微合金化铝镁合金敏化-稳定化转变的微观组织演变规律及机理研究

Considerable work has been performed on the corrosion resistant Al-Mg alloy at home and abroad. The results showed that for Al-Mg alloy with medium to high magnesium content, "stabilization treatment" could be used to obtain long-term corrosion resistant state with moderate strength. But further increasing the magnesium content, although the strength could be improved, the sensitization to stabilization transition temperature was increased beyond the recrystallization temperature, resulting in the impossible stabilization treatment and therefore poor long-term corrosion resistance of the alloy. To solve this problem, the present project is proposed for the Al-Mg alloy with medium to high magnesium content to investigate the evolution of the amount and microstructure of the second phase with the changing of the main alloying element content, deformation degree, annealing temperature and time in the process of heat treatment and the correlation with strength and long-term corrosion resistance, to obtain quantitative diagram of magnesium content, deformation degree and sensitized zone. Meanwhile, the nucleation thermodynamics and growth kinetics of erodible phase will be theoritically analyzed to elucidate the mechanism of various parameters for sensitizing-stablization transition process, and to determine the key parameters affecting the strength and long-term corrosion resistance. On this basis, further research on two types of micro-alloying elements, which aim at reducing the quantity of erodible phase or increasing the recrystallization temperature, will be carried out to study their influence and mechanism on the sensitization - stablization transition process. Based on these results, it will be expected to develop new strengthened corrosion resistant aluminum magnesium alloys and the corresponding key technologies.

目前国内外对铝镁耐蚀合金已进行了大量研究，对于中高镁含量的铝镁合金可采用“稳定化”的处理方式获得中等强度的长期耐蚀态。但继续提高镁含量，虽然可提高强度，但是敏化-稳定化转变温度升高，超过其再结晶温度，导致稳定化无法实现，长期耐蚀性能极差。针对此问题，本项目拟基于中高以及高镁含量的铝镁合金，系统研究热处理过程中主合金元素的不同含量、形变量、退火温度及时间条件下第二相的数量和微观形态的演变，及其与强度和长期耐蚀性的相关性规律，获得不同镁含量、形变量与敏化区的定量关系图，并通过对易蚀相的形核热力学和长大动力学的理论分析，探明各种参数对于敏化-稳定化转变过程的影响机制，发现影响强度和长期耐蚀性的关键特征量。在此基础上，进而研究旨在减少易蚀相数量和提高再结晶温度的两类微合金化元素对于铝镁合金敏化-稳定化转变的过渡态特征量的影响规律及作用机制。这些成果将为开发强化耐蚀的新型铝镁合金及关键工艺提供依据。

针对中高Mg铝合金长期使用时的敏化现象，及高Mg铝合金无法实现稳定化的困局，强化和耐蚀存在突出矛盾的问题，本项目探明了主合金元素含量、退火温度和时间与铝镁合金腐蚀性能的关系；并从理论和实验分析了铝镁合金敏化-稳定化转变的β相(Al3Mg2)的演变规律和腐蚀相关性；探究微合金元素对腐蚀性能和稳定化影响，并通过复合微合金化扩大稳定化窗口。具体内容如下:.1. 探究了Mg含量、退火温度和时间对晶间腐蚀（IGC）的影响规律。结果显示Al-xMg(x=4,5,6,7wt.%)合金敏化-稳定化的转变温度分别为200，220，250和270℃；对应转变的化学驱动力为(6.06±0.51)×107J/m3，可作为敏化-稳定化转化判据。在低于稳定化温度的10-20℃，合金的失重先增加，后下降，最终稳定在耐蚀区。.2.探究了中Mg(5E83）和高Mg(5E06)合金敏化和稳定化退火过程中晶间腐蚀性能和β相的演变规律。发现合金敏化和稳定化温度的转变对应于β相形核位置从晶界向三角晶界转变。经典形核理论计算表明，考虑晶界偏聚对形核驱动力的增加，敏化温度β相在大角晶界形核率(对于5E83和5E06合金)最大可达1013(m-3.s-1)和1016(m-3.s-1)。升高温度到稳定化温度，β相在晶界形核率下降4个数量级，三角晶界形核占据优势。在敏化稳定化转变的过渡温度，合金呈现先敏化，后反转稳定化的特点，是由晶界β相的拉长和球化造成的。由于低于传统稳定化温度，被定义为低温反转稳定化态。根据β相周围Mg的扩散流估算，拉长和球化是由于开始占优的晶界扩散逐渐被界面扩散超过造成。.3. 探究了微合金元素对腐蚀性能和稳定化稳定化影响，在Al-Mg合金中Zn的添加通过降低β相的含量，并在晶界形成分离的τ(Mg32(Al,Zn)49)相提高合金的腐蚀性能。Er，Sc，Zr的添加在铝合金中形成心部富Er，内壳富Sc，外壳富Zr的三层核壳结构的Al3(Er,Sc,Zr)相。这些析出相可以显著提高了合金的再结晶起始温度25-100℃。.本项目对于低温反转稳定化态的发现和微合金化元素对再结晶温度的提高分别使稳定化窗口向低温和高温两个方向扩展，从而实现高Mg含量铝合金的稳定化，可以开发强度更高且耐蚀的新型铝镁合金。