Mechano-chemical coupling during precipitate formation in Al-based alloys

鉴于结构应用对高强度和轻质合金的需求不断增长，铝基合金是重要的工业材料。超细晶(UFG)组织的形成、晶粒度强化和沉淀硬化的结合为生产具有高强度和持久极限、良好的延性和足够的断裂韧性的半成品提供了诱人的途径。在Al-Mg-Sc合金中，UFG组织的形成可以显著影响第二相析出物在Al基体中的结构、化学和分布。含有额外溶质的工业合金会产生额外的现象，如核壳纳米颗粒的形成。因此，本项目的目的是了解和解决这些过程背后的热化学和热力学之间的耦合。为了实现这一目标，在项目的第二阶段，还将进行基于从头算的原子模拟，并伴随着专门和精心挑选的实验。微观结构和外加载荷引起的应力场对局部化学和热力学影响的研究将扩展到多组分体系。此外，为了模拟强应变条件下的沉淀形成，将采用一种新的动力学蒙特卡罗方案，该方案允许包括中程和长程弹性相互作用。热力学-动力学耦合方法不仅可以详细分析大的局部应变场如何影响沉淀物的形成和化学，而且还可以分析相反的途径，即新的化学相(沉淀物)的形成如何影响机械应变场。第二阶段的重点是核-壳-纳米颗粒的形成和晶界的影响。如果没有与精心选择的和项目具体的测量进行仔细的比较和基准，就不可能开发出可靠的方法和理解。压剪变形实验将对沉淀物的分布提供新的见解。用透射电子显微镜及相关方法对UFG合金的显微组织和局部化学进行了深入的分析。同时，载荷下的放射性示踪扩散实验将提供关于机械变形如何影响化学成分和扩散迁移率的数据。这种实验和理论相结合的方法的协同效应将使我们能够系统地探索技术相关材料系统中的机械力-化学耦合，并提高我们对应变、化学、结构、界面动力学、沉淀物的形成及其对机械响应的反向影响之间复杂相互作用的基本理解。

Al-based alloys are important industrial materials in view of a continuously rising demand for high strength and light-weight alloys for structural applications. A combination of ultra-fine grained (UFG) microstructure formation, grain size strengthening and precipitation hardening offers attractive routes to produce semi-finished products with high strength and endurance limit, good ductility and sufficient fracture toughness. In the case of Al-Mg-Sc alloys the formation of an UFG microstructure can significantly affect the structure, chemistry and distribution of second-phase precipitates within the Al matrix. Technical alloys with the additional solutes Zr, Ti, Mg and Mn give rise to additional phenomena like the formation of core-shell nanoparticles. It is therefore the aim of the present project to understand and resolve the coupling between thermo-chemistry and thermo-mechanics underlying these processes.To achieve this goal, ab initio based atomistic simulations, accompanied by dedicated and carefully selected experiments will also be performed in the second period of the project. The investigation of the effect of the stress field caused by the microstructure and external loads on the local chemistry and thermodynamics will be extended to multicomponent systems. Moreover, to simulate precipitate formation under strongly strained conditions a new kinetic Monte-Carlo scheme that allows to include medium and long range elastic interactions will be applied. The coupled thermodynamic-kinetic approach will not only allow a detailed analysis of how large local strain fields affect the formation and chemistry of precipitates, but also the opposite route, i.e. how the formation of a new chemical phase (precipitates) affects the mechanical strain fields. Highlights of the second phase are the formation of core-shell-nanoparticles and the influence of grain boundaries. The development of a reliable method and understanding is not possible without careful comparisons and benchmarks against well-selected and project specific measurements. Compression-shear deformation experiments will provide new insights into the distribution of precipitates. An in-depth analyses of the microstructure and local chemistry in the UFG alloys is obtained by transmission electron microscopy and related methods. In parallel, radio-tracer diffusion experiments under load will provide data on how mechanical deformations affect the chemical composition and diffusion mobilities.The synergy effects of this joined experimental and theoretical approach will allow to systematically explore the mechano-chemical coupling in a technologically relevant materials system and to improve our fundamental understanding of the complex interplay between strain, chemistry, structure, kinetics of interfaces, precipitate formation and their reverse effect on the mechanical response.