Tungsten Grains Research Articles

Enhanced mechanical properties of Co-containing W-Ni3Al alloy by trace Y2O3 addition

Although W-Ni3Al alloys have many promising applications in the civil and military fields, their further development requires better mechanical properties. In this study, based on our reported W-Ni3Al-Co alloy, its mechanical properties were greatly enhanced via adding trace Y2O3. Among them, the 0.1 wt% Y2O3 added alloys exhibited excellent comprehensive properties, whose relative density, hardness, bending strength, and tungsten grain size were 99.86 %, 71.2±0.22 HRA, 1733.32±27.32 MPa, and 3.57±1.15 μm, respectively. By analyzing the relationship between microstructures and mechanical properties of sintered W-8.9Ni3Al-1Co-0.1Y2O3 alloy, its outstanding mechanical properties were mainly attributed to the following factors: Firstly, the binder phase and W phase were solid-solution-strengthened by forming (Ni,Co)3(Al,W) and (W,Co,Al) solid solutions; Secondly, the added Y2O3 and formed Al2O3 particles contributed to producing the dispersion strengthening and fine grain strengthening; Thirdly, the interfaces between Y2O3 and Al2O3 were strengthened by the formed Y4Al2O9 phase. The above factors played important roles in improving the mechanical properties of Co-containing W-Ni3Al alloy, especially the substantial increase in bending strength.

Effect of ZrB2 addition on microstructure and mechanical properties of 95W-HEA alloys

In tungsten heavy alloys, the substitution of conventional binders with novel high entropy alloys and the addition of rare metal borides have demonstrated considerable promise in enhancing comprehensive performance. In this study, 95W-5CoCrFeNiMn -x ZrB2 (x = 0.1, 0.2, 0.4, 0.6, 0.8) was synthesized by powder metallurgy method and characterized. The results show that: the spherical tungsten grains are distributed in binder network, and the added ZrB2 reacts with oxygen to generate ZrO2 and B2O3, which will affect the microstructure and properties of the alloys. An increase of ZrB₂ content results in a reduction in alloy density, accompanied by an enhancement in hardness; the changes in grain size, contiguity, and solid volume fraction present a trend of first falling and then rising, and the minimum value is obtained when the addition is 0.4 wt%. At this time, 95WHEAs-0.4 ZrB2 presents the best comprehensive performance, exhibiting an average grain size of 16.80 μm, a relative density of 96.10 %, a hardness of 411.68 HV1, and a compressive strength of 2457.64 MPa. The primary fracture mode observed in the alloys is W-binder interfacial separation, with a minor occurrence of tungsten cleavage when the ZrB2 addition is within an optimal range.

An understanding of the segregation and migration mechanism of point defects in tungsten grain boundaries: An atomic scale simulation

In this study, molecular dynamics (MD) simulations are employed to investigate the interactions between point defects and grain boundaries (GBs) in tungsten. Firstly, the segregation energy of vacancies (Vs) and self-interstitial atoms (SIAs) to GBs is examined. The results indicate that both Vs and SIAs tend to segregate towards GBs. Notably, the different types of GBs exhibit varying attraction to Vs and SIAs. The interaction radius is applied to investigate the degree of segregation, which is a significant index of GBs’ attraction to Vs and SIAs. The segregation radius of SIAs is typically larger than that of Vs. High-angle grain boundaries (HAGBs) show strong segregation for SIAs, facilitating the aggregation at GBs. Additionally, elastic theory is applied to qualitatively discuss the factors influencing the segregation energy distribution. The differences in atomic free volume caused by hydrostatic stress and the lattice distortion induced by point defects lead to Vs and SIAs occupying different sites. Finally, the nudged elastic band (NEB) method is employed to study the migration of Vs and SIAs near GBs, revealing that migration energy barriers for Vs and SIAs in GBs are much lower than in bulk. GBs facilitate the migration of point defects. Low-angle grain boundaries (LAGBs) present a higher energy barrier for V migration. Vs tend to occupy the compressive region, while SIAs tend to occupy the tensile region. Particularly, in comparison to Vs, SIAs are more likely to segregate to GBs due to higher binding energy, wider interaction radius, and extremely lower diffusion energy barrier. This provides some insights into the segregation and migration of point defects in GBs.

