High Homologous Temperatures Research Articles

On the grain size-thickness correlation for thin films

A meta-analysis of published data in combination with measurements on Al, Cu, CuO, CoCrCuFeNi, Ni90Cr10, TiN, and V sputter deposited thin films, demonstrates that the grain size is correlated to the film thickness by a power law. The growth exponent depends on the homologous temperature which is defined as the ratio between the deposition and the melting temperature of the studied material. An average value of the growth exponent close to 0.4 was found for the growth exponent for a homologous temperature between 0.14 and 0.31. Film growth models that depict an evolutionary overgrowth mechanism obtain the same growth exponent. Above a homologous temperature of approximately 0.3, a slightly higher exponent is observed which agrees with the general idea that at higher homologous temperatures the grain size is also influenced by restructuring mechanisms occurring during film growth. At low homologous temperatures (<0.14), a substantially lower exponent was noticed. From a theoretical point of view the growth exponent’s value should be close to zero. The aforementioned boundaries of the homologous temperatures correspond with those observed in published structure zone models that describe the microstructure of physical vapor deposited thin films. The good agreement suggests that the underlying mechanism for the observed boundaries is the atom mobility. This hypothesis was further investigated by a study on the influence of intentionally added impurities on the power law behavior for Al and Cu thin films. For both materials, a similar decrease of the grain size as a function of an increasing impurity-to-metal impingement flux ratio on the substrate was observed. No change of the growth exponent is observed for Al, while the growth exponent becomes almost zero for Cu at sufficiently high impurity-to-metal flux ratios.

Length-scale-dependent stress relief mechanisms in indium at high homologous temperatures.

Nanoindentation and electron microscopy have been used to examine the length-scale-dependent stress relaxation mechanisms in well-annealed, high-purity indium at a homologous temperature of 0.69. The experimental methods, analysis, and observations serve as a stepping stone in identifying the stress relaxation mechanisms enabling the formation and growth of metallic dendrites originating at the buried interface between a metallic anode and a solid electrolyte separator. Indium’s load–displacement data are found to be very similar to that of high-purity lithium. Residual hardness impressions show two distinct surface morphologies. Based on these morphologies, the measured hardness, and the estimated pile-up volume, it is proposed that residual impressions exhibiting significant pile-up are the result of deformation dominated by interface diffusion. Alternatively, impressions with no significant pile-up are taken to be the result of shear-driven dislocation glide. An analytical model is presented to rationalize the pile-up profile using interface diffusion.

Decelerated grain growth kinetic and effectiveness of Hall-Petch relationship in a cold-rolled non-equiatomic high entropy alloy

The present work deals with the grain growth behavior and mechanical properties of a cold-rolled non-equiatomic metastable Fe50Mn30Co10Cr10 high entropy alloy. The bimodal grain size distribution which was developed during isothermal annealing at the temperature range of 750–1050 °C, was gradually annihilated through increasing the annealing time. This was due to the higher capability of fine grains to grow up compared with coarse grains. The obtained grain growth exponents and grain growth activation energies were well higher than those reported for solid solution alloys and also reported for five component CoCrFeMnNi high entropy alloy. The extremely low grain growth kinetics of the experimented alloy even at high homologous temperature was discussed regarding the solute drag like effect of whole-solute matrix in addition to the sluggish diffusion as an intrinsic properties of high entropy alloys. The variation in mechanical properties of the experimented alloy was mainly discussed relying on the effectiveness of Hall-Petch relationship and the stability of the matrix. It was found that the grain boundaries acted as an effective barrier against dislocation motion, and caused postponement of the strain induced transformation leading to an acceptable strength/ductility balance.

