Stochastic Metallic-Glass Cellular Structures Exhibiting Benchmark Strength

By identifying the key characteristic “structural scales” that dictate the resistance of a porous metallic glass against buckling and fracture, stochastic highly porous metallic-glass structures are designed capable of yielding plastically and inheriting the high plastic yield strength of the amorphous metal. The strengths attainable by the present foams appear to equal or exceed those by highly engineered metal foams such as Ti-6Al-4V or ferrous-metal foams at comparable levels of porosity, placing the present metallic-glass foams among the strongest foams known to date

Compression-compression fatigue of Pd_(43)Ni_(10)Cu_(27)P_(20) metallic glass foam

Compression-compression fatigue testing of metallic-glass foam is performed. A stress-life curve is constructed, which reveals an endurance limit at a fatigue ratio of about 0.1. The origin of fatigue resistance of this foam is identified to be the tendency of intracellular struts to undergo elastic and reversible buckling, while the fatigue process is understood to advance by anelastic strut buckling leading to localized plasticity (shear banding) and ultimate strut fracture. Curves of peak and valley strain versus number of cycles coupled with plots of hysteresis loops and estimates of energy dissipation at various loading cycles confirm the four stages of foam-fatigue

Effect of strain rate on the yielding mechanism of amorphous metal foam

Stochastic amorphous Pd_(43)Ni_(10)Cu_(27)P_(20) foams were tested in quasistatic and dynamic loading. The strength/porosity relations show distinct slopes for the two loading conditions, suggesting a strain-rate-induced change in the foam yielding mechanism. The strength/porosity correlation of the dynamic test data along with microscopy assessments support that dynamic foam yielding is dominated by plasticity rather than elastic buckling, the mechanism previously identified to control quasistatic yielding. The strain-rate-induced shift in the foam yielding mechanism is attributed to the rate of loading approaching the rate of sound wave propagation across intracellular membranes, thereby suppressing elastic buckling and promoting plastic yielding

High porosity metallic glass foam: A powder metallurgy route

A powder metallurgy route to the fabrication of metallic glass foam is introduced. The method involves consolidating metallic glass powder blended with blowing agent particulates to produce expandable precursors, capable of yielding foams with porosities as high as 86%. The foams are found to inherit the strength of the parent metallic glass and to be able to deform heavily toward full densification absorbing high amounts of energy

Accessing thermoplastic processing windows in metallic glasses using rapid capacitive discharge

The ability of the rapid-capacitive discharge approach to access optimal viscosity ranges in metallic glasses for thermoplastic processing is explored. Using high-speed thermal imaging, the heating uniformity and stability against crystallization of Zr_(35)Ti_(30)Cu_7.5Be_(27.5) metallic glass heated deeply into the supercooled region is investigated. The method enables homogeneous volumetric heating of bulk samples throughout the entire supercooled liquid region at high rates (~10^5 K/s) sufficient to bypass crystallization throughout. The crystallization onsets at temperatures in the vicinity of the “crystallization nose” were identified and a Time-Temperature-Transformation diagram is constructed, revealing a “critical heating rate” for the metallic glass of ~1000 K/s. Thermoplastic process windows in the optimal viscosity range of 10^0–10^4 Pa·s are identified, being confined between the glass relaxation and the eutectic crystallization transition. Within this process window, near-net forging of a fine precision metallic glass part is demonstrated

Joining and Assembly of Bulk Metallic Glass Composites Through Capacitive Discharge