Transient liquid phase sintering in spark plasma sintered tungsten heavy alloys with Nb and Mo additives

In the present investigation spark plasma sintered tungsten heavy alloys of composition 90W-7Ni-3Fe, 84W-7Ni-3Fe-6Mo, and 84W-7Ni-3Fe-6Nb were analyzed to determine the effect of Mo and Nb addition on microstructure and phase formation. The alloys were sintered at temperatures ranging from 1150 °C to 1275 °C and at a fixed sintering pressure of 30 MPa. Nb added alloy exhibited a maximum relative density of 99% at sintering temperature of 1200 °C. Whereas the base alloy and Mo added alloy, exhibited a maximum relative density of 97.9% and 95.9% at 1150 °C respectively. Transient liquid phase sintering was found to play a crucial role in the base alloy and Mo added alloy whereas the Nb added alloy was found to undergo persistent liquid phase sintering. Microstructure characterisation by SEM revealed formation of finer tungsten grains with average grain size ranging from 1 μm to 4.3 μm. Mo addition restricted the tungsten grain growth while Nb addition promoted the tungsten grain growth compared to the base alloy. Phase analysis by XRD revealed the formation of Ni4W intermetallic (at lower temperatures) for the base alloy, NbO2 and NbW intermetallics for Nb added alloy and MoNi rich phase for Mo added alloys. Hardness measured by Vickers hardness method revealed a declining trend in hardness with increase in sintering temperature, owing to tungsten grain growth in all the alloys and along with Kirkendall pore formation and growth for the base alloy and Mo added alloy. Nb addition resulted in maximum hardness of 678 HV1 which is highest ever reported for SPSd WHA with 90 wt% concentration of W.

Tungsten heavy alloys for kinetic energy penetrators: a review

Tungsten heavy alloys are preferred materials for kinetic energy penetrators owing to the combination of high density, tensile strength, good impact strength and inert nature. The factors affecting shear band formation in the material and strategies to aid their early emergence by various alloy and process modifications to sustain a self-sharpened head during penetration are the main focus of this review article. Changes in the microstructural features such as tungsten grain size, dihedral angle and contiguity and mechanical properties by the addition of various alloying constituents and post-processing are critically discussed, along with high strain rate deformation behaviour analysis. This article also captures recent developments in the last decade and future prospects in the field of tungsten heavy alloys.

Effect of nanosecond laser irradiation on tungsten grain structure

The laser-induced grain structure of pure tungsten was evaluated using a nanosecond Q-switched Nd:YAG laser. The grain structure, crystal orientation, and grain boundary angular distribution were analyzed through electron backscatter diffraction analysis, both before and after laser processing. The mechanical properties of pure tungsten after laser processing were also measured. The results obtained indicate that laser processing-induced recrystallization can offer micro-nano surface treatment of materials without compromising the mechanical properties.

Improving mechanical properties of W-Ni3Al alloy through Zr element addition: Solid-solution and oxide particle strengthening

In this study, fine Ni3Al flakes facilitated the W-Ni3Al alloy liquid phase sintering, while the added 0.9 wt% Zr element enhanced its mechanical properties, finally obtaining the 1200 °C sintered W-Ni3Al-Zr alloy (WNZ-1200 alloy) with excellent comprehensive properties. Specifically, the relative density, tungsten grain size, bending strength, and macro hardness of WNZ-1200 alloy were 98.26 ± 0.31 %, 3.06 ± 0.65 µm, 1472.56 ± 61.90 MPa, and 76.10 ± 0.82 HRA, respectively. By analyzing the alloy’s phase and microstructure, its excellent mechanical properties were originated from the formed Ni3(Al, W, Zr) solid solution, unique nano-Al2O3 ‘rivet’, the semi-coherent interface of W/t-ZrO2, oxide dispersion strengthening, and the t-ZrO2 phase transformation strengthening.