Microstructural Shift due to Post‐Deformation Annealing in the Upper Mantle

AbstractSyntectonic microstructural evolution is a well‐known phenomenon in the mantle and lower crust associated with two main processes: grain size reduction through dynamic recrystallization and development of crystallographic preferred orientation (CPO). However, the effects of annealing via static recrystallization on grain size and CPO have been largely overlooked. We investigated mantle annealing by analyzing a suite of kimberlite‐hosted garnet peridotite xenoliths from the Wyoming Craton. We focus on five xenoliths that show microstructures reflecting different degrees of recrystallization, with annealed grains characterized by distinctive faceted boundaries crosscutting surrounding, nonfaceted matrix grains. These textures are indicative of discontinuous static recrystallization (DiSRX). Electron backscatter diffraction analysis further demonstrates a ∼10°–20° misorientation between DiSRXed grains and the matrix grains, resulting in an overall weaker CPO. These characteristics are remarkably similar to microstructures observed in samples that were annealed after deformation in the laboratory. Measurements of the thermal conditions and water contents associated with the last equilibration of the xenoliths suggests that high homologous temperatures (T/Tm &gt; 0.9) are necessary to induce DiSRX. We postulate that annealing through DiSRX occurs under high temperatures after a short episode of intense deformation (years to hundreds of years) with timescales for annealing estimated as weeks to years, significantly slower than the timescale of hours expected for a kimberlitic magma ascent. We conclude that microstructural transformation due to DiSRX will occur during transient heating events associated with mantle upwelling, plumes, and lithospheric thinning.

Full crystallographic orientation (&amp;lt;i&amp;gt;c&amp;lt;/i&amp;gt; and &amp;lt;i&amp;gt;a&amp;lt;/i&amp;gt; axes) of warm, coarse-grained ice in a shear-dominated setting: a case study, Storglaciären, Sweden

Abstract. Microstructures provide key insights into understanding the mechanical behavior of ice. Crystallographic preferred orientation (CPO) develops during plastic deformation as ice deforms dominantly by dislocation glide on the basal plane, modified and often intensified by dynamic recrystallization. CPO patterns in fine-grained ice have been relatively well characterized and understood in experiments and nature, whereas CPO patterns in “warm” (T&gt;-10∘C), coarse-grained, natural ice remain enigmatic. Previous microstructural studies of coarse-grained ice have been limited to c-axis orientations using light optical measurements. We present the first study of a axes as well as c axes in such ice by application of cryo-electron backscatter diffraction (EBSD) and do so in a shear-dominated setting. We have done this by developing a new sample preparation technique of constructing composite sections, to allow us to use EBSD to obtain a representative, bulk CPO on coarse-grained ice. We draw attention to the well-known issue of interlocking grains of complex shape and suggest that a grain sampling bias of large, branching crystals that appear multiple times as island grains in thin sections may result in the typical multimaxima CPOs previously identified in warm, coarse-grained ice that has been subjected to prolonged shear. CPOs combined from multiple samples of highly sheared ice from Storglaciären provide a more comprehensive picture of the microstructure and yield a pronounced cluster of c axes sub-normal to the shear plane and elongate or split in a plane normal to the shear direction as well as a concomitant girdle of a axes parallel to the shear plane with a maximum perpendicular to the shear direction. This pattern compares well with patterns produced by subsampling datasets from ice sheared in laboratory experiments at high homologous temperatures up to strains of ∼1.5. Shear strains in the margin of Storglaciären are much higher than those in experimental work. At much lower natural strain rates, dynamic recrystallization, particularly grain boundary migration, may have been more effective so that the CPO represents a small, final fraction of the shear history. A key result of this study is that multimaxima CPOs in coarse-grained ice reported in previous work may be due to limited sample sizes and a sampling bias related to the presence of island grains of a single host that appear several times in a thin section.

Improving method for deterministic treatment of double cross-slip in FCC metals under low homologous temperatures

Despite its importance in crystal plasticity, cross-slip has not yet been completely understood. The widely accepted approach understands cross-slip as a thermally-activated stochastic process, especially at higher homologous temperatures. An alternative approach is based on an experimentally supported observation that, especially at lower homologous temperatures, cross-slip becomes a deterministic stress-driven process governed by the line tension, by the applied stress, and by the short-range interactions among dislocations. The current challenge remains in understanding how the tip of the screw dislocation segment of changes the primary slip plane for the secondary slip plane. We suggest an explanation of this problem available by the mathematical theory of moving parametrized curves. For the dynamical change between the primary plane and the cross-slip plane, two methods are introduced and compared. They are referred to as the tilting projection method (denoted as TPM) and the vertical projection method (denoted as VPM). Main difference between them is how they regularize the motion of the dislocation across the edge between the primary and the cross-slip plane. After the motion strategy is set, this article explains that dislocation motion laws can be treated numerically by means of the flowing finite-volume method, including the redistribution of the discretization points ensuring a long-term stability of the algorithm. Particular computational results of cross-slip obtained by both approaches are mutually compared and evaluated. For the considered material setup of the FCC copper and nickel crystals, good agreement has been observed.

