| In this dissertation, the Vickers Hardness testing, tensile properties testing and exfoliation corrosion experiments were used to investigate the effects of Ge on the mechanical properties, high temperature properties and exfoliation corrosion behavior. And the microstructure evolution and precipitate identification were carried out by Differential Scanning Calorimetry (DSC), Transmission Electron Microscope (TEM and HRTEM) and Atom Probe Tomography (APT). The major conclusions go as follows:(1) With the addition of Ge, the aging response rate of Al-Cu alloy decreased when aged at175℃. The time needed to achieve peak hardness extended from30h to46h, and the peak hardness and tensile strength decreased12HV and48Mpa respectively. While the aging response rate of Al-Cu alloy increased with Ge addition when aged at200℃. The time needed to achieve peak hardness decreased from25h to19.3h, and there is no large difference in peak hardness between the Al-Cu and Al-Cu-Ge alloys.(2) With the addition of0.2wt.%Ge, the ageing response rate of Al-3.5Cu-0.4Mg alloy increased when aged at175℃. The ageing time taken to peak hardness in Ge-containing alloy was25h, while in Ge-free Al-Cu-Mg alloy it was32h. And the peak hardness increased about37HV with0.2Ge addition. There is a notable increase of～60MPa in the UTS,～70MPa in the YS and～44%in the elongation compared with Ge-free Al-Cu-Mg alloy.(3) When aged at175℃, the TEM observations found that in peak aged condition, there are homogeneous and dispersal distribution of rod-liked S’phase and plate-liked0’phase together with very few distribution of Ω phase in the Ge-free alloy, while the microstructure of the Al-Cu-Mg-0.2Ge alloy is dominated by the dispersal distribution of nanoscale precipitates and some0’phases together with very few distribution of θ’Ⅱ and σ phase. The ultimate tensile strength (UTS), yield strength (YS) and elongation of Al-Cu-Mg-0.2Ge alloy in peak aged condition are415.3MPa,347.3MPa and17.8%. (4) The high temperature tensile properties of peak aged Al-3.5Cu-0.4Mg-0.2Ge (wt.%) decreased with increased temperature, and its high temperature tensile properties are better than the traditional heat resisting alloy. At the same time, aged Al-3.5Cu-0.4Mg-0.2Ge (wt.%) alloy revealed good tensile property after exposed at150℃. After exposed to1000h, the UTS of this alloy still was above400MPa, and the elongation was above11%.(5) The Al-3.5Cu-0.4Mg-0.2Ge (wt.%) in different aged conditions shows good exfoliation corrosion resistance. After immersed in EXCO solution for96h, the alloy in under-aged and peak-aged condition was EB+level, and the alloy in over-aged condition was EA+level.(6)A trace addition of Ge could increase the peak hardness and strength of Al-3.5Cu-0.4Mg alloy when aged at200℃. With Ge content increased (≤0.3wt.%), the peak hardness and strength of Ge-containing alloys increased while the ageing time taken to achieve peak hardness and strength decreased. The time taken to peak hardness of the alloys with0.05%,0.2%and0.03%Ge are11h,9h and7h respectively. The peak hardnesses are110.2HV、122.2HV and131.9HV respectively, the peak tensile strength337MPa、369.5MPa and394.6Mpa。(7)The precipitation process in the Al-3.5Cu-0.4Mg-0.2Ge (wt.%) alloy aged at200℃was found to be as follows:The first major precipitates form in the alloy are dense nanoscale X precipitates, subsequently,θ’,θ’Ⅱ and a precipitates appear. X,θ’,θⅡ and σ precipitates can coexist in the alloy for a long time when aged at200℃.(8) The X precipitates in the experimental alloy were investigated in detail after aging at200℃. The X phase is<001>α needle-shaped precipitation and enriches in aluminum, copper, magnesium and germanium. The Mg/(Cu and Ge) ratio in it is close to1. The X phases are homogenously dispersed throughout the matrix and appear to be coherent. Its orientation relationship with Al matrix is<100>x//<100>A1,{010}x//{010}A1.(9) Based on HRTEM observation and APT data, a modified structure model of X precipitate was proposed. Its composition is Al10Mg3Cu3-xGex (0<x<3). The proposed structure has orthorhombic structure with lattice parameters a=0.405nm, b=1.62nm and c=0.405nm and space group Imm2. The simulated HRTEM images and diffraction patterns fit well with observed HRTEM and SAED data.(10) A trace addition of Ge to a base Al-3.5Cu-0.4Mg alloy stimulated the formation of X precipitate and suppressed the formation of S’ precipitate, presumably due to the strong and preferential interaction between Mg and Ge. The nucleation mechanism of the X precipitates might be described as follows:Firstly, preferential Mg-Ge cluster formed. Then, the Cu element entered into Mg-Ge cluster in later ageing stage. Based on the Mg-Ge-Cu clusters, the X precipitates formed quickly with the ageing time increased at200℃. The X precipitates were also largely observed in alloy after ageing at200℃for120h. Its stability should be related to the strong bind energy between Mg and Ge. |
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