Head of the group: Dr.-Ing. Alexander Kauffmann
Dr.-Ing. Sascha Seils
Dipl.-Ing. Hans Chen
M.Sc. Camelia Gombola
M.Sc. Aditya Srinivasan Tirunilai
M.Sc. Susanne Obert
M.Sc. Stephan Laube
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)
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.
Peculiarities of deformation of CoCrFeMnNi at cryogenic temperatures
In cooperation with colleagues from ITEP/CryoMaK, KNMF, IFW Dresden and University of Bayreuth, we have investigated peculiarities of plastic deformation of CoCrFeMnNi at cryogenic temperatures. Special emphasis was placed on the effect of deformation twinning on the work-hardening behavior as well as serrated plastic flow at temperautres of 4.2 and 8 K in this study.
Contribution of Lattice Distortion to Solid Solution Strengthening in a Series of Refractory High Entropy Alloys
In this contribution, we provide evidence for a correlation of atomic size difference and microhardness at room temperature in Al-containing, refractory high entropy alloys. This fundamental correlation which might lead to a better understanding of solid solution hardening in bcc high entropy alloys is further supported by temperature-dependent investigation of mechanical properties.
Phase Evolution in and Creep Properties of Nb-Rich Nb-Si-Cr Eutectics
In this contribution, we exactly determined the location of the ternary eutectic in the Nb-rich part of the Nb-Si-Cr system and investigated the thermal stability and creep properties of ternary, eutectic alloys.