Partial melting effect on tensile elongation behavior and activation energy for plastic flow in high-strain-rate superplastic 2124 Al alloy and 2124 Al composites

Abstract

It is now well recognized that high strain-rate superplasticity (HSRS) can be obtained from powderprocessed aluminum and aluminum matrix composites [1–6]. Though the origin of HSRS is still controversial, it has been proposed that the presence of a liquid phase is important for achievement of HSRS as it serves to help the accommodation process for grain boundary sliding by relaxing the high stress concentration around the matrix/particulate interfaces [7]. Mabuchi and Higashi [8] showed that the optimum superplastic temperature for the MMCs is often close to the onset temperature for partial melting. Mabuchi et al. [8–10] and Koike et al. [11] conducted differential scanning calorimeter (DSC) tests on the 20% Si3N4 2124 Al composites reinforced by particulates or whiskers and discovered that for each material two downward peaks appeared in the DSC trace that were small and sharp, and large and continuous, interpreted to represent the local melting at the reinforcement/matrix interfaces and the melting of the aluminum alloy matrix, respectively. The temperatures for partial melting were 816 K, 784 K and 849 K for the 20% Si3N4 (dp= 0.2μm, where dp is the size of particulate reinforcement), 20% Si3N4 (dp= 1μm) and 20% Si3N4w (whisker) 2124 Al composites. Recently, Zahid et al. [12] reported HSRS in a PM 18 vol% SiCp (dp= 3μm)/2124 Al composite where tensile elongations over 300%, with the maximum value of 450%, were obtained in the temperature range between 743 K and 783 K at 7× 10−2 s−1. Their DSC test, however, did not show the sharp peak for partial melting. The onset temperature for a continuous endothermic curve was 861 K, which is significantly higher than the optimum temperatures where superplasticity was obtained. Kim et al. [13] recently investigated the HSRS in a PM 20 vol% SiCp (dp= 3μm)/2124 Al composite at the temperatures between 773 K and 838 K. Tensile elongations of about 300% could be obtained at temperatures between 793 K and 823 K near 10−1 s−1, with the largest tensile elongation of 400% at 808 K. Kim and Hong [14] also examined the HSRS behavior of a PM 2124 Al alloy where tensile elongations over 300% were achieved at temperatures between 773 K and 823 K at the optimum strain rate of 10−2 s−1, with the tensile elongation as large as 700% at 823 K. The aim of the present study is to look for evidence for partial melting in the HSR superplastic 2124 Al alloy and 20 vol% SiCp/2124 Al composite using a DSC technique, and to compare the DSC results with those reported by other investigators. The tensile ductility and activation energy for superplastic flow are also discussed based on the DSC test results. The chemical composition of the matrix of the composite is 3.5%Cu-1.24%Mg-0.53%Mn-0.14%Fe0.01%Zn-bal Al by weight percent. The 2124 Al alloy and 20% SiC/2124 Al composite were prepared by hot pressing at 843 K and hot extrusion at 723 K. DSC tests were conducted on the as-extruded and the superplastically deformed samples in a ∼50 mg specimen during heating and cooling at a rate of 10 K min−1. Two different DSC schedules were arranged for each material: 1) the sample was heated to 993 K; 2) the sample was heated to 878 K, held there for 30 min, and cooled down to 673 K. Then it was heated again to 993 K. Fig. 1 shows the DSC results for the superplastically deformed 2124 Al alloy (803 K, 10−1 s−1, 280%) and 20% SiCp/2124 Al composite (808 K, 9× 10−2 s−1,