Bulk metallic glasses (BMGs), a class of amorphous metals defined as having a thickness greater than 1 mm, are being broadly investigated by NASA for use in spacecraft hardware. Their unique properties, attained from their non-crystalline structure, motivate several game-changing aerospace applications. BMGs have low melting temperatures so they can be cheaply and repeatedly cast into complex net shapes, such as mirrors or electronic casings. They are extremely strong and wear-resistant, which motivates their use in gears and bearings. Amorphous metal coatings are hard, corrosion-resistant, and have high reflectivity. BMG composites, reinforced with soft second phases, can be fabricated into energy-absorbing cellular panels for orbital debris shielding. One limitation of BMG materials is their inability to be welded, bonded, brazed, or fastened in a convenient method to form larger structures. Cellular structures (which can be classified as trusses, foams, honeycombs, egg boxes, etc.) are useful for many NASA, commercial, and military aerospace applications, including low-density paneling and shields. Although conventional cellular structures exhibit high specific strength, their porous structures make them challenging to fabricate. In particular, metal cellular structures are extremely difficult to fabricate due to their high processing temperatures. Aluminum honeycomb sandwich panels, for example, are used widely as spacecraft shields due to their low density and ease of fabrication, but suffer from low strength. A desirable metal cellular structure is one with high strength, combined with low density and simple fabrication. The thermoplastic joining process described here allows for the fabrication of monolithic BMG truss-like structures that are 90% porous and have no heat-affected zone, weld, bond, or braze. This is accomplished by welding the nodes of stacked BMG composite panels using a localized capacitor discharge, forming a single monolithic structure. This removes many complicated and costly fabrication steps. Moreover, the cellular structures detailed in this work are among the highest- strength and most energy-absorbent materials known. This implies that a fabricated structure made from these materials would have unequaled mechanical properties compared to other metal foams or trusses. The process works by taking advantage of the electrical properties of the matrix material in the metal-matrix composite, which in this case is a metallic glass. Due to the random nanoscale arrangement of atoms (without any grain boundaries), the matrix glass exhibits a near-constant electrical resistivity as a function of temperature. By placing the composite panels between two copper electrode plates and discharging a capacitor, the entire matrix of the panel can be heated to approximately 700 C in 10 milliseconds, which is above the alloy s solidus but below the liquidus. By designing the geometry of the panels into the shape of an egg box, the electrical discharge localizes only in the tips of each pyramidal cell. By applying a forging load during discharge, the nodes of the panels can be fused together into a single piece, which then dissipates heat through radiation back into a glassy state. This means that two panels can be metallurgically fused into one panel with no heat-affected zone, creating a seamless connection between panels. During the process, the soft metal particles (dendrites) that are uniformly distributed in the glassy matrix to increase the toughness are completely unaffected by the thermoplastic joining. The novelty is that a truss (or foam-like) structure can be formed with excellent energy- absorbing capabilities without the need for machining. The technique allows for large-scale fabrication of panels, well-suited for spacecraft shields or military vehicle door panels. Crystalline metal cellular structures cannot be fabricated using the thermoplastic joining technique described here. If metal panels were te assembled into a cellular structure, they would either have to be welded, brazed, bonded, or fastened together, creating a weak spot in the structure at each connection. Welded parts require a welding material to be added to the joint and exhibit a soft and weak heat-affected zone. Brazing and bonding do not form a metallurgical joint and thus exhibit low strengths, especially when the panels are pulled apart and fasteners require high-stress-concentration holes to be drilled. No equivalent rapid heating method exists for assembling metal panels together into cellular structures, and thus, those parts must be foamed, machined, or investment cast if they are to form a monolithic structure. If the crystalline panels were to be joined using capacitive discharge, as with a spot welder, their bond would be very weak, and the panels would have to be extremely thin. In contrast, the strength of joined BMG parts has been demonstrated to have strength comparable to the parent material. This technique opens up the possibility of using large-scale BMG hardware in spacecraft, military, or commercial applications

Metallic-glass-matrix composite structures with benchmark mechanical performance

Metallic-glass-matrix composites demonstrating unusual combination of high strength, high toughness, and excellent processability are utilized to fabricate cellular structures of egg-box topology. Under compressive loading, the egg-box panels are capable of undergoing extensive plastic collapse at very high plateau stresses enabling absorption of large amounts of mechanical energy. In terms of specific mechanical energy absorbed, the present panels far outperform panels of similar topology made of aluminum or fiber-reinforced polymer composites, and even surpass steel structures of highly buckling-resistant topologies, thus emerging among the highest performance structures of any kind

