Radiation effects on material properties

• When materials are used under irradiation of high energy particles (neutrons or ions), radiation defects are formed by sequential collision of atoms. 

• The concentration of radiation defects is much larger than that under thermal equilibrium, and the structure is more complex, often significantly clustered. 

• Such high-concentration complex defects significantly change material properties, such as loss of ductility (i.e., radiation embrittlement), hardening (i.e., radiation hardening), amorphization, decrease in thermal conductivity, etc.

Radiation damage formation and evolution

• Radiation damage is formed by primary knock-on atoms (PKA) that receive excess kinetic energy from collisions with neutrons. PKA collides sequentially with its constituent atoms. As a result, many vacancies and self-interstitial atoms (SIA) are formed.

• If the excess kinetic energy of PKA is high, clustered defects are formed. This phase of atomic collision and defect formation ends at 10-100 ps after the generation of PKA.

• Defects then evolve more slowly through thermal processes. Some defects are recovered, while others cluster to form voids (= vacancy clusters) and dislocation loops (= SIA clusters). These large defects have a significant impact on material properties.

Simulations of radiation damage formation

• Since the radiation damage process is quick (<100 ps), it is difficult to observe by experiments directly. Thus, computer simulations have been widely used.

• Several computational methods, such as binary collision approximation (BCA) and molecular dynamics (MD), are used to simulate radiation damage.

• By MD, radiation damage processes can be simulated at an atomic scale, as in the left video. This video shows radiation damage formation by 1 keV PKA in iron.

Simulation of radiation damage evolution

• Rate models are widely used in simulations of radiation damage evolution, which occurs over a long time, such as days and years. The followings are typical reactions involved in rate models after the formation.

  • Defect recovery by recombination of vacancy and SIA.

  • Defect clustering, such as di-vacancy formation by two mono-vacancies.

  • Defect decomposition, such as di-vacancy split into two mono-vacancies.

  • Defect absorption by defect sinks, such as surfaces, grain boundaries, and voids, on which vacancies and SIAs disappear.