Study of the fundamental role of interfacial mobilities during phase transformations in aluminium alloys

本课题从析出相界面的析出硬化现象出发，研究铝合金析出相界面迁移率和析出初期生长速率的变化规律。界面迁移率的概念被用来解释晶粒生长和大规模转变，因为这些转变不涉及浓度梯度。这些转换被称为是接口控制的。近十年来，一些学者提出固态析出的早期阶段也是界面控制的，在此阶段界面处于非平衡状态。这意味着在转换开始时对生长速率的精确估计需要知道界面迁移率的值。问题是，直到我们最近开发了一种方法，通过差示扫描量热计（DSC）测量的热量和透射电子显微镜（TEM）拍摄的图像分析来评估沉淀的界面迁移率及其温度依赖性，才有了沉淀的界面迁移率的数据。将该方法应用于二元铝合金上。现在，我们想要评估化学成分对沉淀界面迁移率的影响，我们将试图找到在基质中添加非常少量的元素，同时强烈影响界面迁移率。我们希望找到能在高温下增强铝合金组织稳定性的成分。这部分研究计划将由博士候选人进行。

This research program is dedicated to the study precipitation hardening in Aluminium alloys from the phenomena occurring at the interface of precipitates, which regulate the interfacial mobility and so the growth rate during the early stage of precipitation. The concept of interfacial mobility was used to explain grain growth and massive transformations because there is no concentration gradient involved in these transformations. These transformations are said to be being interface controlled. In the last decade, some authors stated that the early stage of solid-state precipitation is also interface controlled, the interface being out of equilibrium during that stage. This implies that a precise estimation of the growth rate at the beginning of the transformation requires to know the value of the interfacial mobility. The problem is that there were no data for the interfacial mobility of precipitates until we have developed recently a methodology for its evaluation and its temperature dependency from the heat measured by a differential scanning calorimeter (DSC) and image analysis of pictures taken in a transmission electron microscope (TEM). The new method was applied on a binary Aluminium alloy. Now, we want to evaluate the influence of chemical composition on the interfacial mobility of precipitates and we will try to find what elements can be added in very small amounts in the matrix while impacting strongly the interfacial mobility. We hope to find compositions enhancing the stability of the microstructure of Aluminium alloys at elevated temperatures. This part of the research program will be conducted by a PhD candidate. Hardness is not only determined by the size of the precipitates but also on the number of precipitates per unit volume, called hereafter number density. We intend to study the evolution of number density during all stages of precipitation. A set of experiments is proposed where alloys aged under different conditions will be submitted to TEM examinations to estimate the number density at specific times of aging. The evolution of number density is well recognized in the literature, particularly during the coarsening stage, but actual measurements are scarce because of the costs associated to sample preparation and TEM examinations. The price is worthy only for those who are motivated to develop a complete microstructure model able to predict the hardness. With the growth and dissolution model we have developed recently for multicomponent precipitates, which includes the interfacial mobility as an important parameter, the measurements of the evolution of the number density will help us to assess the nucleation and coarsening models that will eventually be added to the actual growth and dissolution model. The experimental determination of the number densities would be conducted by a MSc candidate, while the nucleation and coarsening models will by developed by two PhD candidates.