Grain-Boundary Precipitation and Fracture Behavior of an Al-Cu-Li-Mg-Ag Alloy

摘要

RX818 alloy is a precipitate-strengthened Al-Cu-Li alloy with additions of Mg and. Ag for improving the nucleation frequency and distribution of the precipitate phases. In addition to the main strengthening T, (Al_2CuLi) plantlike precipitates,~[1-7] a significant amount of S' (Al_2CuMg) lath-shaped particles~[8,9] precipitates from the Al matrix during extended low-temperature aging (up to 2000 hours at 107 deg C to 163 deg C) after the initial -T8 treatment (stretched 5 pet and aged at 163 deg C for 20 hours)~[10] The combination of high strength and toughness obtained from the T_1 and S' precipitates in RX818 alloy make it a potential airframe material for a high-speed civil transport (HSCT) vehicle, which may operate at speeds of Mach 2 to 2.4. Recently, however, this alloy was reported to display a reduction in fracture toughness after extended thermal exposure.[11] Although the reduced fracture toughness could be due to several phenomena, grain-boundary precipitation was suspected to play a major role in the fracture toughness behavior and therefore needed to be. investigated in detail. The purpose of this communication is to report various grain-boundary T_1 and S' morphologies and their dependence on the boundary orientation and the misorientation between adjoining grains in RX818 alloy. All samples were made from 3-mm-thick RX818 alloy sheet with the composition listed in Table I. The sheets were rolled from 363-kg ingots produced by Reynolds Metals Co. (Richmond, VA).