Abstract
ABSTRACT In this work, we combine a generic alloy-by-design model with a novel concept, the nucleation barrier for the formation of Laves phase to fill the creep cavities, in order to develop multi-component creep resistant steels with kinetically tuned self-healing behaviour. In the model the high-temperature long-term strength is estimated by integrating precipitation strengthening due to M23C6 carbides and solid solution strengthening, while the optimized compositional solutions are determined by employing the coupled thermodynamic and kinetic principles. W-containing Laves phase herein is selected as the self-healing agent to autonomously fill the grain boundary cavities, so as to prolong the creep lifetime. To achieve the effective healing reaction, the nucleation time for Laves precipitates are expected to coincide simultaneously with which creep cavities start to form or reach a healable size. Using experimental data from literature, an empirical relationship to estimate the incubation time for Laves phase formation has been constructed, from which the thermodynamic driving force for onset of precipitation as a function of temperature and intended precipitate nucleation time was derived. Three sample alloys have been selected among the desirable solutions, which are predicted to have the same strength but widely different Laves phase nucleation times. The calculations are also performed for different use temperatures to explore the compatibility between high temperature strength and timely cavity filling behaviour. In its current form the model is not expected to yield the truly optimal composition but to demonstrate how the kinetics of the healing reaction can affect the predicted optimal alloy compositions.
Highlights
Heat-resistant steels 112124374041combining super ior creep strength and great corrosion resistance at high temperature have been employed for decades in automotive, aerospace, fossil and nuclear power plants applications [1,2], but the industrial demand for materials with better performance for higher use tem perature and longer expected lifetimes has led to ongoing research into the development of novel heatresistant steels
Mater. 21 (2020) 642 steels are generally optimised by employing two strengthening mechanisms, precipitation strengthen ing by finely distributed particles and solid solution strengthening by solvent atoms to achieve decent high temperature strength
(1) The volume fraction of Laves phase which can form at the grain boundary cavities should be higher than 1% to sufficiently fill the total volume of cavities
Summary
Heat-resistant steels 112124374041combining super ior creep strength and great corrosion resistance at high temperature have been employed for decades in automotive, aerospace, fossil and nuclear power plants applications [1,2], but the industrial demand for materials with better performance for higher use tem perature and longer expected lifetimes has led to ongoing research into the development of novel heatresistant steels. In the present work we will introduce this kinetic aspect into our earlier genetic algorithm/thermodynamics computational design approach for the compositional optimisation of high performance creep resistant steels [23,24,25,26,27]. A hybrid approach has been developed by considering various potential solutions as suggested by the Genetic algorithm selection and taking the Laves phase nucleation time parameter as a first-order indication for the desired kinetics of the healing reaction to find optimal compositions. Future work will aim to explicitly introduce the nucleation and growth kinetics of the defects, in order to account for the stress dependence of the creep damage evolution, as well as the corresponding healing reaction
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