Rainer Völkl

Phase separation in equiatomic AlCoCrFeNi high-entropy alloy

The microstructure of the as-cast AlCoCrFeNi high entropy alloy has been investigated by transmission electron microscopy and atom probe tomography. The alloy shows a very pronounced microstructure with clearly distinguishable dendrites and interdendrites. In both regions a separation into an Al-Ni rich matrix and Cr-Fe-rich precipitates can be observed. Moreover, fluctuations of single elements within the Cr-Fe rich phase have been singled out by three dimensional atom probe measurements. The results of investigations are discussed in terms of spinodal decomposition of the alloying elements inside the Cr-Fe-rich precipitates.

Economical creep testing of ultrahigh-temperature alloys

Specially designed facilities for creep testing of ultrahigh-temperature alloys are presented. Ohmic heating is chosen for easy access to the sample, quickly attainable heating and cooling rates, and simplicity in design and operation. Strain is measured with a video extensometer by means of the software SuperCreep. SuperCreep allows fast and automated analysis of creep curves. For short but precise representations of creep curves, Bezier splines are used. Creep tests on pure Pt, Pt-10 wt% Rh, and the oxide dispersion strengthened Pt-base alloys Pt DPH and Pt-10 wt% Rh DPH between 1200 and 1600°C have proven the reliability of the equipment. Stationary creep rates follow the Norton power law. Stress exponents, activation energies, and pre-exponential factors are given for the four alloys investigated.

On the Path to Optimizing the Al-Co-Cr-Cu-Fe-Ni-Ti High Entropy Alloy Family for High Temperature Applications

The most commonly investigated high entropy alloy, AlCoCrCuFeNi, has been chosen for optimization of its microstructural and mechanical properties by means of compositional changes and heat treatments. Among the different available optimization paths, the decrease of segregating element Cu, the increase of oxidation protective elements Al and Cr and the approach towards a γ-γ′ microstructure like in Ni-based superalloys have been probed and compared. Microscopical observations have been made for every optimization step. Vickers microhardness measurements and/or tensile/compression test have been carried out when the alloy was appropriate. Five derived alloys AlCoCrFeNi, Al23Co15Cr23Cu8Fe15Ni16, Al8Co17Cr17Cu8Fe17Ni33, Al8Co17Cr14Cu8Fe17Ni34.8Mo0.1Ti1W0.1 and Al10Co25Cr8Fe15Ni36Ti6 (all at.%) have been compared to the original AlCoCrCuFeNi and the most promising one has been selected for further investigation.

Influence of Solid Solution Hardening on Creep Properties of Single-Crystal Nickel-Based Superalloys

Improving the creep resistance of the matrix by alloying with refractory elements is a major strengthening effect in nickel-based superalloy with rhenium as one of the most effective elements. In this work, the influence of rhenium on creep properties of single-phase single crystals with varying rhenium content and matrix-near composition is investigated. The use of single-crystalline material leads to very distinct results which are not deteriorated by grain boundary effects. So the strengthening effect can be solely attributed to the alloying element rhenium and is quantified for the first time. By comparing the creep strength of two matrix compositions with the corresponding single-crystal superalloys using the threshold stress concept, the potential of creep strengthening of the matrix in two-phase single-crystal alloys is quantified.

Microstructural study of boron-doped Co–Re–Cr alloys by means of transmission electron microscopy and electron energy-loss spectroscopy

Abstract The development of corrosion-resistant alloys with sustained thermal and mechanical stabilities is the focus of current research on high-temperature alloys. It has been shown that the creep behavior and oxidation resistance of Co–Re alloys can be improved by small additions of, e.g., B and Zr. To understand the effect of these elements on mechanical properties, we have characterized the phases and microstructures in the Co–Re–Cr system using transmission electron microscopy. Zr forms isolated large crystals in the matrix, while B is accommodated in small intergranular (Cr,Re)2B borides. The incorporation of up to 23 at.% Re into the Cr2B structure results in increased unit cell constants. The elongated borides occur pervasively along grain boundaries, suggesting that they improve the grain boundary cohesion and thereby the mechanical properties of Co–Re–Cr alloys.