Noble Gas Bubbles in Bcc Metals: Ab Initio-Based Theory and Kinetic Monte Carlo Modeling

Understanding the interactions of noble gases with metals is of fundamental importance for the design of radiation-resistant structural materials for fission and fusion nuclear reactors. Here we present a unified theory for describing the energetics of He, Ne, Ar, and Kr bubbles in bcc metals in group 5B (V, Nb, Ta), 6B (Cr, Mo, W) and 8B (Fe). Our predictive analytical model is based on the effective-medium and isotropic elasticity theories, and is parameterized using density functional theory (DFT) calculations of small gas-vacancy clusters. By performing kinetic Monte Carlo (KMC) simulations driven by our analytical model, we have predicted the lifetimes of noble gas bubbles and their coarsening by Ostwald ripening. Our most notable finding is the exceptionally higher thermal stability of Ne, Ar and Kr bubbles than He bubbles in bcc metals, conferring them outstanding resistance to Ostwald ripening. The physical origin of the unexpected stability of bubbles formed by large noble gas atoms has been further elucidated. Our theoretical finding is consistent with the experimental observation of He gas bubble superlattice (GBS) coarsening under thermal annealing, and provides new insights on the exceptional stability of fission GBS in bcc U-Mo up to a high homologous temperature of 0.78.

Creep Deformation of Lithium Foil at Moderate Pressures (< 2 MPa) and Temperatures (30-50°C)

Lithium metal is receiving significant research attention as a potential next generation negative electrode material in lithium ion batteries because it can offer a ~50% increase in cell energy density compared to conventional graphite electrodes. There are, however, a number of mechanical challenges using lithium metal electrodes. Mechanical stack pressure is required to ensure good contact between solid-state electrodes and electrolytes. Moderate to high stack pressures (e.g. < 10 MPa) can also supress void formation during fast charging and discharging.1 Creep, the plastic deformation of a material under constant pressure, improves battery performance by preventing and filling voids, but can also lead to decreased performance if lithium is unable to sustain the required stack pressure, and potentially cell shorting if lithium is e.g. extruded beyond the current collectors.Creep deformation rates are highly dependent on homologous temperature and pressure. Lithium metal has a relatively low melting point (181°C) / high homologous temperature at common cell temperatures which should result in substantial lithium creep deformation. Previous studies have characterized both the tensile creep properties of lithium foils2 and compressive creep properties of bulk lithium rods.3 Significant barrelling of the high aspect ratio lithium rods (initial height:width of 4:1 to 1:1) during compression made it difficult to determine the applied pressure on the lithium, obscuring the relationship between pressure and creep rate.3 These important studies increased our collective understanding of the mechanical properties of lithium, but are not necessarily reflective of the compressive creep of lithium foils in a cell environment. Here, we report on the compressive creep of lithium foil within an electrochemical cell at commercially relevant stack pressures and temperatures.Compressive creep deformation of low aspect ratio (1:40 to 1:10; initially 0.3 – 2.5 mm thick, 13 mm diameter) lithium metal foils was measured in hermetically sealed, rigid but flexible cells at temperatures between 30 - 110°C and applied pressures between 1 - 2 MPa. Creep testing was performed by integrating a Conflat-style electrochemical cell4 featuring welded bellows and an optical window with an Instron 5966 Universal Testing System (which measures / controls force and displacement). Creep rates were determined using the time-dependent displacement data after reaching a constant applied force. Two methods to convert force to pressure will be described, including the use of an inline camera to measure the lithium-compressive piston contact area.Creep deformation was observed at all tested temperatures, and all pressures above 1 MPa. Creep rates were on the order of a few tenths of a micron per hour, which while small, implies very limited device lifetime with thin lithium layers. Lithium metal may not be physically strong enough for use in high energy density all solid-state devices. Figure 1

Solute repositioning to tune the multiple microalloying effects in an Al–Cu alloy with minor Sc, Fe and Si addition

