S. Mitsche

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Papers by S. Mitsche

Quantitative Metallography

Imaging & Microscopy, 2006

ABSTRACT

Improved Creep and Oxidation Behavior of a Martensitic 9Cr Steel by the Controlled Addition of Boron and Nitrogen

Advanced Microstructures for Increased Creep Rupture Strength of MARBN Steels

Materials Science Forum, 2014

Over the past three decades a lot of effort was made to optimize the chemical compositionof 9% Cr... more Over the past three decades a lot of effort was made to optimize the chemical compositionof 9% Cr martensitic steels, aiming to increase the operating temperature up to 923K and thus im-proving the efficiency of thermal power plants. Under these service conditions (high temperature andstress exposure), the creep strength of such steels is closely related to the long term stability of theirmicrostructure. The time to rupture can also be understood as an equivalent to the time of microstruc-ture deterioration. Optimization of the initial microstructure and understanding of the microstructureevolution during creep exposure are therefore decisive to improve the creep behavior of 9% Cr steels.Selected chemical compositions of MarBN steels (Martensitic 9% Cr steels strengthened by Car-bides, Nitrides and Boron) were subjected to different heat treatments to produce an optimized mi-crostructure to improve the creep rupture time. The initial microstructure before creep exposure wasinvestiga...

Assessment of dynamic softening mechanisms in Allvac® 718Plus™ by EBSD analysis

Materials Science and Engineering: A, 2011

The nickel-based superalloy Allvac ® 718Plus TM is a future candidate for turbine disc applicatio... more The nickel-based superalloy Allvac ® 718Plus TM is a future candidate for turbine disc applications, as this new material combines the formability and cost advantages of Alloy 718 with the higher temperature capability of Waspaloy. Due to the strong influence of the microstructure on the final mechanical properties, a fundamental knowledge of the dynamic recrystallization mechanism of Allvac ® 718Plus TM is of great importance in order to develop precise microstructure evolution models for this material. Compression tests were performed at temperatures of 900 • C to 1050 • C and strain rates between 0.1 s −1 and 10 s −1. The microstructures obtained were analyzed by electron backscatter diffraction (EBSD) to evaluate the influence of the different hot forming parameters on the dynamic softening processes of Allvac ® 718Plus TM .

$Fig. 1. Adiabatic and isothermal flow curves of the superalloy Allvac® 718Plus™ in dependence on the temperature and strain rate The results of the recrystallized grains as a function of temper- ature, strain and strain rate of Allvac® 71 in Fig. 4. It can be seen that the recrysta 8Plus™ are summarized lized fraction depend on the strain, strain rate and temperature whereas the strain seems to be the most dominant. In general a d ecrease of these param- eters is accompanied by a decrease in the recrystallized fraction. The marginal increase in the recrystallized fraction at a strain rate of 0.1s~! and T= 1050 °C between a strain of 0.4 and 0.9 might be con- nected with a change in the dynamic softening mechanism which is discussed in a later section. The size of the recrystallized grains was determined with twin removal and only a slight influence of The measured (adiabatic) flow curves were transformed into isothermal curves [13] using the scheme from Laasraoui and Jonas [16]. These curves are plotted in Fig. 1 and exhibit the influence of$

$Recrystallized fraction and fraction of twin boundaries of the recrystallized grains as a function of strain. Table 3$

$Fraction of twin boundaries of the recrystallized grains as a function of temperature (strain rate 1s~', strain 0.9). grains. This recrystallization behaviour is quite similar to that of the nickel base Alloy 80A reported by Poelt et al. [17]. In contrast, Wang et al. [18] observed that Alloy 718 also shows recrystalliza- tion in the interior of the grains at temperatures of 1050°C and higher. These papers revealed one distinct difference between the two alloys in their propensity to form twins in the recrystallized grains. Whereas only a slight tendency for twinning was found for Alloy 718, the recrystallized grains of Alloy 80A are highly twinned. Table 2 shows that the recrystallized grains of Allvac® 718Plus™ contain a high amount of twin boundaries in specimens strained up to 0.9 ata strain rate of 1 s~!. In order to express twin boundaries the coincidence site lattice notation was used and for face centred cubic materials the Sigma 3 boundaries represent the twin boundaries. The permitted angular deviation was calculated using Brandon’s criterion [19]. For the calculation of the twin fraction all high angle boundaries between recrystallized grains and between recrystal- lized and deformed grains were considered. The fraction of twin boundaries is relatively constant at forming temperatures between 950°C and 1050°. The decrease in twinning at 900°C might be con- nected with the appearance of precipitation at this temperature.$

