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At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Defects and 3D structural inhomogeneity in electron beam additively manufactured Inconel 718

Highlights: • A protocol for 3D characterization of structure and defects in additively manufactured EBM 718 is presented. • Lack of fusion defects disrupt local microstructure, creating isolated regions of equiaxed grains. • Clustering of micron-scale nitrides is attributed to a particle rafting process. • Normalized size-number density relationship of carbides matches that of industrial ingots. • Removal of porosity via post-processing will not correct microstructural inhomogeneities created by lack of fusion defects. - Abstract: Material structure and defects created during additive manufacturing contribute to the variability in mechanical properties of these parts, limiting their use for critical applications. Three-dimensional crystallographic, structural, and chemical information in electron beam melted Inconel 718 was collected from the sub-micron to the millimeter scale using the TriBeam tomography system. The relationship of defects to the surrounding microstructure has been studied in detail. Lack of fusion defects created sharp discontinuities in the grain morphology that extended several build layers beyond the layer where the defect originated. Nitrides and carbides in the size range of 1.25 μm to 10 μm were observed throughout the volume, with pronounced nitride clustering. The similarities between solidification in additive manufacturing and traditional manufacturing processes are demonstrated by the comparable size and distribution of carbides measured for industrial-scale samples of 718. Defects created during additive manufacturing cannot be easily remedied via standard post-processing techniques, and will likely diminish the mechanical performance of additively manufactured parts.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of Sr, Ce and P on the microstructure and mechanical properties of rapidly solidified Al7Si alloys

Highlights: • Rapid solidification caused formation of nanoscaled Si particle chains. • Sr or Ce induced the columnar transition zone only at extremely high cooling rate. • Enhancement of hardness is attributed to fine dispersion of eutectic Si, grain refinement and the supersaturation of Si. • Sr or Ce caused further enhancement of hardness, attributing to Si modification. - Abstract: A series of Al7Si based alloys with trace addition of Sr, Ce or P were prepared using melt-spinning at two different high cooling rates, and the microstructure as well as the hardness of the ribbons was comprehensively studied. The results show that Sr or Ce induces the formation of columnar transition zone and modifies the morphology of eutectic Si phase from dispersed particle chains to fine fibers only at extremely high cooling rate. However, P has little effect on the microstructure of either α-Al or eutectic Si. Compared with conventional casting, the Al7Si ribbons exhibit extremely high hardness due to the fine dispersion of eutectic Si, the grain refinement and the supersaturation of Si. The hardness is further enhanced by Sr or Ce addition at extremely high cooling rate, especially for columnar and equiaxed zones, attributing to the modification effect on Si phase.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of boron addition on the morphology of silicon phases in Al-Si casting alloys

Highlights: • The influence of boron on Si phase was studied. • Boron addition reduces the length of eutectic Si plates in Al-11Si alloys. • Boron addition promotes the nucleation temperature of primary Si phases in Al-14Si alloys. • Boron addition increases the twin density in the <112>{sub Si} growth direction of eutectic Si in Al-11Si and Al-14Si alloys. - Abstract: The influence of B elements on the morphology of Si phases in Al-11Si and Al-14Si alloys has been investigated employing Optical Microscope, Scanning Electron Microscopy, Transmission Electron Microscopy and Differential Scanning Calorimetry. In Al-11Si alloy, the addition of B element not only refines the primary Al dendrites, but also reduces the length of eutectic Si plates. In Al-14Si alloys, the addition of B element promotes the nucleation temperature of primary Si phases, but shows weak influence on the nucleation temperature of eutectics. The effects of B element on the morphologies of both primary Si particle and eutectics can be attributed to the interaction between B and Si elements during the solidification process.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of Nb and Ta additions on the strength and coarsening resistance of precipitation-strengthened Al-Zr-Sc-Er-Si alloys

