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Results for “boron segregation”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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10 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Effect of Solute Elements on Boron Segregation in Boron-Containing Steels

The addition of boron to steel alloys results in an increase in both hardenability and casting defects. The casting difficulties are predicted to stem from a metatectic reaction, δ + γ → L + γ, where a fully solidified material begins to locally remelt as the temperature decreases. Another possible source of casting defects is a boride-rich phase that is predicted to remain liquid at low temperatures. To experimentally determine which reaction is the likely source of the casting defects, the predicted reactions and the effect of solute elements on those reactions are investigated. Levitation zone melting is used to control segregation in a ternary Fe–C–B alloy and a commercial 22MnB5 alloy. Carbon segregation and a peritectic reaction result in a peritectic jump during directional solidification where the first directionally solidified (DS) zone undergoes δ-bcc solidification followed by a peritectic jump to steady state planar solidification of γ-fcc in the second DS zone. The presence of other solute elements in the zone melted 22MnB5 alloy lead to a breakdown in the planar solidification front before steady state solidification could be achieved in the second DS zone. With a cellular solid/liquid interface, boron-rich intercellular liquid formed low melting iron boro-carbide particles. The controlled solidification conditions in a levitation zone melter were unable to prevent ~0.003 wt% boron from segregating to high enough levels to form boride particles. Therefore, it is likely that during commercial casting, the formation of the low melting boride phase from interdendritic segregation is a key source of the casting issues.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Minor Elements and Solidification Cracking During Laser Powder-Bed Fusion of a High $\gamma ^{\prime }$ CoNi-Base Superalloy

Here, the cracking behavior of a high γ' volume fraction CoNi-base superalloy fabricated via laser powder bed fusion (LPBF) is studied in relation to the content of carbon and boron. Severe cracking occurred with the increase in boron content from 0.08 to 0.16 at. pct (0.015 to 0.029 wt pct), while compositions with 0.1 to 0.36 at. pct C (0.02 to 0.076 wt pct) and 0.08 at. pct B exhibited minimal cracking. Assessment of cracks in the high-boron composition shows a variation in crack density with printing parameters, and alignment of the cracks with the build direction. Scanning electron microscopy (SEM) of the crack surfaces shows evidence of a solidification cracking mode. Differential thermal analysis (DTA) reveals a decreased incipient melting temperature for the high-boron composition, and atom probe tomography (APT) is used to measure the enrichment at grain boundaries, revealing distinct boron segregation. Scheil-Gulliver solidification simulations for the different C and B levels are consistent with the incipient melting behavior observed with DTA. Evaluation of the solidification cracking susceptibility from the simulations allow for comparison of the CoNi alloy behavior to Ni-base superalloys studied for LPBF fabrication and displays how such metrics may aid in the design of new precipitation-strengthened superalloys for additive manufacturing (AM).

36 MATERIALS SCIENCE↗
DOE OSTI2024

Grain boundary engineering for efficient and durable electrocatalysis

Grain boundaries in noble metal catalysts have been identified as critical sites for enhancing catalytic activity in electrochemical reactions such as the oxygen reduction reaction. However, conventional methods to modify grain boundary density often alter particle size, shape, and morphology, obscuring the specific role of grain boundaries in catalytic performance. This study addresses these challenges by employing gold nanoparticle assemblies to control grain boundary density through the manipulation of nanoparticle collision frequency during synthesis. We demonstrate a direct correlation between increased grain boundary density and enhanced two-electron oxygen reduction reaction activity, achieving a significant improvement in both specific and mass activity. Additionally, the gold nanoparticle assemblies with high grain boundary density exhibit remarkable electrochemical stability, attributed to boron segregation at the grain boundaries, which prevents structural degradation. This work provides a promising strategy for optimizing the activity, selectivity, and stability of noble metal catalysts through precise grain boundary engineering.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Applications of analytical electron microscopy to guide the design of boron carbide

Compositional analysis of boron carbide on nanometer length scales to examine or interpret atomic mechanisms, for example, solid-state amorphization or grain-boundary segregation, is challenging. This work reviews advancements in high-resolution microanalysis to characterize multiple generations of boron carbide. First, ζ-factor microanalysis will be introduced as a powerful (scanning) transmission electron microscopy ((S)TEM) analytical framework to accurately characterize boron carbide. Three case studies involving the application of ζ-factor microanalysis will then be presented: (1) accurate stoichiometry determination of B-doped boron carbide using ζ-factor microanalysis and electron energy loss spectroscopy, (2) normalized quantification of silicon grain-boundary segregation in Si-doped boron carbide, and (3) calibration of a scanning electron microscope X-ray energy-dispersive spectroscopy (XEDS) system to measure compositional homogeneity differences of B/Si-doped arc-melted boron carbides in the as-melted and annealed conditions. Overall, the improvement and application of advanced analytical tools have helped better understand processing–microstructure–property relationships and successfully manufacture high-performance ceramics.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Interstitials in f.c.c. High Entropy Alloys

