Head of the group: Dr.-Ing. Alexander Kauffmann
Dr.-Ing. Sascha Seils
Dr.-Ing. Daniel Schliephake
M.Sc. Camelia Gombola
M.Sc. Aditya Srinivasan Tirunilai
M.Sc. Susanne Obert
M.Sc. Stephan Laube
M.Sc. Frauke Hinrichs
we get support by our APT experts at KNMF:
Dr. Torben Boll
Dr.-Ing. Sascha Seils
The Physical Metallurgy group focuses on the development of metallic and intermetallic materials for harsh environments. The investigation and optimization of materials for engines operating at high temperatures is of central interest. Therefore, outstanding high temperature stability (mechanical and microstructural) in conjunction with reasonable toughness at room temperature as well as suitable oxidation resistance are the main objectives of the research. In addition, fundamentals of contact materials for vacuum switches and the deformation behavior of high-entropy alloys under various conditions are studied in detail.
Synthesis of new materials
The synthesis of new materials is based on the following methods that are available in house:
- melting technologies: arc melter and zone melting
- powder metallurgy: attritor grinding mill, planetary ball mill, hot uniaxial pressing
- heat treatments in various atmospheres
Methods of characterization
The characterization of mechanical and thermo-physical properties as well as microstructure of metallic and intermetallic materials is performed by means of:
- standard metallographic procedures
- mechanical testing under various loading conditions (tension, compression, cyclic, creep conditions, various atmospheres)
- thermal analysis: thermogravimetry (TGA) and differential scanning calorimetry (DSC)
- focused ion beam (FIB) for microscopic preparation
- analytical scanning electron microscopy: energy-dispersive X-ray spectroscopy (EDX) and electron backscatter diffraction (EBSD)
- X-ray diffraction (XRD)
- 3D atom probe tomography (APT)
Temperature dependent strengthening contributions in austenitic and ferritic ODS steels
Strengthening of oxide dispersion strengthened (ODS), powder metallurgical steels with nanometer-sized oxide particles leads to high-strength, creep-resistant materials, which promise increasing application temperatures compared to classic high-temperature steels. In this study, we investigate the temperature dependent strength of ferritic and austenitic ODS steels. It is demonstrated that using elemental powders for mechanical alloying of austenitic ODS steels is preferred. We determine typical strengthening contributions from microstructural information and found that the lower room temperature strength of austenitic ODS steels results from lower contributions by grain boundaries as well as by dislocations compared to ferritic ODS steels. The drop in strength above 500 °C can reasonably be described by annihilation of dislocations at grain boundaries.
Comparison of cryogenic deformation of the concentrated solid solutions CoCrFeMnNi, CoCrNi and CoNi
This contribution on cryogenic deformation of face-centered cubic high entropy alloys was performed in collaboration with colleagues from ITEP at KIT, Ruhr-University Bochum and IFW Dresden. CoCrFeMnNi and CoCrNi are complex concentrated alloys which show significant strengthening and ductility at cryogenic temperatures. The chief contribution for the strength is associated with solute dislocation interaction. A comparison of the yield strength variation with temperature down to 8 K shows that CoCrFeMnNi and CoCrNi have a similarly high solid solution strengthening effect contributing temperature dependent strengthening despite a higher absolute yield strength of CoCrNi at all temperatures. CoCrNi has a reportedly lower SFE leading to formation of ε-martensite at cryogenic temperatures, while CoCrFeMnNi does not. Comparison of work-hardening rates at cryogenic temperatures indicates that both alloys strengthen at similar rates despite the TRIP effect being active in CoCrNi and not in CoCrFeMnNi. The ε-martensite in CoCrNi exists as twin-martensite nano-laminates and strengthens the alloy through dislocation-interface interaction.
Creep of an Oxidation Resistant Coated Mo-9Si-8B Alloy
The previous work of Prof. Perepezko's group at the University of Wisconsin – Madison shows that the oxidation resistance of Mo-Si-B alloys can be improved significantly by applying an SiO2 layer by the pack-cementation process. In collaboration with the colleagues from Madison, we investigated the effect of the layer stability during time-dependent, mechanical loading at 1200 °C. By applying a constant load at high temperatures, the Mo-Si-B substrate starts to elongate while cracks form within the SiO2 layer perpendicular to the loading direction. However, the formation of cracks does not lead to catastrophic failure of the substrate since the layer starts to fill them with new oxides during a self-healing process.
Controlling crystallographic ordering in Mo-Cr-Ti-Al high entropy alloys to enhance ductility
In this study with our colleagues from Siegen University, we utilized our recently proposed methods to reveal ordering phenomena in HEA (Chen et al. in Acta Materialia 176 (2019) 123-133) to enhance ductility of alloys from within the Mo-Cr-Ti-Al system. The composition-depending transistion to disordered alloys is indeed associated to a drop in yield strength. Nevertheless, ductility at room temperature is significantly enhanced when no ordering is present.
