Study Of Strengthening And Toughening On Advanced High Strength Aluminum Alloys

In the present thesis, two advanced high-strength aluminum alloys produced by ingot-metallurgy method were investigated. The strengthening mechanisms and the effect of trace element zirconium in an Al-Zn-Mg-Cu alloy were studied. And the alloying effects and strengthening behaviors of a high-strength and low-density aluminum alloy, Al-Cu-Li system, were also investigated.The influences of element Zr in the solidification and homogenization processes were investigated. It is found that the grain sizes could be significantly refined with the addition of Zr. When the alloy contains more than 0.06%Zr, its grain sizes are smaller about thirty percent than that without Zr element. The primary Al₃Zr compounds (about 50um) are also observed in this study, when the content of Zr reaches 0.16%. The morphology of the Al₃Zr phases similar to that of the compounds containing Fe or Si element should be avoided in the solidification microstructures. After the ingots were homogenized, another metastable type Al₃Zr particles, which precipitate as coherency phases withα(Al) matrix are found in the Zr-bearing alloys and can exist for a long period of time. It is also found that the precipitation speed of the metastable Al₃Zr in Al-Zn-Mg-Cu alloys is much faster than that in Al-Zr binary alloys. The intensity of the Al₃Zr dispersoids and the equilibriumηphases also increase with the volume of Zr increasing.After the alloys forged, the sizes of subgrains are different with the variation of the Zr content. When the alloys were soluted for two hours at 470℃, the difference becomes more significant owing to the extent of recrystallization processing in each alloy. When the content of element Zr reaches 0.06%, the grain sizes could be smaller than 10um. According to the model proposed by Nes and Wert, it is necessary to keep the content of Zr higher than 0.1% in order to retain the unrecrystallized microstructures.In ageing process, Zr atoms in solution suppress the formation of GP( I) regions and the metastable Al₃Zr particles promote the precipitation ofη' phase. Thus, the age-hardening curves of the high strength aluminum alloys with different Zr content can be identified as three types. The mechanical property results show that the strength of the alloys increases with the content of zirconium, but the elongation reaches a peak at 0.06%Zr and then decreases with the content of Zr increasing. This is mainly due to the addition of element Zr can significantly retard the recrystallization and refine the microstructure of the alloys.Based on the study mentioned above, it is suggested that the content of Zr should be in range of 0.10%-0.14% in the Al-Zn-Mg-Cu alloy.The effects of the content of Cu and Li on the mechanical properties of Al-Cu-Li alloys were investigated according to the metastable phase diagram. It is found that the strength of the alloys increases with the Cu content increasing. However, the effect of element of Li is in the reverse direction. Furthermore, the mechanical properties of the Al-Cu-Li alloys can also be affected by the degree of the supersaturation. The higher the Li content is, the more significant the effect of the degree of supersaturation will be. Appropriate contents of Cu and Li are proposed that the Cu element should be in the range of 3.0%-5.0% and the Li element in the range of 1.0%-1.5%.The mechanical anisotropies of Al-Li alloys were also studied in this paper. The results show that the mechanical anisotropies are mainly affected by the deformation character and the crystalline oriental distribution of the materials. However, the grain microstructures have little effect on the anisotropy. The in-plane anisotropy of Mn-bearing alloy is lower than that without Mn because the microstructure and the intensity and component of the crystallize texture were significantly changed by the addition of element Mn.The effects of microalloying elements such as Mg, Ag, and Zn on the mechanical properties of Al-Cu-Li alloys were also studied extensively. The strengthening effects of Mg+Ag, Mg+Zn and Mg+Ag+Zn additions are much higher than the individual addition of Mg, Ag or Zn. The element Mn can also bring some extent of strengthening effects on the alloys whether the other microalloying elements present or not. But the quenching sensitivity becomes more seriously when the alloys contain element Mn. A new high-strength Al-Li alloy containing Mg, Zn and Mn was developed for aircraft applications in this study. Its promising mechanical properties were found to be:σ_b= 657 MPa,σ_0.2= 635 MPa andδ= 4.9%.Finally, the principles of developing high strength Al-Li alloys were proposed that:â‘ the baseline alloy should contain more than 3.0%Cu and less than 1.5%Li;â‘¡the element Mn could be selected to improve the deformation performance and control the grain microstructure and crystalline texture;â‘¢addition of microalloying elements would be necessary to obtain a super-high strength alloy with yield strength higher than 600MPa.