【作者】 李文斌；

【导师】 潘清林；

【作者基本信息】 中南大学 ， 材料物理与化学， 2010， 博士

【摘要】 采用活性熔剂保护熔炼,水冷铜模激冷铸造技术制备了五种不同含Sc量的Al-8.1Zn-2.3Cu-2.05Mg-0.12Zr合金铸锭。该合金铸锭经均匀化-热轧-中间退火-温轧-中间退火-冷轧成2.3 mm厚板材。研究了含Sc量对Al-8.1Zn-2.3Cu-2.05Mg-0.12Zr合金微观组织与性能的影响,发现含Sc量为0.21%时,合金具有最佳的综合力学性能。以此为基础,研究了含0.21% Sc的该合金的均匀化处理制度,合金等温热压缩变形行为以及合金在单级固溶、双级固溶,单级时效、双级时效和回归再时效(RRA)处理状态下的微观组织与力学性能；合金板材在不同时效制度下的晶间腐蚀、剥落腐蚀和应力腐蚀行为；合金在不同时效制度下电化学阻抗谱(EIS)的变化规律与剥落腐蚀行为的关系。探讨了微量Sc在Al-8.1Zn-2.3Cu-2.05Mg-0.12Zr合金中的存在形式与作用机理以及合金板材在不同时效制度下的腐蚀发展过程与腐蚀机理；分析了不同时效制度下合金板材电化学阻抗谱的变化规律。得出了以下主要结论：(1)添加0.11-0.49%的Sc到Al-8.1 Zn-2.3Cu-2.05Mg-0.12Zr合金中,合金的抗拉强度和屈服强度均有明显提高。含0.21% Sc时,合金峰值时效态下的抗拉强度和屈服强度为696 MPa和654 MPa,与未添加Sc的合金相比分别提高94 MPa和110 MPa,其延伸率也保持在11.1%的较高水平。(2)微量Sc在Al-8.1 Zn-2.3Cu-2.05Mg-0.12Zr合金中,可与Al形成Al3Sc或Al3(Sc,Zr)粒子。初生的A13Sc或Al3(Sc,Zr)粒子能显著细化合金的铸态晶粒,消除枝晶组织。次生的Al3Sc或Al3(Sc,Zr)粒子强烈钉扎位错和亚晶界,能有效抑制合金的再结晶,对合金具有直接析出强化作用和亚结构强化作用。(3) Al-8.1Zn-2.3Cu-2.05Mg-0.21Sc-0.12Zr合金适宜的均匀化处理制度为470℃/24h。经此处理后,铸锭中非平衡低熔点共晶相消除,晶界变得细小,各合金元素分布趋于均匀。这一制度与由均匀化动力学方程得到的理论结果470℃/22 h基本相符。(4) Al-8.1Zn-2.3Cu-2.05Mg-0.21Sc-0.12Zr合金高温压缩变形时1nε和ln[sinh(ασ)]之间、ln[sinh(ασ)]和1/T之间满足线性关系,表明合金的高温塑性变形过程是一种类似于高温蠕变的热激活过程。该合金热压缩变形的流变应力方程为：(5)合金热压缩变形的主要软化机制为动态回复和动态再结晶。在相同应变速率下,变形温度410℃以下时主要形成亚晶组织,发生动态回复,其动态回复机制主要为螺型位错的交滑移。变形温度≥440℃时合金发生部分动态再结晶,动态再结晶的形核机制主要是晶界弓出和亚晶合并(6) Al-8.1Zn-2.3Cu-2.05Mg-0.21Sc-0.12Zr合金适宜的固溶-单级时效处理制度为：475℃/40 min水淬+120℃/24 h时效。经此处理后,合金的σb、σ0.2和δ分别为676 MPa、623 MPa和8.1%。其主要强化相为η’(MgZn2)相、A13Sc相、Al3(Sc,Zr)相和少量S’(Al2CuMg)相。双级固溶处理(465℃/40 min+490℃/30 min)后,合金的强度有所提高,且析出相η’更细小弥散。(7)合金适宜的双级时效制度为120℃/8 h+150℃/16h,经此双级时效处理后,合金的σb、σ0.2、δ、硬度和电导率分别为679 MPa、642MPa、7.4%、195HV和30.2% IACS。双级时效处理可提高合金的延伸率、电导率和抗应力腐蚀(SCC)性能。合金晶内组织有所粗化,较大的晶界析出相呈现孤立分布。(8)合金最佳的RRA处理制度为：120℃/24 h预时效+180℃/30 min回归+120℃/24 h终时效。在此条件下,合金的σb、σ0.2、δ和电导率分别为687 MPa、648 MPa、7.4%和30.2% IACS。RRA处理后,合金的强度不降低,电导率和抗应力腐蚀性能得到较大提高。晶内呈现与峰值时效类似的均匀弥散的η’相,晶界较大的平衡相呈孤立分布。(9)晶内η’析出相,晶界η平衡相和无沉淀析出带(PFZ)是引起合金腐蚀敏感性的主要因素。随时效温度升高和时间的延长,合金的抗晶间腐蚀和剥落腐蚀能力提高,其腐蚀敏感性的顺序为：100℃>120℃>140℃>160℃；自然时效>欠时效>峰时效>过时效。峰时效状态的合金强度虽高但抗应力腐蚀性能较差,过时效或双级时效状态的合金抗应力腐蚀能力强但强度降低较大,RRA处理后的合金在获得较高强度的同时也获得较高的抗应力腐蚀能力。(10)所有时效态合金在EXCO溶液中浸泡初期,其电化学阻抗谱由一个高中频容抗弧和低频感抗弧组成,且随浸泡时间的延长,低频感抗部分逐渐减弱消失。一旦发生剥蚀,峰时效、欠时效和自然时效态合金的电化学阻抗谱上出现两个部分重叠的容抗弧,而过时效态合金则主要是由于合金表面蚀坑而导致其电化学阻抗谱上出现第二个容抗弧。依据腐蚀特征和电化学原理设计了等效电路图,对合金腐蚀发展过程的电化学阻抗谱进行了拟合,拟合数据和实验结果一致。(11)对于峰时效、欠时效和自然时效态的合金,随着在EXCO溶液中浸泡时间的延长,剥落腐蚀程度增加,导致剥落腐蚀新界面面积增加,对应的新界面电容C2值随浸泡时间的延长而增加。在腐蚀发展过程中,C2值在C2时域曲线上增加速率越快,剥落腐蚀发展速率越快,对应合金的剥落腐蚀敏感性越大。而过时效态合金由于只出现孔蚀,Q值在整个浸泡过程基本呈下降趋势。更多还原

【Abstract】 Five kinds of Al-8.1Zn-2.3Cu-2.05Mg-0.12Zr containing different quantity of Sc were prepared using water chilling copper mould ingot metallurgy processing which was protected by active flux. These ingots were homogenized, hot rolled, intermediate annealed, warm rolled, annealed and cold rolled into plates with thickness of 2.3 mm. The investigation of effects of minor Sc addition on the microstructure and mechanical properties of the above alloys has revealed that 0.21% addition of Sc corresponds the optimum mechanical properties of Al-8.1Zn-2.3Cu-2.05Mg-0.12Zr alloy. On the basis of this alloy, the homogenization of the alloy, the flow stress behavior of the alloy during hot compression deformation, the microstructure and mechanical properties of the alloy