Cloud II Interface Engineering

Dr. T. Hickel (Max-Planck Institut für Eisenforschung, Düsseldorf), PD Dr. D. Music (Institut für Werkstoffchemie, RWTH Aachen University)

 

A central research topic of the SFB in the third funding period is the impact of interface-dominated processes on the mechanical properties of the investigated steels. This is especially true for twin-boundaries, without which a TWIP effect is not possible at all. Interfaces are not only important for simulations, but are also characterized by high-resolution methods. In addition, grain boundaries are an important component in the formation of microstructures during processing and therefore determine the mechanical properties decisively. They are therefore simulated in particular in phase field simulations, but also by means of ab initio methods, molecular dynamics and constitutive models.

However, the material concepts of the third funding period now show, that particularly heterogeneous interfaces are important for the mechanical properties of the steels. This applies, on the one hand, to the interfaces between k-phase and austenitic matrix material in high manganese steels, but also to the interface between various steel phases such as ferrite, martensite and austenite in medium manganese steels. The scientific question of this Cloud II is:

Do the individual phases in a multiphase structure retain their generic properties or are they influenced by effects at the interfaces?

This includes questions such as: Will a k-phase in an austenitic matrix reach its ferromagnetic state or is the magnetism dominated by the boundary surface to the paramagnetic austenite? Does an austenitic island in a medium manganese steel with mechanical stress react like a pure austenite with the same chemical composition, or is the TWIP effect influenced by the presence of the boundary surface? Such phenomena, which are related to these heterogeneous interfaces, are to be analyzed and understood across the entire project in Cloud II.

A detailed characterization of the interfaces (project area C) is necessary first. This includes both the investigation of the atomic and the chemical order. The segregation of substitutional (manganese, aluminum) and interstitial alloying elements (carbon) are investigated with ab initio calculations as well as experimentally (APT and TEM) (Fig. 18). Furthermore, it is important for the comprehension of the interface structure to understand the influence of defects. These include point defects (e.g., vacancies) as well as extended defects (e.g., misfit dislocations). The results obtained result in interface-energies as required for simulations on the meso-and continuum scale.

When considering the boundary surface between k-phase and matrix material, it will be important to analyze the influence of local residual stresses and the degree of coherence to the narrow and broad austenitic channels. In medium manganese steels additionally the local atomic distributions at interfaces are to be analyzed. Principally, however, the investigation of the interfaces primarily serves to determine the mechanical properties of the microstructure. The stability of the interface under mechanical load is important as well as its interaction with dislocations and twins.

.

 

TEM-Charakterisierung und APT-3D-Elementverteilungskarte
APT Mangan-Anreicherung an Martensitlattengrenzen

 

TEM: Austenit an Korngrenzen

 

APT: Mangananreicherung an Korngrenzen

 

APT: Mangan-Anreicherung an Martensitlattengrenzen

TEM: Austenit an Korngrenzen

APT: Mangananreicherung an Korngrenzen

 

The investigations within the cloud take place in a multifunctional context. All project areas, simulation (area A) and production (area B) and characterization (area C), provide crucial contributions. In addition, both the idealized planar interfaces (experiment: thin films, theory: ab initio treatment) and real interfaces (experiment: solidified structure, theory: continuum theories) play an important role and are correlated. In this way atomic processes as well as macroscopic phenomena are investigated. Furthermore, there is a close link to the other two Clouds: In the case of Cloud I, the connection is given in particular by k-phases, in the case of Cloud III by a Hydrogen Enhanced Decohesion (HEDE) at interfaces.