$Fig. 4. Recrystallized fraction (left) and size of the recrystallized grains (right) in dependence on the strain and the forming parameters; for legend see figure left.$

$Fig.6. Relative frequency of misorientation for Allvac® 718Plus™ formed at 1050 °¢ and 10s~! as a function of strain. Specimens formed at a temperature of 950°C and a strain rate of 1s~! show a completely different behaviour than the specimens formed at 1050°C. At a strain of 0.2 a clear subgrain structure is visible with a high fraction of subgrain boundaries along the ini- tial grain boundaries, but also some in the grain interior (see GSB maps in Fig. 9). No recrystallized grains are observable at this strain. When the strain is increased to 0.4 recrystallization sets in and a few recrystallized grains can be seen along former grain bound- aries. Moreover, the subgrain structure is further developed at both the grain interior and the grain boundaries. The distribution of the subgrain boundaries appears to be inhomogeneous. Some grains contain a high amount of substructure in the interior of the grains and some grains are relatively free of substructure. At a strain of 0.9 the specimen appears to be mostly recrystallized, only a few deformed grains remain. The interesting point is that these deformed grains contain subgrain structures which are in the size range of the recrystallized grains (see zoomed area in of Fig. 9c).$

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Recrystallization and grain growth in the nickel-based superalloy allvac 718Plus

International Journal of Materials Research, 2009

The nickel-based superalloy Allvac 718Plus was developed in order to combine formability and cost... more The nickel-based superalloy Allvac 718Plus was developed in order to combine formability and cost advantages of Alloy 718 with high temperature strength of Alloy 720. Typical applications for this new material are turbine disks that are produced by closed die forging, e. g. by screw pressing. The final mechanical properties are strongly related to the microstructure, which forms during hot forming. It is therefore of great interest to model both the dynamic and static grain structure development during processing. In this work, dynamic and static recrystallization as well as normal grain growth models are described for this material. The calculated grain structure evolution in Allvac 718Plus is compared to that of Alloy 718 in the typical process range of disk forging. Additionally, both the experimental program and structure analysis methodology are specified.

On the modelling of the interaction of materials softening and ductile damage during hot working of Alloy 80A

Journal of Materials Processing Technology, 2006

Ductile fracture at high temperatures was investigated by means of continuum damage mechanics. An... more Ductile fracture at high temperatures was investigated by means of continuum damage mechanics. An enhanced model of the effective stresses considering crack closure effects by splitting the Cauchy stress tensor into a compressive and tensile part is combined with a damage evolution law that has been derived from the potential of dissipation and depends on the damage strain energy release rate. Additionally, a semi-empirical grain structure model had been coupled to describe the materials softening by dynamic recrystallisation. It can be demonstrated that with the onset of recrystallisation the accumulated deformation, i.e. the effective plastic strain, is reduced by recrystallisation and hence the initiation of damage is retarded. EBSD analyses of hot deformed samples were performed in order to validate the model and to investigate the interaction of crack initiation as well as of crack progress with dynamic recrystallisation.

format_quoteHot compression tests revealed critical strains for macro surface cracks, informing on crack growth dynamics and microstructure influences.format_quote

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Ex-situ and In-situ Microstructure Investigations of AM Part and Powders

This paper presents an advanced microstructural analysis of the AlSiMg, Ti64 and N700 powders use... more This paper presents an advanced microstructural analysis of the AlSiMg, Ti64 and N700 powders used for additive manufacturing. The internal microstructure of the regular and irregular powder grains were characterized down to atomic resolution by using scanning electron microscopy and high resolution scanning transmission electron microscopy.The accretionary forms on top of the irregular AlSiMg powder grains exhibit a slightly coarse microstructure with a network of eutectic Si consisting of nano-crystallites, suggestinga slower cooling than the grain itself that contain a predominately amorphous Si network. A nm thin amorphous C layer on the surface of some Ti64 plasma atomized powder grains promoted the attachment of satellites and growth of envelopes. In case of gas atomized N700 powder grains, we identified thin oxide and carbon amorphous layers as well as metal segregations at the interface between the grain body and the accretionary forms.