under single solution, two-stage solution, single-aging, two-stage aging, retrogression and re-aging(RRA), the intergranular corrosion, exfoliation corrosion and stress corrosion behaviour of the alloy have been investigated. The existing forms and acting mechanisms of Sc in Al-8.1Zn-2.3Cu-2.05Mg-0.12Zr alloy, as well as the corrosion process and its mechanism have been discussed. The evolution of electrochemical impedance spectroscopy (EIS) features and the relation with exfoliation corrosion under different aging regimes of the alloy in EXCO solution have been also analyzed. The main results are listed as follows:(1)The additions of 0.11～0.49% Sc in Al-8.1Zn-2.3Cu-2.05Mg-0.12Zr alloy obviously improve the ultimate strength and yield strength of the alloy. The alloy containing 0.21% Sc under peak aging condition exhibits a ultimate strength of 696 MPa, a yield strength of 654 MPa and high elongation of 11.1% which increases 94 MPa and 110 MPa compared with the alloy without Sc.(2) The minor Sc in Al-8.1Zn-2.3Cu-2.05Mg-0.12Zr alloy can form Al3Sc or Al3(Sc,Zr) particles with aluminum. The Al3Sc or Al3(Sc,Zr) particles in the alloy will results in obvious fine grain and the disappearing of dendritic regions. During homogenization and subsequent heat treatment process, the secondary Al3Sc or Al3(Sc,Zr) particles can effectively pin the dislocations and subgrain boundaries, at the same time inhibit the recrystallization of the alloy, producing precipitation strengthening and substructure strengthening.(3) The suitable homogenization regime of the alloy is 470℃/24 h which is consistent with the theoretical result of 470℃/22 h in analysis of homogenization kinetics. In the cast alloy with such homogenization, most of the nonequilibrium eutectic disappeared, the grain boundary tend thin and the alloying elements distribute homogeniously.(4) When Al-8.1Zn-2.3Cu-2.05Mg-0.21Sc-0.12Zr alloy is compressed at high temperature, lnεand ln[sinh(aa)], ln[sinh(ασ)] and the 1/T satisfy linear relationship, indicating that the deformation process of the alloy is similar to a thermal activation process of high temperature creep process. The following formula is the softening stress equation under hot compressed deformation:(5) The main softening mechanisms of the alloy during hot deformation are dynamic recovery and dynamic recrystallization. At the same strain rate and under the temperature of 410℃, the hot deformation structure is mainly characterized as dynamic recovery structure. The predominant dynamic recovery mechanism is found to be cross-slip of screw dislocations. It is showed that when the temperature is higher than 440℃, a part of dynamic recrystallization can only be activated during hot compression. The main nucleation mechanism during dynamic recrystallization is grain boundary protruding and subgrain coalescence.