Abstract For multicomponent Al-based alloys, one of the most valuable approaches of microstructural design is to find a path to maximize the multiple microalloying effects while overcoming their negative counterparts. In this paper, triple Sc-Fe-Si microalloying was performed in an Al–Cu alloy to assemble the co-existence of θ′-Al2Cu and Al3Sc precipitates. Besides, the mutual interactions among triple microalloying elements were utilized to maximize the positive effects on tailoring the dual precipitates, as illustrated in two goals of microstructural design: (i). As to θ′-Al2Cu plates, the multiple Sc-Fe-Si segregation at θ′-Al2Cu/matrix interface was preferentially created in the alloy before creep (as-aged condition). During subsequent high-temperature creep, such interfacial warden will be rapidly reinforced by solute repositioning, as a process of accumulating solutes diffusing from both inner θ′-Al2Cu precipitate and outer matrix to limit the interfacial migration. (ii). For Al3Sc precipitates, the beneficial Si microalloying effect on encouraging the nucleation of Sc-rich entities (precursor of Al3Sc) was successfully acquired during aging, while the detrimental Si effect on accelerating Al3Sc coarsening is generally prohibited by tuning Fe–Si synergy within Al3Sc interior. The establishments of (i) and (ii) enable the satisfactory dispersion as well as the outstanding thermal stability of dual precipitates in the current Al-Cu-Sc-Fe-Si system, leading to a good creep resistance at a high homologous temperature of 0.61Tm ~ 300 °C (where Tm is the melting temperature of the α-Al matrix).

Thermal Aging Reliability of Socketable BGA Packages With Ni–Au-Coated SAC305 Spheres

Land grid arrays (LGAs) and ball grid arrays (BGAs) constitute the board-level interconnect technologies for a large number of packaged ICs. These technologies are used in socketing and surface mount (SMT) applications, respectively. The concept of socketable BGA packages, with solder spheres coated with a barrier layer/noble metal coating, was introduced to develop a single package design for processor ICs that are used in both socketing and SMT applications. While this concept has previously been demonstrated, the feasibility of these packages in socketing conditions has not yet been studied. This article assesses the applicability of these packages in socketing by subjecting the packages to thermal aging at a temperature of 120 °C. The change in microstructure with thermal aging along with the consumption of the barrier layer was understood. The joint shear strength evolution with thermal aging and the contamination of the top surface of the spheres was studied by the X-ray photoelectron spectroscopy (XPS) analysis. It was found that the trend for experimental consumption of the barrier layer closely follows the theoretical model predictions. Solid-state solder wicking of the Sn57.6Bi0.4Ag (SBA) solder joint due to creep was confirmed at high homologous temperature for the solder. This, along with grain coarsening over time, led to a decrease in the joint shear strength from 33.5 to 16 MPa. Polymer collars are suggested as a potential solution to prevent the solder wicking and make the package feasible for socketing applications.

Using a composite flow law to model deformation in the NEEM deep ice core, Greenland – Part 2: The role of grain size and premelting on ice deformation at high homologous temperature

Abstract. The ice microstructure in the lower part of the North Greenland Eemian Ice Drilling (NEEM) ice core consists of relatively fine-grained ice with a single maximum crystallographic preferred orientation (CPO) alternated by much coarser-grained ice with a partial (great circle) girdle or multi-maxima CPO. In this study, the grain-size-sensitive (GSS) composite flow law of Goldsby and Kohlstedt (2001) was used to study the effects of grain size and premelting (liquid-like layer along the grain boundaries) on strain rate in the lower part of the NEEM ice core. The results show that the strain rates predicted in the fine-grained layers are about an order of magnitude higher than in the much coarser-grained layers. The dominant deformation mechanisms, based on the flow relation of Goldsby and Kohlstedt (2001), between the layers is also different, with basal slip rate limited by grain boundary sliding (GBS-limited creep) being the dominant deformation mechanism in the finer-grained layers, while GBS-limited creep and dislocation creep (basal slip rate limited by non-basal slip) contribute both roughly equally to bulk strain in the coarse-grained layers. Due to the large difference in microstructure between finer-grained ice and the coarse-grained ice at premelting temperatures (T&gt;262 K), it is expected that the fine-grained layers deform at high strain rates, while the coarse-grained layers are relatively stagnant. The difference in microstructure, and consequently in viscosity, between impurity-rich and low-impurity ice can have important consequences for ice dynamics close to the bedrock.

