Ralf Rettig

Composition of corrosion layers on a magnesium rare-earth alloy in simulated body fluids.

In this study, the composition of corrosion product layers on a magnesium rare-earth alloy in simulated body fluid (m-SBF) containing albumin in physiological concentration is examined. The time dependence of the composition of the layer is studied. The ions from the body fluid that participate in the corrosion layer formation were identified by analyzing layers formed in different solutions that contain only some of the ions of SBF. The layer composition was analyzed by different complementary methods. We used energy dispersive X-ray analysis, grazing incidence X-ray diffraction, and Fourier transform infrared spectroscopy. The morphology of the corrosion layers was studied using scanning electron microscopy and light microscopy. In m-SBF with and without albumin we found an amorphous layer of carbonated calcium phosphate with some calcium replaced by magnesium. It can be clearly shown that calcium is only deposited in the corrosion layer if phosphates are in the solution. The cross-sections reveal that there are some sharp crevices in the substrate. The work systematically explores the nature of surface layers on magnesium rare-earth alloys formed in complex SBFs, with the aim to elucidate the influence of specific electrolyte components on the morphology, structure, and composition of corrosion layers on Mg alloys.

Verification of a Commercial CALPHAD Database for Re and Ru Containing Nickel-Base Superalloys

The addition of rhenium and ruthenium to single crystal nickel-base superalloys improves the high-temperature properties of the alloys. In this work the applicability of the database TTNi7 (ThermoTech Ltd, UK) for developing 4th generation single crystal superalloys containing rhenium (Re) and ruthenium (Ru) was investigated. We systematically compared experimentally determined alloy properties to the predictions of ThermoCalc with the database TTNi7. The investigated properties were liquidus, solidus and ´ solvus temperature as well as incipient melting point and segregation. Calculations were based on thermodynamic principles with the assumption of either equilibrium or Scheil-Gulliver conditions, i.e. no diffusion in the solid and complete diffusion in the liquid. Furthermore the composition of the  and the  phase of a Re- and Ru-containing superalloy was measured and compared to calculations. Our results show that the database is capable of simulating general trends of 4th generation superalloys up to 6 weight percent (wt.-%) Re and 6 wt.-% Ru. The present work shows that Scheil-Gulliver calculations can only be used as a first approximation for nickel-base superalloys.

Optimization of the Homogenization Heat Treatment of Nickel-Based Superalloys Based on Phase-Field Simulations: Numerical Methods and Experimental Validation

A method for predicting the fastest possible homogenization treatment of the as-cast microstructure of nickel-based superalloys is presented and compared with experimental results for the single-crystal superalloy ERBO/1. The computational prediction method is based on phase-field simulations. Experimentally determined compositional fields of the as-cast microstructure from microprobe measurements are being used as input data. The software program MICRESS is employed to account for multicomponent diffusion, dissolution of the eutectic phases, nucleation, and growth of liquid phase (incipient melting). The optimization itself is performed using an iterative algorithm that increases the temperature in such a way that the microstructural state is always very close to the incipient melting limit. Maps are derived allowing describing the dissolution of primary γ/γ′-islands and the elimination of residual segregation with respect to temperature and time.

Modeling of precipitation kinetics of TCP-phases in single crystal nickel-base superalloys

The precipitation of brittle so-called TCP-phases is critical for the application of Re-containing single crystal superalloys. In this work a fully multicomponent precipitation model is presented, which is capable of simulating the precipitation process of the TCP-phases in superalloys considering complex precipitation sequences with several metastable phases. The model is coupled to multicomponent thermodynamic CALPHAD calculations and relies on multicomponent diffusion models based on the TC-API interface of the software DICTRA. The required mobility database has been newly developed and covers all relevant alloying elements of the Ni-base superalloys including rhenium (Re) and ruthenium (Ru). It is well known that adding Ru strongly reduces TCP-phase precipitation. Based on the developed precipitation model, possible mechanisms are investigated to explain this effect and it is concluded that Ru mostly influences the nucleation rate by a combined influence on interface energy, “reverse partitioning” and γ’-phase fraction.

