WP2 – P1 Summary
MatISSE WP2 – Modelling of irradiation-induced hardening and creep in F/M alloys
Summary of periodic report 1.11.2013 – 30.04.2015
In WP2, electronic structure, atomistic and dislocation calculation techniques are combined in order to provide an explanation in terms of physical mechanisms for the experimental observations concerning irradiation hardening and creep in high-Cr ferritic/martensitic steels. These are modelled as Fe-Cr-X alloys (where X stands for any additional element) in the case of the study of irradiation hardening, and as Fe in the case of the modelling of irradiation creep.
Modelling of irradiation hardening in F/M alloys (MEFISTO):
The microstructural features mainly responsible for hardening in these alloys are dislocation loops and Cr-rich precipitates, that can be classified in two families: radiation-induced CrNiSiP precipitates and radiation-enhanced Cr precipitates. Previous studies showed that the CrNiSiP precipitates seem to be main responsible for radiation hardening in these types of alloys, thus the MEFISTO research area focuses on developing models that describe the formation of these precipitates as a consequence of precise physical mechanisms that are investigated using electronic structure calculation and atomistic models, as well as experiments. The results of the investigation are meant to lead to the development of models that can predict the nanostructure evolution under irradiation in Fe-Cr-X alloys, in particular the formation of loops and both types of precipitates and then, via dislocation dynamics simulations, provide an assessment of the hardening produced.
At the moment extensive electronic structure calculations have been performed, that allowed a semi-empirical interatomic potential to be fitted for the Fe-Cr-Ni bcc system, currently being validated and to be used for dislocation/defect interaction studies. Moreover, the basis has been set for kinetic Monte Carlo models meant to describe the microchemical and nanostructural evolution of these alloys under irradiation. The experimental programme has been designed and the neutron and ion irradiations performed.
Modelling of irradiation creep in F/M alloys (MOIRA):
Irradiation creep is a complex phenomenon that leads to stress relaxation and irreversible deformation under irradiation that occurs even if the load applied to the material is below the yield (limit to swtich from elastic to plastic deformation regime) and the temperature is too low for thermal creep to occur. Several atomic level mechanisms can be invoked to explain this phenomenon, all of them related to the biased way for dislocation loops to grow under simultaneous irradiation and application of stress, as well as for dislocation lines to climb and overcome obstacles, i.e. in essence to the bias when absorbing radiation-defects. Atomistic techniques are therefore going to be applied to explore in detail these mechanisms and assess their likeliness. Eventually, these mechanisms will be introduced in a set of equations that describe the evolution of the creep strain as a function of irradiation dose.
At the moment an exhaustive literature review of creep mechanisms has been produced, the basis for atomistic studies of the fundamental mechanisms leading to creep have been set and first equations have been written.
List of abbreviations:
AKMC Atomistic kinetic Monte Carlo
APT Atom-probe tomography
Bcc Body-centred cubic
DFT Density functional theory
Fcc Face-centred cubic
FE Finite elements
F/M Ferritic/martensitic
KMC Kinetic Monte Carlo
OKMC Object kinetic Monte Carlo
PAS Positron annihilation spectroscopy
SCMF Self-consistent mean field
SANS Small angle neutron scattering
TEM Transmission electron microscopy