Electron Microscopy Investigation Of Precipitates In Al-Cu-Mg-Ag(-Ce) Alloys

Al-Cu-Mg series alloy is a hardenable alloy. For an Al-Cu-Mg alloy with high Cu:Mg ratio, a small addition of Ag to the alloy promotes the formation of a kind of plate-like precipitate which is fine, hexagonal-shaped and uniformly distributed on {111}αmatrix planes. This precipitate was named as ? phase, and its formation can change the precipitation process and further enhance the age-hardening effect for the Al-Cu-Mg alloy.In this paper, the major precipitates in Al-Cu-Mg-Ag alloys subjected to different aging treatments have been studied by a combination of various advanced electron microscopy techniques and theoretical image calculations, with the aims to investigate characteristic electron diffraction and structures of the precipitates, to reveal imaging characteristics of the ?/α-Al interface due to the existence of Ag in it and to clarify the nature of the preferential precipitation phenomenon. In addition, the effects of presentation of small amount of rare earth Ce on the microstructures in as-cast and rapidly solidified Al-Cu-Mg-Ag alloys were also studied.Energy dispersive X-ray spectroscopy (EDS) analysis using nano-sized electron probe has directly confirmed the existence of Ag atoms at the ?/α-Al interface. HRTEM image calculations show that the distribution of Ag on the ?/α-Al interface has sensitive influence on the imaging contrasts of the interface. However, because HRTEM contrast forming is intricate, how Ag atoms distribute on the interface need study by means of other analysis methods. The existence of Ag element greatly influences the nucleation and precipitation behaviors of the ? precipitate in Al-Cu-Mg-Ag alloys. This can be attributed to the fact that Ag element can change the thermodynamic factors for nucleation and precipitation of the ? structure. In other words, the existence of Ag can accelerate the diffusion speed of Cu atoms and reduce the growth activation energy of the ? precipitate. This will promote the precipitation ability of the ? precipitate which is a Cu-rich structure and therefore enhance alloy strength.HREM observations of Al-Cu-Mg-Ag alloys subjected to different aging treatments show the existence of long and straight arrays of precipitates, which can be interpreted as preferential precipitation on small-angle grain boundaries or subgrain...