Materialdesign

How does the microstructure influence the behavior of structural materials?

We want to understand, how the microstructure and behavior of structural materials is influenced by altering external parameters. Structural materials are high performance alloys (or superalloys) which are mainly used for high temperature applications such as parts in airplane engines. Our goal is to improve or develop new materials which can be exposed to even harsher enviroments and thereby, reduce emissions (e.g. CO2). We also contribute to this area of research by developing new methods, to study structural materials on the smallest length scales.

In the subsequent sections, some recent reseach projects are introduced:

Comparison of the micro- and nanostructure of the Ni superalloys Inconel 718 and Haynes 282. (a, b) are SEM images of the microstructure; (c, d) TEM images of the nanostructure.

Research on nickel-based superalloys

Influence of the microstructure on the formation of residual stresses

The aim of this research project is the study of residual stresses in different nickel-based superalloys and their correlation to the microstructure of the alloys by a micromechanical model, which is based on a description of the local micromechanical behavior on the grain level. Residual stresses are mechanical that remain within a material without external forces. They can be both, beneficial or detrimental, for the life of a component, depending on their sign and the type of stresses during operation. In order to avoid material failure and to optimize components, it is it is essential to know the micromechanical material behavior - or the residual stress state - before and during service.

CPFEM simulations of the micro-mechanical material behavior: cubic unit cell with 22x22x22 elements and 100 grains in the undeformed (above) and the state after tensile deformation (below). (a, c) show the grain orientations and (b, d) the distribution of internal strains in the simulated cell.

In cooperation with the Institute of Nanotechnology (KIT) and the Institute of Materials Science and Mechanics of Materials (TU München), we utilise neutrons to investigate lattice strains deep inside components during mechanical deformation. For this porpose, measurements were carried out atthe FRM II research reactor at the Heinz-Maier Leibnitz Zentrum (MLZ) in Garching. The results of the neutron diffraction experiments are correlated to accompanying high-resolution microscopy data of the microstructure and serve as input parameters for modelling the material behavior by a crystal-plasticity based finite element model (CPFEM). The model allows to predict the micromechanical material behavior, so that future components can be better designed according to the loads they experience during service.

Quantification of precipitates in alloy Inconel 718 by SANS: Size distribution of precipitatesusing TEM (a) and chemical information from reconstructed data using APT (b) enable the complete interpretation of SANS data (c).

New method to characterize smallest precipitates in the alloy Inconel 718

In the alloy Inconel 718, the two different phases γ‘- and γ‘‘ have a significant influence on the alloys mechanical properties. However, the differentiation and quantification of the precipitates is experimentally very challenging and has so far only been acchieved when investigating nanoscopic sample volumes. We have developed a new method, which enables to  quantify the different precipitates also in macroscopic sample volumes by means of small-angle neutron scattering (SANS). For interpretation of the SANS- data, the techniques TEM and APT are used complementary. Electron microscopy is used to determine the size distributions of the precipitates, while atom probe tomography (APT) provides information about the local chemical composition which is necessary to evaluate the scattering contrast quantitatively. With these data, a structural model can be set up, that, by adapting to the SANS data, provides statistically significant information about the size and amount of the precipitates.

Lawitzki, R. et al. Acta Mater. 163, 28–39 (2019)

Atomsondenrekonstruktionen der Nanostruktur einer ferritischen Superlegierung nach unterschiedlichen Wärmebehandlungen.

Research on iron-based superalloys

Iron-based ferritic superalloys are promising candidate materials for high temperature applications such as parts in aircraft and gas turbines. Ferritic alloys are characterized by lower thermal expansion coefficients and higher thermal conductivity. At the same time, they are far more cost-effective than Ni-based alloys or austenitic steels, which are traditionally used for these applications. The aim of the alloy development of ferritic steels is their use in so-called “ultra supercritical” (USC) power plants.

(a-e) EM images of the microstructure of the alloy FBB-8 with Ti and after (partial) replacement of Ti by Nb; (f) Influence of the modification to the alloy on its creep performance.

Design of new alloys

Previous studies have shown that in the model alloy FBB-8 with the composition Fe-6.5Al-10Cr-10Ni-3.4Mo-0.25Zr-0.005B (in wt .-%), so-called hierarchical precipitates - Ni2TiAl (L21)- phases which are pervaded by lamellar B2 (NiAl) phases - form after addition of 2-3.5 wt. -% Ti. These phases contribute to the alloy's exceptionally good creep properties; however, at the expense of room temperature ductility and the long-term stability of the alloy. Currently, we are investigating whether the material properties can be further increased by (partially) replacing the titanium with other L21- phase-forming elements (V, Ta, Nb) (see figure).

We manufacture new model alloys in vacuum induction furnaces. After subsequent heat treatments, the material performance is tested in high-temperature creep experiments and correlated to the microstructure (analyzed by SEM, TEM and APT) of the respective alloys.

In the future and together with the Institute for Materials Testing, Materials Science and Strength of Materials (IMWF), we will produce the alloy additively. This will help to avoid the problems associated with the materials' brittleness during shaping. Additionally, it alloys to adjust the alloys' microstructure in a targeted manner.

 

Visualization of precipitates in an atom probe reconstruction (above) and the principle of the modified cluster search method (below).

Advanced "cluster-search" algorithm for the detection of smallest precipitates

Since the diffusion of elements such as Al and Ni in ferritic alloys is faster than the diffusion in Ni-based alloys, the smallest precipitates are already form during cooling after typical heat treatments. The higher the temperature of the heat treatment, the more and larger precipitates are formed after the cooling, which has a considerable influence on the hardness of the alloy at room temperature.

Our goal is to obtain chemical information of these small precipitates. For this purpose, we apply atom probe tomography. To localize the precipitates in the reconstructed data sets, often containing several 100 million atoms, 
we use the cluster search algorithm "Maximum Separation Method", which is most frequently used in the APT community. We have expanded this cluster algorithm in order to now also capture the often complicated shape of the precipitates and thereby, we are able to characterize the composition of the precipitates more precisely.

Lawitzki, R. et al. Mater. Charact. 171, 110722 (2021)

Effect of local magnifications in atom probe tomography on the morphology and density of a field evaporated β‘- precipitate with higher and lower evaporation field than the surrounding α- matrix atoms.

Atom probe tomography is one, if not the best, method for determining highly spatially resolved chemical information 
(in the sub-nanometer range) of smallest phases. In contrast to this, it is so far almost impossible to obtain exact geometric information about these phases by APT, since measurement artifacts often lead to incomprehensible 
distortions in the reconstruction. Using a new approach, we were able to circumvent this problem an could demonstrate a new approach on the example of very the small precipitates in the alloy FBB-8. For this purpose, numerical simulations of the atom probe tomography measurement were carried out and a model was developed from the results that enables the correction of the precipitate sizes which appear distorted in the APT reconstruction.

Lawitzki, R. et al. Microsc. Microanal. 27, 499–510 (2021)

This image shows Robert Lawitzki

Robert Lawitzki

Dr.

Transmission Electron Microscopy

This image shows Guido Schmitz

Guido Schmitz

Prof. Dr. Dr. h.c.

Chair Professor

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