Abstract

    A new software programme, JMatPro, is used to obtain steady state creep rates and creep rupture life for multi-component commercial nickel based alloys. A key feature of the programme is that overall properties are obtained through calculation and combination of the properties of individual phases. This includes thermodynamic properties, thermo-physical and physical properties, mechanical properties, and derivative properties such as anti-phase boundary and stacking fault energies. Access to such properties allows the self-consistent calculation of the required input parameters for a standard dislocation creep equation. This is in contrast to many previous attempts that have had to use empirical values for various critical parameters, such as stacking fault energies, anti-phase boundary energies and various elastic moduli, which are all dependent on temperature as well as composition. It is shown that calculations can be made for any desired alloy by entering only the compo-sition of the alloy, the size(s) of γ′ and/or γ″ (if present), the creep temperature and the applied stress. Good agreement has been obtained between the calculated secondary creep rates/rupture life and the observed results for many commercial  nickel based superalloys.

Introduction

   Understanding the factors affecting the performance of Ni-based superalloys is vital to the industrial gas turbine industry and many formulations have been proposed to calculate the secondary creep rates.1,2,3,4,5 While engineering requirements require the properties of the alloy as a whole to be described, it is the properties of individual phases and microstructural features that play the fundamental role in determining overall proper-ties and which are a necessary input into physically based models. However, values for critical input parameters referring to individual  phases are often missing, especially for complex multi-component industrial alloys. In the absence of data related to specific alloys and temperatures, previ-ous treatments have often had to use empirically determined values for important parameters such as the modulus, stacking fault  energy or APB energies. This makes it difficult to judge whether a particular approach would still be applicable outside the limited range of alloy composition or temperatures used to justify the initial equations. More importantly, the effect of changing composition and other variables  within or outside speci-fication cannot be estimated properly when input parameters are assumed independent of these variables. The present paper has several objec-tives. Firstly to show that it is now possible to systematically calculate secondary creep rates and stress rupture life, not only for the wide range of superalloys currently available, but also for new combination of elements that are beingsuggested for the design of the next generation of alloys. It will be shown that most of the required parameters can be calculated, thus leaving far fewer factors to be empirically determined by comparing theory and experiment. The present approach does not remove the necessity of making experiments, but it does sub-stantially reduce the degree of empiricism inherent in many previous treatments. This can markedly reduce the number of trial experiments when developing new alloys, testing the effect of variations within specification limits, and determining the permissible range of heat-treatments.

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