Fast interpolation algorithm for the calculation of thermodynamic property maps of microstructures

A fast method for the calculation of thermodynamic property mappings of alloy microstructures is presented. It uses surrogate interpolation models instead of the direct CALPHAD calculation of each data point. With the best algorithm, a speed-up of nearly 30 can be achieved compared to the direct method. If repeated calculations of similar microstructures are required, a speed-up of around 300 can even be realized. Different surrogate models have been evaluated. The universal kriging method gives the most accurate results, while polynomial surface response models turn out to generate larger interpolation errors. In order to minimize the number of calculation points of the surrogate interpolation model, the microstructure is classified into similar regions, in which the design points are chosen randomly. The algorithm is applicable independent of alloy composition. Within the paper, we present the application of the algorithm for two single-crystalline nickel-based superalloys.

Creep rupture strength of Re and Ru containing experimental nickel-base superalloys

The influence of Re and Ru on creep rupture strength has been investigated using a new in-house designed alloy-series comprising 9 experimental nickel-base superalloys with stepwise increased Re and Ru additions. The presented creep data reveals a significant increase in creep rupture strength by additions of Re. For additions of Ru an increase of creep rupture strength can only be found for low Re contents. The present article, which is part of an extensive and systematic investigation on Re and Ru influences, shows, that an improved creep resistance by an influence of Re and Ru on the γ’-solvus temperature is rather improbable. Likewise, the influence of Re and Ru on liquidus temperature is not expected to play an important role. However, the creep rupture strength is suggested to be highly modified by γ/γ’-microstructure changes.

Effect of B, Zr, and C on Hot Tearing of a Directionally Solidified Nickel-Based Superalloy

The effect of the minor elements B, Zr, and C on the castability of a Nickel-based γ′-strengthened superalloy has been investigated. Tube-like specimens were prepared by directional solidification where the rigid ceramic core leads to hoop stresses and grain boundary cracking. It was found that an important improvement in castability can be achieved by adjusting the minor elemental composition. Too low C (≤0.15 pct) and too high B and Zr contents (≥0.05 pct) lead to material that is very prone to solidification cracking and should be avoided. The results cannot be rationalized on the basis of the current models for solidification cracking. Instead, pronounced hot tearing is observed to occur at high amounts of γ/γ′-eutectic and high Zr contents. The critical film stage where dendrites at the end of solidification do not touch and are separated by thin liquid films must be avoided. How Zr promotes the film stage will be discussed in the paper.

Effects of Solid Solution Strengthening Elements Mo, Re, Ru, and W on Transition Temperatures in Nickel-Based Superalloys with High γ′-Volume Fraction: Comparison of Experiment and CALPHAD Calculations

We prepared 41 different superalloy compositions by an arc melting, casting, and heat treatment process. Alloy solid solution strengthening elements were added in graded amounts, and we measured the solidus, liquidus, and γ′-solvus temperatures of the samples by DSC. The γ′-phase fraction increased as the W, Mo, and Re contents were increased, and W showed the most pronounced effect. Ru decreased the γ′-phase fraction. Melting temperatures (i.e., solidus and liquidus) were increased by addition of Re, W, and Ru (the effect increased in that order). Addition of Mo decreased the melting temperature. W was effective as a strengthening element because it acted as a solid solution strengthener and increased the fraction of fine γ′-precipitates, thus improving precipitation strengthening. Experimentally determined values were compared with calculated values based on the CALPHAD software tools Thermo-Calc (databases: TTNI8 and TCNI6) and MatCalc (database ME-NI). The ME-NI database, which was specially adapted to the present investigation, showed good agreement. TTNI8 also showed good results. The TCNI6 database is suitable for computational design of complex nickel-based superalloys. However, a large deviation remained between the experiment results and calculations based on this database. It also erroneously predicted γ′-phase separations and failed to describe the Ru-effect on transition temperatures.

Development of Single-Crystal Ni-Base Superalloys Based on Multi-criteria Numerical Optimization and Efficient Use of Refractory Elements

The development of new Ni-base superalloys with a complex composition consisting of eight or more alloying elements is a challenging task. The experimental state-of-the-art development cycle is based on the adaption of already existing compositions. Although new alloy compositions with potentially improved material properties are expected to be similar to already known superalloys, this procedure impedes efficiently finding these compositions in the large multi-dimensional design-space of all alloying elements. Modern alloy development combines numerical optimization methods with experimental validation to guide the development towards promising compositions. In this work, an improved numerical multi-criteria optimization tool using CALPHAD calculations and semi-empirical models for alloy development is presented. The model improvements to its predecessor are described and the successful application for the development of rhenium-free single-crystal Ni-base superalloys ERBO/13 and ERBO/15 is revisited. The optimization tool is described and the designed alloys are discussed regarding phase stability. Finally, a possible phase stability model extending the optimization tool and improving the alloy composition predictions is presented.