【作者】 周明哲；

【导师】 易丹青；

【作者基本信息】 中南大学 ， 材料学， 2010， 博士

【摘要】 合金的热处理工艺与其微观组织演变规律及疲劳损伤行为有着密切的联系,通过热处理工艺优化及新型处理工艺开发对合金耐损伤微结构进行调控具有重要价值。本文利用光学显微镜、扫描电镜、透射电镜、X射线衍射仪、背散射电子衍射技术、正电子湮没技术、疲劳试验机、示差量热仪等手段系统的考察了固溶处理、形变处理、电场时效处理及热效应影响后的航空用2E12铝合金拉伸性能、疲劳性能的变化规律,分析和探讨了不同热处理状态下合金微观组织演变对合金耐损伤行为的影响规律及其机理,得到了以下主要结论：(1)2E12铝合金中粗大残留相在循环应力作用下将与基体脱粘而成为疲劳裂纹萌生位置,并在裂纹扩展过程中起到桥接作用,从而降低合金疲劳寿命、加速疲劳裂纹扩展速率。固溶处理可大幅提高合金疲劳性能,适当提高固溶温度、延长保温时间可提高合金综合力学性能,固溶温度应低于Al-Al2Cu-Al2CuMg共晶转变温度509℃。合金固溶后经室温水及沸水淬火后DSC曲线存在明显GPB区向S"转变峰；经油及室温水淬火后存在TypeⅠ及TypeⅡS相两相共存现象；空冷及沸水淬火后合金中存在明显的析出相结构,前者析出相体积更大,数量更多；导致合金析出相结构各异的原因主要是淬火介质冷却速率及冷却温度不一。(2)2E12合金在180℃时效表现为双阶段时效硬化过程,时效前冷变形削弱了合金第一阶时效段硬化趋势。预变形加速了合金第二阶段时效过程,缩短了峰值时效时间。冷变形显著细化了合金中沉淀析出相,随冷变形量的增加,强化相S相愈弥散,愈细小；预变形过程引入大量位错组织,成为合金析出相有利形核位置。(3)轧制变形后合金的晶粒在轧制方向尺寸增加,长宽比增大,产生由S型织构{123}<634>、黄铜型织构{011}<112>和铜型织构{112}<111>组成的形变织构。轧制变形可提高2E12铝合金表面残余压应力,拉伸变形则使得合金表面产生残余拉应力,即使变形量很小,对合金试样表面残余应力的性质及大小产生的影响都要高于轧制变形。(4)合金的疲劳裂纹扩展过程受到晶界的阻碍,扩展方式为穿晶扩展,疲劳过程中产生二次裂纹,二次裂纹萌生位置为裂纹前端附近的取向差较大晶界处。主裂纹扩展面为{111}面,裂纹扩展发生分叉位置为裂纹前端可同时开动两个或两个以上{111}<110>滑移系的晶粒处。形变处理后合金中出现{110}<112>,{123}<634>及{112}<111>织构,导致合金中{111}面倾向于偏离至循环应力加载最大方向,一定程度上抑制了裂纹的扩展,从而导致形变处理后短裂纹扩展阶段扩展速率降低。合金裂纹扩展速率与表面残余应力、裂纹尖端晶粒取向、裂纹尖端塑性区尺寸及塑性区吸收形变能的能力等因素有关。在短裂纹扩展阶段,裂纹尖端塑性区尺寸仅为单个晶粒大小时,表面残余压应力及形变引起的织构可抑制裂纹扩展。在裂纹稳态扩展区,加工硬化引取的裂纹尖端塑性区吸收形变储能的能力降低成为加速裂纹扩展的主要因素。(5)190℃时效过程中施加9kV/cm电场导致合金时效硬化曲线硬度峰值提前。施加9kV/cm强度电场时效合金试样电导率随时间的变化与不加电场时效的合金试样具有相同的变化规律,但其电导率值在时效各个阶段均较后者有所提高。电场时效可明显降低2E12铝合金中S相的形成激活能,Kissinger方法和普适积分法计算结果施加电场后合金S相形成激活能分别降低了7.9kJ/mol～12.7kJ/mol和6.8kJ/mol～22.6kJ/mol。电场可促进2E12铝合金中TypeⅠS相向TypeⅡS相转变,未施加电场时效的合金样品具有更高的过饱和度,其析出体积百分数及析出速率均高于施加电场的样品。合金经190℃强静电场时效,析出相数量明显增加,且分布更为弥散。这是由于淬火时产生的过饱和空位在电场作用下跳动几率增加,从而促进了溶质原子脱溶,增加第二相形核位置。时效不同时间后合金拉伸流变应力由高到低依次为10h、24h、5h。电场时效后合金中更为细小弥散的第二相增加了合金的屈服强度,对于时效过程中施加9kV/cm强度电场的试样,相同时效时间的试样均表现出较未施加电场更高的变形流变应力。经190℃/10h电场时效后合金疲劳寿命较未施加电场时效试样提高了约20%,且前者疲劳裂纹扩展速率在裂纹扩展各阶段均低于后者。(6)合金在150℃下进行热暴露,随暴露时间的增加,合金中第二相经历GPB长大→S"相形成→S’相形成的过程,析出第二相数量不断增加,导致硬度值呈线性增加。随热暴露时间的增加,合金的疲劳寿命呈现先增后降的趋势,暴露10h后合金具有最高疲劳寿命,当暴露1000h后合金疲劳寿命急剧降低。暴露100h及1000h合金中形成的S"相及S’相由于共格程度较GPB区低,使得疲劳过程可逆滑移位错数量减少,加之晶界处析出的第二相易成为疲劳微裂纹源,两者疲劳寿命不同程度地降低。合金的疲劳裂纹扩展速率随热循环时间的延长而加快；合金中GPB区结构尺寸受到疲劳过程中的位错切割作用和温度影响导致长大两方面因素影响,温度影响效果更为显著；高温影响下,当疲劳裂纹形成后,裂纹表面暴露在高温有氧的环境下,氧气较容易扩散进入裂纹尖端,促使裂尖表面形成AlxOy氧化膜,加快了合金试样的裂纹扩展速率。合金的疲劳过程可被看作热激活过程,随着循环应力值的增加,激活能不断降低。其激活过程可用公式βσ=Q0-RTln(Nf/N0)表示,式中Q0和β分别为8.54和1.52。更多还原

【Abstract】 The microstructure evolution and fatigue damage of alloys depend on the heat-treatment. Thus, it is very important to develop and optimize heat-treatment technology for better fatigue performance microstructure. By means of OM, SEM, TEM, XRD, EBSD, PAL, fatigue tester, DSC, etc. the microstructure evolution and mechanical properties of aerostatic aluminum alloy 2E12 after solution heat treatment, deformation, ageing and heat exposion were studied. The relationship between microtructure and fatigue behavior of 2E12 aluminum alloys