稀土铈Ce改性A7N01铝合金耐腐蚀性能研究

发布时间：2018-08-25 16:35

【摘要】：本课题以A7N01铝合金的标准成分为基础,通过自行设计合金元素的成分范围以及梯度掺杂微量Ce元素的试验方法设计5种不同稀土含量的A7N01-T5铝合金。对各合金的铸态和挤压棒材进行显微组织观察、晶粒尺寸比较、析出相成分分析以及各项基本力学性能分析。通过剥落腐蚀、开路电位、动电位极化以及交流阻抗等试验研究稀土铝合金无应力条件下的局部腐蚀行为。并采用电化学噪声技术监测稀土铝合金C环应力腐蚀的开裂过程。试验结果表明,Ce元素能明显细化A7N01铝合金的晶粒大小,影响第二相的分布和形态,使铸态晶粒尺寸由183.7μm细化至54.2μm,由于Ce元素在铝合金中的固溶度小,偏聚在晶界处,形成一种含Ce的脆性结晶相,在轧制过程中被挤碎后沿着晶界零散分布,因其尺寸较大,易使晶格扭曲从而形成大量位错,阻碍晶界的运动,加大该处的过冷度,提升了晶粒的形核速率,同时在已成形的晶粒表面生成一层活性膜,阻碍晶粒的继续长大,进而达到细化晶粒的效果。但当Ce元素加入过量时,使其首先在合金中形成大量的金属间化合物,降低了 Ce元素在晶界处的偏聚量,减弱了这种成分过冷的作用,合金组织又开始逐渐粗化。而且该稀土结晶相是硬脆相,在塑性变形过程中作为裂纹源,引发裂纹生长,严重的降低合金的韧性以及延伸率。所以未添加Ce元素的C1合金的基本力学性能均为最好,而添加0.3%Ce元素的C4合金,因细晶强化和第二相强化作用使其抗拉强度、屈服强度以及硬度仅次于C1合金,但同时因为晶内析出相以及含Ce稀土相对位错的钉扎作用,会使位错在这些相周围塞积严重,使晶内存在大量的位错对和位错缠结,应力集中较大,降低了其韧塑性,使C4合金的延伸率以及冲击韧性都达到最小值。在A7N01铝合金中,η相的电位比铝基体的电位低,作为阳极发生溶解,腐蚀严重。适量Ce元素的添加,减少了晶界上η相的数量,晶界析出相变得更加粗化和断续分布,不易形成连续的粗大链状物,使合金的PFZ窄化,有效地阻止了腐蚀活性通道,降低了腐蚀的敏感性。且Ce元素极易与腐蚀液中的O2-发生化学反应,生成致密的含Ce钝化层,降低了 Cl-对铝合金表面的点蚀破坏,能有效的阻止合金的进一步腐蚀。同时Ce元素与H有较大的亲和力,能大量的吸附以及溶解H,减轻H原子在缺陷处聚积,从而降低了 SCC敏感性。而当Ce元素添加过量时,形成的含Ce金属间化合物缺失了 Ce元素的保护性。适量Ce元素的添加能使剥蚀的等级从EC+级提高到PC级,开路电位从-0.9227V提高到-0.9003V,极化曲线腐蚀速率从0.940mm/a降低到0.235mm/a,点蚀电阻从4394Ω·cm2提高到16260Ω.cm2,有效的提高了稀土铝合金无应力条件下的耐腐蚀性能。电化学噪声电位信号能准确的表征试样表面点蚀的发生、钝化膜的修复以及裂纹的萌生和扩展过程。时域谱分析表明C1、C5合金在NaCl-HCl溶液中浸泡62小时后开始出现规律的、等时间间距的噪声峰,且随着时间的继续延长,这种规律的阶跃变得更加明显和规律,而PDS曲线上也开始对应在高频部分由闪烁噪声转变为白噪声,表明裂纹开始生长并扩展,而C2、C3、C4合金未出现这种阶跃和白噪声,同时结合每个时间段的体视形貌验证了试验结果的可靠性。A7N01稀土铝合金的应力腐蚀开裂是阳极溶解为主,氢脆加速的共同结果。
[Abstract]:Based on the standard composition of A7N01 aluminium alloy, five kinds of A7N01-T5 aluminium alloys with different rare earth contents were designed by means of self-designed composition range of alloy elements and gradient doping method of trace Ce. The microstructure of as-cast and extruded bars were observed, the grain size was compared, and the precipitated phase composition was analyzed. The local corrosion behavior of RE-Al alloy under stress-free condition was studied by stripping corrosion, open circuit potential, potentiodynamic polarization and AC impedance. The cracking process of RE-Al alloy C-ring stress corrosion was monitored by electrochemical noise technique. The results show that Ce can refine A7N01 aluminum obviously. The grain size of the alloy affects the distribution and morphology of the second phase and refines the grain size from 183.7 micron to 54.2 micron. Because of the small solid solubility of Ce element in the aluminum alloy, it is segregated at the grain boundary and forms a brittle crystalline phase containing Ce. It is dispersed along the grain boundary after being crushed during rolling because of its large size and easy to distort the lattice. As a result, a large number of dislocations are formed, the movement of grain boundaries is hindered, the undercooling of the grain is increased, the nucleation rate of the grain is increased, and a layer of active film is formed on the surface of the formed grain, which hinders the continuous growth of the grain and refines the grain. Compounds reduce the segregation of Ce at grain boundaries, weaken the supercooling effect of this component, and the alloy microstructure begins to coarsen gradually. Moreover, the rare earth crystalline phase is hard and brittle, which acts as a crack source during plastic deformation, causing crack growth and seriously reducing the toughness and elongation of the alloy. The basic mechanical properties of gold are all the best. The tensile strength, yield strength and hardness of C4 alloy with 0.3% Ce addition are only inferior to those of C1 alloy because of fine grain strengthening and second phase strengthening, but the dislocation around these phases will be heavily plugged due to the precipitation and pinning effect of rare earth containing Ce. In A7N01 aluminum alloy, the potential of_phase is lower than that of the aluminum matrix, which is used as anode to dissolve and corrode seriously. The boundary precipitation phase transformation is coarser and more discontinuous, and it is difficult to form continuous coarse chains, which narrows the PFZ of the alloy, effectively prevents the corrosive active channel and reduces the corrosion sensitivity. At the same time, Ce has a greater affinity with H, can adsorb and dissolve H, reduce the accumulation of H atoms in the defect, and thus reduce the SCC sensitivity. The corrosion resistance of RE-Al alloy under stress-free condition is improved effectively by raising the grade of denudation from EC + to PC, increasing the open-circuit potential from -0.9227V to -0.9003V, decreasing the corrosion rate of polarization curve from 0.940mm/a to 0.235mm/a, and increasing the pitting resistance from 4394_.cm2 to 16260.cm2. Time domain spectroscopic analysis showed that after soaking in NaCl-HCl solution for 62 hours, the noise peaks of C1 and C5 alloys appeared regularly with equal time intervals, and the step of the regularity became more obvious and regular with the time prolonging, while the PDS curves were observed. At the same time, the experimental results were verified by the stereo morphology of each time period. The stress corrosion cracking of A7N01 rare earth aluminum alloy was mainly caused by anodic dissolution, while hydrogen was mainly caused by hydrogen. The common result of brittle acceleration.
【学位授予单位】：西南交通大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG146.21

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