Nucleation in metals and metal alloys – An atomistic view

• Applicant: Privatdozentin Dr. Jutta Rogal
• Subject Area: Theoretical Condensed Matter Physics ; Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: from 2014 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 262052203

• Final Report Year: 2021

Final Report Abstract

Crystal nucleation is a central step during solidification and key in a large number of phenomena in areas ranging from physics to biology. For both experiment and computation, the investigation of nucleation processes with atomistic resolution is extremely challenging due to the involved time and length scales. In materials science, a microscopic understanding of the underlying mechanisms is highly desirable, as it can significantly aid the prediction and control of materials properties. In this research project, we employed molecular simulations to study the initial stages of nucleation and growth during solidification in metals and alloys. Specifically, we investigated homogeneous nucleation in the face-centred cubic (fcc) metal Ni and the body-centred cubic (bcc) metal Mo, heterogeneous nucleation in fcc Ni, and nucleation in a binary Ni-Al alloy. Since nucleation processes are typically associated with rather high energy barriers, a straightforward sampling with standard molecular dynamics, the workhorse of molecular simulations, is unfeasible. Instead, the application of enhanced sampling approaches is required, that can efficiently explore these so-called rare events. In our studies, we employed transition path sampling to harvest an ensemble of dynamical trajectories. The analysis of the transition path ensemble reveals information about the kinetic and thermodynamic properties as well as the atomistic mechanisms of the nucleation processes. For homogeneous nucleation in Ni and Mo, our simulations demonstrated that the emergence of the crystalline nucleus is preceded by the formation of pre-ordered regions in the supercooled liquid. These precursor zones are distinct from the liquid and their formation is associated with a seizable increase in energy, contributing to the overall nucleation barrier. The crystal structure of the final bulk polymorph is already determined in this very early stage of the nucleation process. The structural features of the pre-ordered liquid in Ni and Mo were remarkably different, promoting the emergence of fcc and bcc, respectively. The importance of pre-ordering in the description of the nucleation mechanisms could also be confirmed by a quantitative analysis of the corresponding reaction coordinates. Our findings for the nucleation mechanism during homogeneous nucleation has implications for our understanding of heterogeneous nucleation. By adding small seeds with different structures in the supercooled liquid, we were able to study the effect of these seeds on the nucleation mechanisms and barriers. The efficiency of a seed to promote nucleation is directly correlated with its ability to facilitate pre-ordering in the liquid, instead of simply providing a structural template. The crystalline phase again emerges from the pre-ordered regions, but the formation of these precursor zones is strongly impacted by the seeds. The nucleation mechanism in binary Ni3 Al is significantly more complex than in unary metals. In our simulations, we identified competing nucleation pathways towards the formation of either fcc or bcc. In addition to the structure of the growing nucleus, the chemical short-range order is decisive in the description of the nucleation process. Solid clusters with the same size and crystallinity are stabilised by a chemical short-range order towards L12 ordered Ni3 Al. A detailed analysis of a large number of parameters revealed, that a full description of the nucleation mechanism in Ni3 Al requires to take into account the interplay between the size, crystallinity, and chemical short-range order of the growing cluster. With our simulations, were were able to provide new insight into the atomistic mechanisms during the initial stages of nucleation and growth in metals and alloys. Specifically, we clarified the role of pre-ordering in the nucleation mechanism and the selection of polymorphs, the impact of seeding on precursor formation and polymorph selectivity in heterogeneous nucleation, and the importance of chemical ordering in binary alloys.

Publications

• Comparison of minimum-action and steepest-descent paths in gradient systems. Phys. Rev. E, 93:022 307, 2016
  G. Diaz Leines and J. Rogal
  
• Atomistic insight into the non-classical nucleation mechanism during solidification in Ni. J. Chem. Phys., 146:154 702, 2017
  G. Diaz Leines, R. Drautz, and J. Rogal
  
• Maximum likelihood analysis of reaction coordinates during solidification in Ni. J. Phys. Chem. B, 122:10 934–10 942, 2018
  G. Diaz Leines and J. Rogal
  
• pyscal: A python module for structural analysis of atomic environments. J. Open Source Softw., 4:1824, 2019
  S. Menon, G. Diaz Leines, and J. Rogal
  
• Identification of a multi-dimensional reaction coordinate for crystal nucleation in Ni3 Al. J. Chem. Phys., 152:224 504, 2020
  Y. Liang, G. Diaz Leines, R. Drautz, and J. Rogal
  
• Role of pre-ordered liquid in the selection mechanism of crystal polymorphs during nucleation. J. Chem. Phys., 153:104 508, 2020
  S. Menon, G. Diaz Leines, R. Drautz, and J. Rogal
  
• Template induced precursor formation in heterogeneous nucleation – Controlling polymorph selection and nucleation efficiency. arXiv:2106.00147 [cond-mat.mtrl-sci], 2021.
  G. Diaz Leines and J. Rogal