(6) The favorable solution treatment and single aging treatment of the alloy is 475℃/40 min+quenching+120℃/24 h aging. Under such condition, theσh,σ0.2 and 8 are 676 MPa,623 MPa and 8.1% respectively. The strengthening phase of the alloy isη’(MgZn2)、Al3Sc、Al3(Sc,Zr) and S’(Al2CuMg) phase. Following double solution(465℃/40 min+490℃/30 min), the strength of alloy increases and the precipitatη’ phase distributes finely and dispersively. (7) The proper two-stage aging treatment is 120℃/8 h+150℃/16 h. The correspondingσh,σ0.2,δ, HV hardness and conductivity are 679 MPa,642 MPa,7.4%,195 HV and 30.2% IACS respectively. The two-stage aging can improve elongation, conductivity and the resistance of stress corrosion cracking. The microstructures within the grain coarsen obviously and the precipitates on the grainboundary distribute discontinuously.(8) The optimal RRA(120℃/24 h+180℃/30 min+120℃/24 h) treatment has been concluded. The correspondingσh,σ0.2,δand conductivity are 687 MPa,648 MPa,7.4% and 30.2% IACS respectively. After RRA treatment, the strength declines slightly, but the conductivity increases greatly which reflects a high resistance of stress corrosion cracking of the alloy. The precipitates whthin grain consist of fine and dispersiveη’phase which is similar with those under peak aging condition. The coarsened precipitates on grain boundary tend to be discontinuous.(9) The 77’phase whthin grain, the coarsed 7 phase on grain boundary and the precipitate free zone(PFZ) are the main factors of corrosion susceptibility of the alloy. While elevating the aging temperatures and increasing the aging time, the resistance of intergranular corrosion and exfoliation corrosion of the alloy rises. The order of corrosion susceptibility rank as follows:100℃>120℃140℃>160℃, Natural aging>Under aging>Peak aging>Over aging. The peak-aged alloy displays high strength and a poor performance of stress corrosion cracking (SCC). The over aging or two-stage aging obviously improve the resistance of stress corrosion cracking and decrease the strength apparently. The alloy undertaking RRA treatment shows better resistance of stress corrosion cracking and remains similar high strength of the peak-aged condition.(10)At the beginning of immersion in EXCO solution, the EIS of the alloy under various aging conditions is comprised of a capacitive arc at high-mediate frequency and an inductive arc at mediate-low frequency, and the inductive component is decreased or disappeared with time. Once exfoliation occurs on the surfaces of peak-aged, under-aged and natural-aged alloys, the EIS patterns are comprised of two overlapping capacitive arcs. However, on the over-aged alloy, two capacitive arcs appear in the EIS patterns due to the occurrence of grievous pitting corrosion. The equivalent circuit is designed according to the structure of corrosion and the mechanism of electrochemical corrosion, and all EIS patterns are simulated after the appearance of two capacitive arcs, and the good agreement between the experiment results and the simulated results has been obtained.(11)In peak-aged, under-aged and nature-aged alloys, with prolonging the immersion time in EXCO solution, exfoliation is deteriorated, leading to increased exposed area, and the corresponding capacitance (C2) is increased. The high increasing rate of C2 value in C2vs time curve is related to more sensitive exfoliation susceptibility of the corresponding alloy. In over-aged alloy, C2 is decreased with immersion time, and no exfoliation has been observed.更多还原