Implementierung eines strukturabhängigen Werkstoffmodells für die Superlegierung IN738LC in das Finite Elemente Programm ABAQUS

Superalloys, mainly consisting of nickel, are used for applications in aerospace as well as in stationary gas turbines. In the temperature range above 800°C the blades, which are manufactured of these superalloys, are subjected to high centrifugal forces and thermal induced loads. For computer based analysis of the thermo-mechanical behaviour of the blades models for the stress-strain behaviour are necessary. These models have to give a reliable description of the stress-strainbehaviour, with emphasis on inelastic effects. The implementation of the model in finite element codes requires a numerical treatment of the constitutive equations with respect to the given interface of the used code. In this paper constitutive equations for the superalloy IN738LC are presented and the implementation in the finite element code ABAQUS with the numerical preparation of the model is described. In order to validate the model calculations were performed for simple uniaxial loading conditions as well as for a complete cross section of a turbine blade under combined thermal and mechanical loading. The achieved results were compared with those of additional calculations by using ABAQUS, including Norton's law, which was already implemented in this code

Untersuchung des Verformungsverhaltens von stengelkristallinen und einkristallinen Superlegierungen bei Temperaturen oberhalb von 900 Celsius

This work concerns the microstructure and the anisotropic mechanical behaviour of several, chosen nickel-base superalloys in case of directional solidification. The uni- and multiaxial, mainly strain-rate controlled experiments were run with single crystal and columnar grained, solid, cylindrical specimen (orientation ) at temperatures above 900°C. These reality-close testing conditions refer to intended future applications as material for the first row of cooled turbine blades in stationary gas turbines. The changes of the microstructure ($\gamma$'coarsening) due to pure thermal exposure and to thermal and mechanical load were analysed.The relation between axial and/or torsional stationary strain-rate and corresponding stress response was described mathematically. This description is based on a creep-potential, which is modified for anisotropic behaviour using a formalism formulated by HILL. The analysis of stationary strain-rates and measured stresses leads to a creep law, which implicates the microstructure-based unified model of PENKALLA as special case

Beitrag zur Phasenkinetik in neuen Superlegierungen

At the development of new ODS ($\textbf{O}$xide-$\textbf{D}$ispersion-$\textbf{S}$trengthened) alloys for components being exposed to hot-gas temperatures nickel-base alloys and ODS alloys have been investigated. The experimental work mainly referred to phase investigations of thermally loaded samples. The addition of alloying elements will have an decisive influence on the $\gamma$'-phase fraction, $\gamma$'-lattice parameter, the morphology as well as the coarsening behaviour of the $\gamma$'-phase under temperature load. The main characteristics of ODS alloys are Y,Al-mixed oxides and microstructural inhomogenities. As for MA6000 a Time-Temperature-Precipitation diagram has been elaborated. By means of the improved computer programme $\textit{PHASCALC}$ for phase and N$_{y}$ (= average electron vacancy concentration) -value calculation many microstructural parameters such as the $\gamma$/$\gamma$'-misfit, the solution of the $\gamma$'-phase and melting temperatures can be determined. Tensile tests made at temperatures above 900°C have shown that the strengthening parameters are dependent on alloy composition, temperature conditions, $\gamma$'-phase fraction and oxide dispersion content

Erarbeitung einer gefügeabhängigen Beschreibung des Hochtemperaturverformungsverhaltens des Turbinenschaufelwerkstoffes in 738 LC

Since more than 15 years, the nickel-base alloy IN 738 LC is used for stationary gas-turbine blades because of its good high temperature strength coupled with acceptable hot corrosion resistance. The high strength in the temperature range 700° to 1000°C results primarily from the intermetallic phase $\gamma$'= Ni$_{3}$(Al, Ti). The $\gamma$' precipitates restrict the motion of the dislocations. The high temperature deformation behaviour has been correlated with the morphology of the $\gamma$' precipitates. It was found that the deformation rate in short term creep experiments (up to 1000 hours) depended on the initial microstructure of the cast alloy. The microstructure could be characterized by $\gamma$' particle size and distribution in the matrix. A heat treatment was applied with the aim of producing a more stable microstructure, so that the long time creep behaviour does not depend so strongly on the initial microstructure. The structure development in the experiments could be described by the growth of the $\gamma$' precipitates, calculated from the diffusion rates of aluminium and titanium in the nickel matrix. During operation, the turbine blade is loaded with a multi axial stress comprising the centrifugal force, the thermally induced stationary and instationary stresses, and the bending stresses from the gas flow. It was shown in strain-rate controlled tension-torsion test on hollow round samples, that the multiaxial stress-strain behaviour could be calculated using the invariant theory for isotropie materials with a comparative stress value from the uniaxial creep experiment, applying the von Mises theory. This mathematical model is useful for calculating the life time for a-turbine blade which is multiaxially loaded