Investigation of Radiation Temperature and Straining Temperature Effects on the Screw Dislocation Mobility Evolution in Irradiated Ferritic Grains Using 3D Dislocation Dynamics

Abstract

Nuclear structural materials subjected to neutron irradiation accumulate dose-dependent, disperse defect clusters populations. Subsequent dislocation/defect interactions induce material mechanical property degradations, including hardening and embrittlement. Our goal in this work is to evaluate the effect of disperse defect clusters population on the effective dislocation mobility in ferritic Fe-Cr grains, using 3D dislocation dynamics simulations. The defect induced changes of the grain-scale mechanical response are evaluated using the recently proposed Defect-Induced Apparent Temperature (DIAT) shift concept. It is found that the DIAT shift associated with a given defect dispersion scales with the ductile to brittle transition temperature (DBTT) shift associated with exactly the same, observed defect population. The dose-dependent evolutions associated with broad irradiation temperature and straining temperature changes are investigated herein, for further exploration and validation of the DIAT shift concept.

Dose-dependent strain localization and embrittlement in ferritic materials: A predictive approach based on sub-grain plasticity modelling

“…( 11) also works/applies in presence of a heterogeneous crack stress field and therefore, correctly captures the relevant (grain-scale), fracture-related plasticity mechanisms [27]. It is also essential to note that the 𝐷𝐼𝐴𝑇 level associated with a fixed reference temperature and defect dispersion (𝑛, 𝐷) closely follows the actual 𝐷𝐵𝑇𝑇 magnitude achieved in resilience specimens holding a comparable (𝑛, 𝐷) defect population [29]. This means the (dose-dependent) ductile to brittle transition temperature shift critically depends on the statistical variations of quantities  and 𝑡 𝑆𝐵 , characterizing the internal stress landscape in the highly deformed grains, sitting near the specimen fracture surface.…”

“…This paper presents a plastic-strain spreading model based on crucial sub-grain plasticity mechanisms supported by recent DD simulation results [28][29], The present model was first developed for FCC [32] and then BCC materials [20], for straining temperature T = 300K and single slip conditions, in consistence with observations of post-irradiated, strained specimens [23,24,33].…”

“…Shear band thickness is  1/𝐷 and therefore directly depends on the local stress landscape, at the scale of individual irradiation defect clusters [28]. mechanisms modelling [28,29].…”

“…Straining temperature changes likewise affect the dislocation mobility; so it is in principle possible to find a straining temperature (down) shift yielding exactly the same dislocation mobility changes as a given defect population does. It can be shown that those defect-induced changes are correctly captured by a Defect-Induced Apparent straining Temperature shift, noted 𝐷𝐼𝐴𝑇 [27][28][29], that can be calculated using the following, semianalytical expression:…”

“…2) the number of shear bands per unit grain surface decreases with the dose, for a given defect size and plastic strain level [25][26]. Strain localisation effect generates high local stress conditions on internal material interfaces (at grain boundaries, for example), thereby degrading the post-irradiation ductility of polycrystalline materials; 3) disperse defect population significantly affects the effective dislocation mobility, depending on the irradiation defect size and number densities [27][28][29]; 4) mobile dislocations tend to follow a path of least resistance, by way of cross-slip mechanism: this situation significantly lowers the effective defect strength, no matter the defect type involved [30][31].…”