LC_4高强度铝合金应力腐蚀断裂机理的研究(一)

本文用DCB试样研究了LC4高强度铝合金在pH=6～6.5,35℃±1℃的3.5％NaCl溶液中应力腐蚀断裂机理。 文中分析了断口表面的特征,并且指出靠近裂纹尖端的断口表面上被一层薄的钝化膜所复盖。它是一种非常有效的防止氢进入金属的阻挡层。 用金相跟踪照相的方法得到的应力腐蚀断裂试验结果证实裂纹尖端前沿的塑性区大小取决于裂纹尖端的K_1,而不取决于其它因素。 用恒电位技术得到的IgV—ψ曲线,表明阴极极化作用会抑制裂纹扩展,而阳极极化作用会促进裂纹扩展速率。 作者论证了LC4铝合金在试验条件下的裂纹扩展不是由于氢脆引起的。     

论铝合金应力腐蚀DCB试样中的残留应力

测定了不同热处理状态LC4合金DCB试样中的残留应力。测定和计算了残留应力对裂纹尖端应力强度因子的贡献K_(1R)。试验证明,当K_(1R)值高于材料的K_(1SCC)值时,无外载荷的DCB试样中的裂纹能够应力腐蚀扩展到试样末端。进行160℃时效或予变形,能够消除淬火残留应力的大部分。     

Nb/α-Ti3Al体系的互扩散行为

采用反应扩散偶的方法,研究了Nb元素在α-Ti₃Al中的扩散行为.反应扩散偶经过120 h扩散退火,在高温(1 573 K)下生成了均匀的β-(Ti,Nb)扩散层;低温(1 273,1 423 K)下生成不均匀的扩散区,相组成为Ti₃Al(Nb),β-(Ti,Nb)及Nb₂Al.采用Dayananda方法计算了体系的扩散系数,即根据一个扩散偶的浓度分布曲线计算体系的扩散系数.采用平均扩散系数来表征体系的扩散系数,得出扩散模型和相关扩散机理.结果表明:Ti和Al的主扩散系数比Nb的主扩散系数大5个数量级;Ti的主扩散系数约是Al的2倍.Nb通过占据Ti的空位进行扩散,Nb的掺入阻碍了Ti元素扩散,从而提高了α-Ti₃Al体系的高温抗氧化性能.     

304不锈钢的应力腐蚀断裂

用慢应变速率技术研究了敏化的304不锈钢在氧饱和的NaCl溶液中的应力腐蚀断裂(SCC)行为以及浓度、温度和电位的影响。结果表明,在90℃1000ppmNaCl溶液中,在5.5×10~(-6)s~(-1)应变速率下,电位大于-0.5V(相对于SCE,下同)有应力腐蚀敏感性。随着应变速率提高、浓度减小或温度降低,断裂电位范围移向较正的电位(-0.2V以上)。在较慢的应变速率下,在-0.5～-0.2V产生单裂纹;高于-0.2V对于所用的两种应变速率都产生多裂纹;低于-0.6V未发现裂纹。该体系的自腐蚀电位在-0.25V附近,处于SCC敏感区。恒电位下动态应变试验中电流增量与平均裂纹速度在双对数座标上呈正比关系,表明该体系的沿晶SCC本质上受溶解控制。     

模拟服役环境中航空导管连接件耐蚀性能研究

采用自制的航空导管连接件腐蚀试验系统,研究了航空镀镉45^#钢导管连接件在模拟服役环境中的气密性、油压气密性以及应力腐蚀、表面腐蚀等性能.对该导管连接件在高温盐水溶液6%NaCl(90±5)℃浸泡与空气烘烤(60±5)℃循环200次条件下的表面腐蚀及应力腐蚀行为进行了评价;探讨了在模拟服役环境中,45^#钢发生腐蚀的机理.10^＃航空油不对镀镉45^＃钢造成腐蚀.镀镉45^#钢导管连接件在模拟服役环境中只是表面镀镉层发生腐蚀破坏,基体45^#钢只发生轻微腐蚀,表明45^＃钢导管连接件表面的镀镉磷化层可作为阳极性镀层而对基体金属进行保护.在最小屈服强度0.85的弯曲应力水平作用下,无应力腐蚀倾向.导管材料铝合金LF2-M与连接件45^#钢在6％NaCl水溶液中只有很小的电偶腐蚀电流,平均电偶电流密度为0.185μA/cm²,证明该类导管及导管连接件能安全地进行偶接.     

