Evolution of strengthening phases under in-service stresses and temperatures: phase-field and experimental study

析出是铝合金中最重要的硬化机制。然而，在相分离过程中，析出相的大小和间距会发生变化。在降水过程中，系统中的化学变化在很大程度上与沉淀周围的内应力重叠。因此，扩散和机械松弛之间的耦合相互作用预计在系统内。在本项目的第一期，研究了二元（Al-Li, Al-Cu）和三元（Al-Cu- li）合金由于成分依赖的弹性常数而产生的强化学-力学交叉耦合，并通过力学和显微组织实验进行了支持。在这种化学-力学交叉耦合下，揭示了沉淀逆成熟和重排的范式改变机制。此外，外载荷对微观结构演化影响的理论和实验研究正在进行中。在本项目的第二阶段，我们将重点研究蠕变条件下的微观结构演变。建立了多组分系统中空位扩散的模型。我们的研究将集中在点缺陷（溶质原子和空位）的意义上。化学-力学耦合模型将被扩展到包括成分依赖的应变（Vegard定律），这将被添加到现有的成分依赖的弹性常数模型中。我们将研究不同应力环境下缺陷的再分布，即靠近(i)沉淀（了解空位对逆成熟的影响），（ii）晶界（了解空洞成核的标准）和（iii）位错旁边（了解攀爬机制）。通过这些研究获得的见解将与表面/界面扩散的新模型（如上所述）相结合，以解决蠕变后期阶段空洞的形成问题。与之前的项目一样，模拟将通过我们的高纯度Al-4Cu-1Li-0.25Mn合金实验进行验证。采用热处理来改变空位和位错密度。通过TEM， HRTEM和SAXS测量等显微组织研究和量化对析出相成核和粗化过程的影响。通过对二级和三级蠕变的不同阶段进行蠕变实验，然后进行光学显微结构分析和微型计算机断层扫描（CT）实验，研究蠕变过程中孔隙的形成和演化。

Precipitation is the most important hardening mechanism in Al alloys. The size and spacing of the precipitates, however, are subject to change during the phase separation. During precipitation, chemical variations in the system largely overlap with internal stresses around the precipitate. Thus, a coupled interaction between the diffusion and mechanical relaxation is expected within the system. In the first period of this project, a strong chemo-mechanical cross-coupling due to composition-dependent elastic constants has been investigated on binary (Al-Li, Al-Cu) and ternary (Al-Cu-Li) alloys and supported by mechanical and microstructural experiments. A paradigm-changing mechanism of inverse ripening and rearrangement of precipitates was uncovered under this chemo-mechanical cross-coupling. Furthermore, theoretical and experimental work on the effect of external load on the microstructure evolution are in progress.In the second period of this project, we will focus on the microstructure evolution under creep conditions. A model for diffusion of vacancies in multicomponent system will be developed. Our studies will focus on the significance of point defects (solute atoms and vacancies). The chemomechanical coupling model will be extended to include the composition-dependent strains (Vegard's law) that will be added to the existing model for composition-dependent elastic constants. We will investigate the redistribution of defects in different stressed environments, i.e. next to (i) precipitates (to understand the effect of vacancies on the inverse ripening), (ii) grain boundaries (to understand the criteria for void nucleation) and (iii) next to the dislocations (to understand the mechanism of climb). The insight gained by these investigations will be combined with the new model of surface/interface diffusion (mentioned-above) to address formation of voids in the later stages of creep.As in the previous project, the simulations will be complemented by experiments on our high purity Al-4Cu-1Li-0.25Mn alloy for validation. Thermomechanical treatments will be applied to vary the vacancy and the dislocation density. The resulting effect on the nucleation and coarsening processes of the precipitates will be studied and quantified by microstructural investigations involving TEM, HRTEM and SAXS measurements. The formation and evolution of voids during creep will be studied by performing creep experiments to different stages of secondary and tertiary creep followed by optical microstructural analysis and micro computer tomography (CT) experiments.