Influence of processing route on the alloying behavior, microstructural evolution and thermal stability of CrMoNbTiW refractory high-entropy alloy

Abstract

Characterization of γ′ Precipitates in Cast Ni-Based Superalloy and Their Behaviour at High-Homologous Temperatures Studied by TEM and in Situ XRD

In situ X-ray diffraction and transmission electron microscopy has been used to investigate René 108 Ni-based superalloy after short-term annealing at high-homologous temperatures. Current work is focused on characterisation of γ′ precipitates, their volume fraction, evolution of the lattice parameter of γ and γ′ phases and misfit parameter of γ′ in the matrix. Material in the initial condition is characterised by a high-volume fraction (over 63%) of γ′ precipitates. Irregular distribution of alloying elements was observed. Matrix channels were strongly enriched in Cr, Co, W and Mo, whereas precipitates contain large amount of Al, Ti, Ta and Hf. Exposure to high-homologous temperatures in the range 1100–1250 °C led to the dissolution of the precipitates, which influenced the change of lattice parameter of both γ and γ′ phases. The lattice parameter of the matrix continuously grew during holding at high temperatures, which had a dominant influence on the more negative misfit coefficient.

Interdiffusion Studies in the Co-Sb System

CoSb based compounds have gained much importance in the fields of thermoelectric devices. In this work, we have conducted the solid–state conventional bulk diffusion couple experiments. To study the phase evolutions, Co/Sb diffusion couples are annealed at 450–550 °C. The interdiffusion zone is analysed using field emission gun equipped scanning electron microscope and the composition measurements are done in electron probe micro−analyser to confirm the growth of various product phases. The marker experiment indicates that the CoSb3 phase grows mainly by diffusion of Sb in the binary Co–Sb system. Growth of the CoSb3 phase is discussed based on assessment correlating the difference in mobilities of species with the high homologous temperature, crystal structure of the phase, and the concept of sublattice diffusion mechanism in line compounds.

To what extent does friction-stir welding deteriorate the properties of powder metallurgy Al?

Powder metallurgy (PM) fabricated HITEMAL is a near- and sub-micrometer aluminum (Al) material that is stabilized by a small fraction of a nano alumina (Al2O3) secondary phase. This phase forms in situ from surface Al2O3 films present on as-atomized Al powders during compaction. HITEMAL showed unique mechanical properties and creep performance at high homologous temperatures owing to its ultra-fine Al grain structure effectively stabilized by Al2O3. However, conventional fusion welding is generally not applicable to PM Al. This study reports on friction stir welding (FSW) that was applied to join bars extruded from Al powders of different particle size. As PM Al performance crucially depends on the Al2O3 phase, we focused on changes to this phase that were, induced by FSW. Thermal stability, mechanical properties, and deformation behavior of the weld materials were studied versus changes in the morphology and distribution of the Al2O3 phase during FSW. The study confirmed that FSW can be successfully used to join PM Al, although some detrimental transformation of the Al2O3 phase occurred upon FSW. This transformed Al2O3 phase did not provide sufficient stabilization under severe plastic deformation induced by a rotating tool action. The transformation involved dynamic recrystallization and minor Al grain growth in weld zones. This led to a decrease in the mechanical strength of the weld compared with that of the respective base materials. While this negative effect was pronounced for HITEMAL with the finest Al grain size, it was negligible in the coarse-grained material produced from the coarsest Al powder.

Microstructural modification of additively manufactured metals by electropulsing

Additive manufacturing (AM) promises rapid development cycles and fabrication of ready-to-use, geometrically-complex parts. The metallic parts produced by AM often contain highly non-equilibrium microstructures, e.g. chemical microsegregation and residual dislocation networks. While such microstructures can enhance some material properties, they are often undesirable. Many AM parts are thus heat-treated after fabrication, a process that significantly slows production. This study investigated if electropulsing, the process of sending high-current-density electrical pulses through a metallic part, could be used to modify the microstructures of AM 316 L stainless steel (SS) and AlSi10Mg parts fabricated by selective laser melting (SLM) more rapidly than thermal annealing. Electropulsing has shown promise as a rapid postprocessing method for materials fabricated using conventional methods, e.g. casting and rolling, but has never been applied to AM materials. For both the materials used in this study, as-fabricated SLM parts contained significant chemical heterogeneity, either chemical microsegregation (316 L SS) or a cellular interdendritic phase (AlSi10Mg). In both cases, annealing times on the order of hours at high homologous temperatures are necessary for homogenization. Using electropulsing, chemical microsegregation was eliminated in 316 L SS samples after 10, 16 ms electrical pulses. In AlSi10Mg parts, electropulsing produced spheroidized Si-rich particles after as few as 15, 16 ms electrical pulses with a corresponding increase in ductility. This study demonstrated that electropulsing can be used to modify the microstructures of AM metals.