Design of heat-treatment and its effect on Ni-rich matrix of a W-Ni-Co alloy

The aim of this investigation is to optimize the heat-treatment for tailoring a suitable microstructure, especially that of the matrix phase in a W-Ni-Co alloy. Different microstructural features that have been focused on are grain size, fine W particles, intermetallics and short-range ordering. 92W-5.3Ni-2.7Co alloy prepared through powder metallurgy route was imparted a cyclic heat-treatment to obtain a microstructure comprising finely distributed tungsten particles in an FCC matrix that envelopes the large tungsten grains. While the fine W particle influences the mechanical behaviour of W-Ni-Co alloys, the grain size of the matrix and short-range ordering are also expected to play a significant role. The effect of different heat-treatments on the microstructure of the matrix was studied using transmission electron microscope (TEM) and electron back scatter diffraction (EBSD). It is observed that increasing the quenching rate diminishes short-range ordering in the matrix. The formation of intermetallic phase at intermediate temperature range (between 800 °Cand 900 °C) and its subsequent dissolution at higher temperature influences the final microstructure of the alloy. In the present study, the effect of the intermetallic phase on the formation of randomly orientated fine equiaxed grains in the matrix has been elucidated.

Hot Hydrogen Testing of W-Coated UN Kernels in a Mo30W Matrix

Ceramic uranium mononitride (UN) is being considered as a reactor fuel for nuclear thermal propulsion. To avoid or reduce the dissociation of UN at the high temperatures needed, embedding it in a metallic matrix (cermet) has been proposed. To assess the viability of this concept, hot hydrogen testing of tungsten-coated UN kernels embedded in a Mo-30 wt% W (Mo30W) alloy matrix has been performed at temperatures from 1800°C to 2300°C. Both the isolated kernels and kernels consolidated by spark plasma sintering in the Mo30W matrix were tested. In addition to direct observations and mass loss measurements, the samples were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM)/energy dispersive X-ray spectroscopy (EDS) after each run. The decomposition of UN started at 1800°C despite the coating and matrix, and increased at 2000°C. Uranium seeped through the tungsten grain boundaries of the coating at all temperatures. The consolidated sample expanded irregularly at 2000°C through the formation of voids, and SEM/EDS analysis showed uranium-containing veins in the matrix consisting of U2Mo according to the XRD data. The observed pore generation at 2000°C was explained by the formation of water vapor from residual oxides and diffused hydrogen. At 2200°C and above, both the kernels and the consolidated samples melted through the formation of uranium or low–melting point uranium-molybdenum alloys.

Unraveling grain growth of metallic tungsten: Investigating the nanoscale realm of hydrogen reduction of tungsten oxides

This paper focuses on the experimental observation of tungsten grain evolution during the process of hydrogen reduction of tungsten trioxide. The objective is to gain insights into the grain size distribution (GSD) evolution and morphology changes of metallic tungsten. The study employs an amalgamation of conventional characterization techniques – such as TGA, SEM-EDS, TEM and XRD – alongside cutting-edge techniques like phase contrast nano-tomography from synchrotron sources. Conducted as a quasi in-situ study, this research offers the opportunity to observe the transient evolution of the formed metallic tungsten phase within its precursor WO2 particles. The paper presents and discusses quantitative results concerning the evolution of GSD and morphological changes, encompassing growth rates of both crystallites and grains of the synthesized metallic phase. Additionally, the quasi in-situ study highlights the dependence of grain size on water concentration during the reduction process. The qualitative and quantitative findings contribute to a more nuanced understanding of the kinetics of grain growth of metallic tungsten and offer insights into the intricate interplay between reduction parameters, GSD evolution and morphological changes. Gaining a deeper understanding of the underlying mechanisms driving the changes in GSD establishes a foundation for predictive theory with immediate academic and industrial impact.

