Project C3: Local Mechanical Properties of the Fe-Mn-C System

Local Mechanical Properties

 

Prof. Schneider, Ph.D. (Materials Chemistry, RWTH Aachen Univeristy)

Music, Ph.D. (Materials Chemistry, RWTH Aachen Univeristy)

The goal of the part project C3 is to investigate the influence of interfacial phenomena on the mechanical properties of multiphase Mn-steels. Model systems with chemically graded 2D-interfaces (γ/α and γ/κ) are synthesized by means of thin film technology and characterized by transmission electron microscopy (C1) and atom probe tomography (C3: thin films, C8: bulk samples). Based on the ab initio interfacial models from the part projects A1 and A2, we will calculate the ab initio work of separation to evaluate its predictive capability for the mechanical properties of nanostructured materials.

 

 

 

 

Previous Phase

Design of novel steels has often been carried by experimental means. Only recently, efforts have been made towards a modeling-based design process. The structure-composition-elastic property relationship within the Fe-Mn-C system has never been investigated by ab initio calculations.

We will study local mechanical properties of Fe-Mn-C steels using both experimental and theoretical methods. The elastic modulus and hardness values will be determined by nanoindentation. Theoretically, we intend to use the Vienna ab initio software package (VASP), based on the density functional theory, as well as exact muffin-tin orbitals (EMTO). We will calculate elastic constants and compare the results with our experimental elastic modulus data. This will be carried out together with A7 (continuum model). Our elastic tensor will be used in A7 to obtain elastic modulus values, which will then be used for further modeling in A5, A6, and B3. We will also synthesize Fe-Mn-C-x thin films using the combinatorial approach and determine the correlation between the structure, chemical composition, and elastic properties. This correlation will be used in B1 to optimize the chemical composition. This synthesis pathway allows for synthesis of chemically well defined thin films and enables the determination of the exact correlation between the chemical composition (alloying element x) and the elastic properties.

It is our ambition to contribute towards understanding the structure-composition-elastic property-relationship of Fe-Mn-C steels. Our specific goals are:

1. To explore the elastic properties of Fe-Mn-C steels by nanoindentation and ab initio calculations.

2. To systematically investigate the role of alloying elements on the elastic properties of Fe-Mn-C-x steels (x = Al and Si) by ab initio calculations and in part by nanoindentation.

 

 

Hitherto work and selected results of TP C3

In TP C3 the combinatorial materials synthesis is used to grow Fe-Mn-X (X: further alloying elements) thin films on suitable substrates by means of physical vapor deposition. By virtue of the geometrical arrangement of the cathodes in the deposition chamber the grown thin films exhibit a gradient in chemical composition. For this reason the combinatorial materials synthesis displays an efficient approach to investigate the influence of the chemical composition on the elastic properties of high-Mn steels.

Fig. 1
Fig. 1: Experimental setup in the deposition chamber. Fig. 2: Mn gradient of a ternary Fe-Mn-Si sample.
 

As an example, the experimental setup for the deposition of ternary Fe-Mn-Si thin films is shown in figure 1. Three cathodes have been used, which were equipped with elemental Fe and Si targets as well as with a Fe-Mn compound target. The fourth cathode, which was not used in this case, was shut. In this way 2 µm thick films have been grown on heated sapphire wafer with a diameter of 50 mm. For further space-resolved characterization of the graded thin films a pattern with 145 positions with a lateral spacing of 3.5 mm was superimposed. The chemical composition was determined by means of energy dispersive X-ray analysis. Figure 2 shows the sample holder with an overlay of the Mn distribution as well as schematically the superimposed pattern of a representative sample. The presented Fe-Mn-Si thin film exhibits a gradient with Mn contents from 25 to 36 at.-% with Si additions up to 6 at.‑% auf. Subsequently the phase constitution of the thin films has been investigated by means of micro-diffraction with an X-ray diffractometer with an area detector for high-throughput analysis. The synthesis of austenitic thin films is aimed at. The elastic properties as a function of the chemical composition have been measured with nanoindentation.

These experiments serve to validate our ab initio calculations of the elastic properties of high-Mn steels. Figure 3 shows the calculated elastic constants and local magnetic moments of paramagnetic binary austenitic Fe-Mn alloys as a function of the Mn content [1]. The C44 values are in a range from 135 to 138 GPa, and are hence nearly independent of the Mn content. (C11‑C12)/2 is also independent of the Mn content, while C11 und C12 exhibit drastically different behaviors. There is a slight increase of C11 and C12 from 198 and 138 GPa to 211 and 153 GPa as the Mn content increases from 5 to 10 at.-%. Further increase of the Mn content to 40 at.-% results in a drop to 157 and 93 GPa. The local magnetic moment for Fe and Mn decreases from 1.62 to 1.33 µB and from 1.43 to 0.87 µB, as the Mn content increases from 5 auf 40 at.-%. The behavior of the elastic constants can be understood based on the so-called magnetovolume effect. Furthermore, we can state that the magnetic effects in Fe-Mn alloys have a strong influence on the elastic properties, even above the Neél temperature.

Fig. 3
Fig. 3:Calculated elastic constants as well as local magnetic moments of Fe and Mn.

 

 

[1] D. Music et al., "Elastic properties of Fe-Mn random alloys studied by ab initio calculations," Appl. Phys. Lett. 91 (19), 191904 (2007)