【作者】 刘晓艳；

【导师】 潘清林；

【作者基本信息】 中南大学 ， 材料物理与化学， 2011， 博士

【摘要】 传统的2219和2618耐热铝合金由于具有较高的强度和良好的耐热性能,被广泛应用于航空航天领域。但当工作温度超过200℃以后,主要强化相的粗化使其力学性能显著下降,难以满足新一代飞行器、推进器等结构部件对使用温度的要求。与传统耐热铝合金相比,添加Ag的Al-Cu-Mg系耐热铝合金不仅具有较高的室温强度和较好的耐损伤性能,而且还具有优异的耐热性能和热稳定性。这种新型耐热铝合金有望满足超音速飞机的耐热性能及经济性要求,具有广阔的应用前景。为此,采用活性熔剂保护熔炼,水冷铜模激冷铸造技术制备了含Ag和不含Ag的两种Al-Cu-Mg合金,研究了微量Ag对Al-Cu-Mg合金组织与性能的影响,并在此基础上,研究了含Ag的Al-Cu-Mg耐热铝合金的均匀化处理,热压缩变形的流变应力行为和组织演变,固溶和时效处理及其组织与性能的变化规律,热暴露及高温蠕变行为,为该系合金的成分设计、热加工工艺和热处理制度的制定以及合金的工业应用提供了依据。采用维氏硬度测试、室温和高温拉伸性能测试及金相(OM)、扫描电镜(SEM)和透射电镜(TEM)分析技术,研究了微量Ag对Al-Cu-Mg合金组织与性能的影响。发现微量Ag的添加在一定程度上抑制了Al-Cu-Mg合金中θ’相的析出,促进了Ω相的析出,提高了合金的时效硬化速率,缩短了峰时效时间,增大了合金的峰值硬度,使Al-Cu-Mg合金中的强化相由θ’相和少量S’相转变为Q相和少量θ’相,显著提高了合金的室温和高温强度。研究了均匀化处理对含Ag的Al-Cu-Mg耐热铝合金组织的影响,优化了合金的均匀化制度。含Ag的Al-Cu-Mg耐热铝合金铸态组织中存在严重的枝晶偏析,合金中各元素在晶内和晶界分布不均匀。随均匀化温度的升高或均匀化时间的延长,合金中的非平衡相逐渐溶解,元素分布趋于均匀。该合金适宜的均匀化制度为500℃/16h,这一制度与均匀化动力学分析得出的结果基本相符。采用热压缩模拟实验研究了含Ag的Al-Cu-Mg耐热铝合金在热变形过程中的流变应力行为,并利用OM、SEM和TEM分析技术研究了合金在热压缩变形过程中的组织演变。发现含Ag的Al-Cu-Mg耐热铝合金在高温压缩变形时的流变应力随应变速率的增大而增大,随变形温度的升高而减小,ln[sinh(ασ)]与Inε以及ln[sinh(ασ)]与1/T之间满足线性关系。合金在热压缩变形时的流变应力本构方程为ε=1.89×1013[sinh(0.0116σ)]7.29exp(-196000/RT)。采用人工神经网络对合金高温变形的流变行为进行了预测,预测结果与实验结果吻合得较好。随热变形温度的升高或应变速率的减小,合金晶粒发生长大,位错密度减小,再结晶体积分数增大,合金的软化机制由动态回复转变为动态再结晶。加工图表明合金存在两个失稳区,温度为300-400℃、应变速率为0.1-10s-1和温度为460-500℃、应变速率为1.0-10s-1。产生失稳的主要原因是局部流变和断裂。合金较适宜的加工条件是温度为385-460℃、应变速率为0.001-0.003s-1。研究了固溶、单级时效、形变时效和断续时效对含Ag的Al-Cu-Mg耐热铝合金组织与性能的影响,优化了合金的固溶时效处理制度。合金适宜的固溶一单级时效处理制度为515℃/1.5h水淬+185℃/4h时效,在此条件下,合金的抗拉强度、屈服强度和伸长率分别为505MPa、443MPa和12.2%。形变时效能够提高合金的时效硬化速率,促进Q相和θ’相在位错处形核,细化晶内和晶界析出相；合金的强度随着预变形量的增加先减小后增大,合金适宜的预变形量为4%；经4%预变形+185℃/2h时效后,合金的抗拉强度为516MPa,屈服强度为453MPa,伸长率为12.1%。断续时效在二次时效温度较低时能够促进θ’相的析出,提高合金的伸长率；二次时效温度较高时能够促进Ω相的析出并细化Ω相,改善析出相在晶界的分布,提高合金的强度和塑性；合金适宜的断续时效制度为185℃/2h+150℃/6h,经此时效制度处理后,合金的抗拉强度为518MPa,屈服强度为454MPa,伸长率为13.8%。采用热暴露实验,对比研究了欠时效态和峰时效态含Ag的Al-Cu-Mg耐热铝合金组织与性能随温度和时间的变化,并探讨了合金的热稳定机制。欠时效态和峰时效态合金在250℃以下均具有良好的热稳定性。随热暴露时间的延长或温度的升高,峰时效态合金中的主要强化相Ω相和θ’相尺寸逐渐增大,数量逐渐减少,合金强度逐渐降低。在100-150℃下,欠时效态合金中Ω相和θ’相发生了二次析出,合金强度随热暴露时间的延长先增大后缓慢减小；在200-300℃下,欠时效态合金强度随热暴露时间的延长或温度的升高逐渐减小对比研究了欠时效态和峰时效态含Ag的Al-Cu-Mg耐热铝合金的高温蠕变行为,并采用SEM和TEM分析技术研究了合金在蠕变过程中的组织演变。含Ag的Al-Cu-Mg耐热铝合金具有良好的抗高温蠕变性能。欠时效态合金在蠕变过程中Ω相和θ’相均发生了动态析出,其稳态蠕变速率和强化相的长大速率均低于峰时效态合金。合金的稳态蠕变速率随蠕变温度的升高或蠕变应力的增大而增大,三者之间的本构关系可描述为ε=7.60×10-4σ3.60exp(-102000/RT)。随着蠕变的进行,蠕变机制由初期的位错机制逐渐转变为晶内扩散机制。更多还原

【Abstract】 The conventional Al-Cu-Mg series heat-resistant aluminum alloys such as2219and2618, are widely used in aeronautic and astronautic industries due to their high strength and better heat-resistant properties. But the working temperature is usually under200℃otherwise the mechanical properties of these alloys would decrease dramatically due to the coarsening of the strengthening phases, which can not meet the working temperature for the structural parts of new-generation supersonic aircraft and thrusters. Compared to the conventional Al-Cu-Mg series heat-resistant aluminum alloys, the Al-Cu-Mg heat-resistant aluminum alloys containing Ag possess higher strength at room temperature, better damage-resistant property as well as the excellent heat-resistant property and thermal stability. The new heat-resistant aluminum alloys could meet the heat-resistant property and the economical efficiency of the supersonic aircraft and would be used widely. As a result, Al-Cu-Mg alloys with and without Ag were prepared using water chilling copper mould ingot metallurgy processing which was protected by active flux. The effect of trace addition of Ag on the microstructure and properties