Study of changes in the surface structure of tungsten irradiated by helium plasma

One of the important aspects is the interaction of plasma with the surface of a material, especially in the conditions of a fusion facility. The current work presents the preliminary results of the study of tungsten surface structure modification under helium plasma irradiation. A small-sized linear simulator KAZ-PSI with a plasma-beam setup was designed and assembled, where helium was used as a working gas. The main elements of the linear plasma simulator are an electron beam gun with a LaB6 cathode, a plasma-beam discharge chamber, an interaction chamber, a target device, and an electromagnetic system consisting of electromagnetic coils. It was revealed that under irradiation on the surface of the samples there is a relief with defective structure consisting of chaotically arranged ledges and pits of various shapes with average size (100‒600) nm and pore sizes (0.1‒1.5) μm with visible areas of flaking and sputtering. It was found that when the negative potential on the target is varied by –500V/–1000V/–1500V, the formation of dislocation with chaotic and cellular structure of tungsten with an average grain size of (1‒25) μm is observed; it was revealed that the total values of elastic and plastic components of deformation across the tungsten grain differ from each other by about 2.5 times.

Tungsten heavy alloy: Nano-crystallinity and alloying induced low temperature sintering, microstructure and mechanical properties

In this study, a tungsten heavy alloy with a composition of 90 W-4.9Ni-2.1Fe-3.0Cu was processed using powder metallurgy. Pure elements were ball-milled to develop a mechanically alloyed nano-grained solid solution to reduce the required sintering temperature. The alloy was then liquid-phase sintered in a hydrogenated argon atmosphere at 1250 °C. Microstructural characterization revealed that the high-energy ball milling process successfully developed a tungsten solid solution with a reduced tungsten grain size to a level of 15 nm, facilitating effective low-temperature (i.e., 1250 °C) liquid-phase sintering with 88% relative density. The tungsten grains grew mostly in a polygonal shape, coalesced to a large size, and had strong interfacial integrity with the nickel‑iron‑copper matrix. Mechanical properties characterization revealed a significantly high microhardness level of 380 HV, compressive yield strength of 910 MPa, and ultimate compressive strength of 1326 MPa. The applied compressive stress induced intergranular crack propagation leading to the fracture of the sample with a failure strain of 25.6%.

Micro-Mechanical Fracture Investigations on Grain Size Tailored Tungsten-Copper Nanocomposites

Tungsten-copper composites are used in harsh environments because of their superior material properties. This work addresses a tungsten-copper composite made of 20 wt.% copper, which was subjected to grain refinement by high-pressure torsion, whereby the deformation temperature was varied between room temperature and 400 °C to tailor the grain size. Deformation was performed up to microstructural saturation and verified by hardness measurement and scanning electron microscopy. From the refined nanostructured material, micro-cantilever bending beams with cross-sections spanning from 5 × 5 to 35 × 35 µm2 were cut to examine possible size effects and the grain size influence on the fracture behavior. Fracture experiments were performed in situ inside a scanning electron microscope by applying a quasi-static loading protocol with partial unloading steps. Inspection of the fracture surfaces showed that all cantilevers failed in an inter-crystalline fashion. Nevertheless, remaining coarser tungsten grains impacted the resultant fracture toughness and morphology. Cantilevers fabricated from the 400 °C specimen exhibited a fracture toughness of 220 ±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm $$\\end{document} 50 Jm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\frac{{\ ext{J}}}{{{\ ext{m}}}^{2}}$$\\end{document}. For the room temperature cantilevers, a fracture toughness of 410 ±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm $$\\end{document} 50 Jm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\frac{{\ ext{J}}}{{{\ ext{m}}}^{2}}$$\\end{document} was observed, which declined to 340 ±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm $$\\end{document} 30 Jm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\frac{{\ ext{J}}}{{{\ ext{m}}}^{2}}$$\\end{document} for cantilevers < 10 × 10 µm2, confirming a size effect. The increased fracture toughness is attributed to the delamination-like structures formed in the room temperature sample.

Fabrication of spherical W–ZrC powder for additive manufacturing by spray granulation combined with plasma spheroidization

In this study, a W–1%ZrC spherical powder for additive manufacturing was successfully fabricated by spray granulation combined with radio frequency (RF) plasma spheroidization. 3 μm tungsten powder and the nano-sized ZrC powder were spray granulated into a porous agglomerated powder and then spheroidized to form a dense spherical powder by RF plasma. Spherical W–ZrC powder with a narrow particle size distribution and a uniform distribution of Zr element was obtained, whose flowability and apparent density was 7.5 s/50g and 9.36 g/cm3, respectively. Furthermore, ZrC enters the tungsten grains after plasma spheroidization, which causes dislocations in the tungsten grain. The spherical W–ZrC composite powder is expected to suppress the crack formation of W parts during the laser powder bed fusion process.

