Control of the microstructure of thin multilayer systems by ultrashort pulsed laser irradiation - process understanding by complementary in situ and ex situ characterizations and multiscale simulations

激光辅助技术属于材料加工领域的前沿技术，被广泛应用于材料的激光烧蚀和结构制造等诸多领域。伴随激光消融的过程通常使用流体力学方法和分子动力学相结合的方法来模拟。然而，在纳米尺度上，高强度激光的应用还会导致材料微观结构的改变，这些变化与局部熔化、凝固、相关物种的互扩散、组织缺陷的形成和再结晶有关。这些过程可以用于有针对性地操纵材料的微观结构，但由于激光照射引起的微观结构变化也会影响激光与材料之间的相互作用，因此在纳米尺度上调整所需的微观结构是一项非常复杂的任务。由于描述微观结构对激光与材料相互作用的影响的整体模型仍然缺乏，目前在许多情况下，激光工艺参数的调整是采用试错法进行的。本项目的目的是有助于理解激光引起的金属薄膜微观结构的变化，并描述微观结构对材料特性的影响，这些特性与激光过程有关，如激光束的吸收，电子-声子耦合，热传递，这一目标应该通过将原位(激光照射时的超快椭偏和反射)和非原位实验(扫描和透射电子显微镜、X射线和电子能谱)与使用介观(流体动力学)和微观(分子动力学)方法的模拟相结合来实现。对电子显微照片的评价将得到基于深度学习的多模式分析的支持。分子动力学模拟所获得的信息将为非原位微结构研究提供补充，例如，为激光辐照过程中的原位微结构分析提供原子位置。本研究所用的材料是单层(Cr，Mo，Ti，Fe)和双层(Au/Cr，Mo/Ti，Au/Fe)，它们由不同熔点的一元金属相组成，不同的高温和高压相序列，以及不同的相互溶解度和扩散系数。实验观察到的相变和浓度分布将被用作激光照射引起的温度和压力的“传感器”。在原始状态下具有不同的晶粒度和择优取向的样品中，研究显微组织对激光加工的影响。

Laser-assisted techniques belong to state of the art in the materials processing, and are widely used for laser ablation and structuring of materials in many fields of applications. The processes accompanying the laser ablation are typically simulated using hydrodynamic approaches in combination with molecular dynamics. On the nanometer scale, however, the application of high-intensity laser radiation leads additionally to the modifications of the materials microstructure, which are related to local melting, solidification, interdiffusion of involved species, formation of microstructure defects and recrystallization. These processes could be used for a targeted manipulation of the materials microstructure, but because the microstructure changes that are induced by the laser irradiation also affect the interaction between the laser beam and the material, the adjustment of a desired microstructure on the nanoscale is a very complex task. As a holistic model describing the effect of the microstructure on the interaction between the laser beam and the material is still missing, the adjustment of the parameters of the laser process is currently carried out using a trial-and-error approach in many cases.The aim of this project is to contribute to the understanding of laser-induced changes in the microstructure of thin metallic films and to the description of the effect of microstructure on the materials characteristics, which are relevant for the laser processes, e.g., absorption of the laser beam, electron-phonon coupling, heat transfer, etc. This aim should be achieved by combining in situ (ultrafast ellipsometry and reflectometry during the laser irradiation) and ex situ experiments (scanning and transmission electron microscopy, X-ray and electron spectroscopy) with simulations using mesoscopic (hydrodynamics) and microscopic (molecular dynamics) approaches. The evaluation of the electron micrographs will be supported by a multimodal analysis based on deep learning. The information obtained from the molecular dynamic simulations will complement the ex situ microstructure studies by providing, e.g., the atomic positions for the in situ microstructure analyses during the laser irradiation.The materials proposed for this study are single layers (Cr, Mo, Ti, Fe) and bilayers (Au/Cr, Mo/Ti, Au/Fe) consisting of unary metallic phases with different melting points, different sequences of high-temperature and high-pressure phases, and with different mutual solubilities and diffusivities in the respective binary system. Experimentally observed phase transitions and concentration profiles will be used as “sensors” for the temperatures and pressures induced by the laser irradiation. The effect of the microstructure on the laser process will be studied in samples having different grain size and preferred orientation of crystallites in the original state.