应力腐蚀断裂过程中的裂纹表观形态及断裂机理

通过金相跟踪照相等技术,获得了30CrMnSiNi22A超高强度钢在不同应力腐蚀环境条件下裂纹扩展的一系列动态照片及数据,由此可以看出:(1)该钢的应力腐蚀断裂过程中,氢并非致塑因素,与此相反,它常常引起基体脆化,起到抑制塑性变形的作用;(2)该钢的应力腐蚀断裂行为和机制往往因环境不同而异,即“多因素致裂”的“综合机理”。     

外加应力下的LY12CZ电化学行为

利用动电位极化和电化学阻抗谱技术对应力加载下的LY12CZ铝合金在3％NaCl水溶液中的腐蚀行为进行了研究,考察了应力作用对LY12CZ在3％NaCl水溶液中的阳极极化行为、自腐蚀电位和破裂电位以及极化电阻和双电层电容大小的影响.结果表明,在加载应力时LY12CZ的阳极极化曲线和自腐蚀电位、破裂电位均明显负移;在浸泡时间相同的情况下,极化电阻也随应力的增加而显著降低,在应变强化阶段力学化学效应达到最大.破裂电位的负移和极化电阻的降低说明应力对LY12CZ的局部腐蚀有明显的影响.      

载荷作用下5A06铝合金焊接试样的腐蚀敏感性

针对5A06铝合金焊接对腐蚀敏感性的影响,研究了载荷作用下的不同焊缝方向（垂直、穿越）的5A06铝合金试样在50℃的3.5% NaCl溶液中的腐蚀行为的影响和焊接接头的电化学腐蚀行为。结果表明,经过110 d的盐水溶液浸泡后,2种焊缝试样均是热影响区首先发生腐蚀,表面的腐蚀坑两侧由于材料组织的不同分别呈台阶式和韧窝状。穿越焊缝试样应力腐蚀敏感性较低,腐蚀坑顶端应力集中,有滑移台阶;垂直焊缝试样应力腐蚀敏感性较高,腐蚀坑沿焊缝贯穿试样表面,腐蚀坑两侧呈平行撕裂状态。电化学极化曲线表明,热影响区、母材区和焊缝区的自腐蚀电位分别为-1.369、-0.791和-0.740 V。      

Nb／α-Ti Al体系的互扩散行为

采用反应扩散偶的方法,研究了Nb元素在a-Ti3 Al中的扩散行为.反应扩散偶经过120 h扩散退火,在高温（1 573 K）下生成了均匀的阶（ Ti , Nb）扩散层;低温（1273,1 423 K）下生成不均匀的扩散区,相组成为Ti3Al（ Nb） ,β-（Ti , Nb）及Nb2Al.采用Dayananda方法计算了体系的扩散系数,即根据一个扩散偶的浓度分布曲线计算体系的扩散系数.采用平均扩散系数来表征体系的扩散系数,得出扩散模型和相关扩散机理.结果表明：Ti和A1的主扩散系数比Nb的主扩散系数大5个数量级;Ti的主扩散系数是Al的2倍.Nb通过占据Ti的空位进行扩散,Nb的掺入阻碍了Ti元素扩散,从而提高了a-Tip A1体系的高温抗氧化性能     

LEO原子氧对空间材料侵蚀的数值模拟

提出了吸附作用(包括物理吸附和化学吸附)是引起空间材料原子氧腐蚀的主要机理.以此为依据,建立了相应的腐蚀量的反应-扩散方程.应用分子动力学经典碰撞理论的估算表明了方程中的物理作用项对于扩散效应为一小量,从而进一步简化了方程.在所建立的模型方程中,选用了基于Eyring绝对速率理论的扩散系数.对空间表面材料Kapton在近地轨道LEO(Low Earth Orbit)环境受原子氧侵蚀的过程进行了数值模拟,计算结果与飞行试验数据在误差允许的范围内符合得较好.     

高强合金与钛合金的电偶腐蚀行为

采用电偶腐蚀实验方法、扫描电镜和能谱方法等研究了高强合金与钛合金异接电偶腐蚀行为.从不同材料、不同成分对电偶腐蚀的影响,研究了电偶腐蚀的规律及电偶对偶接的可能性.同时比较了铝合金和高强度钢与钛合金电偶腐蚀行为的差异,探讨了自腐蚀电位差与电偶电流密度的关系.实验表明,铝合金LY12、铝合金LC4、高强度钢30CrMnSiA均不能与TC2钛合金偶接,高强度钢1Cr17Ni2则可以与TC2偶接,且电偶电流密度随自腐蚀电位差增大而增大.      

