Quantitative study of the precipitates evolution in stress aged Al-Cu-Mg-Ag alloy single crystal
Al-1.96Cu-0.24Mg-0.18Ag alloy single crystal with (15) plane orientation has been used to investigate the effect of external stresses (50, 100, 134, 160 and 200 MPa) on the precipitation behavior of Ω, θ′ and S precipitates. The microstructure of the aged specimens was observed by transmission electron microscopy (TEM), selected area electron diffraction (SAED) and scanning transmission electron microscopy (STEM). X-ray elemental mapping under high-angle annular dark-field (HAADF) was used to analyze the elements in the precipitates. TEM images showed that the amount and distribution of the three types of precipitates are different in specimens aged under different external stresses. Along with the SAED and STEM images, the type, amount and distribution of Ω, θ′ and S precipitates were quantitatively measured. Compared to the results on stress-free aged specimen, the amount of Ω precipitates decreases in stress aged samples, while that of the S precipitates increases, and the amount of θ′ precipitates is constant with low value. The orienting effect of stress on Ω precipitates gets serious at 100 MPa, but it decreases at 200 MPa. The orienting effect of stress on S precipitates is quite similar to that in the case of the Ω phase. As the competition of acquiring Mg and Cu atoms between S and Ω precipitates, the orienting effect of stress on S precipitates is serious from 100 to 160 MPa. Both the orienting effect of stress on Ω and S precipitates decreased because of the inhibiting effect of dislocations at 200 MPa. The orienting effect on θ′ variants is different from that of Ω precipitates. It is serious at 50 MPa and gets weaker at 100–200 MPa.