The effects of interstitials on the mechanical properties of single-phase f.c.c. high entropy alloys (HEAs) have been assessed based on a review of the literature. It is found that in nearly all studies, carbon increases the yield strength, in some cases by more than in traditional alloys. This suggests that carbon can be an excellent way to strengthen HEAs. This strength increase is related to the lattice expansion from the carbon. The effects on other mechanical behavior is mixed. Most studies show a slight reduction in ductility due to carbon, but a few show increases in ductility accompanying the yield strength increase. Similarly, some studies show little or modest increases in work-hardening rate (WHR) due to carbon, whereas a few show a substantial increase. These latter effects are due to changes in deformation mode. For both undoped and carbon doped CoCrFeMnNi, the room temperature ductility decreases slightly with decreasing grain size until ~2–5 µm, below which the ductility appears to decrease rapidly. The room temperature WHR also appears to decrease with decreasing grain size in both undoped and carbon-doped CoCrFeMnNi and in nitrogen-doped medium entropy alloy NiCoCr, and, at least for the undoped HEA, shows a sharp decrease at grain sizes <2 µm. Interestingly, carbon has been shown to almost double the Hall–Petch strengthening in CoCrFeMnNi, suggesting the segregation of carbon to the grain boundaries. There have been few studies on the effects of other interstitials such as boron, nitrogen and hydrogen. It is clear that more research is needed on interstitials both to understand their effects on mechanical properties and to optimize their use.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Multi-microscopy characterization of threading dislocations in CVD-grown diamond films

We present site-coincident imaging of a high dislocation density boron-doped chemical vapor deposition-grown homoepitaxial (001) diamond film using electron channeling contrast imaging (ECCI), cathodoluminescence, transmission electron microscopy (TEM), and scanning secondary ion mass spectroscopy (SIMS). With growing interest in large area heteroepitaxial substrates, we show that ECCI is a promising technique to accurately evaluate dislocations in diamond films. We find the electron backscatter yield is sufficient to distinguish individual threading dislocations in ECCI despite the low atomic number of carbon, and we have generally good agreement between dislocations observed in ECCI and TEM of the same region. Importantly, relying on luminescence in the 430 nm dislocation-related A-band alone results in an underestimation of the defect density as we find only 20%–40% of the threading dislocations luminesce. Here, we further show that dislocations do not perturb the spatial uniformity of boron doping measured by SIMS, even when the dislocations are clustered with high density, and we can tentatively rule out strong segregation effects at the dislocations. Our results establish the complementary use of microscopy and microanalysis methods to rapidly characterize and understand the impact of dislocations in diamond thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Understanding the influence of boron in additively manufactured GammaPrint®-700 CoNi-based superalloy

Boron is commonly added to superalloys in small amounts to enhance creep resistance, but can lead to cracking at high concentrations, especially during the additive manufacturing process. Two variants of CoNi-based GammaPrint®-700 superalloy with different B contents (0.08 at% vs 0.16 at%) were printed via laser powder bed fusion (LPBF) with the same printing parameters, with only the high B alloy exhibiting solidification cracking. Atom probe tomography (APT) revealed stronger segregation behaviors in the high B alloy compared to the low B alloy at both the inter-dendritic regions and grain boundaries (GBs). The segregation behavior at inter-dendritic regions was well captured with Scheil simulation and can correlate with the existing cracking susceptibility index (CSI) on cracking tendencies, although high angle GBs are where cracking occurs according to electron backscatter diffraction (EBSD) measurements. Additionally, the extent of GB segregation was compared between the high B and low B alloy. Higher B additions led to significantly more GB B segregation in the high B alloy compared to the low B alloy. Further, for the high B alloy, the cracked region of one GB exhibited higher levels of B compared to the uncracked region of the same GB. However, much higher B contents were also found in two other uncracked GBs in the high B alloy, which demonstrates that higher GB B concentrations are not fully responsible for the cracking. A much larger variance in GB B segregation content was found in the high B alloy compared to the low B alloy. These phenomena were explained with a solidification model with the GB segregation content expressed explicitly by a modified Langmuir-McLean equation. This model linked the GB segregation content with solidification undercooling, which can be used as quantitative cracking criteria for future builds.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Non-Equilibrium Doping of Poly-Si:Ga/SiOx Passivating Contacts Solar Cells

In this contribution, we show that Ga-hyperdoped poly-Si/SiOx contacts are realized by pulsed laser annealing and post-hydrogenation. The motivation behind this work is to improve the inferior performance of boron-doped poly- Si/SiOx contacts when compared to the phosphorus-doped contacts. Here we used Ga as a novel p-type dopant to replace B, as B is known to accumulate at the interface between the oxide and the crystalline silicon and cause recombination. Due to a high diffusivity of Ga in oxide and a much higher segregation coefficient (Si:SiOx), Ga is shown to be immune to the oxide pileup phenomenon. However, Ga has a low solid solubility limit in Si (5x10^19 cm-3), which is not enough to form a low resistivity contact. To overcome this problem, we explored pulsed laser annealing via excimer laser by varying the laser energy densities and pulses. The passivation and electrical properties are studied systematically using photoluminescence (PL), secondary ion mass spectrometry (SIMS), and Van der Pauw-Hall measurements. We show an active Ga doping concentration above 1x10^20 cm^-3, which is above the solid solubility limit of Ga in Si. Good passivation quality is demonstrated with the highest implied Voc (iVoc) of 721 mV on single side polished samples. Furthermore, a low diode contact resistivity of ~22 m..omega.. cm^2 is obtained using Ti/Ag contacts, with minimal metal-to-semiconductor contact resistivity of 0.9 m..omega.. cm^2.

non-equilibrium doping↗
DOE OSTI2021

The detrimental effect of elemental contaminants when using B additions to improve the creep properties of a Ni-based superalloy

The effect of Si contamination when using B to improve the creep properties of a Ni-based superalloy was investigated using advanced characterization techniques and first-principles simulations on alloys with high and low B levels with varying Si contents. The positive effect of B segregation along grain boundaries on the creep properties was mitigated by the presence of Si which showed a similar segregation preference. Density functional theory calculations were used for validation by calculating grain boundary cleavage energies. Silicon was shown to decrease grain boundary cohesion which offsets the positive effect of B.

36 MATERIALS SCIENCE↗