几种航空铝材动态力学性能实验

利用分离式Hopkinson压杆实验设备测定了4种航空铝合金材料的动态力学性能,根据材料的应力应变曲线特性,选择Cowper-Symonds模型来拟合4种材料的动态本构.在此基础上从实验结果中分析和比较了这4种铝合金在动态冲击下的破坏形式和动态力学特性.实验结果表明LF6R和LF21M具有良好的塑性,而LY12CZ和LC4CS具有一定的脆性;随着4种材料屈服应力的提高,材料切线模量也增加,二者呈线性关系;实验的4种航空铝材中LC4CS应变率敏感程度最小,LF6R敏感程度最大,LF21M,LY12CZ和LC4CS的应变率因子曲线形状相同.      

INTERFACIAL MASS TRANSPORT IN OXIDE CRYSTAL GROWTH

A space high temperature in situobservation instrument (SHITISOI) is dedicated to visualize and record the whole growth process of oxide crystal in high temperature melts and solutions. Model experiments using transparent liquids such as KNbO3,Li2B4O7+KNbO3 were chosen to investigate effects of interracial mass transport in oxide crystal growth. For the scaling of the coupled velocity, heat and concentration fields in KNbO3 crystal growth, a rotating crystal growth process was performed and the widths of interfacial concentration, heat and momentum transition zones (The "boundary layers") are obtained, which are 7.5×10-a, 8.6×10-2 and 4.4×10-1 cm,respectively. Hence one can expect that interfacial concentration gradient will be confined to a narrow layer and in region of major concentration change at the in terface. In order to study a mechanism based on the interfacial mass transport resulting from hydrodynamics, the growth of KNbO3 grain in high temperature Li2B4O7 and KNbO3 solutin was studied. The result shows that the pivotal feature in the KNbO3 crystal growth is the initiated by KNbO3 solute surface tension gra dient which is caused by the slow diffusion of KNbO3 solutes. Direct comparison of the model predictions and experimental observed phenomena demonstrate the predictive capability of this model.     

高超声速飞行器在中间大气层内的飞行速度研究

基于FLUENT软件, 采用Sp-A湍流模型并运用AUSM计算格式,通过对球头模型在高超声速来流下的外流场模拟,得到了该模型在30km,45km,53km,60km和75km高度处,满足氧化铝陶瓷最大使用温度的极限飞行马赫数。结果表明：在53km以下时,极限马赫数随高度增加而减小,之后再增大,变化趋势符合气温变化规律,静温对驻点处的最高温影响巨大。     

氯离子和温度对铝合金在冷却液中腐蚀的影响

分别采用了动电位极化法、电化学阻抗法研究了不同温度下、不同氯离子浓度下防锈铝在模拟冷却液中的腐蚀行为,采用金相显微镜和扫描电镜对极化后防锈铝的表面特征进行了观察和记录。结果表明,氯离子吸附在防锈铝氧化膜表面,对氧化膜表面造成破坏。在一定温度下（30℃）,当氯离子浓度超过0.01mol/L时,中间产物吸附作用就会增强,阻抗谱中低频区出现感抗弧,点蚀萌生。随着氯离子浓度增高（超过0.01mol/L）,防锈铝的腐蚀加剧,耐腐蚀性能下降。随着温度的升高,阴极和阳极反应阻力变小,阳极金属铝的溶解和阴极氧还原速度增大,腐蚀速度增大。温度升高的同时,溶解氧含量下降,阴极反应受到抑制。与此同时扩散过程在整个腐蚀反应过程中的支配性降低,中间产物在氧化膜表面的吸附作用增强,从而导致低频容抗弧的出现。温度对点蚀形貌的影响主要体现在影响单个点蚀坑的几何大小,促进点蚀坑的生长。      

Microscopic approach to analyze solar-sail space-environment effects

Near-sun space-environment effects on metallic thin films solar sails as well as hollow-body sails with inflation fill gas are considered. Analysis of the interaction of the solar radiation with the solar-sail materials is presented. This analysis evaluates worst-case solar radiation effects during solar-radiation-pressure acceleration. The dependence of the thickness of solar sail on temperature and on wavelength of the electromagnetic spectrum of solar radiation is investigated. Physical processes of the interactions of photons, electrons, protons and α-particles with sail material atoms and nuclei, and inflation fill gas molecules are analyzed. Calculations utilized conservative assumptions with the highest values for the available cross sections for interactions of solar photons, electrons and protons with atoms, nuclei and hydrogen molecules. It is shown that for high-energy photons, electrons and protons the beryllium sail is mostly transparent. Sail material will be partially ionized by solar UV and low-energy solar electrons. For a hollow-body photon sail effects including hydrogen diffusion through the solar-sail walls, and electrostatic pressure is considered. Electrostatic pressure caused by the electrically charged sail’s electric field may require mitigation since sail material tensile strength decreases with elevated temperature. It also can substitute inflation-gas pressure loss due to gas diffusion and perforation by micrometeoroids impact to keep the sail inflated.     