of Al-Cu-Mg alloy was studied. And on this basis, the homogeneous treatment, the flow behavior and the corresponding microstructural evolution during hot compression deformation, the solution treatment, the aging treatment and the corresponding microstructure and property changes as well as the thermal-exposure and creep behavior were studied. This could provide basis for the composition design, the formulation of hot working process as well as the heat treatment process and its industrial applicationThe effects of trace addition of Ag to Al-Cu-Mg alloy on the microstructure and mechanical properties were studied by means of Vickers hardness tests, tensile testing at room and elevated temperatures, optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. The results show that trace addition of Ag inhibited the precipitation of θ’ phase and accelerated the precipitation of Ω phase. And it can also accelerate the age-hardening effect of the alloy, reduce the time to the peak-aging, enhance the peak hardness, make the major strengthening phases of the Al-Cu-Mg alloy turn θ’ and less S’ phases into Ω and less θ’ phases and improve the mechanical properties of the alloy both at room temperature and at elevated temperatures.The effects of homogenization treatment on the microstructure of the Al-Cu-Mg heat-resistant aluminum alloy containing Ag were studied. And the homogenization process of the alloy was also optimized. The results show that severe dendritic segregation existed in the ingot and the alloy elements were unevenly distributed both inside the grains and along the grain boundaries. With increasing the homogenization temperature or prolonging the holding time, the residual phases dissolved into the matrix gradually and all elements became more homogenized. The proper homogenizing process was500℃/16h, which was consistent with the results of homogenization kinetic analysis.The flow behavior of Al-Cu-Mg heat-resistant aluminum alloy containing Ag was studied by thermal simulation test. And the microstructural evolution during hot compression deformation was also studied by OM, SEM and TEM, respectively. The results show that the flow stress increased with increasing the strain rate or decreasing the deforming temperature. The relationship between ln[sinh(ασ)] and le ε, ln[sinh(ασ)] and1/T satisfied linear type. The constitute equation of flow behavior of the alloy can be expressed as ε=1.89×1013[sinh(0.0116σ)]7.29exp(-196000/RT). The flow stress during high temperature deformation was predicted by artificial neural network and the predicted stress-strain curves were in good agreement with the experimental results. With increasing the strain rate or decreasing the deformation temperature, the density of the dislocation decreased while the size of the grain as well as the volume fraction of the recrystallization increased and the main soften mechanism of the alloy transformed from dynamic recovery to dynamic recrystallization. According to the processing map analysis, flow instability occurs at two regions, the low temperature300～400℃with a strain rate of0.1～10s-1and the high temperature460～500℃with a strain rate of1.0～10s-1. The flow localization and the fracture are responsible for it. The suitable deformation condition for the alloy is385～460℃and0.001～0.003s-1.The effects of the solution treatment, single aging, the deformation aging and the interrupted aging on the microstructure and the properties of the Al-Cu-Mg