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Defect-Interface Interactions in Irradiated Cu/Ag Nanocomposites

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Defect-Interface Interactions in Irradiated Cu/Ag Nanocomposites Acta Materialia 160 (2018) 211e223 Contents lists available at ScienceDirect Acta Materialia journal homepage: www.elsevier.com/locate/actamat Full length article Defect-interface interactions in irradiated Cu/Ag nanocomposites * Min Wang a, Irene J. Beyerlein b, Jian Zhang c, Wei-Zhong Han a, a Center for Advancing Materials Performance from the Nanoscale (CAMP-Nano), State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an, 710049, China b Mechanical Engineering Department, Materials Department, University of California, Santa Barbara, CA, 93106-5070, USA c College of Energy, Xiamen University, Xiamen, 361005, China article info abstract Article history: In this work, we employ transmission electron microscopy and helium ion irradiation to study the Received 20 July 2018 response of biphase interfaces to radiation induced point defect fluxes from the two adjoining phases. Received in revised form Analysis of interface-affected defect accumulation was carried out over a wide range of radiation damage 23 August 2018 levels from near zero displacement per atom (dpa) to 16 dpa and helium concentrations of 0 at.% to Accepted 3 September 2018 8 at.%. Results show a strong interface density dependence in which Cu/Ag interfaces in the nanolayered Available online 5 September 2018 regions spaced <500 nm were remarkably microstructural stable over the entire range without accu- mulating micro-scale defects, while those spaced >1 mm apart were destroyed. We report the concom- Keywords: Interface itant development of a bubble-free zone in Cu that was independent of defect levels and interface- fi Radiation defects contacting bubbles zone in Ag. This nding is explained by bias segregation to the interface of in- Helium bubbles terstitials from Ag and vacancies to misfit dislocation nodes in the interface from Cu. The point defect Vacancy pump transfer across the interface can be explained by the spatial variation in interface pressure within the Dislocations interface and gradient in pressure across the interface, both originating from the lattice mismatch and surface energy difference between the two crystals. © 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. 1. Introduction Interface engineering is becoming a recognized and widely adopted method for designing radiation tolerant materials [14e22]. Structural materials used in nuclear reactors are subjected to a Under the same service conditions, material radiation tolerance can high level of irradiation, an extreme environment that over time be dramatically enhanced by introducing a large number of in- causes defects to form and accumulate inside the material and terfaces, either homophase or biphase, into its microstructure. The eventually lead to internal damage [1e4]. Radiation-induced de- basic strategy exploits the idea that interfaces are efficient defect fects are first produced in the form of atomic scale point defects, “sinks”; that is, they are preferable regions within the material, vacancies and interstitials, which evolve into larger point defect where the interstitial and vacancy combination rates can be clusters, such as dislocation loops, voids, and bubbles [1e4]. significantly enhanced relative to the adjoining bulk crystals Accumulation of these radiation defects degrades mechanical per- [14e22]. The sink properties of free surfaces [23e26], grain formance typically in the form of significant increases in hardening boundaries [27e34] and interfaces [16,35e37] have been studied and embrittlement [5e10]. In order to reduce radiation damage, the extensively. By microscopic quantification of the width of defect- key is to enhance the recombination/annihilation rate of radiation free-zone formed along these interfaces, the sink efficiency of defects as soon as they are produced [10e18]. It is well known that different interfaces or grain boundaries can be measured, and the recombination processes of radiation defects are influenced by therefore, they can be ranked [16,33]. It becomes clear from such a number of factors, such as, radiation dose, the nature of energized analyses that not all these interfaces exhibit similar sink effi- particles, radiation flux, radiation temperature, the diffusion and ciencies, and that the differences can be rationalized based on migration behavior of defects, the intrinsic properties and micro- differences in their atomic structures and interaction energies with structures of materials, etc [1e4]. point defects. In addition to selecting or engineering the optimal interface structure, in order to achieve high radiation tolerance, it is also of high importance to increase the volume fraction of in- * Corresponding author. terfaces. Several methods have been used to fabricate interface- E-mail address: [email protected] (W.-Z. Han). https://doi.org/10.1016/j.actamat.2018.09.003 1359-6454/© 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. 