Effect of Ag additions on the lengthening rate of Ω plates and formation of σ phase in Al-Cu-Mg alloys during thermal exposure

Effect of Ag additions on the mechanical properties and microstructures of the peak-aged Al-Cu-Mg alloys during prolonged thermal exposure at 150 degrees C, was investigated by tensile testing, conventional transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The results showed that after exposure for 500 h, >85% of the peak strength remained. Microstructure observations indicated that increasing the Ag content from 0.14 to 0.57% promoted the precipitation of a fine and uniform Omega phase and suppressed the formation of the theta' phase, leading to a notable improvement of the strength properties and thermal stability of the studied alloys. Quantitative TEM analysis showed that the coarsening of Omega phase was predominated by plate lengthening rather than thickening, while its lengthening rate was independent of various Ag additions during exposure at 150 degrees C. In addition, an increase of Ag also facilitated the formation of a cubic sigma phase, which was further supported by STEM results. (C) 2016 Elsevier Inc. All rights reserved.