The effect of excess atomic volume on He bubble formation at fcc–bcc interfaces

Abstract

Atomistic modeling shows that Cu-Nb and Cu-V interfaces contain high excess atomic volume due to constitutional vacancy concentrations of ~5%at. and ~0.8%at., respectively. This finding is supported by experiments demonstrating that a ~5-fold higher He concentration is required to observe He bubbles via throughfocus transmission electron microscopy at Cu-Nb interfaces than in Cu-V interfaces.Interfaces with structures tailored to minimize precipitation and growth of He bubbles may be used to design damage-resistant composites for fusion reactors.Unlike pure metals 1 , some materials contain constitutional vacancies that are thermodynamically stable at arbitrarily low temperature, for example grain boundaries (GBs) in ceramics 2 and semiconductors 3 , compounds with wide phase fields like NiAl 4 , and certain metal hydrides 5 . We use atomistic modeling to show that Cu-Nb and Cu-V interfaces contain high constitutional vacancy concentrations.Indirect experimental verification of this prediction is obtained by measuring the critical He concentration at which bubbles become detectable at these interfaces in transmission electron microscopy (TEM).

Formation, migration, and clustering of delocalized vacancies and interstitials at a solid-state semicoherent interface

“…Because clusters containing up to 3 vacancies are tightly confined to just one single MDI, we find the interaction energies between vacancy clusters at adjacent MDIs to be negligible. Therefore, in agreement with previous studies 13,16,56 , the Cu-Nb interface in its ground state must contain 2 or 3 vacancies at each MDI.…”

Structure, shear resistance and interaction with point defects of interfaces in Cu–Nb nanocomposites synthesized by severe plastic deformation

“…To find the ground state interface configuration, vacancies and interstitials must be iteratively added until no negative formation energy sites remain. Using this approach or related ones, the ground states of some interfaces have been shown to have local densities and compositions markedly different from the neighboring crystals [8,37,45,46]. In the interface studied here, however, no negative vacancy or interstitial formation energy sites were found.…”

“…Crystallographically, Cu-NbSPD and Cu-NbMSP only differ in the orientation of their habit planes: {112}fcc||{112}bcc for Cu-NbSPD and {111}fcc||{110}bcc for Cu-NbMSP. Cu-NbMSP has been exhaustively studied through atomistic modeling, including its structure [8,27,53], shear resistance [27][28][29], interaction with point defects [8,27,37,41], as well as other properties, such as interaction with climbing dislocations [54]. Selected findings from these studies will be described below as needed.…”

“…10. Unlike in certain other Cu-Nb interfaces [8,27,37], all vacancy formation energies are positive, meaning that no constitutional vacancies are present in the ground state configuration of this Cu-Nb interface. Furthermore, all vacancies-even ones with lowest formation energiesretain the form of compact point defects and do not delocalize upon relaxation.…”

“…By trapping and accelerating recombination of vacancies and interstitials, interfaces may mitigate radiation-induced degradation phenomena such as hardening and swelling [8,27,35,36]. Interfaces with high free volume have been shown to trap implanted He impurities, thereby delaying bubble formation [36][37][38].…”