Microstructure dependent burst behavior of oxide dispersion–strengthened FeCrAl cladding
Not Available
36 MATERIALS SCIENCE↗
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Electro-strengthening of the additively manufactured Ti–6Al–4V alloy
Structure-property-processing relationship has been studied in additively manufactured Ti–6Al–4V alloy. The processing was performed using in-situ electron microscope (EM) at a moderate current density of 5 × 105 A/cm2 applied for 5 min, and by suppressing Joule heating with massive heat sinks such that the temperature rise was <180 °C and the mechanical properties were not compromised. Overall, the results show that while the grain size increased by ~15%, the nanohardness increased by 16%. This is attributed to the pronounced dislocation generation, regeneration, and clustering as well as defect healing. Ultimately, there is a reduction in the residual strain and a significant increase in the intrinsic strength as evidenced by the high Taylor factor of the electric current processed specimen. This novel processing technique represents an alternative pathway for active controlling of microstructure and internal defects for parts that might be sensitive to high-temperature processing or conventional methods.
36 MATERIALS SCIENCE↗
Dynamic properties of FeCrMnNi, a high entropy alloy
The goal of this paper was to assess the dynamic properties of a new class of materials, High entropy alloys (HEAs). Specifically, gas gun experiments coupled with recovery were performed on a specific HEA, FeCrMnNi, to measure its equation of state (EOS) and its spall strength, which is a measure of the stress required to nucleate voids under shock loading conditions. While there has been a plethora of work performed to assess the mechanical properties of these HEAs under uniaxial stress conditions as a function of strain rate, investigation of its properties in dynamic extremes remains rare. The current work fills this gap in knowledge for this novel class of materials. Our results show that the measured EOS for this material at one velocity was in reasonable agreement with an estimated Hugoniot. Furthermore, while the spall strength for this material was found to be ~1.9 GPa, with some variation based on sample location, the main failure mode was brittle. This brittle failure leading to formation of cracks in the material is different than the ductile failure observed in pure iron and its alloys.
36 MATERIALS SCIENCE↗
Microstructural control of additively manufactured Ti6Al4V via in-situ large-area laser annealing
Not Available
36 MATERIALS SCIENCE↗
Steady detonation propagation in thin channels with strong confinement
We examine asymptotically the dynamics of two-dimensional, steady detonation wave propagation and failure for a strongly confined high explosive (HE), in which the width of the explosive is small relative to the reaction zone length. An energy balance equation is derived, which shows how the longitudinal acceleration of subsonic flow behind the detonation shock is influenced both by chemical reaction and by the effects of HE boundary streamline deflection, specifically via the induced rate of change of mass flux through the detonation wave. The latter serves to either counteract or reinforce the acceleration of longitudinal flow, depending on the sign of the gradient of the boundary streamline deflection at the detonation shock. The analysis is valid for general equations of state and chemical reaction rates in the HE. The asymptotically derived form of the energy equation represents an eigenvalue problem for the determination of the steady detonation propagation speed, solved via a shooting method. We explore specific results for ideal and stiffened equations of state, along with a pressure-dependent reaction rate for which changes in the pressure exponent and reaction order are also studied. We consider the influences of both straight and curved HE boundary streamline shapes. The asymptotic analysis reveals significant physical insights into how detonation propagation and failure are affected by strong confinement.