Microstructure Investigations of Powders and Additive Manufactured Parts

BHM Berg- und Hüttenmännische Monatshefte

This paper presents an advanced microstructural analysis of the AlSiMg, Ti64, and N700 powders us... more This paper presents an advanced microstructural analysis of the AlSiMg, Ti64, and N700 powders used for additive manufacturing. The internal microstructure of the regular and irregular powder grains were characterized down to atomic resolution by using scanning electron microscopy and high resolution scanning transmission electron microscopy.The accretionary forms on top of the irregular AlSiMg powder grains exhibit a slightly coarse microstructure with a network of eutectic Si consisting of nano-crystallites, suggesting a slower cooling than the grain itself that contain a predominately amorphous Si network.A nm thin amorphous C layer on the surface of some Ti64 plasma atomized powder grains promoted the attachment of satellites and growth of envelopes. In case of gas atomized N700 powder grains, we identified thin oxide and carbon amorphous layers as well as metal segregations at the interface between the grain body and the accretionary forms.

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Quantification of the Recrystallized Fraction in a Nickelbase-Alloy from EBSD-Data

Microscopy and Microanalysis

Creep and damage investigation of advanced martensitic chromium steel weldments for high temperature applications in thermal power plants

Science and Technology of Welding and Joining

DualBeam FIB application of 3D EDXS for superalloy δ-phase characterization

EMC 2008 14th European Microscopy Congress 1–5 September 2008, Aachen, Germany, 2000

Optimizing the functionality of materials often depends on a precise control of the size, shape, ... more Optimizing the functionality of materials often depends on a precise control of the size, shape, crystal structure and composition of the material being synthesized. But in the age of micro- and nanotechnology it can be a significant challenge to characterize solids in an appropriate way. Therefore many sophisticated analysing methods were established and combined in the past [1, 2]. Automated serial sectioning in combination with physical analytics achieved by using a script-able Dual Beam -Focused Ion Beam system (DB-FIB) equipped with an energy dispersive X-ray spectrometer (EDXS) is only one of the possible characterization opportunities mentioned above [3].

Characterisation of the subgrain structure of the aluminium alloy AA6082 after homogenization and hot forming by EBSD

EMC 2008 14th European Microscopy Congress 1–5 September 2008, Aachen, Germany, 2008

The properties of a hot rolled aluminium alloy are strongly dependent on the homogenization and t... more The properties of a hot rolled aluminium alloy are strongly dependent on the homogenization and the forming conditions [1,2], because the microstructure of the specimen will be changed by the various processing steps [3]. A powerful tool to investigate the obtained grain — subgrain structure is the backscatter electron diffraction (EBSD) technique [4]. The evolution of the microstructure of the

Experimental determination of the total scattering cross section of water vapour and of the effective beam gas path length in a low vacuum scanning electron microscope

EMC 2008 14th European Microscopy Congress 1–5 September 2008, Aachen, Germany, 2008

ABSTRACT Low Vacuum Electron Microscopy enables the investigation of non-conductive samples witho... more ABSTRACT Low Vacuum Electron Microscopy enables the investigation of non-conductive samples without special preparation procedures. The imaging gas inside the specimen chamber is responsible for the contrast formation by gas amplification and the generated positive gas ions suppress charging of the sample. But the gaseous environment inside the chamber is limiting the capability of the microscope by elastic and inelastic collisions of the primary beam electrons (PEs) with the gas molecules. This so called skirt effect degrades the signal to noise ratio by generating gaseous secondary electrons (SEs) as well as SEs from regions far away from the focused probe. Therefore the primary beam loses exponentially electrons to a broadly dispersed skirt along the beam gas path length (BGPL) (1).

