Stephan Huth

Microsegregation and precipitates of an as-cast Co-based superalloy—microstructural characterization and phase stability modelling

The demand for increased efficiency of industrial gas turbines and aero engines drives the search for the next generation of materials. Promising candidates for such new materials are Co-based superalloys. We characterize the microsegregation and solidification of a multi-component Co-based superalloy and compare it to a ternary Co–Al–W compound and to two exemplary Ni-based superalloys by combining the experimental characterization of the as-cast microstructures with complementary modelling of phase stability. On the experimental side, we characterize the microstructure and precipitates by electron microscopy and energy-dispersive X-ray spectroscopy and determine the element distributions and microsegregation coefficients by electron probe microanalysis (EPMA). On the modelling side, we carry out solidification simulations and a structure map analysis in order to relate the local chemical composition with phase stability. We find that the microsegregation coefficients for the individual elements are very similar in the investigated Co-based and Ni-based superalloys. By interpreting the local chemical composition from EPMA with the structure map, we effectively unite the set of element distribution maps to compound maps with very good contrast of the dendritic microstructure. The resulting compound maps of the microstructure in terms of average band filling and atomic-size difference explain the formation of topologically close-packed phases in the interdendritic regions. We identify B2, C14, and D024 precipitates with chemical compositions that are in line with the structure map.

Microsegregation and Secondary Phase Formation During Directional Solidification of the Single-Crystal Ni-Based Superalloy LEK94

A multicomponent phase-field method coupled to thermodynamic calculations according to the CALPHAD method was used to simulate microstructural evolution during directional solidification of the LEK94 commercial single-crystal Ni-based superalloy using a two-dimensional unit cell approximation. We demonstrate quantitative agreement of calculated microsegregation profiles and profiles determined from casting experiments as well as calculated fraction solid curves with those determined in differential thermal analysis (DTA) measurements. Finally, the role of solidification rate on dendrite morphology and precipitation of the secondary phases is investigated and a new measure of the dendrite morphology is presented to quantify the effect of back diffusion on the amount of secondary phases.

Diffusion alloying – a new manufacturing method for PM tool steels

Abstract To increase the wear resistance of tool steels, high contents of super-hard MC type carbides formed by Ti, V or Nb are desirable. Unfortunately, these carbides precipitate primarily from the melt at high temperatures. Thus, atomising such steels is not feasible with current technology because carbides precipitating in the melt may clog the nozzle. This problem can be avoided by atomising a melt that contains high amounts of carbide forming elements but no carbon. Subsequently the powder is mixed with graphite to provide the carbon necessary to form carbides and for the hardenability of the matrix. During hipping, graphite dissolves and the carbon is distributed evenly in the material. The phase distribution of a candidate cold work tool steel consisting of a stainless steel matrix with fine, well distributed niobium carbide precipitates is reported and compared with equilibrium calculations.

Effect of HIP Parameters on the Micro-Structural Evolution of a Single Crystal Ni-Based Superalloy

For reducing the porosity of single crystal (SX) nickel-based superalloys, Hot Isostatic Pressing (HIP) is used. High pressures of about 100-170 MPa lead to local deformation, which close the pores. However, since HIP also requires high temperatures (1000-1200°C) it has a pronounced effect on the microstructure and the local distribution of elements. This contribution analyses the effect of different HIP treatments on both the microstructure and the segregation of the SX superalloy LEK94 in the as-precipitation-hardened state. In addition, the effects of rapid or slow cooling are analyzed. To distinguish the effect of pressure from those of temperature, the HIPed samples are compared with specimens annealed at atmospheric pressure.

Reciprocating sliding wear of case-hardened spheroidal cast iron against 100Cr6 under boundary lubrication

