【作者】 王建；

【导师】 丁桦；

【作者基本信息】 东北大学 ， 材料加工工程， 2018， 硕士

【摘要】 7xxx系列铝合金具有优良的综合力学性能,被广泛应用于航空航天零件制造领域。飞行器所需一体化零件结构复杂,对材料的强度和成形性能都提出了高的要求。研究7xxx系列铝合金的超塑变形机理和组织演变规律,可以为制定热加工工艺和控制组织缺陷提供参考和依据。本文以7B04铝合金为研究对象,选用细晶和粗晶(平均晶粒尺寸分别为8μm、16μm)两种板材进行不同变形条件和多个变形量的高温拉伸实验。通过真应力-真应变曲线对高温拉伸变形行为进行分析和计算,结合超塑变形显微组织和断口形貌,判断和分析超塑变形机理,并对该合金的空洞行为进行表征。主要结果如下:(1)通过对细晶板材进行不同条件下的高温拉伸实验,得出最佳超塑变形条件为530℃、3×10-4s-1,最大延伸率为1663%。温度一定时,随着初始应变速率的降低,延伸率逐渐增大。初始应变速率较高时,延伸率随温度变化不大;初始应变速率较低时,随温度的升高,延伸率逐渐增大。当温度一定时,随着初始应变速率的增加,峰值应力逐渐增大;当初始应变速率一定时,随着温度的升高,峰值应力逐渐减小。(2)应变速率敏感性指数m随温度的升高而增大,在530℃时,m值达到0.63。所选初始应变速率条件下,变形激活能Q分别为158.44 kJ/mol、188.13 kJ/mol、177.78 kJ/mol、250.54 kJ/mol。建立了高温变形的本构方程和不同温度下的R-W-S变形机理图。在实验条件下,7B04铝合金的变形机理为晶格扩散控制、位错滑移协调的晶界滑动。(3)通过对细晶板材多个变形量试样进行显微组织分析,发现变形段组织发生了变形诱发晶粒长大现象。晶粒的等轴化和圆弧化可以为超塑变形过程的进行创造良好条件。高温拉伸断口处存在“须状物”,其含量随温度的升高而增加,随初始应变速率的增大而减少,表明变形过程中存在液相协调机制。(4)通过对细晶和粗晶板材高温拉伸过程中的空洞行为进行表征,发现空洞平均直径基本呈线性增长,空洞体积分数呈指数规律增长。细晶板材中空洞的发展较为缓慢。该7B04铝合金对空洞的容忍度约为16%。绘制了空洞长大机理图,求得扩散控制和塑性控制的空洞长大机理间的转换半径约为1.64μm。更多还原

【Abstract】 7xxx aluminum alloys possess excellent and comprehensive mechanical properties.They are widely used in the aerospace industry and parts manufacturing.The parts used in aircraft have complicated structures and high requirements are brought on the strength and the formability of the materials.Research on the superplastic deformation mechanism and the laws of microstructure evolution can provide bases and references for hot working processes and controlling deficiencies.The thesis takes a 7B04 aluminum alloy as the research object.Sheets with fine and coarse grains(the average grain sizes are 8μm and 16μm)were chosen to conduct the high temperature tensile tests in different deformation conditions and different amount of deformation.The hot tensile deformation behavior was analyzed and calculated by the true stress-true strain curves.Superplastic deformation mechanism was obtained and analyzed by considering the microstructure evolution and the fracture appearances.Cavity behavior of the aluminum alloy was presented and analyzed.The main results are as follows:(1)The maximum elongation of 1663%and the optimal superplastic deformation condition 530℃,3×10-4s-1 were obtained through the high temperature tensile tests of the fine grained sheet materials in different conditions.At certain temperatures,the elongation was gradually increasing with initial strain rate decreasing.The elongations showed little change with temperature changing at high initial strain rates.And the elongation was gradually increasing with temperature increasing at low initial strain rates.At certain temperatures,the peak stress was gradually increasing with initial strain rate increasing.At certain initial strain rates,the peak stress was gradually decreasing with temperature increasing.(2)The strain rate sensitivity index m was gradually increasing with temperature increasing.The maximum m was 0.63 at 530℃.The values of deformation activation energy Q were 158.44 kJ/mol,188.13 kJ/mol,177.78 kJ/mol,250.54 kJ/mol at the initial strain rates chosen in the tests.A constitutive equation was established to describe the hot deformation process.R-W-S deformation mechanism maps at different temperatures of the alloy were established.The superplastic deformation mechanism in the experimental conditions of the 7B04 aluminum alloy was dislocation slip accommodated grain boundary sliding controlled by lattice diffusion.(3)It was found that the deformation induced grain growth existed in deformed section by analyzing the microstructures of several samples from fine grained sheet material with different amount of deformation.Equiaxial grains and arc-shaped grain boundaries could provide good conditions for the superplastic deformation.Substances like whisker existed on the fracture surface of high temperature tensile tests.Their content was increasing with temperature increasing and it was decreasing with initial strain rate increasing.It was indicated that liquid phase coordinated mechanism existed in the deformation process.(4)Cavity behavior of the fine grained and coarse grained sheet materials in the high temperature tensile tests was presented and analyzed.It was found that the average diameter of cavities was increasing linearly and the volume fraction was increasing exponentially.The development of cavities in fine grained sheet material was slower.The tolerability for cavities of the 7B04 aluminum alloy was about 16%.Cavities growth mechanism map was obtained.The converting radius between diffusion controlled and plasticity controlled cavity growth mechanisms is～1.64μm.更多还原