Highlights: • Nb and Ta micro-additions are made to a dilute Al-Er-Sc-Zr-Si alloys. • Zr-enriched primary L1{sub 2} precipitates form on solidification. • Secondary L1{sub 2} precipitates form on aging, and show improved coarsening resistance. • Nb and Ta are enriched at the secondary nanoprecipitate/matrix interface. • Micro-additions of Nb and Ta have similar effects as V, previously studied. - Abstract: A dilute Al-0.07Zr-0.02Sc-0.005Er-0.06Si (at.%) alloy was microalloyed with 0.08 at.% Nb or Ta. Atom-probe tomography reveals that, upon aging, Nb and Ta partition to the coherent L1{sub 2}-Al{sub 3}(Zr,Sc,Er) nanoprecipitates (with average concentrations of 0.2 and 0.08 at.%, respectively), with both segregating at the matrix/nanoprecipitate heterophase interface. This is consistent with the Nb- and Ta-modified alloys exhibiting, as compared to the unmodified alloy: (i) higher peak microhardness, from a higher nanoprecipitate volume fraction and/or lattice parameter mismatch; and (ii) improved aging resistance, from slower nanoprecipitate coarsening due to the small diffusivities of niobium and tantalum in aluminum. Analogous results were previously reported for a V-modified alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2018

A comparison of Ti-6Al-4V in-situ alloying in Selective Laser Melting using simply-mixed and satellited powder blend feedstocks

Highlights: • Satelliting mixing is proposed for preparing powder feedstocks for AM. • Satelliting allows the creation of blended feedstocks of improved homogeneity. • Sample homogeneity is affected by particle size distribution and powder mixing technique. - Abstract: In-situ alloying within laser powder-bed fusion, specifically Selective Laser Melting (SLM), has been investigated for the formulation of novel alloys from elemental powders to extend the benefits offered by these technologies. Inadequate preparation of the powder feedstock prior to in-situ alloying can yield inhomogeneous microstructures, often deteriorating the mechanical performance of the deposited parts. The present work was designed to assess the use of the ‘satelliting’ method to create powder feedstocks for in-situ laser powder-bed fusion. The research was carried out on Ti-6Al-4V. Ti-6Al-4V feedstocks obtained by mixing or satelliting elemental powders were investigated and compared to a reference pre-alloyed material. The processability of the different feedstocks was assessed by comparing the microstructure of the deposits originating from the different powder blends. Results show that the powder percolation and particle size distribution of the feedstocks translate in deposited microstructures exhibiting different porosity, elemental segregation, and average grain size, revealing the impact of the powder blend characteristics on the laser energy absorbance and solidification of the alloy. This study provides fundamental insights of how to formulate powder feedstock and aims to support future research activities on the design and development of new alloys for use in powder-based additive manufacturing.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Analyzing the microstructural evolution and hardening response of an Al-Si-Mg casting alloy with Cr addition

Highlights: • The formation of β-Al{sub 5}FeSi intermetallics was suppressed since impurity Fe has been consumed by Al{sub 13}Cr{sub 4}Si{sub 4} intermetallics; • A large number of Al{sub 13}Cr{sub 4}Si{sub 4} dispersoids were observed in Al dendrites in the as-quenched and aged alloy; • The precipitation of Cr-bearing dispersoids harden the as-quenched alloy by over 10HV; • The current thermodynamic database underestimate the solubility of Fe in Al{sub 13}Cr{sub 4}Si{sub 4} intermetallics during solidification simulation. - Abstract: Chromium is added to Al-Si based casting alloys for the mitigation of die soldering. In the present work, EBSD, EDS and TEM characterizations are combined with thermodynamic simulations to study the microstructural evolution and hardening response of an Al-Si-Mg-Cr casting alloy during heat treatments. It is shown that the Fe-containing intermetallic compounds formed in the inter-dendritic region were acicular π-Al{sub 8}(Fe, Cr)Mg{sub 3}Si{sub 6} phase and polyhedral Al-(Cr, Fe)-Si phase which is structurally similar to face-centered cubic Al{sub 13}Cr{sub 4}Si{sub 4}. The polyhedral Al-(Cr, Fe)-Si intermetallics that crystallized prior to β-Al{sub 5}FeSi consumed impurity Fe, suppressing the formation of β phase during gravity die casting process. Also, Cr-addition enables the precipitation of nano-scale Cr-bearing dispersoids in Al dendrite during solution treatment at 813 K. Hardness increment of over 10 HV can be induced by the Cr-bearing dispersoids for as-quenched alloy. Though the dispersoids have a moderate hardening effect on the as-quenched alloy, they may impair the ageing hardening response by acting as heterogeneous nucleation sites for coarse Mg-Si rods and Si particles when quenching into water at 338 K.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Variation improvement of mechanical properties of Mg-9Al-1Zn alloy with melt conditioned high pressure die casting