heat-resistant aluminum alloy containing Ag were studied. And the solution and the aging process of the alloy were also optimized. The results show that the suitable solution and single aging process was515℃/1.5h water-quenching+185℃/4h, treated by which, the tensile strength of the alloy was505MPa, the yield strength was443MPa and the elongation was12.2%. The deformation aging for the Al-Cu-Mg heat-resistant aluminum alloy containing Ag accelerated the aging process and the nucleation of Ω and θ’ at dislocations and refined the precipitations both in the grains and on the grain boundaries. With increasing the deformation amount, the strength of the alloy decreased and then increased. And the suitable pre-deformation amount was4%. Treated by4%pre-deformation+185℃/2h aging, the tensile strength of the alloy was516MPa, the yield strength was453MPa and the elongation was12.1%. Lower secondary aging temperature for interrupted aging process accelerated the precipitations of θ’ and increased the elongation of the alloy while higher secondary aging temperature accelerated the precipitations of Ω and refined its size, modified the distribution of the particles on the grain boundaries which led to the enhanced strength and plasticity of the alloy. The suitable interrupted aging process for Al-Cu-Mg heat-resistant aluminum alloy containing Ag was185℃/2h+150℃/6h, treated by which, the tensile strength of the alloy was518MPa, the yield strength was454MPa and the elongation was13.8%.The thermal exposure experiments of the peak-aged and the under-aged Al-Cu-Mg heat-resistant aluminum alloy containing Ag alloys were carried out and the corresponding changes of the microstructure and the properties with varying the thermal exposure temperature and time were studied. And the mechanism of the thermal stability of the alloys was also discussed. Both under-aged and peak-aged Al-Cu-Mg heat-resistant aluminum alloy containing Ag possessed excellent heat-resistant properties below250℃. With increasing the thermal exposure time or the temperature, the precipitations in the peak-aged alloy coarsed and the number of that decreased. Correspondingly, the strength of the peak-aged alloy decreased. The main strengthening phases Ω and θ’ precipitated secondarily in the under-aged alloy exposed at100～150℃and the strength of the alloy increased and then decreased with prolonging the thermal exposure time. The strength of the under-aged alloy exposed at200～300℃decreased with increasing the thermal exposure time or the temperature.The creep behavior at elevated temperatures of both under-aged and peak-aged Al-Cu-Mg heat-resistant aluminum alloy containing Ag were studied. And the microstructural evolution was analysised by means of SEM and TEM. The Al-Cu-Mg heat-resistant aluminum alloy containing Ag possessed excellent creep resistant properties at elevated temperatures. The main strengthening phases Ω and θ’ in the under-aged alloy precipitated dynamically during the creep process. And the coarsening rate of the strengthening phases and the steady creep rate of the under-aged alloy were much lower than that of the peak-aged alloy. The steady creep rate increased with increasing the temperature or the applied stress, which can be described by a constitutive equation ε=7.60x10-4σ3.60exp(-102000/RT). The main creep mechanism was dislocation mechanism at the early stage of creep and changed to the intracrystalline diffusion mechanism as the creep proceeded.更多还原