双相不锈钢内界面分布特征及演变机理研究

It is well recognized that many properties of polycrystalline materials, especially those with fine-grained microstructure, are determined to a large extent by their internal interfaces. The features of interfacial plane distribution (IPD) will be investigated extensively by means of five-parameter analysis(FPA) based on electron back-scatter diffraction(EBSD) measurements and other techniques such as TEM, atom force microscopy(AFM), XRD in duplex stainless steels(DSSs), which are initially processed by various strain and annealing treatments. First, relatively fine-grained microstructure will be produced by different large strain and subsequent annealing and the resulted grain boundary plane distribution (GBPD) and phase boundary plane distribution (PBPD) are to be analyzed at different levels of grain size. In the meanwhile, the GBPDs with various misorientation and PBPDs with various orientation relationships (ORs) will be further studied as well. Second, microstructural development will be traced during several typical thermomechanical processing which have introduced different IPDs. And also, correlations between IPD features and the related microstructure including pre-strained sub-microstructure, grain size, grain boundary character distribution (GBCD), ORs, length/number fractions of grain boundary and phase boundary, volume fractions of ferrite and austenite and etc., are to be studied systematically. Third, based on the experimental results, the IPD evolution mechanism will be clarified by the metallography theories associated with crystallography, plastic deformation, recovery and recrystallization, phase transformation and grain growth. Finally, after mechanical properties and corrosion resistance testing in DSSs as processed, the desired thermomechanical processing will be found to improve certain mechanical properties or corrosion resistance as the favorable IPDs are achieved, which will be meaningful for the optimization of IPDs in duplex alloys.

内界面（晶界和相界）分布特征对多晶材料(尤其是细晶材料)的性能和行为具有重要影响。本项目拟以双相不锈钢为研究对象，主要采用基于EBSD分析的界面五参数法并结合TEM，AFM，XRD等技术系统研究经多种大变形退火处理后合金组织中内界面分布（IPD）特征及其演变机理。首先，通过不同大形变退火工艺在合金中得到相对细晶组织，统计不同晶粒水平下的晶界面分布（GBPD）和相界面分布（PBPD）,并进一步分析特定晶界取向差下的GBPD和两相取向关系(OR)下的PBPD,获得可改善某种或多种合金性能的IPD特征及其诱导工艺组合；其次，针对产生不同IPD的典型工艺，追踪合金的组织演变情况，探讨双相合金退火过程中再结晶和相变耦合作用与IPD演变之间的关联性。最后运用金属晶体学、塑性变形、回复再结晶、相变和晶粒长大等理论来阐释双相不锈钢IPD的演变机理。

不锈钢具有良好的力学性能和优异的耐腐蚀性能，被广泛地应用于各种工业和环境中，但不锈钢的力学性能和耐蚀抗力在很多情况下是一对矛盾对立体。 在合金（尤其是细晶组织）中制造大量的“好”界面，以同步改善合金腐蚀抗力和力学性能具有重要意义。 本项目以双相不锈钢（UNS S31803， S32303和S32750）为研究对象，首先通过改变初始组织、变形方式、变形量、退火时间等工艺参数，采用SEM-EBSD和五参数分析法等系统研究了合金的晶界（面）特征分布和相界（面）特征分布的演变过程。然后结合固态相变、形变再结晶和晶体学等理论阐明了各类界面的演变规律及形成机理。最后对具有不同界面组成的合金进行了腐蚀和力学性能检测和评价。 研究发现，第一，通过高温固溶、提高变形量或多向变形等方法可在合金中制备均匀细小且包含大量低能内界面的两相组织；第二，两相均匀相间分布组织的奥氏体中包含大量的小角度晶界，长度分数占到30%左右，这类奥氏体大都在5º范围内与铁素体基体满足K-S或N-W晶体学位向关系，而团簇化的奥氏体晶粒中小角度晶界比例较少（不足5%），主要以孪晶界sigma3为主，大都与基体偏离特定的晶体学位向关系(K-S和N-W)；第三，再结晶过程中形成的奥氏体比相变过程中形成的奥氏体更容易发生孪晶,而相变过程中形成的奥氏体会以激发形核的方式生成大量小角度晶界。第四，随着晶粒长大孪晶数量不断增加，初始形成的相界面匹配关系为{110}F∥{111}A，当晶粒充分长大后，由于孪晶的作用，该匹配关系调整为{541}F∥{533}A。从这个角度看，奥氏体析出过程中，小角度晶界和孪晶界的生成形成了一种竞争关系,但各种内界面特征的演变最终朝向总界面能降低的方向。第五，性能检测结果表明，调控界面组成可以同步改善合金的晶间腐蚀抗力和力学性能，这对双相合金的组织调控和性能改善提供了重要理论依据。