Effects Of Ag, Cu, Mg On Microstructure And Aging Hardening Behavior Of Al-Cu-Mg-Ag Alloys

With excellent comprehensive performance, Al-Cu-Mg alloys are widely used inaerospace and other fields. The coarse strengthening phases can lead to bad mechanicalproperties obviously when the work temperature is higher than150℃. Adding trace Ag intoAl-Cu-Mg alloys can cause a new strengthening phase to precipitate, therefore the roomand elevated temperature strengths increase as well as the thermal stability is strengthened.In this paper, Al-Cu-Mg-Ag alloys with different component were prepared，and thensolid solution and aging treatments were proceeded. The effects of Ag, Cu, Mg and theircontents on microstructure, mechanical properties and aging hardening behavior werestudied with the methods of microstructure observation, BH measurement, room andelevated temperature tensile test. Based on the EET theory, the micro mechanism of theeffects of Ag, Cu and Mg on aging strengthening and precipitation nucleation were analyzed.It is show that with the increase of Ag content, the peak aging time shortens, peakhardness and strength first increase and then decrease, and the thermal stability isstrengthened. With the increase of Cu content, the peak aging time keeps basically changeless,peak hardness and strength increase, but elevated-temperature strength decreases obviously.With the increase of Mg content, the peak aging time shortens, peak hardness, strength andthermal stability all first increase and then decrease.After calculating the valence electron structure (VES) of the matrix in solid solution, it isfound that the reason for Ag can promote the precipitation of phase lies in the formation ofMg-Ag-cluster easier than that of Mg-Cu-cluster. The precipitation of phase not onlyconsumes Mg and Cu, but also inhibits the diffusion of Mg, therefore the precipitation of Sphase is inhibited while that of θ′phase is promoted to some extent.The micro mechanism that Ag, Cu, Mg and their contents can change aging hardeningbehavior lies in that they change the VES of the matrix, affect the precipitation mechanism ofthe strengthening phases and change the amount and type of precipitated phases. Thecalculations of the bonding ability of the strengthening phase, the bonding force of thestrongest bond and the VES of interface between the strengthening phase and the matrix showthat the strengthening effect of phase is the best, θ′phase is second, S phase is the weakest.phase is easier to precipitate than θ′phase and S phase, and its thermal stability is betterthan that of the two phases. So the element which can promote the formation of phase willaccelerate aging process and improve the effect of age hardening and thermal stability.It is significant for the optimization of alloy component and aging treat process toresearch the micro mechanism of the nucleation of the precipitation phases and the aginghardening behavior in Al-Cu-Mg-Ag alloys.