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

本研究致力于从析出物界面上的现象来研究铝合金中的析出硬化，在析出早期阶段，析出物界面上的现象调节着界面的流动性，从而控制着生长速度。界面迁移率的概念被用来解释晶粒生长和大量相变，因为这些相变不涉及浓度梯度。这些转换据说是由接口控制的。在过去的十年里，一些作者指出，固相沉淀的早期阶段也是界面控制的，在那个阶段界面是不平衡的。这意味着，在相变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来评估沉淀物的界面迁移率及其与温度的关系之前，还没有关于沉淀物界面迁移率的数据，方法是通过差示扫描量热仪(DSC)测量的热和在透射电子显微镜(TEM)中拍摄的图像分析。并将该方法应用于一种二元铝合金。现在，我们想要评估化学成分对沉淀物界面迁移率的影响，我们将试图找出在强烈影响界面迁移率的同时，可以在基质中添加非常少量的元素。我们希望找到能提高铝合金高温组织稳定性的成分。研究计划的这一部分将由一名博士生进行。 硬度不仅取决于析出物的大小，还取决于单位体积内的析出物数量，以下称为数密度。我们打算研究在降水的所有阶段数密度的演变。提出了一套不同条件下时效的合金进行透射电子显微镜检查的实验，以估算特定时效时间的数密度。数密度的演变在文献中得到了很好的认识，特别是在粗化阶段，但由于与样品制备和透射电子显微镜检查相关的成本，实际测量很少。只有那些有兴趣开发一种能够预测硬度的完整微结构模型的人，这个价格才值得。利用我们最近建立的多组分沉淀的生长和溶解模型，将界面迁移率作为一个重要参数，对数密度演化的测量将有助于我们评估最终将被添加到实际的生长和溶解模型中的形核和粗化模型。数密度的实验测定将由一名硕士候选人进行，而成核和粗化模型将由两名博士生建立。

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.