Highlights: • The variation improvement of mechanical properties of AZ91D was achieved via application of high shear melt conditioning. • The elongation variation of AZ91D with MC-HPDC has a significant reduction from 31.2% for tradition HPDC to 19.7%. • High shear melt conditioning can result in grain refinement of primary α-Mg formed in both shot sleeve and die cavity. - Abstract: Melt conditioning treatment via high shearing on Mg-9Al-1Zn melt was employed to disperse the oxide inclusions, and to harness the well-dispersed particles as the effective heterogeneous nucleation sites for primary α-Mg grains during solidification in high pressure die casting. To reveal the mechanism of property improvement, Prefil pressure filtration cleanliness analyzer, field-emission scanning electron microscopy with energy dispersive X-ray spectroscopy and high-resolution transmission electron microscopy were employed in present study. The experimental results showed that the application of high shearing not only improved elongation, ultimate tensile strength and yield strength simultaneously, but also reduced the variation of the mechanical properties of Mg-9Al-1Zn alloy. The improvement was ascribed to the refinement of the primary α-Mg grains formed in both shot sleeve and die cavity, and to the reduction of porosity as well as refinement of Mg{sub 17}Al{sub 12} phases.

36 MATERIALS SCIENCE↗
DOE OSTI2018

In-situ microstructural observation of Ti-Cu alloys for semi-solid processing

The semi-solid processing of metallic alloys strongly depends on the microstructural features exhibited by the raw material. Consequently, many characterization techniques have been used to elucidate semi-solid microstructures. In the present work, high-temperature laser-scanning confocal microscopy (HT-LSCM) was used to characterize Ti-Cu alloys intended for thixoforming. During the reheating stage, this in-situ technique enabled us to observe the Ti-β grains formed through the reverse eutectoid transformation. Furthermore, the results showed that thermomechanical treatment was primarily responsible for breaking up the dendritic microstructure. The liquid formation and peritectic temperature were easily determined via the HT-LSCM investigation. Prior to the reverse peritectic reaction, which corresponds to the onset of equilibrium melting, we obtained experimental evidence of the melting of non-equilibrium phases that originated via segregation during solidification. Finally, it was possible to qualitatively study the coarsening mechanisms that occurred in the semi-solid state. Irrespective of the alloy composition (liquid fraction), both Ostwald ripening and coalescence occurred during the isothermal heat treatments. Based on the obtained results, HT-LCSM can be considered a valuable technique for the characterization of thixotropic alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Interaction between laser radiation and metallic powder of 316L austenitic steel during selective laser melting

Highlights: • Single and multiple tracks on a loose bulk powder substrate were generated. • Understanding of the microstructure development during SLM was achieved. • Single tracks reveal a nearly symmetrical crystallography with elongated grains. • Multiple tracks show elongated grains perpendicular to the build direction. - Abstract: During selective laser melting (SLM), a complex heat state develops that leads to a characteristic crystallography and microstructure of the processed materials. Depending on the geometry of the processed part, most scan tracks of a new layer, so-called hatches, are located above a dense substrate or already solidified structures whereas others are located above loose powder. This is, inter alia, the case for overhanging structures. Attributable to the lower thermal conductivity of loose powder, temperature gradients and cooling rates of the melt pool differentiate in these areas, resulting in a different microstructural build-up. In this work, the microstructure and the crystallographic orientation of grade 316L austenitic stainless steel processed by SLM was investigated to understand the interaction between the laser radiation and the metallic powder during SLM-processing and to investigate the remelting of a track on a previous SLM-densified track. Single and multiple tracks on a loose bulk powder substrate, as well as single tracks on a dense substrate plate, were generated. A parameter study revealed that high energy densities are necessary to build continuous tracks on a loose bulk powder substrate. In addition, the amount of adhered particles, which are sintered on the fully melted and solidified tracks, is determined in comparison to the melted powder because the sintered particles strongly influence the surface roughness. To understand the microstructure development and, particularly, the influence of adjacent hatches during SLM-processing, investigations on the resulting microstructure and crystallographic orientation of a single track and two connected multiple tracks were carried out. During SLM processing of the tracks, the substrate plate and the solidified structures influence the temperature gradient and cooling rate of the melt pool, thus directionally solidified and elongated grains occur. Furthermore, the solidification is characterized by an epitaxial growth due to a distinct thermal gradient between the melt pool and the surrounding elements.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Scan strategies in EBM-printed IN718 and the physics of bulk 3D microstructure development

