Radiation damage in nanocrystalline iron

Gregory Alan Vetterick

doi:10.17918/etd-7056

There is strong evidence that grain boundaries act as recombination sites for interstitials and vacancies in a polycrystalline material. The prevailing theory is that grain boundaries act to absorb freely mobile interstitials and vacancies as well as sub-microscopic defect clusters, thereby depleting the region adjacent to the grain boundary of sufficient point defects to produce visible defect structures (e.g. stacking fault tetrahedra, voids, and dislocation loops). This theory is the basis for the design of radiation tolerant materials engineered to remove the large non-equilibrium point defect concentration created in cascades under irradiation by high energy particles by introducing a large number of grain boundary sinks. This thesis presents direct experimental evidence for a number of mechanisms operating to remove radiation damage at grain boundaries by in-situ transmission electron microscopy of free standing nanocrystalline iron films. It was found that the size of dislocation loops found in irradiated iron decreases with smaller grain size until a minimum cluster size is reached (about 2-5nm). The number density of defect clusters appears less affected by the presence of a high number of grain boundary sinks, but does vary strongly with the sink strength of the particular boundaries. If a small grain is defined by grain boundaries are capable of producing a very strong denuded zone, the cluster density can be very small. Therefore, the magnitude of this effect is dependent on the grain boundary character well into the nanocrystalline grain size regime. This work also shows that, in addition to the ability of a grain boundary to absorb sub-microscopic defects, the mobility and absorption of microscopic defect structures (i.e. defect clusters and dislocation loops) at grain boundaries has a strong influence on the response of a material to irradiation. Using in-situ TEM we examined the behavior of these microscopic DCs in nanocrystalline iron. The one-dimensional loop hop of b=1/2<111> DCs was found totransport DCs to close proximity to GBs where they could annihilate, suggesting a contribution to the long range flux of interstitials to GB sinks. This process had marked effects on the morphology of the irradiated microstructure in nanocrystalline iron, limiting the length of DC strings and reducing the coalescence of DCs into larger defect loops. Furthermore, the research presented in this thesis showed that when large dislocation loops are able to form they may just as easily be lost to grain boundaries: a process which enhances the effect that grain boundary sinks have on the microstructure of the irradiated material. In-situ TEM irradiations in nanocrystalline iron at temperatures from 50K to 773K show that as the temperature of the specimen is increased from cryogenic temperatures (e.g. 50K), the mobility of first b=1/2<111> defect clusters, then b=1/2<111> dislocation loops, and finally b=<100> dislocation loops reaches sufficient levels to enable climb to grain boundaries resulting in absorption. In nanocrystalline materials with a high density of grain boundary sinks this activity results in a small downward shift in the transition temperature between a b=1/2<111> dominated microstructure and one that consists primarily of b=<100> dislocation loops. At 773K, the microstructure is largely free of any dislocation loops in nanocrystalline iron, a stark change from previous work in microcrystalline iron where the b=<100> loops remain stable. The shift in the transition temperature agrees well with initial hypotheses in literature that the nature of the loops remaining in irradiated iron depends on the relative stabilities of the dislocation loops arising due to the elastic anisotropy of iron from thermal magnetic fluctuations, and the high mobility of b=1/2<111> dislocation loops compared to b=<100> dislocation loops. Using in-situ transmission electron microscopy, the activity of dislocation loops in nanocrystalline iron were directly observed and analyzed using orientation mapping. Bycomparing in-situ TEM results to molecular dynamics simulations, the process of absorption was elucidated for microscopic defect clusters, b=1/2<111> dislocation loops, and b=<100> dislocation loops.

Radiation damage in nanocrystalline iron

Files and links (1)

Abstract

Metrics

Details

Radiation damage in nanocrystalline iron

Files and links (1)

Abstract

Metrics

Details

Drexel University Social media