Glassy steel optimized for glass-forming ability and toughness

An alloy development strategy coupled with toughness assessments and ultrasonic measurements is implemented to design a series of iron-based glass-forming alloys that demonstrate improved glass-forming ability and toughness. The combination of good glass-forming ability and high toughness demonstrated by the present alloys is uncommon in Fe-based systems, and is attributed to the ability of these compositions to form stable glass configurations associated with low activation barriers for shear flow, which tend to promote plastic flow and give rise to a toughness higher than other known Fe-based bulk-glass-forming systems

Viral suppression and HIV-1 drug resistance 1 year after pragmatic transitioning to dolutegravir first-line therapy in Malawi: a prospective cohort study.

BACKGROUND Many countries are now replacing non-nucleoside reverse transcriptase inhibitor (NNRTI)-based first-line antiretroviral therapy (ART) with a regimen containing tenofovir disoproxil fumarate, lamivudine, and dolutegravir (TLD). Recognising laboratory limitations, Malawi opted to transition those on NNRTI-based first-line ART to TLD without viral load testing. We aimed to assess viral load and HIV drug resistance during 1 year following transition to TLD without previous viral load testing. METHODS In this prospective cohort study, we monitored 1892 adults transitioning from NNRTI-based first-line ART to the TLD regimen in the Médecins Sans Frontières-supported decentralised HIV programme in Chiradzulu District, Malawi. Eligible adults were enrolled between Jan 17 and May 11, 2019, at Ndunde and Milepa health centres, and between March 8 and May 11, 2019, at the Boma clinic. Viral load at the start of the TLD regimen was assessed retrospectively and measured at month 3, 6, and 12, and additionally at month 18 for those ever viraemic (viral load ≥50 copies per mL). Dolutegravir minimal plasma concentrations (Cmin) were determined for individuals with viraemia. Drug-resistance testing was done at the start of TLD regimen and at viral failure (viral load ≥50 copies per mL, followed by viral load ≥500 copies per mL; resistance defined as Stanford score ≥15). FINDINGS Of 1892 participants who transitioned to the TLD regimen, 101 (5·3%) were viraemic at TLD start. 89 of 101 had drug-resistance testing with 31 participants (34·8%) with Lys65Arg mutation, 48 (53·9%) with Met184Val/Ile, and 42 (40·4%) with lamivudine and tenofovir disoproxil fumerate dual resistance. At month 12 (in the per-protocol population), 1725 (97·9% [95% CI 97·1-98·5]) of 1762 had viral loads of less than 50 copies per mL, including 83 (88·3% [80·0-94·0]) of 94 of those who were viraemic at baseline. At month 18, 35 (97·2% [85·5-99·9]) of 36 who were viraemic at TLD start with lamivudine and tenofovir disoproxil fumarate resistance and 27 (81·8% [64·5-93·0]) of 33 of those viraemic at baseline without resistance had viral load suppression. 14 of 1838 with at least two viral load tests upon transitioning had viral failure (all with at least one dolutegravir Cmin value <640 ng/mL; active threshold), suggesting suboptimal adherence. High baseline viral load was associated with viral failure (adjusted odds ratio [aOR] 14·1 [2·3-87·4] for 1000 to <10 000 copies per mL; aOR 64·4 [19·3-215·4] for ≥10 000 copies per mL). Two people with viral failure had dolutegravir resistance at 6 months (Arg263Lys or Gly118Arg mutation), both were viraemic with lamivudine and tenofovir disoproxil fumarate resistance at baseline. INTERPRETATION High viral load suppression 1 year after introduction of the TLD regimen supports the unconditional transition strategy in Malawi. However, high pre-transition viral load, ongoing adherence challenges, and possibly existing nucleoside reverse transcriptase inhibitor resistance can lead to rapid development of dolutegravir resistance in a few individuals. This finding highlights the importance of viral load monitoring and dolutegravir-resistance surveillance after mass transitioning to the TLD regimen. FUNDING Médecins Sans Frontières. TRANSLATIONS For the French and Portuguese translations of the abstract see Supplementary Materials section