Solid solution strengthening and deformation behavior of single-phase Cu-base alloys under tribological load
The influence of solid solution strengthening on the evolution of the microstructure under cyclic dry sliding is still not fully rationalized. One reason is that alloying needed for solid solution strengthening alters the stacking fault energy at the same time and, hence, the mode of dislocation slip in face-centered cubic metals and alloys. Both aspects determine the details of plastic deformation and, therefore, lead to different results under tribological load. A series of Cu-Mn alloys was investigated in the present investigation, which exhibit wavy slip mode and an almost constant stacking fault energy over a wide solute concentration range. Solid solution strengthening is the main contribution to the hardness in these alloys. The sole impact of changing strength and hardness on the tribological response along with microstructure evolution during tribological load is assessed. After the reciprocating, tribological loading a linear correlation between the wear track width and hardness could be ascertained. Electron microscopy reveals a horizontal discontinuity of the dislocation structure beneath the surface in all alloys at a similar depth. An evaluation of the Hamiltonian elastic stress field model indicates that the depth of the dislocation feature after one sliding pass correlates with the stress distribution as well as the critical stress for dislocation motion. The subsurface microstructure features a transition from the dislocation feature to subgrain formation after about five to ten cycles. Beyond ten cycles, oxide clusters are formed on the sliding surface and the grains elongate in the sliding direction.
Characterisation of the oxidation and creep behaviour of novel Mo-Si-Ti alloys
The oxidation and creep behaviour of novel eutectic-eutectoid Mo-Si-Ti alloys were studied and compared to entirely eutectic and eutectoid reference alloys. While the reference alloys showed either outstanding oxidation behaviour (eutectic alloy) or reasonable creep resistance (eutectoid alloy), a combination of both was successfully achieved in a Ti-rich alloy variant (Mo-21Si-43.4Ti). The ubiquitous catastrophic oxidation ("pesting2) of Mo-based alloys at 800°C is suppressed in this alloy and reasonable oxidation resistance at higher temperatures is observed. For the first time, the unexpected oxidation resistance of the alloys exhibiting eutectic volume fractions of more than 50 vol% is rationalised by a systematic deconvolution of mass gain by scale formation and mass loss by evaporation of volatile species. Furthermore, creep is revealed to be based on similar creep mechanisms throughout the alloy series and differences can be quantitatively related to changing solidus temperatures.
Crystallographic ordering in a series of Al-containing refractory high entropy alloys Ta-Nb-Mo-Cr-Ti-Al alloys
Crystallographic ordering might play an important role in the ductility of body-centered cubic high entropy alloys at low and intermediate temperatures. Therefore, we investigated the appearance and detection of crystallographic order in alloys from the Ta-Nb-Mo-Cr-Ti-Al system together with colleagues from Brown University (Providence, USA), MPIE Düsseldorf, ITEP at KIT, KNMF, and University of Siegen. Due to the various possibilities of lattice site occupation in multi component systems, laboratory diffraction methods do not necessarily provide unambiguous evidence for the absence or appearance of ordered crystal structures. Rather, the anomalies of specific heat during order-disorder transformations should be analyzed for more reliable evidence and comparably fast assessment of this peculiar feature in near-equiatomic high entropy alloys.
Microstructural evolution during creep of lamellar eutectoid and off-eutectoid FeAl/FeAl2 alloys
In order to reveal the fundamental mechanisms during the complex creep response of eutectoid FeAl/FeAl2, we performed detailed microstructural analysis of crept material together with Prof. Kumar (Brown University, USA). In the early stages of creep, the FeAl phase primarily carries creep deformation by dislocation motion, whereas FeAl2 remains mostly plastically undeformed, except in certain locations near colony boundaries where the lamellar structure is disrupted/absent. Within the colonies, where the lamellae are intact, deformation is accommodated at the FeAl/FeAl2 interface. This continues to be the case at the minimum creep rate. With further progression in creep, FeAl2 begins to participate in the process of plastic deformation in a more substantive manner through unexpected deformation twinning and dislocation slip, while FeAl continues to plastically deform and dynamically recover. Further beyond the minimum, the lamellar structure adjacent to the colony boundaries breaks down, and these areas become the primary contributors to creep and result in a continuous loss in creep resistance. These investigations will further support our activities with Prof. Böhlke’s group (ITM) to quantitatively describe the complex creep response of lamellar intermetallic materials.
Constitution, oxidation and creep of eutectic and eutectoid Mo-Si-Ti alloys
In collaboration with colleagues from University of Siegen, we compared the microstructure evolution, oxidation and creep of (almost) two-phase Mo-Si-Ti alloys. We developed a eutectic alloy Mo-20.0Si-52.8Ti (at.%) which shows for the first time - despite a large volume fraction of Mo-rich solid solution (~ 50 vol.%) - pesting-resistant oxidation behavior. The evaporation of MoO3 is prevented during oxidation at 800 °C. In combination with its rather low density of only 6.2 g/cm³, a promising candidate for future development of high temperature materials is therefore introduced.