Three-dimensional (3D) characterization provides opportunities for understanding processing-structure relationships in additively manufactured (AM) materials. Bulk samples of Inconel 718 were fabricated via electron beam melting (EBM) in order to study microstructural development as a function of energy input and beam scan strategy. TriBeam tomography of bulk Inconel 718 microstructures built under steady-state growth conditions reveals the sensitivity of microstructure formation and evolution to machine process parameters. Here, samples manufactured using a narrow range of energy input per unit build area result in varied grain morphologies and crystallographic textures. Using TRUCHAS, a thermal simulation software, the thermal history of bulk scan strategies was predicted, and combined with a calibrated microstructure-processing map to accurately predict bulk grain morphologies. The solidification parameters and the 3D measured nucleation density are used to predict the transition between columnar and equiaxed grain morphologies, providing a process map to guide AM parameter choices to locally control as-printed microstructure. A two-dimensional metric for characterizing bulk grain morphology was also found to agree well with predictions from the process map calibrated by 3D data. Combined with 3D tomography and thermal modelling, the physics of structure development were understood at a new level of detail with respect to the competing processes of grain nucleation and epitaxial growth.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Microstructural characterization of dendritic evolution using two-point statistics

Characterizing large four-dimensional materials datasets is difficult due to the presence of complex mi- crostructures and time-varying length scales. We showcase the use of two-point statistics as an efficient and un-biased way of extracting materials parameters from an Al-Cu alloy during solidification. The evo- lution of dendrite primary arm thickness, average secondary arm spacing, and average tip-to-tip spacing were tracked using two-point Pearson auto-correlations of scaled mean curvatures. Insights into compet- itive side-branching are also reported. We show both visually and quantitatively that most length scales change rapidly during early stages of dendritic growth, but slow as diffusion fields of dendrites overlap.

Aluminum alloys↗
DOE OSTI2013

Time-Resolved SAXS/WAXS Study of the Phase Behavior and Microstructural Evolution of Drug/PEG Solid Dispersions

Simultaneous small-angle X-ray scattering/wide-angle X-ray scattering (SAXS/WAXS) was employed to elucidate the physical state and location of various small molecule drugs blended with polyethylene glycol (PEG), as well as the time dependent microstructural evolution of the systems. Samples were prepared by comelting physical mixtures of the drug and PEG, followed by solidification at 25 C. The model drugs selected encompassed a wide variety of physicochemical properties in terms of crystallization tendency and potential for interaction with PEG. It was observed that compounds which crystallized rapidly and had weak interactions with PEG tended to be excluded from the interlamellar region of the PEG matrix. In contrast, drugs which had favorable interactions with PEG were incorporated into the interlamellar regions of the polymer up until the point at which the drug crystallized whereby phase separation occurred. These factors are likely to impact the effectiveness of drug/PEG systems as drug delivery systems.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2013

Microwires fabricated by glass-coated melt spinning

The glass-coated melt spinning method offers a route for the manufacture of metal filaments with a few micrometers in diameter in a single operation directly from the melt. Cobalt-based amorphous wires, Cu-15.0 atomic percent (at. %) Sn shape-memory wires, and Ni{sub 2}MnGa (atomic percent) ferromagnetic wires were successfully produced by this method. The cobalt-based amorphous wire is flexible, and Cu-15.0 at. % Sn shape-memory wires have the tensile elongation of 14%. However, because of chemical reaction with glass and oxidation, it is hard to make Cu–Al–Ni shape-memory wires and Ni–Nb–Sn amorphous wires. Conditions for preparing these materials were summarized, and the differences of the solidification processes among glass-coated amorphous cobalt-based wires, Cu-15.0 at. % Sn shape-memory wires, and Ni{sub 2}MnGa wires were analyzed and discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Time-resolved temperature measurement and numerical simulation of millisecond laser irradiated silicon

