aluminium alloys reinforced with volume fractions of 0, 5, 10, and 15 vol.-% of SiC particles were fabricated from the mechanically alloyed powders by an optimised hot compactions and prestraining process. Fine and equiaxed grain structures with grai n sizes of <1 mu m were observed within the matrix of each alloy. The composite specimens were compressed at temperatures between 733 and 81 3 K with a wide strain range from 10(-3) to 10 s(-1). Two strain rate regions with different slopes from similar to 5 x 10(-1) s(-1) were fo und 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 sigma(th) for each alloy was estimated using an extrapolation procedure. A linear relationship was found bet ween V-f(0.5) and sigma(th) where V-f is the volume fraction of SiC pa rticles. In the higher stain rate region of each alloy, m values great er 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 pr ocessing of the specimens compressed at 773 K. Thus, it was found that the optimum temperature for the high stain rate superplastic processi ng of the composites was just below the solidus temperature of the 202 4 aluminium alloy. The grain coarsening resulted in the decrease of po st-deformation strength and ductility as well as the m value in hot co mpression above the solidus temperature. (C) 1997 The Institute of Mat erials.