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

该研究项目致力于从沉淀物界面处发生的现象研究铝合金的沉淀硬化，这些现象调节沉淀早期阶段的界面流动性和生长速率。界面迁移率的概念被用来解释晶粒生长和大量的转变，因为没有浓度梯度参与这些转变。据说这些转换是由界面控制的。在过去的十年中，一些作者指出，固态沉淀的早期阶段也是界面控制的，界面在该阶段处于不平衡状态。这意味着，一个精确的估计的增长率在开始的转变需要知道的界面迁移率的值。问题是，有没有数据的沉淀物的界面流动性，直到我们最近开发了一种方法，它的评估和它的温度依赖性从热测量的差示扫描量热仪（DSC）和图像分析的照片中拍摄的透射电子显微镜（TEM）。并将该方法应用于二元铝合金。现在，我们想评估化学成分对沉淀物界面流动性的影响，我们将尝试找到哪些元素可以以非常少量的方式添加到基质中，同时强烈影响界面流动性。我们希望找到在高温下增强铝合金微观结构稳定性的组合物。研究计划的这一部分将由博士候选人进行。 硬度不仅取决于沉淀物的尺寸，而且取决于每单位体积沉淀物的数量，下文称为数量密度。我们打算研究在降水的所有阶段的数密度的演变。提出了一组实验，其中在不同条件下老化的合金将被提交给TEM检查，以估计在特定的老化时间的数密度。数密度的演变在文献中得到了很好的认可，特别是在粗化阶段，但由于与样品制备和TEM检查相关的成本，实际测量很少。这个价格只适合那些有动力开发能够预测硬度的完整微观结构模型的人。随着我们最近开发的多组分沉淀物的生长和溶解模型，其中包括界面迁移率作为一个重要参数，测量的数量密度的演变将帮助我们评估的成核和粗化模型，最终将被添加到实际的生长和溶解模型。数密度的实验测定将由一名理学硕士候选人进行，而成核和粗化模型将由两名博士候选人开发。

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.