On the validity of the aluminum equivalent approximation in space radiation shielding applications

The origin of the aluminum equivalent shield approximation in space radiation analysis can be traced back to its roots in the early years of the NASA space programs (Mercury, Gemini and Apollo) wherein the primary radiobiological concern was the intense sources of ionizing radiation causing short term effects which was thought to jeopardize the safety of the crew and hence the mission. Herein, it is shown that the aluminum equivalent shield approximation, although reasonably well suited for that time period and to the application for which it was developed, is of questionable usefulness to the radiobiological concerns of routine space operations of the 21st century which will include long stays onboard the International Space Station (ISS) and perhaps the moon. This is especially true for a risk based protection system, as appears imminent for deep space exploration where the long-term effects of Galactic Cosmic Ray (GCR) exposure is of primary concern. The present analysis demonstrates that sufficiently large errors in the interior particle environment of a spacecraft result from the use of the aluminum equivalent approximation, and such approximations should be avoided in future astronaut risk estimates. In this study, the aluminum equivalent approximation is evaluated as a means for estimating the particle environment within a spacecraft structure induced by the GCR radiation field. For comparison, the two extremes of the GCR environment, the 1977 solar minimum and the 2001 solar maximum, are considered. These environments are coupled to the Langley Research Center (LaRC) deterministic ionized particle transport code High charge (Z) and Energy TRaNsport (HZETRN), which propagates the GCR spectra for elements with charges (Z) in the range 1 ? Z ? 28 (H–Ni) and secondary neutrons through selected target materials. The coupling of the GCR extremes to HZETRN allows for the examination of the induced environment within the interior of an idealized spacecraft as approximated by a spherical shell shield, and the effects of the aluminum equivalent approximation for a good polymeric shield material such as generic polyethylene (PE). The shield thickness is represented by a 25 g/cm² spherical shell. Although, one could imagine the progression to greater thickness, the current range will be sufficient to evaluate the qualitative usefulness of the aluminum equivalent approximation. Upon establishing the inaccuracies of the aluminum equivalent approximation through numerical simulations of the GCR radiation field attenuation for PE and aluminum equivalent PE spherical shells, we further present results for a limited set of commercially available, hydrogen rich, multifunctional polymeric constituents to assess the effect of the aluminum equivalent approximation on their radiation attenuation response as compared to the generic PE.     

Chapman函数在实际模式大气中的数值积分计算

本文对Chapman阳光掠射函数[Ch(z_p,χ)]进行了数值积分,求得了其在实际模式大气中随观测高度z_p及天顶角χ的变化.计算并讨论了低热层高温度梯度、分子与湍流扩散、重力场及太阳活动对 Ch(z_p,χ)的影响.结果表明,在150 km以下,Ch(z_p,χ)与前人用等标高模式及等标高梯度模式的计算结果差别较大.其中高温度梯度的影响起主导作用.特别是太阳活动对Chapman函数影响较显著,高年与低年之间可变化10—40％(在大天顶角时),这有可能推动热层大气中辐射-光化学-动力学耦合关系的变化.     

航空铝合金系列材料裂纹扩展性能的温度效应

高低温裂纹扩展性能是航空金属结构损伤容限设计的前提,为此,试验测定了3种系列的6种航空铝合金材料(2024-T351、2397-T8、6061-T651、7050-T7451、7050-T7452和7475-T761)在5种温度环境(-70、-54、25、125和150℃)下的裂纹扩展性能,观测了试验现象,并进行了性能对比分析和疲劳断口扫描电子显微镜(SEM)分析,研究了温度对航空铝合金材料裂纹扩展性能的影响机制,获得了具有工程参考价值的结果与结论:与25℃相比,低温下裂纹扩展阻力系数的对数值降低7%~15%,而高温下却增大5%~23%;低温下裂纹扩展指数增大7%~21%,而高温下却减少5%~34%;氢脆效应和高温氧化作用是导致裂纹扩展速率随温度升高而加快的主要原因。