42 ENGINEERING↗
Ex Situ Photoelectron Emission Microscopy of Polycrystalline Bismuth and Antimony Telluride Surfaces Exposed to Ambient Oxidation
The surfaces of textured polycrystalline N-type bismuth telluride and P-type antimony telluride materials were investigated using ex situ photoelectron emission microscopy (PEEM). PEEM enabled imaging of the work function for different oxidation times due to exposure to air across sample surfaces. The spatially averaged work function was also tracked as a function of air exposure time. N-type bismuth telluride showed an increase in the work function around grain boundaries relative to grain interiors during the early stages of air exposure-driven oxidation. At longer time exposure to air, the surface became homogenous after a ~5 nm-thick oxide formed. X-ray photoemission spectroscopy was used to correlate changes in PEEM imaging in real space and work function evolution to the progressive growth of an oxide layer. The observed work function contrast is consistent with the pinning of electronic surface states due to the defects at a grain boundary.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
On the elastic anisotropy of the entropy-stabilized oxide (Mg, Co, Ni, Cu, Zn)O compound
Herein, we study the elastic properties of the entropy-stabilized oxide (Mg, Co, Ni, Cu, Zn)O using experimental and first principles techniques. Our measurements of the indentation modulus on grains with a wide range of crystallographic orientations of the entropy-stabilized oxide revealed a high degree of elastic isotropy at ambient conditions. First principles calculations predict mild elastic anisotropy for the paramagnetic structure, which decreases when the system is considered to be non-magnetic. When the antiferromagnetic state of CoO, CuO, and NiO is accounted for in the calculations, a slight increase in elastic anisotropy is observed, suggesting a coupling between magnetic ordering and the orientation dependent elastic properties. Furthermore, an examination of the local structure reveals that the isotropy is favored through local ionic distortions of Cu and Zn—due to their tendencies to form tenorite and wurtzite phases. The relationships between the elastic properties of the multicomponent oxide and those of its constituent binary oxides are reviewed. These insights open up new avenues for controlling isotropy for technological applications through tuning composition and structure in the entropy-stabilized oxide or the high-entropy compounds in general.
Entropy↗
Crystallographic analysis of nano-tendril bundle vs fuzz growth on tungsten exposed to helicon wave-coupled helium plasma
The propensity for nano-tendril bundle (NTB) vs widespread nano-tendril growth (i.e., fuzz) on polycrystalline W under varying ion energy modulation conditions, from DC to peak-to-peak energy modulation of 42 eV at 13.56 MHz, is correlated with the crystal orientation of the underlying grains. Grains that are vicinal to crystal orientations with high surface diffusivity (e.g., {101} for a body centered cubic crystal structure) exhibit NTB growth at lower ion energy modulation amplitude than grains that are vicinal to low surface diffusivity orientations, such as {100}. Adatom mobility considerations are presented to describe the experimental observations. These results support that surface diffusion or W adatom mobility enhanced by ion bombardment plays a key role in the surface morphology evolution of W under He irradiation.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Oblique angle deposition of boron carbide films by magnetron sputtering
Many applications of boron carbide (B4C) films entail deposition on non-planar substrates, necessitating a better understanding of oblique angle deposition phenomena. Here, we systematically study the effect of substrate tilt on properties of B 4 C films with thicknesses up to 10 μm deposited by direct current magnetron sputtering. Results show that all films are amorphous and columnar with an average column width of ~100 nm, independent of substrate tilt. Column tilt angles are limited to ~20° even for substrate tilt of 80°. Film density, residual stress, and the refractive index weakly (within ≲20%) depend on substrate tilt. Furthermore, oxygen impurities bond preferentially with carbon atoms in inter-columnar regions. Substrate tilt has a major effect on mechanical properties that decrease by ~50%, suggesting weak interconnection between nano-columns. Implications of these observations for the deposition onto non-planar substrates are discussed.
36 MATERIALS SCIENCE↗
Effect of grain size on damage and failure in two-phase materials: Homogenized CuPb
It is well known that spall failure strongly depends on the microstructure of a material. There have been numerous studies to study the effect of grain size on the overall spall strength and the total amount of damage in single element metals like copper. However, such systematic studies remain rare in two-phase materials and alloys. In this work, two incipient spall experiments were performed on a Cu–1%Pb alloy to understand the effect of grain size on the damage and failure in a two-phase material. Overall, these results showed that even though the spall strength did not change as a function of grain size, there were significant differences in the total amount of damage as a function of grain size. A clear increase in the total damage present in the material was seen as the grain size was increased from 32 to either 70 or 75 μm in either of the experiments. Furthermore, this difference was attributed to variations in the void growth rate as the grain size was increased.