at different status was analysied, main conclusions were presented as followed:(1) During fatigue process, residual particals were easily debonded from the matrix, which caused initial sites there. With the propagation of fatigue crack, those residual particles were bridged by the main crack. The fatigue crack propagation rate of the alloy was accerlerated by those particles. By introducing solution heat-treatment, fatigue properties of the alloy could be improved significantly. Propriate increasing the heating temperature and holding time could enhance the mechanical properties of the alloy. The solution heat temperature limit was 509℃. There was a indentical transformation peak from GPB zone to S" phase of room temperature and boil water quenched alloys. The coexistence of type I and type II S phase was observed in room temperature water and oil quenched alloys. Visible precipitates appeared in air and boil water quenched alloys. The precipitates in air quenched alloy were bigger in size and more in quantity than those in boil water quenched alloy. The reason caused difference of precipitates was cooling rate and cooling temperature.(2) There were two stages ageing process of 2E12 aluminum alloy at 180℃. Pre-deformation weakened the first ageing hardening process but accelerated the second. The precipitates became finer and more dispersed with the pre-deformation. The dislocations caused by pre-deformation provided beneficial sites for the nuleations of precipitates.(3) After the cold deformation, the grain size of the alloy increased along the rolling direction with the aspect ratio. There were three main texture of cold rolled alloy, S texture {123}<634>, Brass texture{011}<112> and Copper texture{112}<111>. Cold rolling and extension could increase surface residual stress and tensile stress respectively. The effect of extension on residual stress was stronger than cold rolling.(4) Fatigue crack propagation was obstructed by the grain boundaries. The crack propagation was transgranular. During the fatigue process, secondary cracks formed at the misorientation grain boundaries infront of the main crack. The spread plane of main fatigue crack was {111} plane, two or more{111}<110> slip systems started infront of the crack bifurcation. The cold rolled texture{110}<112>,{123}<634> and {112}<111> induced {111} plane deviated to the loading direction resulting lower fatigue crack propagation rate. The fatigue crack propagation rate could be affected by residual stress, orientation of the grains near main crack, size and the deformation torlerence of plastic zone. At the short crack propagation stage, the plastic zone size was at the grain size level, the residual stress and texture could suppress the crack propagation. At the steady propagation area, the crack propagation rate increased by the work hardening.