Publications

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Reactive rare-earth/transition metal multilayers exhibit a variety of complex reaction behaviors depending on surrounding gaseous environment and material design. Small period (< 100 nm bilayer), 5 μm-thick Sc/Ag multilayers undergo self-sustained formation reactions when ignited in air or in vacuum. High-speed videography reveals unstable reaction waves in these samples, characterized by the repeated, transverse passage of narrow, spin bands. Intermediate Sc/Ag designs – with multilayer period between 100 and 200 nm – only react in air. These multilayers exhibit propagating reactions with alternating unstable and stable characteristics. Narrow, spin bands advance the reaction front stepwise. Soon after the passage of a transverse band, a trailing oxidation wave encroaches on the intermetallic reaction front temporarily pushing the stalled wave forward in a uniform manner. Viewed in full, these events repeat giving rise to a new oscillatory behavior. Sc/Ag multilayers having a large period (> 200 nm bilayer) also react exclusively in air but exhibit a different propagating mode. The oxidation of Sc combined with the exothermic reaction of metal species results in continually-stable waves characterized by a smooth wavefront morphology and uniform velocity. The flame temperatures associated with propagating waves are estimated using measured heats of reaction and enthalpy-temperature relationships in order to provide insight into the possible phase transformations that occur during these different exothermic reactions.

This report outlines the fiscal year (FY) 2019 status of an ongoing multi-year effort to develop a general, microstructurally-aware, continuum-level model for representing the dynamic response of material with complex microstructures. This work has focused on accurately representing the response of both conventionally wrought processed and additively manufactured (AM) 304L stainless steel (SS) as a test case. Additive manufacturing, or 3D printing, is an emerging technology capable of enabling shortened design and certification cycles for stockpile components through rapid prototyping. However, there is not an understanding of how the complex and unique microstructures of AM materials affect their mechanical response at high strain rates. To achieve our project goal, an upscaling technique was developed to bridge the gap between the microstructural and continuum scales to represent AM microstructures on a Finite Element (FE) mesh. This process involves the simulations of the additive process using the Sandia developed kinetic Monte Carlo (KMC) code SPPARKS. These SPPARKS microstructures are characterized using clustering algorithms from machine learning and used to populate the quadrature points of a FE mesh. Additionally, a spall kinetic model (SKM) was developed to more accurately represent the dynamic failure of AM materials. Validation experiments were performed using both pulsed power machines and projectile launchers. These experiments have provided equation of state (EOS) and flow strength measurements of both wrought and AM 304L SS to above Mbar pressures. In some experiments, multi-point interferometry was used to quantify the variation is observed material response of the AM 304L SS. Analysis of these experiments is ongoing, but preliminary comparisons of our upscaling technique and SKM to experimental data were performed as a validation exercise. Moving forward, this project will advance and further validate our computational framework, using advanced theory and additional high-fidelity experiments. ACKNOWLEDGEMENTS The authors greatly appreciate the support of Mike Saavedra in machining the experimental samples. The authors would also like to thank the Dynamic Integrated Compression facility (DICE) staff for executing the Thor experiments: Brian Stoltzfus, Randy Hickman, Keith Hodge, Joshua Usher, Lena Pacheco, and Eric Breden. The authors would also like to thank the staff at the Shock Thermodynamics Applied Research (STAR) facility for executing the plate impact experiments: Scott Alexander, Bill Reinhart, Bernardo Farfan, Rocky Palomino, John Martinez, and Rafael Sanchez. Lastly, the authors would like to acknowledge the development support of Jason Sanchez in ALEGRA to incorporate our upscaling method and Michael Powell for helping with post processing scripts for results analysis.

