This study investigates the mechanical and corrosion properties of as-built and annealed equiatomic CoCrFeMnNi alloy produced by laser-based directed energy deposition (DED) Additive Manufacturing (AM). The high cooling rates of DED produced a single-phase, cellular microstructure with cells on the order of 4 μm in diameter and inter-cellular regions that were enriched in Mn and Ni. Annealing created a chemically homogeneous recrystallized microstructure with a high density of annealing twins. The average yield strength of the as-built condition was 424 MPa and exceeded the annealed condition (232 MPa), however; the strain hardening rate was lower for the as-built material stemming from higher dislocation density associated with DED parts and the fine cell size. In general, the yield strength, ultimate tensile strength, and elongation-to-failure for the as-built material exceeded values from previous studies that explored other AM techniques to produce the CoCrFeMnNi alloy. Ductile fracture occurred for all specimens with dimple initiation associated with nanoscale oxide inclusions. The breakdown potential (onset of pitting corrosion) was similar for the as-built and annealed conditions at 0.40 VAg/AgCl when immersed in 0.6 M NaCl. Pit morphology/propagation for the as-built condition exhibited preferential corrosion of inter-cellular Ni/Mn regions leading to a tortuous pit bottom and cover, while the annealed conditions pits resembled lacy pits similar to 304 L steel. A passive oxide film depleted in Cr cations with substantial incorporation of Mn cations is proposed as the primary mechanism for local corrosion susceptibility of the CoCrFeMnNi alloy.
This research objective of this EELDRD study was to learn to electrodeposit Pt Au alloys with independently controlled composition and grain size. What was accomplished was the capability to electrodeposit PtAu alloys with controlled composition and a nanocrysolline grain size. Nanocrystalline metals as a class and, specifically, the Pt0.9Au0.1 alloy developed in 2015-17 via sputtering at Sandia National Labs have clear advantages in strength, wear resistance, and fatigue tolerance over commercially-available structural alloys. With this capability befitting coating of complex components and implementable at existing vendors, we can upgrade the electrical contact component reliability of selected Labs systems.
Contact probing of gaging surfaces is used throughout dimensional metrology. Probe tips such as ruby, sapphire, or diamond are commonly employed as styli for universal length measuring machines (ULMs) and coordinate measuring machines (CMMs) due to the hardness, durability, and wear resistance. Gaging surfaces of gage blocks are precision ground or lapped, with very low surface roughness to enable wringing. Damage or contamination of these surfaces can prevent wringing and lead to measurement error. Experimental investigations using a horizontal ULM and CMM have revealed that even at low force settings (≤0.16 N), probe materials such as ruby and sapphire can cause plastic deformation to hardened carbon chrome steel (such as AISI 52,100) gage block surfaces at the microscale, likely attributed to fretting-associated wear. Under some conditions, permanent transfer of material from the probe stylus to the gaging surface is possible. Results demonstrate irreversible changes and damage to gaging surfaces with repeated probe contact on a ULM and CMM. Optical microscopy, optical profilometry, and scanning electron microscopy (SEM) provide a semi-quantitative assessment of microscale plastic deformation and material transfer. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and Raman techniques confirm chemical constituency of reference materials used (gage blocks and probes) and also identify makeup of deposits on gaging surfaces following probe contact.
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.
As of 2018, renewable energy sources such as wind and solar have the lowest unsubsidized levelized cost of energy, and grid-scale storage solutions are being aggressively developed and deployed. However, for a carbon-free energy generation paradigm to be realistic, any impediments to scalability must be addressed. In the wind industry, dependence on rare-earth (RE) magnets in direct-drive generators represents a significant roadblock to widespread technology proliferation. Sandia’s Twistact technology is a fundamentally new rotary electrical contact with only rolling metal-to- metal contact that eliminates the need for RE magnets by enabling a wire-wound generator architecture with no efficiency or cost penalties. This report summarizes work funded by an LDRD in FY16—18, in which we advanced the technology readiness level (TRL) of Twistact technology to TRL 5 and proved the viability of a Twistact-based generator for utility-scale, direct-drive wind turbines. We conducted coupon-level tests of rolling metallic contacts and developed a tribological model that predicts operation in either a low-wear or high-wear regime. We also built device-level testing apparatuses and observed operation of full-scale Twistact devices, which corroborated the predictions of the tribological model and demonstrated over 50 million rotation cycles (30-year lifetime in a direct-drive generator). Indeed, the present work demonstrated that Twistact technology has potential to be an enabling technology that eliminates RE magnet dependence in the wind industry. The next logical step is commercialization of Twistact technology (currently at TRL 5) in partnership with a generator original equipment manufacturer that already has an established presence in the wind power industry.
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.