Today cast iron with spheroidal graphite is used in a wide range of applications with a high production capacity per year. Due to optimized and well-controlled casting technology, the production of ductile cast iron became economic in such way that ductile cast iron replaced cast or wrought steel in many machinery components like crankshafts, piston rods, and engine mounts. These examples represent technical tribosystems of the automobile industry. Here, current political, economic, and ecological guidelines also demand downsizing combined with high power densities in order to minimize internal friction and reduce fuel consumption and satisfying CO2-emission limits. These guidelines can change the tribological loads and, therefore, result in more severe conditions. One example is the shift of the lubrication regime from hydrodynamic to mixed or boundary lubrication for larger periods of time. In these regimes, the applied load is partially or fully carried by the asperities. Still the need for maintaining as low as possible wear towards the ultra-mild sliding wear regime an integral approach is needed, which has to regard contact conditions, surface topography, interface chemistry, and sub-surface properties. One way to low wear can aim at lowering the run-in phase by e.g. optimizing the topography by means of adjusted machining processes. For this study, reciprocating sliding wear tests were conducted with grinded, milled, polished, and finished samples of case-hardened spheroidal cast iron slid against a 100Cr6 ball of a 5 mm radius. The boundary lubrication was provided by a commercial combustion engine lubricant at 80℃. After predefined test cycles, 3D surface topographies were measured by means of confocal white-light microscopy within each wear test in order to analyse the development of the contact conditions over time. In combination with the measured forces and displacements, the tribological loads are calculated by means of a 3D elastic-ideal plastic contact model. Additionally the wear mechanism was analyzed by means of scanning electron microscopy. The overall wear rates and the coefficients of friction depend strongly on the initial surface topography and, therefore, on the machining process. This is also true for the development of a reaction layer (tribomaterial) allowing for ultra-mild siding wear even under boundary lubrication.

Gas-Solid Interactions During Nonisothermal Heat Treatment of a High-Strength CrMnCN Austenitic Steel Powder: Influence of Atmospheric Conditions and Heating Rate on the Densification Behavior

This work deals with gas–solid interactions between a high-alloyed steel powder and the surrounding atmosphere during continuous heating. It is motivated by the recently developed corrosion-resistant CrMnCN austenitic cast steels. Here, powder metallurgical processing would be desirable to manufacture highly homogeneous parts and/or novel corrosion-resistant metal-matrix composites. However, the successful use of this new production route calls for a comprehensive investigation of interactions between the sintering atmosphere and the metallic powder to prevent undesirable changes to the chemical composition, e.g., degassing of nitrogen or evaporation of manganese. In this study, dilatometric measurements combined with residual gas analysis, high-temperature X-ray diffraction (XRD) measurements, and thermodynamic equilibrium calculations provided detailed information about the influence of different atmospheric conditions on the microstructure, constitution, and densification behavior of a gas-atomized CrMnCN steel powder during continuous heating. Intensive desorption of nitrogen led to the conclusion that a vacuum atmosphere is not suitable for powder metallurgical (PM) processing. Exposure to an N2-containing atmosphere resulted in the formation of nitrides and lattice expansion. Experimental findings have shown that the N content can be controlled by the nitrogen partial pressure. Furthermore, the reduction of surface oxides because of a carbothermal reaction at elevated temperatures and the resulting enhancement of the powder’s densification behavior are discussed in this work.

Comprehensive investigation of phase transformation during diffusion alloying of Nb-rich powder metallurgical tool steels

Abstract Stainless tool steels highly alloyed in niobium can be produced by powder metallurgy using diffusion alloying. Steel powder atomised without carbon is subsequently mixed with graphite and hot isostatically pressed. The atomised powder contains the intermetallic Laves phase NbFe2 that transforms into MC-type carbides during HIP when graphite has been added. The obtained structure features a fine distribution of carbides to increase wear resistance and chromium fully dissolved in the matrix to provide corrosion resistance. X-ray diffraction (XRD) measurements and reflection position analysis with additional scanning electron microscopy (SEM) have been conducted to study the phase transition of NbFe2 Laves phase into NbC carbides in two high Nb alloyed stainless tool steels. The results show that carburisation starts at 1000–1050°C and also confirm the correlation between oxide reduction and carburisation. The formed carbides are distinctly understoichiometric, which leads to an overestimation of the suitable quantitiy of added carbon in the thermodynamic calculations.

Simulation of the External Pressure Influence on the Micro-Structural Evolution of a Single Crystal Ni-Based Superalloy

The phase field method has been applied to simulate the microstructural evolution of a commercial single crystal Ni-based superalloy during both, HIP and annealing treatments. The effects of applying high isostatic pressure on the microstructural evolution, which mainly retards the diffusion of the alloying elements causing the loss of the orientational coherency between the phases is demonstrated by the simulation and experimental results