Study On The Microstructures And Mechanical Properties And The Fracture Behavior Of The Al-Zn-Mg-Cu-Zr Alloys

Al-Zn-Mg-Cu age-hardening alloys with high strength and low density are widely used in aero field. On the basis of analysis of Al-Zn-Mg-Cu-Zr alloys made in America and China, two types of Al-Zn-Mg-Cu-Zr alloys with different copper content were designed in the paper: A1 alloy (Al-6.3wt%Zn-2.3wt%Mg-1.7wt%Cu), A2 alloy (Al-6.3wt%Zn-2.4wt%Mg-2.3wt%Cu). The evolution of microstructure and mechanical properties were studied. The evolution of coarse second phases and precipitates during the heat treatment were investigated by using TEM, SEM, DSC and XRD. The strengthening mechanism and fracture mechanism of the alloys under different hot working conditions were discussed. A novel phase transformation from the Mg(Zn, Cu, Al)2 phase to Al2CuMg phase is observed during the homogenization.The development of microstructure of the alloys for as cast, homogenized, hot extruded and solid solutioned conditions were studied, especially for the coarse second phases. The eutectic structures in as cast alloys were identified asα(Al)+Mg(Zn,Cu,Al)2, and coarse Al7Cu2Fe particles formed in the eutectic structures during solidification. The evolution of primary eutectic structure consists of three processes as follows: dissolving of Mg(Zn,Cu,Al)2, nucleation and growth of Al2CuMg particles while its fraction increases, coarsening of Al2CuMg together with the decreasing of its fraction. A novel phase transformation of the primary eutectic structure from Mg(Zn,Cu,Al)2 to Al2CuMg is observed during homogenization. The Al2CuMg particles formed in the primary Mg(Zn,Cu,Al)2 phase, and grew along the eutectic microstructure.The ageing hardening and precipitation behavior were studied. The two type alloys exhibited similar ageing hardening curves. During the single ageing treatment at 120℃and 140℃, the alloys exhibited high hardness, the peak hardness is above 200 (HV). The main precipitates are different under different ageing conditions. The main precipitates are GP zones andη' phase at 120℃. The main precipitates areη' phase at 140℃. The main precipitates areηphase andη' phase at 160℃. During the aging at 120℃and 140℃, the evolution of precipitate size can be well described using LSW theory, R∝t1/3. The operated mechanism of coarsening is diffusion. During the two step ageing, the pre-ageing at 120℃enhanced the mechanical properties of the alloys aged at higher temperatures. The existing temperature and stability of the GP zones were increased under the two step ageing treatment. During the three step ageing, good combination of hardness and electrical conductivity was achieved with the retrogressing temperature range from 180℃to 200℃.The mechanical properties and fracture behavior were studied. The two type alloys exhibited the same strengthening behavior, and the strength of the alloy with higher copper content is 5MPa-30MPa higher than that of the alloy with lower copper content. The yield strength of the alloy with higher copper content is above 600 MPa after ageing at 120℃from 28h to 60h or at 140℃from 6 h to 36h. The yield strength and ultimate strength of the alloy aged at 140℃for 20h is 622 MPa and 659 MPa, respectively. The fracture modes are mainly influenced by the size of the precipitate. The main mode in the alloy aged at 120℃is shearing fracture mechanism, while the fracture modes in the alloy aged at 140℃contained transgranular fracture and intergranular fracture mechanisms. When the ageing temperature is above 160℃, the fracture were caused by the coarse voiding at intermetallic particles. The fracture process is influenced by the second phases, which is in the order according to the detrimental effect: precipitates within the grain, precipitates in the grain boundaries, coarse intermetallic particles, Fe-bearing dispersoids and Al3Zr dispersoids.The effect of ageing treatment and copper content on the fracture toughness were studied. Both materials exhibited a combination of high strength and fracture toughness under suitable heat treatments. The toughness of A1 alloy is 10%-17% higer than that of A2 alloy. The main factor influenced the fracture toughness is the difference between the strength of the matrix and the strength of the grain boundaries. The precipitates at the grain boundaries decreased the strength of grain boundary, leading to the lower fracture toughness. The alloy with higher copper content contained a greater amount of coarse Cu-bearing particles, which deteriorate the fracture behavior and decrease the ageing hardening ability of the alloy.