Microstructure evolution and mechanical property of W-Ni3Al alloy by adding minor Co element

In this study, W-Ni3Al alloys with fine grain size and outstanding integrated mechanical properties were fabricated based on mechanical alloying and spark plasma sintering (SPS), modified by adding a small amount of cobalt powder. It was found that the alloy was strengthened by the diffusion of Co and W elements into the Ni3Al lattice to form (Ni,Co)3(Al,W) solid solution during the sintering, where Al2O3 particles in-situ formed at the interface between binder phase and tungsten grains, and therefore became obstacles to impeding the dislocation movement. Such obstacles would lead to restraints of dissolution-reprecipitation of tungsten grains, resulting in refined tungsten grains and strengthening effect in the sintered alloys. The synergistic effect of these two kinds of strengthening significantly improved the mechanical properties of W-Ni3Al alloy. As a result, the W-9Ni3Al-1Co alloy sintered at a low temperature of 1250 °C had the best room-temperature mechanical properties, and its bending strength, tensile strength and hardness were 1519.48 MPa, 783.24 MPa and 71.1 HRA, respectively.

Effects of Sintering Temperature on the Microstructure and Properties of a W-Cu Pseudo-Alloy

This paper studies the feasibility of fabricating pseudo-alloys based on a W-Cu system through vacuum sintering of spherical bimetallic particles synthesized using the electric explosion of copper–tungsten wires in argon. The effects of the sintering temperature on the structure and hardness of the fabricated composites was studied. In terms of the structure of the samples, tungsten particles of predominantly spherical shapes with sizes ranging from submicrons to 80–90 µm were uniformly distributed throughout the copper matrix. Based on the analysis, the volume fractions of tungsten and copper were approximately equal. The calculated average phase compositions for all the samples were 58.9 wt% for W, 27.3 wt% for Cu, and 13.8 wt% WO2. When the annealing temperature increased from 1100 °C to 1250 °C, the wetting of tungsten by molten copper improved, which resulted in the porosity of the copper matrix being at the minimum, as observed in the contact zone. Due to good wetting and a decrease in the viscosity of copper, rearrangement of the solid phase of the tungsten in the bulk of the composites improved, and the density and hardness of the pseudo-alloy increased. The formation of coarse tungsten grains is caused by the fact that submicron and micron particles are growing in size and merging into agglomerates during the course of liquid-phase sintering, and this happens because of the high surface activity of ultrafine particles. Further research will be devoted to solving the discovered problems.

Deciphering the role of W content, triple junctions, and heat treatment on the corrosion performance of Ni–W alloy coatings used for automotive applications

In this study we focus on understanding the influence of tungsten (W) content, heat treatment and grain size on the corrosion behaviour of Ni–W coatings. Using pulse and pulse reverse electrodeposition techniques, Ni–W coatings with different W contents (3, 7.5, 12, 17, 20 and 25 at.%) were deposited from a standard ammonia-citrate bath. Coatings were heat treated at 700 °C in vacuum for 1 h and then characterized to determine the surface morphology, grain size, and phase transformations. To evaluate the corrosion performance of the coatings, potentiodynamic polarization tests were performed in 0.6 M NaCl solution. Polarization test results indicated that as-deposited Ni–W coatings had better corrosion resistance compared to heat treated coatings. In the as-deposited condition, corrosion resistance increased up to 17 at.% W. However, further addition of W decreased the corrosion resistance due to high volume fraction of triple junctions and intercrystalline regions. On the other hand, heat treated coatings suffered from micro-galvanic corrosion due to the precipitation of Ni4W and NiW phases, and therefore showed higher corrosion rates compared to as-deposited coatings. The changes observed in the corrosion behaviour of as-deposited and heat treated Ni–W coatings were rationalized based on surface composition, mixed potential theory, and triple junction corrosion theory.

