Multiscale modelling for fusion and fission materials: the M4F project

The M4F project brings together the fusion and fission materials communities working on the prediction of radiation damage production and evolution and its effects on the mechanical behaviour of irradiated ferritic/martensitic (F/M) steels. It is a multidisciplinary project in which several different experimental and computational materials science tools are integrated to understand and model the complex phenomena associated with the formation and evolution of irradiation induced defects and their effects on the macroscopic behaviour of the target materials. In particular the project focuses on two specific aspects: (1) To develop physical understanding and predictive models of the origin and consequences of localised deformation under irradiation in F/M steels; (2) To develop good practices and possibly advance towards the definition of protocols for the use of ion irradiation as a tool to evaluate radiation effects on materials. Nineteen modelling codes across different scales are being used and developed and an experimental validation programme based on the examination of materials irradiated with neutrons and ions is being carried out. The project enters now its 4th year and is close to delivering high-quality results. This paper overviews the work performed so far within the project, highlighting its impact for fission and fusion materials science.This work has received funding from the Euratom research and training programme 2014-2018 under grant agreement No. 755039 (M4F project)

Peierls potential and kink-pair mechanism in high-pressure MgSiO 3 perovskite: An atomic scale study

International audienceThe motion of [100](010) screw dislocations via a kink-pair mechanism is investigated in high-pressure MgSiO3 perovskite by means of atomistic calculations and an elastic interaction model for kink nucleation. Atomistic calculations based on the nudged elastic band method provide the Peierls potential, which is shown to be dynamically asymmetric and stress dependent. The elastic interaction model adjusted to match kink width computed atomistically, is used to evaluate the critical nucleation enthalpy. We demonstrate that the kink-pair mechanism in MgSiO3 perovskite is controlled by the nucleation of kinks along the [100] screw dislocation

Descriptors for mechanical strength and slip-induced crack-blunting in refractory ceramics

Understanding the competition between brittleness and plasticity in refractory ceramics is of importance for aiding design of hard materials with enhanced fracture resistance. We carry out ab initio and classical molecular dynamics (AIMD and CMD) investigations at temperatures between 300 and 1200 K to identify atomic-scale mechanisms responsible for brittle fracture vs slip-induced crack-blunting in Ti-N ceramics. AIMD simulations of single-crystal and notched TiN lattices (1100 atoms) subject to tensile and shear deformation serve to verify predictions of mechanical properties separately obtained by CMD. Benchmarked by AIMD results, CMD is thus confidently used to probe the mechanical response of large (40000 atoms) notched TiN and TiNx models under mode-I tension. Although crack growth occurs in most cases, CMD simulations reveal that typically-brittle TiN and TiNx ceramics can - for comparable rates of accumulation of tensile and shear stress around a flaw - prevent fracture via nucleation and emission of dislocations from the notch tip. Furthermore, we identify descriptors based on properties calculated for ideal single-crystals which reproduce trends in mechanical behavior of flawed lattices. Specifically: (i) the probability of notched ceramics to resist fracture via slip-induced plasticity exhibits linear relationship with ideal tensile-to-shear strength ratios (Iplasticity), (ii) at parity of Iplasticity values, the effective strength (fracture stress) of defective systems ranks according to the tensile strength of corresponding single-crystal phases. The descriptors proposed in this work pave the way for high-throughput screening of ceramics that may combine high strength to superior fracture resistance at room and elevated temperature

Elucidating dislocation core structures in titanium nitride through high-resolution imaging and atomistic simulations

Although titanium nitride (TiN) is among the most extensively studied and thoroughly characterizedthin-film ceramic materials, detailed knowledge of relevant dislocation core structures is lacking. Byhigh-resolution scanning transmission electron microscopy (STEM) of epitaxial single crystal (001)-oriented TiN films, we identify different dislocation types and their core structures. These include, besidesthe expected primary a/2{110}h110i dislocation, Shockley partial dislocations a/6{111}h112i and sessileLomer edge dislocations a/2{100}h011i. Density-functional theory and classical interatomic potentialsimulations complement STEM observations by recovering the atomic structure of the different disloca-tion types, estimating Peierls stresses, and providing insights on the chemical bonding nature at the core.The generated models of the dislocation cores suggest locally enhanced metal–metal bonding, weakenedTi-N bonds, and N vacancy-pinning that effectively reduces the mobilities of {110}h110i and {111}h112idislocations. Our findings underscore that the presence of different dislocation types and their effects onchemical bonding should be considered in the design and interpretations of nanoscale and macroscopicproperties of TiN.FunMat-I