Precipitation strengthening of stress-aged Al-Cu-Mg-Ag alloy single crystals

Abstract

Al-Cu-Mg-Ag single crystals with five different orientations were used to study the precipitation behavior during stress aging under a stress range from 0 to 150 MPa. TEM characterization of the aged samples indicated that three kinds of precipitates Ω, S and θ′ formed, and the number density of S precipitates increased with the applied stress. The distribution of two Ω variants Ω1/Ω2 behaved preferentially oriented under higher applied stress and the anisotropic degree was various between 0.3 and 1.4, which depends on the plane orientation of single crystal. As the stress loading directions of single crystals with (6 5‾ 10), (3‾ 1 3) and (2 1 2) plane orientations are more perpendicular to the dislocation slide planes {111}, the distribution of Ω1/Ω2 in these single crystals is more homogeneous with the number ratio between 1.0 and 1.2. Considering the coupling effects of crystallographic anisotropy, stress-oriented precipitates and stress induced dislocations on precipitates, the size and distribution of Ω, S and θ’ were quantitatively counted and analyzed. The increments of precipitation strengthening were calculated and compared with the measured hardness results, indicating that the hardness of single crystal with (6 5‾ 10) orientation increase by most with 15 HV after stress aging. The trend of hardness changes with applied stress and plane orientations was well modeled in stress-aged Al-Cu-Mg-Ag single crystals.