Continuum damage mechanics modelling based on simulations of microstructural evolution kinetics

Abstract

Continuum creep damage mechanics modelling of creep has been combined with the recently developed Monte Carlo simulations of precipitation kinetics in power plant ferritic steels. The precipitation simulation has been validated by applying it to a 9 wt-%Cr ferritic steel, P92 and using the microstructural data published in the literature. The microstructural evolution results of precipitation kinetics simulation are used in the continuum creep damage mechanics model to calculate the damage evolution instead of the model equations developed for specific creep damage. This approach has been applied to P92 to study the effects of different creep damage mechanisms on the creep behaviour. Reasonable agreement with experimental creep rupture data and creep curves has been obtained. The new approach shows promise to provide a useful tool for both new alloy development and component life prediction.

Characterisation of microstructure and creep properties of alloy 617 for high-temperature applications

“…In particular, the parameters of interest in the present investigation are the type of phases present in each specimen, their volume fractions, particle size and inter-particle spacing (IPS), as a function of time, temperature and stress [13,15].…”

“…Our knowledge of the microstructural evolution on this alloy is based on experimental evidence presented in this paper and the knowledge of microstructural evolution in other nickel based alloys, like IN740 [14], Nimonic 80A [23] and high alloy ferritic steels, like P92 [13]. The general picture in nickel based alloys is that γ' forms at an early stage after ageing at 650-750ºC, accompanied by chromium-based carbides, like M 23 C 6 , M 6 C and, eventually, M 7 C 3 .…”

“…0 is a temperature dependent constant defining the base level strain rate and σ 0 is a "composite" parameter containing several microstructural parameters [13]. σ is the applied stress.…”

“…The approach follows the reasoning of Dyson [15], adapted more specifically for high alloy ferritic steels by Yin and Faulkner [13]. The four microstructural damage parameters are as follows: 1) D d , a dislocation damage parameter based on the accumulation of a dislocation distribution occurring up until the end of primary creep; 2) D s , a solute hardening parameter based on the changes in solute hardening caused by various matrix element partitioning effects occurring during the various precipitation sequences that takes place during exposure at high temperature; 3) D p , a precipitation hardening parameter describing the hardening caused by precipitates during creep exposure at high temperature, which is essentially related to the inter-particle spacing where: D p = 1-(P i /P), P i being the initial inter-particle spacing and P being the inter-particle spacing after time t; 4) D n , a cavity density term describing the structural integrity of the material in the creep fracture crack path, which is usually the grain boundary.…”

“…The quantitative data obtained serve as input and validation for the model predictions. The model combines a continuum damage mechanics (CDM) approach which has been successfully used for the prediction of creep rupture properties of 9 wt.%-Cr ferritic steel [13], and which is now being adapted to the case of Ni-base superalloys systems.…”