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Nanocrystalline metals offer significant improvements in structural performance over conventional alloys. However, their performance is limited by grain boundary instability and limited ductility. Solute segregation has been proposed as a stabilization mechanism, however the solute atoms can embrittle grain boundaries and further degrade the toughness. In the present study, we confirm the embrittling effect of solute segregation in Pt-Au alloys. However, more importantly, we show that inhomogeneous chemical segregation to the grain boundary can lead to a new toughening mechanism termed compositional crack arrest. Energy dissipation is facilitated by the formation of nanocrack networks formed when cracks arrested at regions of the grain boundaries that were starved in the embrittling element. This mechanism, in concert with triple junction crack arrest, provides pathways to optimize both thermal stability and energy dissipation. A combination of in situ tensile deformation experiments and molecular dynamics simulations elucidate both the embrittling and toughening processes that can occur as a function of solute content.

Diamond-like carbon (DLC) films were tribochemically formed from ambient hydrocarbons on the surface of a highly stable nanocrystalline Pt-Au alloy. A sliding contact between an alumina sphere and Pt-Au coated steel exhibited friction coefficients as low as μ = 0.01 after dry sliding in environments containing trace (ppb) organics. Ex situ analysis indicated that the change in friction coefficient was due to the formation of amorphous carbon films, and Raman spectroscopy and elastic recoil analysis showed that these films consist of sp2/sp3 amorphous carbon with as much as 20% hydrogen. Transmission electron microscopy indicated these films had thicknesses exceeding 100 nm, and were enhanced by the incorporation of worn Pt-Au nanoparticles. The result was highly wear-resistant, low-friction DLC/Pt-Au nanocomposites. Atomistic simulations of hydrocarbons under shear between rigid Pt slabs using a reactive force field showed stress-induced changes in bonding through chain scission, a likely route towards the formation of these coatings. This novel demonstration of in situ tribochemical formation of self-lubricating films has significant impact potential in a wide range of engineering applications.

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Recent work suggests that thermally stable nanocrystallinity in metals is achievable in several binary alloys by modifying grain boundary energies via solute segregation. The remarkable thermal stability of these alloys has been demonstrated in recent reports, with many alloys exhibiting negligible grain growth during prolonged exposure to near-melting temperatures. Pt–Au, a proposed stable alloy consisting of two noble metals, is shown to exhibit extraordinary resistance to wear. Ultralow wear rates, less than a monolayer of material removed per sliding pass, are measured for Pt–Au thin films at a maximum Hertz contact stress of up to 1.1 GPa. This is the first instance of an all-metallic material exhibiting a specific wear rate on the order of 10−9 mm3 N−1 m−1, comparable to diamond-like carbon (DLC) and sapphire. Remarkably, the wear rate of sapphire and silicon nitride probes used in wear experiments are either higher or comparable to that of the Pt–Au alloy, despite the substantially higher hardness of the ceramic probe materials. High-resolution microscopy shows negligible surface microstructural evolution in the wear tracks after 100k sliding passes. Mitigation of fatigue-driven delamination enables a transition to wear by atomic attrition, a regime previously limited to highly wear-resistant materials such as DLC.

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Direct laser irradiation of sputter deposited Al/Pt nanolaminate multilayers results in rapid local heating and exothermic mixing of reactant layers. Milli- and microsecond pulsed laser irradiation under certain test conditions leads to single-point ignition of rapid, self-propagating, formation reactions. Multilayers having bilayer thicknesses of 328 nm, 164 nm, and 65 nm are characterized by their ignition onset times and temperatures. Smaller bilayer thickness multilayers require less laser intensity for ignition compared with larger bilayer designs (when utilizing a particular pulse duration). The relationship between laser intensity and ignition onset time is used to calibrate an activation energy for ignition within a finite element reactive heat transport model. The local heating rate is varied from 104 K/s to 106 K/s by selecting a laser intensity. Kissinger analysis was performed on the heating rate-dependent ignition temperatures measured with high speed pyrometry to experimentally determine an activation energy in the foils of (6.2 ± 1.6 × 104 J/mole atoms). This value is then compared to an activation energy produced from model fits to an ignition onset time of 7.2 × 104 J/mole atoms.

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