The Effect of Quench Rate on the Microstructure and Properties of AF/C-458 and AF/C-489 Al-Li-Cu-X Alloys

Abstract

The Air Force recently developed two isotropic Al-Li-Cu-X alloys with 1.8W/oLi and 2.1w/oLi designated AF/C-458 and AF/C-489, respectively. The objective of this investigation was to determine the effect of quench rate on the microstructure and mechanical properties of the AF/C-458 and AF/C-489 alloys. TEM, SEM, microhardness, and tensile testing were utilized to ascertain these microstructure/property relationships for both alloys in the T4, T6, and T86 tempers as a function of quench rate. Subsequent losses in ductility for both alloys in all tempers with decreasing quench rate were determined to be due to the precipitation of the equilibrium Al 2 CuLi (T 1 ) phase along subgrain and grain boundaries which promoted intergranular fracture. Furthermore, yield and tensile strengths increased for both alloys in the T4 temper but decreased in the T6 and T86 tempers with decreasing quench rate. The increased strengths for the T4 condition resulted from the heterogeneous precipitation of coarse T 1 and naturally aged δ' phases. The decrease in yield and tensile strengths for the T6 and T86 tempers were also due to the coarse heterogeneous precipitation of T 1 which denuded regions of Cu thereby reducing the number density of fine matrix θ (T6) and T, (T86). Finally, a comparison of the quench sensitivity for both the AF/C-458 and AF/C-489 alloys indicates that the mechanical properties for both alloys were less quench rate sensitive than other typical Al-Li-Cu-X alloys.