Study of Radiation Damage in Metallic Systems using Molecular Dynamics Simulations

Abstract

Radiation damage in the metallic materials is a stochastic event both in temporal and spatial scales. The cladding and the structural components of the nuclear reactors are exposed to extreme conditions like high-energy particle collisions (kilo electron Volt to Mega electron Volt range), elevated temperature, thermal shocks, and chemically corrosive environments. Consequently, the structural components suffer transmutation in metallic microstructure, mechanical behavior, and thermodynamic characteristics respectively. The nuclear power plants, reliability, sustainability, and safety of nuclear reactor components are major concerning factors. The real-time evolution of the initial radiation-induced defect caused under the impact of highly-energetic neutron or ionic bombardment, which originates within few picoseconds in atomistic range, is difficult to record and investigate. However, literature suggests computational modelling via molecular dynamics (MD) simulations is reliable and precise technique to study the underlying dynamic radiation-induced defect evolution mechanisms. The thesis work is dedicated to understand and draw insights of the evolution of various defects evolved within the irradiated metallic systems, for example, vacancies, interstitials, dislocations segments, Frank loops, point-defects clusters etc. The original contribution of this thesis includes the study of radiation response of the irradiated metallic systems and its alloys at varying grain boundary orientations, grain architecture, primary knock-on atom (PKA) energy magnitude, PKA direction, PKA positions from interfaces within crystallographic lattice and temperature regime closer to nuclear plant operating temperatures respectively. The amalgamation of the grain boundary (GB) engineering concepts to the radiation cascade simulations have been implemented in this research work. Nanostructured materials strongly arrest the radiation-induced defects as the grain boundaries serve efficient defect sink sites. Firstly, this thesis work includes comparative study of radiation damage of single crystal (SC) Cu specimen and two nanocrystalline (NC) specimens: hexagonal columnar grain Cu (CG Cu) specimen with Ʃ3 and Ʃ9 GBs and Cu specimen randomly oriented grain boundary (RG Cu) were irradiated at 600 K at primary knock-on atom (PKA) energy magnitudes, EPKA = 10 keV, 20 keV, 30 keV respectively. By investigating the evolution of point defects, defect cluster distribution, and dislocation analysis it was observed that the irradiated nanostructured Cu specimens survived with comparatively lower defects at the end of cascade simulations. Also, sessile dislocations with networks (consisting dislocation locks and loops) were observed in the irradiated SC Cu specimens. Secondly, we employed radiation-based molecular dynamics (MD) numerical simulations in bi-crystal Nb specimen with Ʃ 13, Ʃ 29 and Ʃ 85 symmetric tilt grain boundaries (STGB) models respectively at varying magnitudes of primary-knock-on atom (PKA) energies, EPKA = 10 keV, 20 keV, and 30 keV attemperature regimes: 300 K, 600 K and 900 K, respectively. This study reveals that Nb-Ʃ 29 GB model with highest misorientation angle survived with the lowest number of residual defects. Also, the recombination rate of the irradiated Nb specimens increases with the increase in temperature and PKA energy magnitude due to enhanced atomic mobility of the dislodged atoms. Third research work in the thesis includes study of irradiated Nb Ʃ 5 STGB model at two high-angled grain boundaries (HAGB) with misorientation angle: 53.13 ° (Ʃ 5(2-10)/ (120)) and 36.86 ° (Ʃ 5(3-10)/ (130)) respectively. Also, both the irradiated bi-crystal Nb models were compared with bulk Nb specimen. It is reported in the thesis that the Nb system with greater misorientation angle i.e. Nb Ʃ 5 (ɵ = 53.13 ° ) survived with lower number point defects at the end of cascade simulations as well as the population small-sized interstitial clusters. Also, in comparison to both the Nb Ʃ 5 STGB models, higher numbers of peak damage and residual defects were recorded in irradiated SC Nb. Finally, radiation simulation studies were carried to study primary radiation damage of the FeNiCrCoCu high entropy alloys (HEAs) with crystalline-amorphous (SC/MG) interface at varying PKA energies of magnitude, EPKA = 10 keV, 20 keV, and 40 keV and varying PKA positions from the interface respectively. In this irradiated nanolaminate specimen we observed significant recovery of the Frenkel pairs by the SC/MG HEA interface. The varying interatomic distance between the imparted PKA and the crystalline amorphous interface also plays role in trapping the defects. The synopsis of the present thesis work elucidates the study and futuristic application of nanostructured and nanolaminated metallic systems as favourable radiation tolerant material in next-generation reactors.