Simulation of cathode spots crater formation on electrodes with micro-scale and nano-scale alloy materials in a vacuum arc

In the field of vacuum interrupting, an important research is to improve the erosion resistance properties of contacts. In this paper, a two-dimensional axisymmetric swirl model based on basic parameters has been proposed to describe the formation and development of cathode spots of micro-scale alloy and nano-scale alloy electrode in a vacuum arc. In this model, the peak ion density of the plasma cloud, the mean charge state in near-cathode region, the electron temperature, and the near-cathode voltage drop are adopted as external parameters. Ions and electrons originating from the plasma cloud, thermo-field (T-F) emission, vaporization of metal atoms, backing ions and back-diffused electrons are all taken into consideration. The morphology and temperature of cathode spots crater and the liquid metal velocity of pure copper electrode (Cu), copper-chromium (Cu-Cr) electrode and tungsten-copper (W-Cu) electrode under the conditions of micro-scale grains and nano-scale grains are compared, respectively. Simulation results show that the cathode spot crater of the Cu-Cr electrode is smaller than that of the pure copper electrode. Moreover, it can hold more current and inhibit the formation of new cathode spots nearby. Cathode spots appearing on tungsten grains are smaller in size and have a lower probability of forming liquid metal droplets than Cu-Cr electrodes. Contacts made of nano-scale grain alloys have stronger erosion resistance properties than that of micro-scale alloy contacts and can reduce the maximum temperature of the cathode. The simulation results are in good agreement with other researchers’ results.

Preparing W-Ni3Al alloy at low sintering temperature by adding MoO3: Microstructure, mechanical properties, and salt spray corrosion

Aiming at poor comprehensive properties and high sintering temperature in the reported W-Ni3Al alloys as well as considering its future application in the marine environment, the W-Ni3Al-MoO3 alloys with good mechanical properties and fine tungsten grain were sintered at a low temperature of 1200 °C, and then we comparatively investigated the salt spray corrosion properties of the W-10Ni3Al (0WNM) and W-8.5Ni3Al-1.5MoO3 (1.5WNM) alloys. The MoO3 reacted with Al and released additional heat in sintering process, favoring the melting of flaky Ni3Al, boosting generation of α-Al2O3 phase, and promoting formation of Ni3(Al, W, Mo) solid solution. With increasing MoO3 content, the tungsten grain size, relative density, and bending strength of sintered alloys increased first and then decreased, while their hardness always increased. Among them, the 1.5WNM alloy exhibited optimal mechanical properties with a bending strength of 1173.11 ± 31.6 MPa and a hardness of 75.5 ± 0.83 HRA (593.5 ± 27.06 HV1). In the salt spray corrosion process, due to uniform distribution of the tungsten grain among binder phases, the formation of loose porous corrosion layer, and the dissolution of MoO3 in local basicity environment in 1.5WNM alloy, its anti-salt spray corrosion property was worse than that of 0WNM alloy.

Grain boundary softening from stress assisted helium cavity coalescence in ultrafine-grained tungsten

The formation of helium cavities in coarse-grained materials produces hardening proportional to the number density and size of the cavities and due to the interaction of dislocations with intragranular helium defects. In nanostructured metals containing a high density of interfacial sinks, preferential cavity formation in the grain boundaries instead produces softening that is often attributed to enhanced interfacial plasticity. Employing two grades of ultrafine-grained tungsten, we explore this effect using targeted implantation studies to map cavity evolution as a function of the irradiation conditions and quantify its impact on the mechanical response through nanoindentation. Softening is reported at implantation temperatures above the threshold for preferential grain boundary cavity formation but at a sufficiently low fluence prior to the growth of intragranular cavities. Collective changes in the mean cavity size, density, and morphology beneath a residual impression on an implanted surface indicate that cavity coalescence accompanied the reduction in hardness. Complementary atomistic simulations demonstrate that, in tungsten grain structures exhibiting softening, grain boundary bubble coalescence is driven by stress concentrations that further act to localize strain in the grain boundaries through cooperative deformation processes involving local atomic shuffling and sliding, dislocation emission, and even the nucleation of unstable twinning events.