212 M. Wang et al. / Acta Materialia 160 (2018) 211e223 dominated materials, such as physical vapor deposition [38], interface froms a bubble-free zone in Cu and interface-contacting accumulative roll bonding [39] and sintering [40] etc. With these bubbles zone in Ag. We rationalize this interface-driven phenom- techniques, the radiation tolerance of multilayers consisting of enon as evidence of point defect transfer across the interface, either two alternating immiscible or miscible phases have been driven by spatial variation in interface pressure within the interface widely studied either by in situ or ex situ radiation techniques. The and change in compression/tensile pressure across the interface high volume fraction of interfaces gives rise to superior irradiation from Cu to Ag. tolerance and mechanical properties [17,22]. With this success, in the past decade, a large number of in- 2. Experimental design and procedures vestigations have been dedicated to the radiation behavior of face- centered cubic (FCC)/body-centered cubic (BCC) nanolaminates Bulk Cu/Ag nanocomposites were prepared through quenching [41e43]. BCC metals usually have higher melting temperature and of a molten Cu/Ag alloy with a eutectic composition, i.e., a 2:3 better radiation resistances than FCC metals, and therefore, the atomic ratio. The starting materials were high-purity Cu (99.99%) FCC/BCC nanolaminates demonstrate greater radiation tolerance and Ag (99.99%). They were first placed into an Al2O3 tube, and then and thermal stability than its FCC component. As such, the contri- sealed with pure Argon to a pressure slightly higher than 1 atm. To bution of BCC phase on radiation resistance of BCC/FCC interface is melt and mix them, the tube was heated slowly to 1150 Cina significant, and much experimental evidence exist demonstrating vertical furnace. After holding at 1150 C for 1 h, the tube containing that these interfaces can heal the radiation defects in FCC layers the liquid mixture of Cu and Ag was immediately quenched in a [16]. In addition, the defect dynamics in BCC and FCC metals are water tank. The dimensions of the ingots produced are 10 mm in markedly different. The mobility of defects in an FCC phase are diameter and 40 mm in length. Two types of samples were made higher than that in a BCC phase [41e43]. Consequently, the FCC/BCC from the ingots: “bulk” disk samples and transmission electron interface under such radiation conditions actually only interact microscopy (TEM) samples. The bulk samples were cut from the with a defect flux from the FCC layer. This may be one explanation ingot by spark-cutting technique, then ground and polished to a why the FCC/BCC interfaces show much better radiation resistance mirror surface. The TEM foils were cut from the ingots and ground and stability than their grain boundaries. However, how a bimetal to about 50 mm in thickness. These thinned TEM foils were dimpled interface would respond to the defect fluxes from the two dissim- by an M200 Dimpling Grinder, and further thinned using an ilar, adjoining crystals has not been given as much study to date. In Ar þ ion milling on an M1050 TEM Mill, operated at 3.5 kV and with order to achieve such a model system, FCC/FCC nanolaminates is a final polishing beam angle of 4. good choice. Several FCC metals have a similar melting tempera- Helium implantation was performed on these two types of tures around 1000 C, such as Cu, Ag and Au, etc., and thus, would samples using helium ions with an energy of 400 keV at 400 Cby have similar point defect dynamics in response to radiation. Hence, using a NEC 400kV Implanter. The ion fluence is 2 Â 1017 ions/cm2, biphase FCC/FCC interfaces found in these nanolaminates made and the implantation lasted for 200 min (corresponding to a flux rate with these combinations of materials would experience similar of 1.67 Â 1013 ions/cm2/s). The helium beam was tuned to implant defect fluxes from both sides. In this case, the radiation behavior of near perpendicular (7 off to avoid channeling effect) to the top FCC/FCC bimetal interface should be different from the FCC/BCC surface of the bulk samples and to the surface of the TEM foils, as interfaces and from homo-phase, i.e., grain boundaries, studied in illustrated in Fig. 1(a) and (b). The radiation damage (in units of dpa) previous works. The irradiation response of biphase interfaces in for each phase and the helium concentration as a function of depth such cases remains largely unexplored [44e46]. from the top surface can be estimated by Monte Carlo simulation, In this study, we aim to obtain a better understanding of the role specifically using the Stopping and Range of Ions in Solids (SRIM) FCC/FCC biphase interfaces play in radiation damage development [52]. Fig. 1 shows the depth variation in radiation damage (dpa) for when exposed to comparable radiation defect fluxes from both Cu (the magenta line) and Ag (the red line) and the helium con- sides. Further, by changing the concentration of radiation damage centration (black line) when using an average threshold displace- and helium concentration, we pursue a second objective to probe ment energy of 29 eV for Cu and 39 eV for Ag, respectively [2].
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