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

析出是铝合金最重要的硬化机制。然而，析出物的大小和间距在相分离过程中会发生变化。在降水过程中，体系中的化学变化与沉淀物周围的内应力大部分重叠。因此，扩散和机械松弛之间的耦合作用在系统内是可望的。在本项目的第一阶段，在力学和微观结构实验的支持下，研究了二元(Al-Li，Al-Cu)和三元(Al-Cu-Li)合金由于成分相关的弹性常数而产生的强烈的化学-机械交叉耦合。在这种化学-机械交叉耦合下，发现了沉淀物反向成熟和重排的范式改变机制。此外，外载荷对组织演化的影响的理论和实验工作正在进行中。在本项目的第二阶段，我们将重点研究蠕变条件下的组织演化。建立了多组分体系中空位扩散的模型。我们的研究将集中于点缺陷(溶质原子和空位)的重要性。化学-机械耦合模型将被扩展以包括成分相关的应变(维加德定律)，这将被添加到现有的成分相关的弹性常数模型中。我们将研究缺陷在不同应力环境中的再分布，即(I)析出物(以了解空位对反向熟化的影响)、(Ii)晶界(以了解空位形核的标准)和(Iii)位错(以了解攀移机制)。通过这些研究获得的洞察力将与新的表面/界面扩散模型(如上所述)相结合，以解决蠕变后期空洞的形成问题。与先前的项目一样，模拟将通过我们的高纯Al-4Cu-1Li-0.25Mn合金的实验来补充以进行验证。热机械处理将用于改变空位和位错密度。由此产生的对析出物形核和粗化过程的影响将通过包括TEM、HRTEM和SAXS测量的微观结构研究来研究和量化。通过对二次蠕变和三次蠕变的不同阶段进行蠕变实验，然后进行光学组织分析和CT实验，研究蠕变过程中空洞的形成和演化。

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.