36 MATERIALS SCIENCE↗
The influence of pearlite fraction on the shock properties of ferrite–pearlite steel microstructures: Insight into the effect of second-phase particles
Here, the goal of this work is to investigate the effect of varying phase fractions on the overall spall strength and damage behavior of a material. Specifically, two plain carbon, ferrite–pearlite steels (1045 and A283) were subjected to spall recovery experiments to investigate the effect of pearlite fraction on spall strength and total damage. The A283 (20% pearlite) alloy exhibited a higher Hugoniot elastic limit and spall strength compared with 1045 (60% pearlite). Discontinuous and continuous yielding behaviors were observed at quasi-static and dynamic rates for A283 and 1045, respectively. The yielding behavior was connected to pearlite fraction and the prevalence of dislocation-emitting, ferrite/cementite interfaces. Postmortem characterization revealed cementite lamellae cracking within pearlite of 1045, suggesting that pearlite reduces spall strength by providing low-energy damage nucleation sites. The rate of damage growth and coalescence was similar between the two alloys; however, 1045 exhibited more continuous cracks than A283, which exhibited a greater prevalence of discrete voids.
36 MATERIALS SCIENCE↗
High-yield magnetic recoil neutron spectrometer on the National Ignition Facility for operation up to 60 MJ
We report that recent progress at the National Ignition Facility (NIF), with neutron yields of order 1 x 10 17 , places new constraints on diagnostics used to characterize implosion performance. The Magnetic Recoil neutron Spectrometer (MRS), which is routinely used to measure yield, ion temperature (T ion ), and down-scatter ratio (dsr), has been adapted to allow measurements of dsr up to 5 x 10 17 , and yield and T ion up to 2 x 10 18 in the near term with new data processing techniques and conversion foil solutions. This paper presents a solution for extending MRS operation up to a yield of 2 x 10 19 (60 MJ) by moving the spectrometer outside of the NIF shield wall. This will not only enhance the upper yield limit by 10x but also improve signal-to-background by 5x.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Constrained model calibration of grain structure dependent spall dynamics in shock-loaded tantalum
Here, we perform a gas gun experiment by shock loading tantalum samples of varying grain structures to assess the suitability of a numerical model for simulating spall behavior. The observed differences in spall strength, as well spallation and re-compression history, are not captured in uncalibrated hydrodynamic simulations. An optimization is performed on the Johnson spall model to determine the best parameters that fit the observed trends. Linear stability analysis is employed to motivate bounds on those parameters. Herein, optimized simulations agree well with the experimental results, reproducing pullback depth and recompression timescales across the different samples tested. Further, the observed pullback time of the single crystal sample was found to imply, via the stability analysis, a percolation threshold in good agreement with the theoretical value for a body centered cubic lattice. Therefore, the combined linear stability and percolation analysis shows promise and may be applied to other materials with diverse microstructures. Collectively, the findings demonstrate that the model is suitable for reproducing spall-induced free surface behavior across various microstructures, but also points to caution in using model coefficients for uncalibrated microstructures.
