Effects of Compositional Variation on the Thermal Stability of \(\theta\)′-Al₂Cu Precipitates and Elevated-Temperature Strengths in Al-Cu 206 Alloys

This study explored the effects of compositional variation of Al-Cu 206 alloys (Si and Mn concentrations, 0.12 versus 0.3 wt.% each) on the precipitation and thermal stability of θ′ precipitates, and the associated elevated-temperature strengths during long-term thermal exposure at 300 °C. The results indicated that both alloys under the T7 condition exhibited comparable yield strengths at 20 and 300 °C, which were predominantly controlled by θ′ precipitates. Upon exposure at 300 °C for up to 1000 h, the θ′ precipitates in the alloy with low Si and Mn concentrations remained stable and resistant to transformation into the equilibrium θ phase. The coarsening of θ′ proceeded through both lengthening and thickening, resulting in the continuing decline in YS at 300 °C from 93 MPa after 300 °C/100 h to 78 MPa after 300 °C/1000 h. Conversely, the θ′ precipitates in the alloy with high Si and Mn concentrations underwent almost complete transformation into deleterious θ after only 100 h at 300 °C, thus imparting a significantly lower YS of 69 MPa after 300 °C/100 h. The possible sources of these microstructure and property changes with the compositional variation were explored. This study has implications for strategic design of Al-Cu alloys for high-temperature applications.