Nucleation in metals and metal alloys – An atomistic view

金属和金属合金构成了各种复杂材料类别的基础，这些材料在能源和运输领域具有重要应用。为了推进特殊用途新材料的开发，必须基于原子水平上获得的洞察力来理解宏观材料特性。获得有关凝固和熔化过程中原子过程的知识是提高我们对加工过程和使用条件下材料行为的理解的基本步骤之一。在固-液相变中，特定多晶型物的选择性和微观结构的发展在很大程度上已经在成核的初始阶段确定。实验和理论上的原子描述的成核过程是非常具有挑战性的，但至关重要的理解原子机制，发生在成核和生长。在这项研究中，我们采用先进的模拟技术，特别是过渡路径采样，调查在凝固和熔化过程中的成核过程。作为一个材料系统，我们专注于镍和镍合金，它们构成了几种技术相关材料的关键成分。基于第一个资助期内获得的结果，我们确定了当前研究项目的三个主要目标：把对纯金属（Ni）形核过程的认识扩展到二元合金（Ni-Al）特别关注化学顺序如何影响成核机制以及需要什么样的顺序参数来捕获这种行为。第二方面是研究缺陷如何影响液体中的预排序以促进特定多晶型物的生长。与某些环境条件（温度、压力）一起，这种成核晶种可用于触发特定晶体结构的形成。第三个方面是在熔化期间，特别是在过热晶界处的非均质成核的延伸，提供了在固化和熔化期间的原子过程之间的直接比较。

Metals and metal alloys form the basis for a wide range of complex materials classes that have important applications in the energy and transportation sector. To advance the development of new materials for special purpose applications it becomes essential to understand macroscopic materials properties based on insight gained on the atomistic level. Obtaining knowledge about atomistic processes during solidification and melting constitutes one of the basic steps to improve our understanding of the materials behaviour during processing and under service conditions. In solid-liquid phase transformations the selectivity of specific polymorphs and the development of the microstructure are determined to a large extend already during the initial stages of nucleation. Both experimentally and theoretically an atomistic description of nucleation processes is very challenging but crucial to understand the atomistic mechanisms that take place during nucleation and growth. In this study we employ advanced simulation techniques, in particular transition path sampling, to investigate nucleation processes during solidification and melting. As a material system we focus on nickel and nickel alloys that constitute key components in several technological relevant materials.Based on the results obtained during the first funding period we have identified three primary objectives for the current research project: the insight gained on nucleation processes in pure metals (Ni) is extended to binary alloys (Ni-Al) with a particular focus on how the chemical order influences the nucleation mechanisms and what kind of order parameters are needed to capture this behaviour. A second aspect is the investigation of how defects affect the pre-ordering in the liquid to promote the growth of particular polymorphs. Together with certain environmental conditions (temperature, pressure) such nucleation seeds might be used to trigger the formation of specific crystal structures. A third aspect is the extension to heterogeneous nucleation during melting, in particular at superheated grain boundaries, providing a direct comparison between atomistic processes during solidification and melting.