58 GEOSCIENCES↗
An assessment of spall failure modes in laser powder bed fusion fabricated stainless steel 316L with low-volume intentional porosity
Here, this paper reports on spall failure and damage modes in Laser Powder Bed Fusion fabricated Stainless Steel 316L (SS316L) with intentional levels of low-volume (1–5 vol. %) porosity and pore sizes of 200, 350, and 500 μm. The fabricated specimens were subjected to uniaxial-strain plate-impact loading at ~4.5 GPa, to initiate incipient spall failure. Analysis of velocimetry profiles measured using multi-probe photon-Doppler velocimetry coupled with post-mortem analysis of soft-recovered samples reveals local suppression of spall failure (termed as spall-dominated) as a function of porosity, as the failure mechanism transitions from spall-centered tensile stress dominated to a pore-centered microstructure-dominated damage mode involving void/crack nucleation and growth at pre-existing pores. The critical porosity level where the suppression of spall failure is first observed, as well as the spall location, is dependent on both the volume fraction and the size of the initially fabricated pores. In samples of 500 μm pore size, the suppression of spall failure is observed with as little as 1 vol. % porosity, while samples with smaller pores (200 μm) still experience spall-centered tensile stress dominated failure with higher levels (5 vol. %) of porosity. In the case of pore-centered microstructure-dominated failure, spall damage can occur but the spall plane is shifted toward the rear free surface, or more generally in areas further away from the region with pores. Highly heterogeneous deformation twinning, shear banding, grain rotation, and cracking are observed in the vicinity of pre-existing pores and expected spall failure sites.
36 MATERIALS SCIENCE↗
Spall strength of additively repaired 304L stainless steel
Additive manufacturing has the potential to repair damaged parts, but the performance of additive materials under high strain rate loading is still uncertain—especially with the added complexity of an interface with an existing wrought material. In this work, 304L stainless steel samples were intentionally damaged and then repaired with wire-fed laser additive manufacturing. The samples were subjected to shock loading to generate incipient spall. Velocimetry and post-mortem metallography results show that when the additive repair process parameters are optimized to reduce porosity and match the equation of state of the original material, the influence of the repair region on the shock propagation is negligible. In conclusion, the free-surface velocity profile and internal damage morphology of the repaired sample are shown to be practically identical to the pristine material.
36 MATERIALS SCIENCE↗
Dynamic and quasi-static strength of additively repaired aluminum
Additive manufacturing has the potential to repair high value components, saving significant time and resources; however, the level of reliability and performance of additive repairs is still relatively unknown. In this work, the structure–property and performance of laser wire additive manufacturing repairs in 1100 aluminum are investigated. Two types of intentional damage are inflicted on the samples and subsequently repaired with pulsed laser deposition additive manufacturing. Quasi-static (10 –3 s –1 ) and high strain-rate (10 –3 s –1 ) mechanical testing is carried out with in situ diagnostics and post-mortem imaging. The results show that while the quasi-static strength and ductility of samples with a repaired region are lower than a pristine sample, the dynamic strength under shock loading is comparable. This work highlights both the potential utility of additive manufacturing for repair purposes, the significant risk of compromised performance of additive parts under specific conditions, and the need to test at varying strain rates to fully characterize material performance.
36 MATERIALS SCIENCE↗
Dynamic compression and spallation behavior of NbTaTiVZr high-entropy alloy
High-entropy alloys are a new class of materials with promising properties for aerospace and defense applications. The unique behavior of these materials is driven by the complex interactions between dissimilar atoms in a crystal and requires combined theoretical and experimental efforts to unlock their full potential. Here, we evaluate the relationship between the microstructure and the dynamic response of the equiatomic NbTaTiVZr alloy. Specifically, the shock Hugoniot, or equation of state, was measured up to a particle velocity of 0.5 mm μs −1 using gas gun plate impacts. Shock wave profile and incipient spallation experiments were used to characterize wave propagation and damage formation with post-mortem recovery experiments. Molecular dynamics simulations confirm the experimental findings and extend them up to 2.1 mm μs −1 particle velocity. Computational thermodynamic calculation of phase diagram simulations explain details of the material microstructure, which explains the measured strength and experimental damage patterns. Overall, this work provides detailed high-strain-rate characterization of a refractory high-entropy alloy, and more importantly, a framework and demonstration of the utility and necessity of a combined theoretical and experimental approach, outlining the importance of considering processing and manufacturing conditions when evaluating the performance of new materials.
36 MATERIALS SCIENCE↗
Understanding creep behaviors of additively manufactured 316L stainless steel via void characterization
Not Available
36 MATERIALS SCIENCE↗