Bernd Böttger

Phase-field simulation of microstructure formation in technical castings - A self-consistent homoenthalpic approach to the micro-macro problem

Performing microstructure simulation of technical casting processes suffers from the strong interdependency between latent heat release due to local microstructure formation and heat diffusion on the macroscopic scale: local microstructure formation depends on the macroscopic heat fluxes and, in turn, the macroscopic temperature solution depends on the latent heat release, and therefore on the microstructure formation, in all parts of the casting. A self-consistent homoenthalpic approximation to this micro-macro problem is proposed, based on the assumption of a common enthalpy-temperature relation for the whole casting which is used for the description of latent heat production on the macroscale. This enthalpy-temperature relation is iteratively obtained by phase-field simulations on the microscale, thus taking into account the specific morphological impact on the latent heat production. This new approach is discussed and compared to other approximations for the coupling of the macroscopic heat flux to complex microstructure models. Simulations are performed for the binary alloy Al-3at%Cu, using a multiphase-field solidification model which is coupled to a thermodynamic database. Microstructure formation is simulated for several positions in a simple model plate casting, using a one-dimensional macroscopic temperature solver which can be directly coupled to the microscopic phase-field simulation tool.

Towards a metadata scheme for the description of materials – the description of microstructures

Abstract The property of any material is essentially determined by its microstructure. Numerical models are increasingly the focus of modern engineering as helpful tools for tailoring and optimization of custom-designed microstructures by suitable processing and alloy design. A huge variety of software tools is available to predict various microstructural aspects for different materials. In the general frame of an integrated computational materials engineering (ICME) approach, these microstructure models provide the link between models operating at the atomistic or electronic scales, and models operating on the macroscopic scale of the component and its processing. In view of an improved interoperability of all these different tools it is highly desirable to establish a standardized nomenclature and methodology for the exchange of microstructure data. The scope of this article is to provide a comprehensive system of metadata descriptors for the description of a 3D microstructure. The presented descriptors are limited to a mere geometric description of a static microstructure and have to be complemented by further descriptors, e.g. for properties, numerical representations, kinetic data, and others in the future. Further attributes to each descriptor, e.g. on data origin, data uncertainty, and data validity range are being defined in ongoing work. The proposed descriptors are intended to be independent of any specific numerical representation. The descriptors defined in this article may serve as a first basis for standardization and will simplify the data exchange between different numerical models, as well as promote the integration of experimental data into numerical models of microstructures. An HDF5 template data file for a simple, three phase Al-Cu microstructure being based on the defined descriptors complements this article.

Multi-Phase-Field Modeling of Solidification in Technical Steel Grades

Following a short introduction into multi-phase-field modeling, a short review of applications of the multi-phase-field concept to modeling of solidification of steels is given. Starting from simulations of directional dendritic solidification and peritectic reaction in a simple binary Fe–C system, further extensions to more complex alloys involving e.g. the formation interdendritic MnS or of TiN and to more complex processes are depicted. As a recent example, effects of microstructure features on hot cracking susceptibility in commercial HSLA steel grades are discussed on the basis of multi-phase field simulations. Preliminary results indicate a pronounced effect of precipitates on hot cracking susceptibility.

Eutectic Solidification of Ternary Al-Cu-Ag Alloys: Coupled Growth of α(Al) and Al2Cu in Univariant Reaction

Coupled, regular eutectic growth of α(Al) and Al2Cu from ternary Al-Cu-Ag liquid alloys is investigated with focus on the formation and the characteristics of eutectic cells in unidirectionally solidified, polycrystalline, bulk samples. The topologic anisotropy of the lamellar eutectic leads to destabilization along the lamellae with elongated cells being intermediate to stable cells, irrespective of the crystallographic orientation of the phases. The formation of stable cellular patterns with elongated or regular cell structure is explained with reference to the crystal orientation of the phases α(Al) and Al2Cu, measured by electron backscatter diffraction (EBSD).

Parallelising computational microstructure simulations for metallic materials with OpenMP

This work focuses on the OpenMP parallelisation of an iterative linear equation solver and parallel usage of an explicit solver for the nonlinear phase-field equations. Both solvers are used in microstructure evolution simulations based on the phase-field method. For the latter one, we compare a graph based solution using OpenMP tasks to a first-come-first-serve scheduling using an OpenMP critical section. We discuss how the task solution might benefit from the introduction of OpenMP task dependencies. The concepts are implemented in the software MICRESS which is mainly used by material engineers for the simulation of the evolving material microstructure during processing.

A Multi-phase-field Approach for Solidification with Non-negligible Volumetric Expansion—Application to Graphite Growth in Nodular Cast Iron

Multi-phase field models have become powerful tools for prediction of microstructure evolution in technical alloys. Coupling to CALPHAD databases provides complex multicomponent multiphase thermodynamic data and diffusion matrices. Although many thermodynamic databases contain volume data, this information is by now hardly used for solidification, since a comprehensive modelling of volumetric expansion based on solid- and fluid-mechanics exceeds present computational capacities. A novel multicomponent multi-phase-field approach is presented which handles volumetric expansion in a simplified way. It assumes that temporary pressure caused by local expansion is instantaneously released by matter transport on a time scale much faster than diffusion. The new approach neglects any mechanical and kinetic aspects of expansion, but allows for a change in total volume and for thermodynamically consistent volume fractions. Moreover, expansion-driven matter fluxes and associated advective solute transport are considered. Application is illustrated by the example of spheroidal graphite growth in a hypoeutectic cast iron alloy.

Microstructure Modeling in ICME Settings

The importance of microstructure simulations in ICME settings is discussed with respect to their added value provided to macroscopic process simulations and their contribution to the prediction of materials properties. Their role in integrating the scales from component/process scale down to atomistic scales and also in integrating the experimental and virtual worlds will be highlighted. Practical implications for coupling a heterogeneous variety of codes and tools to microstructure simulations will be discussed using the example of the commercial multi-phase-field software MICRESS®. The paper concludes with some conceptual thoughts about a future standardized format for the description of digital microstructures.