(5) Applying 9kV/cm electric field during artifical ageing at 190℃could decrease the peak ageing time. The conductivity of artificial aged alloy with electric field showed same tendency with the traditional ageing process, however, the former one had a better performance the the latter one. The electric field dould decrease the formation energy of S phase during ageing. The formation energy of S phase were 7.9kJ/mol～12.7kJ/mol and 6.8 kJ/mol～22.6 kJ/mol lower calculated by Kissinger and General integration method. The electric field promoted transformation of typeⅠS phase to typeⅡS phase. The supersaturation of artificial aged alloy without electric field was higher than that with electric field. The precipitation rate of ones without electric field aged alloy was higher than that with. The volumen fraction of electric field aged alloy was higher than that without, and the precipitates were more dispersed of the former one. The electric field increased the quenched vacancies jumping rate, which promoted desolventizing of the solute atoms. The tensile flow stress of aged alloy with different ageing time was 10h>24h>5h. The refinement of precipitates under effection of electric field ageing caused higher yield strength and tensile flow stress of the alloy. The electric field had a positive effect on fatigue property of alloy. The 190℃/10h aged alloy with electric field had a 20% longer fatigue life and lower fatigue crack propagation rate.(6) During the heat exposion at 150℃, the microstructure evolution process was GPB zone→S"→S’, the volume fraction increased with the exposion time, resulting hardness increased linerally. The fatigue life of alloy increased at first than decreased later with the heat exposed time. The 10h exposed alloy had the longest fatigue life, the fatigue life of 1000h exposed alloy decreased severesly. Precipitates in 100h and 1000h exposed alloy were S" and S’ phases, which exhibited worse fatigue performance. It is because that those phases were less coherent than GPB zones, during fatigue process the amount of reversible dislocation decreased. Another reseaon is the deformatiuon coordination of precipitates free zone would caused microcracks near grain boundaries. Therefore, with the increase of exposed time, the fatigue life of alloy decreased. The size of GPB zone was affected by two factors, the cutting effect and temperature, and the latter was dominant. At higher temperature, the oxygen could penetrate through the surface of fatigue crack then caused AlxOy oxide film. Such oxide film accelerated fatigue crack propagation. The fatigue process could be treated as thermal activated, the activation energy decreased with cyclic loading stress. The thermal activated fatigue process of 2E12 aluminum alloy could be treated asβσ-Q0-RTln(N1/N0), where Q0 andβwere 8.54 and 1.52 respectively.更多还原