Elsevier

Acta Materialia

Volume 165, 15 February 2019, Pages 26-39
Acta Materialia

Full length article
Nano-scale microstructure damage by neutron irradiations in a novel Boron-11 enriched TiB2 ultra-high temperature ceramic

https://doi.org/10.1016/j.actamat.2018.11.030Get rights and content

Abstract

Ultra-high temperature transition-metal ceramics are potential candidates for fusion reactor structural/plasma-facing components. We reveal the irradiation damage microstructural phenomena in Boron-11 enriched titanium diboride (TiB2) using mixed-spectrum neutron irradiations, combined with state-of-art characterization using transmission electron microscopy (TEM) and high resolution TEM (HRTEM). Irradiations were performed using High Flux Isotope Reactor at ∼220 and 620 °C up to 2.4 × 1025 n.m−2 (E > 0.1 MeV). Total dose including contribution from residual Boron-10 (10B) transmutation recoils, was ∼4.2 displacements per atom. TiB2 is susceptible to irradiation damage in terms of dislocation loop formation, cavities and anisotropic lattice parameter swelling induced micro-cracking. At both 220 and 620 °C, TEM revealed dislocation loops on basal and prism planes, with nearly two orders of magnitude higher number density of prism-plane loops. HRTEM, electron diffraction and relrod imaging revealed additional defects on {101¯0} prism planes, identified as faulted dislocation loops. High defect cluster density on prism planes explains anisotropic a-lattice parameter swelling of TiB2 reported in literature which caused grain boundary micro-cracking, the extent of which decreased with increasing irradiation temperature. Dominance of irradiation-induced defect clusters on prism planes in TiB2 is different than typical hexagonal ceramics where dislocation loops predominantly form on basal planes causing c-lattice parameter swelling, thereby revealing a potential role of c/a ratio on defect formation/aggregation. Helium generation and temperature rise from residual 10B transmutation caused matrix and grain boundary cavities for the irradiation at 620 °C. The study additionally signifies isotopic enrichment as a viable approach to produce transition-metal diborides for potential nuclear structural applications.

Introduction

Novel high temperature materials combined with advanced manufacturing routes may offer game changing impacts on nuclear safety and engineering, particularly for future fusion reactors where operational conditions are significantly more challenging than those expected inside fission reactors [1]. Increasing the upper operating temperatures of nuclear reactors improves the power plant thermal efficiency [2]. However, the former is primarily limited by the melting temperature of structural materials. Additionally, an important safety criterion for structural materials in nuclear environments is the margin to melting [3], which ideally should be as high as possible. For these reasons, ultra-high temperature ceramic (UHTC) carbides, nitrides and diborides, which have melting temperatures more than 3000 °C, are emerging as promising structural and high heat flux/plasma-facing candidate materials [4,5]. Diborides such as TiB2 have hexagonal crystal structure and have been investigated for various non-nuclear applications ranging from wear resistant parts/coatings, cutting tools and aerospace based on excellent high temperature strength, high hardness, and good thermal shock resistance [6,7]. TiB2 is also chemically inert and has good thermal conductivity [6]. For example, thermal conductivity of polycrystalline TiB2 is 96-78 Wm−1K−1 between 20 and 1220 °C [6]. Further, TiB2 is a medium to low atomic number (Z) material, implying desirable low impact on plasma power loss if used in fusion reactors. Indeed, for fusion reactors, increase in plasma resistivity and power losses due to introduction of plasma contaminants from high Z materials is a critical issue [1,8]. In addition, TiB2 can be chemically vapour deposited on many substrates that have envisaged nuclear applications such as tungsten, high chromium steels and titanium carbide [9], thereby making TiB2 a good plasma facing wall coating candidate.

Successful implementation of new materials in high temperature nuclear environments demands a thorough understanding of irradiation-induced degradation processes. At present, little is known regarding the irradiation tolerance of TiB2. Some results show irradiation-induced macroscopic changes in natural boron-based TiB2 after light ion and neutron irradiations [[10], [11], [12]]. These macroscopic changes are typically light-ion irradiation-induced blistering/exfoliation or swelling and cracking due to in-reactor neutron irradiation. Low energy (5–120 keV) deuterium (D+) and helium (4He+) ion irradiations at room temperature showed the exfoliation erosion yield to increase with ion energy [10,11]. Grinik et al. [12] observed macroscopic swelling of TiB2 after neutron irradiations inside a WWR-M thermal nuclear reactor [13] at temperatures between 100 and 680 °C. Nearly 2–2.5% swelling was observed for a fluence of ∼0.3 × 1024 n.m−2 at 100 °C. Macroscopic swelling was reported to increase with dose, reaching ∼4–5% for ∼2.5 × 1024 n.m−2 fluence. The trend of macroscopic swelling with neutron irradiation temperature was reported to be non-monotonic in TiB2 by Grinik et al. For example, ∼3% swelling occurred at a fluence of 7 × 1023 n.m−2 at 100 °C. With increasing temperature, swelling decreased steadily, reaching ∼2% for the irradiation temperature between 400 and 530 °C for the same fluence. Beyond 600 °C, the authors observed a sharp increase in swelling, reaching 3% at 680 °C. The initial swelling at low temperatures is expected due to the well-known “point defect swelling” regime, prominent between the recovery stage I (onset of interstitial mobility) and stage III (onset of vacancy mobility) in ceramics [14]. This regime is typically characterized by swelling of the lattice parameter due to the accumulation of point-defects (PDs) and small interstitial atom clusters, thereby inducing macroscopic dimensional changes [14]. Maile et al. observed lattice parameter swelling of TiB2 after neutron irradiations to temperatures as low as −166 and 24 °C using X-ray diffraction (XRD) [15]. Both a- and c-lattice parameters were reported to increase; however, with a larger change in the a-parameter as compared to the c-parameter. For example, the authors reported anisotropic a-parameter swelling of 0.46%, compared to 0.25% for c-parameter at −166 °C for the highest neutron fluence of 5 × 1021 n.m−2. Maile et al. also showed that lattice parameter swelling of TiB2 increased with neutron fluence between (0.2–5)x1021 n.m−2. Anisotropic lattice parameter swelling-induced cracking of TiB2 is also known to occur [12] and is a typical issue with hexagonal ceramics [[16], [17], [18]]. Grinik et al. mentioned cracking at the macro-scale with the extent decreasing with increasing irradiation temperature (100–400 °C). Above 500 °C, only grain boundary micro-cracks were reported [12,19]. Unfortunately, the neutron irradiation studies did not describe the associated evolution of defect microstructures, which is the key to understanding the macroscopic changes.

