Atomistic study of grain boundary sink strength under prolonged electron irradiation

Abstract

Grain boundaries (GBs) can act as either sinks or sources of the point defects that are produced in large numbers under irradiation damage. In polycrystalline materials, as the grain size decreases, more of the point defects resulting from irradiation damage annihilate at GBs. It is unknown, however, whether the GB sink efficiency will saturate after prolonged defect annihilation, particularly when the grain size is of nanoscale dimensions. Using a combination of molecular dynamics (MD) simulation and rate theory, the authors show that high-energy GBs in body-centered-cubic (BCC) Mo do not saturate as sinks of point defects. The MD simulations serve to provide direct measurement of defect evolution, and the rate theory serves both to test whether grain boundary sink strength is constant during prolonged defect annihilation, and to extend the MD results to realistic defect production rates.

Introduction

Grain boundaries can act as sinks and sources of mobile defects [1], [2], [3]. Under irradiation, such as neutron irradiation in nuclear reactors or ion irradiation during implantation, mobile defects – vacancies and self-interstitial-atoms (SIAs) and their clusters – form and interact with the GBs [4], [5], [6], [7]. The GBs in a typical polycrystal can be of various natures, ranging from high-energy boundaries with disordered structures to high-symmetry boundaries with highly ordered crystalline structures and low energies [8]. High-energy GBs are quite prevalent in structural materials, and their role as sinks and sources of mobile defects is of both scientific and technological interest.

The GB sink strength, and changes thereof, is less an issue when the grain size is large. For large grains, many mobile defects cluster in the grain interior, annihilate at interior sinks such as dislocations, or undergo mutual recombination of vacancy–SIA pairs [9]. As the grain size decreases to the nanoscale, it approaches the mean free path for mutual recombination. Moreover, interior sinks such as dislocations become scarcer. Consequently, mobile defects have a much higher likelihood of reaching GBs before recombination or annihilation in the grain interior [6]. Due to the high density of GBs that are sinks of irradiation-produced defects, nanograins are expected to be more resistant to irradiation damage than their coarse grain counterparts, and experimental evidence does support this expectation [10], [11], [12], [13], [14]. For example, under the same irradiation dose, the defect accumulation at 300 K in nanocrystalline gold is much lower than that in coarse grain gold [11]; and the tolerance against irradiation-induced amorphization of nanocrystalline Gd₂(Ti_0.65Zr_0.35)₂O₇ decreases with increasing grain size [14]. However, it is also possible that the GBs could saturate as sinks of mobile defects for nanograins, after prolonged periods of defect absorption.

In this paper, we use a combination of rate theory and MD simulations to examine the interaction of mobile defects with GBs, and in particular whether the ability of a GB to act as a sink saturates. As a start, we focus on point defects, i.e., single vacancies and SIAs. We measure the defect accumulation in BCC Mo near high-energy GBs using MD simulations, and use rate theory to reproduce the MD results by assuming constant sink strength of the GBs. The agreement between MD and rate theory is indirect evidence that the GB does not saturate, at least at the MD scale of time and defect production rate. The conclusion is then extended to realistic production rates using rate theory analysis. Further, both the atomic density and the width of the GBs remain unchanged during irradiation simulations, directly showing that the GB structure does not evolve during prolonged absorption of point defects. In the following, we will describe the MD simulations in Section 2, the rate theory in Section 3, and the results in Section 4. Finally we conclude in Section 5.