Effects of proton irradiation on microstructure and mechanical properties of nanocrystalline Cu–10at%Ta alloy

Duplex nanocrystalline Cu–10at%Ta is attractive for extreme environments because of its grain size stability at high homologous temperatures, owing to dispersed Ta nanoclusters. This study is an initial investigation into the microstructural and mechanical stability of Cu–10at%Ta under irradiation. Proton irradiation is carried out to 1 displacement per atom (dpa) at 500 °C. The Cu–10at%Ta exhibits grain size stability throughout irradiation and resistance to the formation of irradiation-induced defects. However, Ta nanoclusters undergo ballistic dissolution. Nanoindentation reveals there is no significant irradiation hardening, consistent with the relatively irradiation-stable microstructure. Ballistic dissolution of Ta nanoclusters is discussed in the context of Orowan strengthening.

In Situ Transmission Electron Microscopy for Ultrahigh Temperature Mechanical Testing of ZrO2.

This work demonstrates a novel approach to ultrahigh-temperature mechanical testing using a combination of in situ nanomechanical testing and localized laser heating. The methodology is applied to characterizing and testing initially nanograined 10 mol % Sc2O3-stabilized ZrO2 up to its melting temperature. The results suggest that the low-temperature strength of nanograined, d < 50 nm, oxides is not influenced by creep. Tensile fracture of ZrO2 bicrystals produce a weak-temperature dependence suggesting that grain boundary energy dominates brittle fracture of grain boundaries even at high homologous temperatures; for example, T = 2050 °C or T ≈ 77% Tmelt. The maximum temperature for mechanical testing in this work is primarily limited by the instability of the sample, due to evaporation or melting, enabling a host of new opportunities for testing materials in the ultrahigh-temperature regime.

Characterization of Electrical Current Stress and Indentation Creep of Carbon Nanotubes- reinforced Low Melting Temperature Sn-58Bi Composites

Sn-58Bi eutectic alloy emerges as one of the potential lead-free solder material candidates due to its low melting temperature that enables low temperature soldering reflow process during the manufacturing of fine-pitch and low z-height advanced microelectronic devices. Its low melting temperature provides feasible solution for a two-step reflow process in the assembly of 3D-stacked advanced packaging technology, and at the same time reduces dynamic warpage-induced quality and reliability issues. However, miniaturization of these microelectronic devices has triggered significant reliability risk to its solder interconnects due to the requirement of high current density that resulted in severe physical failures, such as electromigration (EM), and thermomigration (TM). To mitigate the risk, carbon nanotubes (CNT) are incorporated into elemental Sn and Bi powders using planetary ball milling technique, and subject to liquid phase melting to form low melting temperature CNT- reinforced eutectic Sn-58Bi composites. Electrical resistance and current density of the eutectic alloy and its composites were investigated, and results show that Sn-58Bi-0.03CNT composite has the lowest electrical resistance and was able to survive 26 times longer duration of current stressing, even with the supply of higher stressed current. Creep performance of eutectic Sn-58Bi system was also examined in this study due to its high homologous temperature of 0.72 at room temperature. Marginal improvement in indentation creep performance was seen in the composites.

Effects of Proton Irradiation on Microstructure and Mechanical Properties of Nanocrystalline Cu-10at%Ta Alloy

Duplex nanocrystalline Cu-10at%Ta is attractive for extreme environments because of its grain size stability at high homologous temperatures, owing to dispersed Ta nanoclusters. This study is an initial investigation into the microstructural and mechanical stability of Cu-10at%Ta under irradiation. Proton irradiation is carried out to 1 displacement per atom (dpa) at 500°C. The Cu-10at%Ta exhibits grain size stability throughout irradiation and resistance to the formation of irradiation-induced defects. However, Ta nanoclusters undergo ballistic dissolution. Nanoindentation reveals there is no significant irradiation hardening, consistent with the relatively irradiation-stable microstructure. Ballistic dissolution of Ta nanoclusters is discussed in the context of Orowan strengthening.