To the best of our knowledge, only one study has reported microstructural evolution due to irradiation in TiB2 [20]. The authors reported that 200 keV electron irradiation in a transmission electron microscope (TEM) was sufficient to produce dislocation loops in electron transparent TiB2 thin foils at room temperature. This was attributed to boron displacements, which have a low displacement threshold of ∼20 eV [20]. Estimated net dose was 7 displacements per atom (dpa) of boron, at a dose rate of 10−3 dpa/s. The dislocation loops were less than 10 nm in diameter, lying on the basal (0001) planes.

The lack of understanding of the microstructure evolution of TiB2 during irradiation is a fundamental knowledge gap that needs to be addressed to fully exploit its potential for high temperature nuclear applications. In this context, we have performed dedicated neutron irradiation of polycrystalline TiB2 inside the mixed-spectrum High Flux Isotope Reactor (HFIR) at 210–230 and 610–630 °C, coupled with state-of-art post-irradiation microstructure characterization. The temperature evolution of irradiation-induced defects was studied using conventional TEM and scanning TEM (STEM). Additionally, high resolution TEM (HRTEM) imaging was also conducted to characterize irradiation-induced defects at the atomic scale.

It is important to highlight that natural boron consists of ∼20% Boron-10 (10B), which is well known to transmute by (n, α) reaction producing helium. This reaction cross-section is particularly large for thermal neutrons. Helium generation is deleterious for metals and ceramics because it induces matrix and grain boundary cavities/bubbles, leading to an early onset of void-swelling and high temperature helium embrittlement [[21], [22], [23], [24], [25], [26]]. Additionally, progressive 10B loss due to transmutation is also expected to compromise structural integrity. For example, Grinik et al. [12]. observed that surface layers gradually crumbled, representing loosely bound powders, after a neutron fluence of 2.5 × 1024 n.m−2 at temperatures greater than 530 °C in their natural boron-based TiB2. However, if isotopically enriched Boron-11 (11B) were used, the negative effects of 10B transmutation could be avoided due to the much lower transmutation cross-section of 11B to thermal neutrons [27]. Using this approach, polycrystalline TiB2 samples were produced with 99.31 at.% isotopic enrichment in 11B for the present study.

Section snippets

Material and neutron irradiations

Polycrystalline TiB2 produced using isotopically enriched 11B was fabricated by reaction hot pressing of ball-milled TiH2 and 99.31 at.% 11B (Ceradyne Inc. Boron Products) powders. The net concentration of boron in the samples was 28.4 wt%, obtained from chemical analysis. The as-produced specimens consisted of predominantly TiB2 grains, with titanium carbide inclusions (TixC) on the grain boundaries. The average grain size of TiB2 was 6.5 μm. Impurity element concentration in this material

Conventional TEM analysis of dislocation loops

Conventional diffraction contrast TEM characterization revealed irradiation-induced extended defects such as dislocation loops and black-dot damage after irradiations at both 220 and 620 °C. Fig. 2 presents bright field and low angle annular dark field (LAADF) STEM images of the same zone for the specimen irradiated at 220 °C. Imaging was performed using [0001] type diffraction vector close to the [12¯10] zone axis. The extended defects were identified to be dislocation loops on the basal

Dislocation loop microstructure evolution

Conventional TEM revealed irradiation-induced dislocation loops lying along the basal (0001) planes with Burgers vector parallel to the c-axis and prism planes after neutron irradiations in HFIR at 220 and 620 °C. At both irradiation temperatures, basal loops were larger than the prism plane loops. However, number density of the prism plane loops was nearly two orders of magnitude higher. The prism plane loops were typical black-dot damage, while well-developed ring shaped/coffee-bean contrast

Conclusions

Neutron irradiation response of 11B enriched TiB2 was studied at nominal temperatures of ∼220 and 620 °C and neutron fluences up to 2.4 × 1025 n.m−2 (E > 0.1 MeV). The dose, including the contribution from residual 10B transmutation recoils, was ∼4.2 dpa. Microstructure characterization using TEM revealed irradiation-induced extended defects such as dislocation loops and cavities. At 220 and 620 °C, dislocation loops on basal and prism planes were identified, with the number density of prism

Acknowledgements

The study was supported by the Office of Fusion Energy Sciences, US DOE and IMR Tohoku University under contract DE-AC05-00OR22725 and NFE-13-04416 with UT-Battelle, LLC, respectively. A portion of this research used resources at the HFIR, a DOE Office of Science User Facility operated by ORNL. This research was performed, in part, using instrumentation (FEI Talos F200X STEM) provided by the Department of Energy, Office of Nuclear Energy, Fuel Cycle R&D Program and the Nuclear Science User

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    Note: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

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