Isothermal Oxidation Behavior of a Precipitation-Hardened Pt-Base Alloy with Additions of Al, Cr and Ni

Abstract The oxidation behavior of a precipitation-hardened Pt-base alloy with 12 at.% Al, 6 at.% Cr and 5 at.% Ni was examined. At high temperatures in air this alloy forms an alumina scale and a precipitate-free layer underneath, both growing according to parabolic functions of time. Small amounts of Ni embedded in the oxide scale accelerate its growth in the initial phase of oxidation. With time, oxidation is retarded due to decreasing impurity concentration and grain coarsening in the oxide scale. The interdiffusion coefficients of the scale-forming element Al in the Pt-rich solid solution matrix are estimated.

Enrichment of boron at grain boundaries of platinum-based alloys determined by electron energy loss spectroscopy in a transmission electron microscope

Abstract Polycrystalline platinum-based alloys show very good creep properties at high temperatures. Small amounts of boron (less than 1 at.%) considerably increase the creep strength. Transmission electron microscopy measurements were conducted to localize the element boron in the samples. With image electron energy loss spectroscopy spectra were extracted revealing an enrichment of B at grain boundaries. The boron distribution was plotted with the three window method.

Building a Thermodynamic Database for Platinum-Based Superalloys: Part III

Work has been ongoing in building a thermo-dynamic database for the prediction of phaseequilibria in Pt-based superalloys (1–7). The alloys arebeing developed for high-temperature applications inaggressive environments. The database will aid thedesign of alloys by enabling the calculation of thecomposition and proportions of phases present inalloys of different compositions. This paper is arevised account of work presented at the conference:Southern African Institute of Mining and Metallurgy‘Platinum Surges Ahead’ at Sun City, South Africa,from 8th to 12th October 2006 (7).Part I, describing initial results for the Pt-Al-Rusystem from the compound energy formalism model,was published in the July 2007 issue of

Finite-element modelling of anisotropic single-crystal superalloy creep deformation based on dislocation densities of individual slip systems

Abstract A finite-element model is proposed which describes the creep behaviour of a two-phase single-crystal superalloy. For the matrix phase, a user-defined material model is used. It describes the creep behaviour based on evolution equations for dislocation densities on individual slip systems. An interaction matrix determines the influence of one glide system on the other. Due to the face-centred cubic (fcc) symmetry, 9 independent parameters of the interaction matrix can be distinguished, describing slip on octahedral and cube glide planes with a Burgers vector of the type a/2 . The finite-element method (FEM) was used to describe the stress– strain behaviour as well as the dislocation evolution in matrix channels during creep deformation. Uniaxial stress in , , and directions was considered. The evolution of the calculated dislocation densities and the resulting creep curve are compared with experimental observations.

High Temperature Strengthening Mechanisms in the Alloy Platinum- 5% Rhodium DPH

To improve the high temperature properties, platinum can be hardened by solid solution and/or oxide parti-cles. The investigated alloy, dispersion hardened plati-num-5% rhodium (Pt-5%Rh DPH), was produced via melting and subsequent annealing of the semi-fi nished product in order to obtain an oxide particle dispersion. Despite the relatively large oxide particles formed in this process, the creep strength is much higher in com-parison to conventional Pt-5%Rh. The aim of this paper is to study the strengthening mechanisms in the alloy Pt-5%Rh DPH by transmission and scanning electron microscopy. The size distribution of oxide particles shows a bimodal distribution, and the average oxide particle diameter is 315 nm for particles larger than 150 nm. For particles between 25 nm and 150 nm the average diameter is 49 nm. The size ranges of oxide par-ticles are not substantially affected by high temperature creep deformation, but particles of