Influence of grain boundary character on void formation in nanotwinned copper
Authors: LaGrange Thomas 
Published: December 2016

Most materials used in nuclear reactors are prone to radiation damage and their mechanical properties degrade with time, limiting their service life. Defects produced by high energy particle radiation can be highly mobile at high temperatures with their mobility being influenced by the local stress fields associated pre‐existing defects and grain boundaries. The relaxation of radiation defects can produce clusters and induce defect diffusion to interfaces and other pre‐existing defects, where they can be absorbed and alter the microstructure that can detrimental to the material's mechanical properties. Radiation mechanisms that promote biased point defect diffusion can induce significant microstructural change and degrade properties. For instance, interstitials, being more mobile than the vacancies, are quickly absorbed by nearby dislocations, inducing creep by dislocation climb and dislocation multiplication that results in work hardening and embrittlement. Likewise, a small excess of remnant vacancies can agglomerate, leading to the formation of voids that cause swelling, increased residual stresses, the formation of microcracks, and the eventual failure of the material. The long‐term stability of a microstructure under irradiation depends on its neutrality towards biased point defect dynamics. Materials with a high density of sites that can act as sinks for the point defects produced by high‐energy particles would thus be an enabling technology for reliable and clean nuclear energy.

We have explored radiation damage mechanisms in one such material, nanotwinned copper, which has a microstructure comprising a high density of coherent twin boundaries distributed in a “latter‐like” morphology within columnar grains having high misorientation angle intercolumnar boundaries. By performing in‐situ particle irradiation in a MeV TEM and characterizing the evolution in the microstructure using the Nanomegas ASTAR system, we have studied the irradiation induced damage and correlated the radiation induced grain boundary migration and void preferential void formation to the local grain boundary character and network. In‐situ observations were made at two different temperatures, room temperature and temperature above 573 K that stimulate void nucleation and growth. Figure 1 shows an example montage of the irradiation induced void formation and grain boundary migration with increased time and dose. The radiation induced grain boundary migration (RIGM) of several high‐angle random and low–angle grain boundaries within the irradiated zone was observed at both temperatures. However, low‐energy CSL type boundaries were not observed to migrate even at elevated temperatures as well as grain boundaries connected to junctions that contain one or more Σ3 boundaries. Thus, coherent twins are thought to stabilize the microstructure from radiation induced coarsening.

At temperature above 573 K during high‐energy electron irradiations, copious amounts of voids were observed to nucleate and growth. They were predominantly observed to nucleate and grow in the regions with a high density of the coherent twin boundaries (CTBs). Very few voids were observed around HARGBs and LAGBs as shown in Figure 2, and only a limited of number of small voids were observed in the vicinity of incoherent twin boundaries. Non‐coordinated boundaries having excess free volume are efficient sinks for both vacancies and interstitials, and thus do not stimulate biased point defect diffusion that promote void formation, explaining the observed lack of voids in the vicinity of HARGBs. On the other hand, the observed high density of large voids in regions with finely spaced CTBs may arise from the biased diffusion of point defects. Interstitials being more mobile that are able to diffuse through CTBs can rapidly migrate and annihilate at free surfaces or other sinks such HARGBs. This leaves an excess of vacancies in the vicinity of the CTBs which can coalesce and cause the nucleation of voids. Given these observations, a nanocrystalline material having a high fraction of CTBs, though being resistant to radiation induced coarsening, may not possess the ideal topology for radiation resilience and may be prone to increased void formation and swelling.