不全冶金结合粉末原始边界的再结晶面棱隅形核的竞争机制研究

Powder metallurgy superalloys have been widely used in turbine engines because of their superior mechanical properties and high temperature capability. However, boundary precipitates and thermal-induced micro-hole along the previous particle boundaries (PPBs) are detrimental to the mechanical properties. In the past decades, several hot processing methods have been developed to improve or eliminate the PPBs by controlling the microstructure evolution during recrystallization process. Recrystallization is a process by which a crystalline metallic material can lower its free energy. Meanwhile, the deformed grains are replaced by a new set of undeformed grains that nucleate and grow until the original grains have been entirely consumed. However, the nucleation repeatability becomes the critical and scientific problem. With the rapid development of modern materials science, hetergenous recrystallization nucleation of PPBs incorporating the grain boundary, edge and junction has been paid more and more attention since incomplete metallurgical bonding in PPBs. In present research, micro-deformation processes have been taken carried on PPBs with typical boundary, edge and junction. Dislocation motion in micro-deformed zone has been systematically investigated based on the atomic structure observation. SEM, EBSD and HR-TEM are used to characterize the microstructural evolution from micro-deformed structure to recrystallization nucleus, and a new in-situ method that can characterize the whole nucleation process has been developed. Meanwhile, nucleation dynamic and 1st nucleation have also been explored based on the investigation on microstructure controlling of the transition zone in gradient microstructure. Research findings on asynchronism and repeatability will beneficial to exploring the physical performance of gradient microstructure. Finally, hetergenous nucleation of PPBs has been verified and deepened through striving the technological breakthrough of integrated manufacturing. Innovative research findings on hetergenous nucleation of the PPBs will beneficial to controlling the microstructure evolution during powder forming and 3D printing processes.

粉末原始边界是粉末成型过程中由于诱导孔洞等微观缺陷破坏了固体原子作用力而形成的一种不全冶金结合微结构，其存在不仅降低了粉末合金的力学性能而且增大了服役失效的风险，通常需采用热塑性变形加以改善和消除。因此，热力耦合作用下粉末原始边界的再结晶机理与组织演变机制成为研究重点。本项目将粉末原始边界细化成面、棱、隅三种典型微结构，研究面棱隅微观样品的位错运动，揭示面棱隅再结晶形核的结构条件；发展以静态组织映射动态演变的实验方法，创新从变形结构到新晶核的“准原位”表征手段；探索受面棱隅微结构影响的形核动力学，建立引入面棱隅因子的再结晶模型；探索最先形核、顺序形核、重复形核的微观结构演变规律，揭示面棱隅形核之间的竞争关系。发展不全冶金结合粉末原始边界的再结晶理论，对粉末合金热加工过程的微观结构控制具有重要的科学意义和应用价值。

再结晶是金属在特定温度与能量条件下微观结构演变成无畸变新晶核的过程。伴随科技发展与技术创新，新材料和新结构层出不穷，再结晶也被赋予了新的研究内涵。本研究以不全冶金结合粉末原始边界为研究对象，围绕再结晶面棱隅形核竞争机制开展实验、理论和数模研究。采用微制造技术加工了不同尺寸的粉末原始边界微样品，利用纳米压痕实验平台进行了微观压缩变形，计算获得了微样品变形量和应力应变关系，定量表征了晶面和晶棱的位错密度变化，发展了从变形位错结构到新晶核的准原位微观表征方法。树枝晶、柱状晶、等轴晶再结晶前后微观结构表征首次证明，高温合金再结晶恰是晶面、晶棱、晶隅不同位置的新晶核形成与长大的结果。面棱隅再结晶动力学研究发现，晶面临界形核功最大，是晶棱形核的1.95倍，达晶隅形核的3.10倍。显然，晶隅再结晶最易发生，但受奥氏体晶隅尺寸数量影响，无法区分晶隅新晶核和晶棱新晶核。晶棱再结晶具有显著方向性，即优先沿棱向发展、而后沿晶面拓展、最后朝晶内演化。晶棱再结晶以后晶面激活弓出形核机制，即从晶棱向晶面发展形核或在软硬取向差较大的晶面独立形核，并在形核晶面辐射演变最后向变形晶内部发展。值得注意的是，晶面再结晶不完全时便形成“项链组织”，造成高温合金综合机械性能降低，需进行后热加工处理以获得均匀晶粒。本项目研究成功揭示了高温合金再结晶形核最先在何处发生、微观结构条件、形核动力大小、新核结构特点、新核长大方式诸科学问题，阶段性研究成果已经在新一代航空发动机涡轮盘组织性能调控中开展了工业应用。鉴于工业应用中的问题，建议尽快发展完善镍基高温合金再结晶统一理论，深入挖掘高温合金奥氏体晶粒细化、粗化和梯度化机理，突破高温合金均匀细晶、梯度晶粒、弯曲晶界制备技术。避免不同牌号高温合金研制过程中的重复动作，缩短研制周期和降低研究成本，切实为新一代发动机提供材料储备和技术支撑。