Thermal process of 1064 nm millisecond pulsed Nd:YAG laser irradiated silicon was time-resolved temperature measured by an infrared radiation pyrometer, temperature evolutions of the spot center for wide range of laser energy densities were presented. The waveforms of temperature evolution curves contained much information about phase change, melting, solidification and vaporization. An axisymmetric numerical model was established for millisecond laser heating silicon. The transient temperature fields were obtained by using the finite element method. The numerical results of temperature evolutions of the spot center are in good agreement with the experimental results. Furthermore, the axial temperature distributions of the numerical results give a better understanding of the waveforms in the experimental results. The melting threshold, vaporizing threshold, melting duration, and melting depth were better identified by analyzing two kinds of results.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Numerical study of the effect of the shape of the phase diagram on the eutectic freezing temperature

To evaluate the reliability of metal-carbon eutectic systems as fixed points for the next generation of high-temperature standards the effect of thermodynamic properties related to the shape of eutectic phase diagram on the freezing temperature is investigated within the context of the numerical multi-phase-field model. The partition coefficient and liquidus slopes of the two solids involved in the eutectic reaction are varied deliberately and independently. The difference between the eutectic temperature and the freezing temperature is determined in dependence of the solid/liquid (s/l) interface shape and concentration. Where appropriate reference is made to the Jackson-Hunt analytical theory. It is shown that there are mainly two typical conditions to decrease the undercooling: 1) a small liquidus slope and 2) the associated difference between the eutectic composition and the liquid composition during solidification.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Giant magnetostriction in Tb-doped Fe{sub 83}Ga{sub 17} melt-spun ribbons

Giant magnetostriction is achieved in lightly Tb-doped Fe{sub 83}Ga{sub 17} melt-spun ribbons. The average perpendicular magnetostriction λ{sub ⊥} is −886 ppm along the melt-spun ribbon direction in the Fe{sub 82.89}Ga{sub 16.88}Tb{sub 0.23} alloy and the calculated parallel magnetostriction λ{sub ‖‖} is 1772 ppm. These values are more than four times as large as those found in binary Fe{sub 83}Ga{sub 17}. The enhanced magnetostriction is attributed to a small amount of Tb entering solution in the A2 matrix phase during rapid solidification. The strong localized magnetocrystalline anisotropy of terbium is thought to cause the giant magnetostriction.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Thin-thick hydrogen target for nuclear physics

In spectroscopic studies of unstable nuclei, hydrogen targets are of key importance. The CHyMENE Project aims to provide to the nuclear physics community a thin and pure solid windowless hydrogen or deuterium target. CHyMENE project must respond to this request for the production of solid Hydrogen. The solid hydrogen target is produced in a continuous flow (1 cm/s) by an extrusion technique (developed with the PELIN laboratory) in a vacuum chamber. The shape of the target is determined by the design of the nozzle at the extrusion process. For the purpose, the choice is a rectangular shape with a width of 10 mm and a thickness in the range of 30-50 microns necessary for the physics objectives. The cryostat is equipped with a GM Cryocooler with sufficient power for the solidification of the hydrogen in the lower portion of the extruder. In the higher part of the cryostat, the hydrogen gas is first liquefied and partially solidified. It is then compressed at 100 bars in the cooled extruder before expulsion of the film through the nozzle at the center of the reaction vacuum chamber. After the previous step, the solid hydrogen ribbon falls by gravity into a dedicated chamber where it sublimes and the gas is pumped and evacuated in a exhaust line. This paper deals with the design of the cryostat with its equipment, with the sizing of the thermal bridge (Aluminum and copper), with the results regarding the contact resistance as well as with the vacuum computations of the reaction and recovery hydrogen gas chambers.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