Automated particle analysis of aerosols formed during biomass combustion by SEM/EDXS

If only the particle size is of interest, uncoated particles and the use of the in-lens detector ... more If only the particle size is of interest, uncoated particles and the use of the in-lens detector is recommended. In case of carbon coated particles the average of the sizes obtained from images of the in-lens and the BSE-detector have to be used Automated analysis of aerosols with SEM/EDXS is possible down to particle diameters of about 50-100 nm depending on the chemical composition

format_quoteCoating significantly affects measured particle size in automated particle analyses.format_quote

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Investigation of Friction Stir Welding of Stainless Steel Using a Stop-Action-Technique

Advanced Materials Research, 2011

Especially for aluminium and its alloys friction stir welding (FSW) has become an established wel... more Especially for aluminium and its alloys friction stir welding (FSW) has become an established welding process. In contrast FSW for steel is still challenging and in basic research. Some reasons are the high price of the tungsten based tools, the durability of the tools and the low welding speeds. For further development of the process, it is necessary to understand the metallurgical changes in the stirred material during welding. In this work, a 4mm thick stainless steel plate (1.4301) was welded with different types of tungsten-alloyed tools. A so called stop-action-technique was used at the end of the weld and the sheet was quenched immediately to prevent metallurgical changes caused by slow cooling. During the process, the temperatures on the top of the welded plate, close to the tool shoulder (10 mm beside the weld centre line) and at the bottom of the plate, directly below the weld centre were measured. On the bottom side, the temperature was also measured 35mm in front of the ...

Geopolymerization of coal fly ash in the presence of electric arc furnace dust

Minerals Engineering, 2013

Microstructure Modeling of the Dynamic Recrystallization Kinetics during Turbine Disc Forging of the Nickel Based Superalloy Allvac 718 Plus

Superalloys 2008 (Eleventh International Symposium), 2008

FE modelling of microstructure evolution during friction stir spot welding in AA6082-T6

Welding in the World, 2013

ABSTRACT Friction stir spot welding (FSSW) is a solid-state joining method, which is a variant of... more ABSTRACT Friction stir spot welding (FSSW) is a solid-state joining method, which is a variant of friction-stir welding. Microstructure analysis shows that the FSSW joint contains four different zones, namely the stir zone, thermo-mechanical affected zone, heat-affected zone and base metal, respectively. In this paper, the results of a FE analysis of the FSSW process of AA6082-T6 considering geometric dynamic recrystallization are presented. A physically based model taking into account three internal state variables was implemented into the commercial FE package DEFORM-3D to describe the microstructure evolution during FSSW. This model allows predicting the dislocation density, grain size, temperature, effective strain, and strain rate during FSSW. The microstructure in stir zone was analysed by electron backscattered diffraction. Experimental and simulation results have been compared to validate the model.

Substrate structure dependence of the growth modes of p-quaterphenyl thin films on gold

Thin Solid Films, 2005

ABSTRACT The variably oriented crystallite surfaces of a recrystallized polycrystalline gold samp... more ABSTRACT The variably oriented crystallite surfaces of a recrystallized polycrystalline gold sample served as substrates for the investigation of the structure dependence of p-quaterphenyl (4P) thin film growth. The films were prepared in ultrahigh vacuum by organic molecular beam evaporation. Optical microscopy, scanning electron microscopy, combined with laterally resolved electron backscatter diffraction and scanning tunnelling microscopy have been applied to determine the correlation between the substrate surface structure and 4P film morphology. Crystallite surfaces consisting of (110) terraces favour highly anisotropic needle-like 4P growth with the needle orientation normal to the Au 〈11¯0〉 directions. Atomic steps on vicinal planes with narrow terraces (&lt; 2 nm) can also induce anisotropy in the 4P thin film growth, in particular elongated 4P islands normal to the step direction. In contrast to that, a nearly isotropic distribution of the needle orientations is observed on Au grains terminated by highly symmetric (111) or (100) crystal planes. Additionally, patches of continuous 4P layers can be found on these surfaces. There is strong evidence that the 4P molecules within the needle-like crystallites are oriented parallel to the Au surface, whereas for the continuous layers the 4P molecules are oriented nearly upright on the surface.

A method to measure the total scattering cross section and effective beam gas path length in a low-vacuum SEM

Scanning, 2009

A method is presented to determine the total scattering cross section of imaging gases used in lo... more A method is presented to determine the total scattering cross section of imaging gases used in low-vacuum scanning electron microscopy or environmental scanning electron microscopy. Experimental results are presented for water vapor, nitrogen gas and ambient air for primary beam electron energies between 5 and 30 keV. The measured results are compared and discussed with calculated values. This method allows the effective beam gas path length (BGPL) to be determined. The variations of the effective BGPL with varying chamber pressure are presented. SCANNING 31: 107-113, 2009.

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