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At least 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Manufacturing Processes for Permanent Magnets: Part I—Sintering and Casting

Abstract Permanent magnets (PMs) produce magnetic fields and maintain the field even in the presence of an opposing magnetic field. Electrical machines using permanent magnets are more efficient than those without. Currently, all known strong magnets contain rare earth (RE) elements, and they are core components of a wide range of applications including electric vehicles and wind turbines. RE elements such as Nd and Dy have become critical materials due to the growing demand and constrained supply. Improving the manufacturing process is effective in mitigating the RE criticality issue by reducing waste and improving parts consistency. In this article, the state of the industry for PM is reviewed in detail considering both the technical and economic drivers. The importance of RE elements is discussed along with their economic importance to green energy. The conventional sintering and casting manufacturing processes for commercial magnets, including Nd-Fe-B, Sm-Co, Alnico, and ferrite, are described in detail.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Unravelling the sintering temperature-induced phase transformations in Ba(Fe 0.7 Ta 0.3 )O 3-δ ceramics

This work reports on the fundamental details of the crystal structure and phase transformations in Ba(Fe 0.7 Ta 0.3 )O 3-δ , the best known temperature-independent oxygen-sensing ceramic material for applications in extreme environments. Ba(Fe 0.7 Ta 0.3 )O 3-δ ceramics were synthesized using conventional solid-state ceramic reaction under variable sintering temperatures (T s = 1200–1350 °C). Combined X-ray diffraction (XRD) and high-resolution transmission electron microscopy (TEM) measurements revealed the T s -induced phase transformations and their origin in Ba(Fe 0.7 Ta 0.3 )O 3-δ . Associated with phase transformations, pseudo-cubic (PC) reflections, such as {200} PC , {211} PC , and {220} PC , exhibited distinct anomalies with increasing T s . At T s = 1200 °C, Ba(Fe 0.7 Ta 0.3 )O 3-δ stabilized in mixed orthorhombic + rhombohedral phases (Amm2 + R3m). With increasing T s (≥1250 °C), Ba(Fe 0.7 Ta 0.3 )O 3-δ ceramics stabilized in tetragonal/rhombohedral [P4mm + R3m] mixed phases, while variations in the quantity of the respective phases were observed. Because both structure and crystal chemistry play key roles in achieving enhanced performance in chemical sensing and catalytic converters, detailed understanding of the phase transformations and crystal structure of Ba(Fe 0.7 Ta 0.3 )O 3-δ ceramics, as derived in this work, will be useful to develop chemical sensors with optimum performance for high-temperature and corrosive environments.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Analysis of microstructural facet fatigue failure in ultra-fine grained powder metallurgy Ti-6Al-4V produced through hydrogen sintering

The hydrogen sintering and phase transformation (HSPT) process is a powder metallurgy method that produces Ti-6Al-4V with fatigue properties rivaling wrought processed Ti-6Al-4V. Previous fatigue work on HSPT Ti-6Al-4V has shown failure to be frequently caused by microstructural facets, which were believed to be grain boundary (GB) α phase. This belief conflicts with other reports as fatigue failure typically occurs across α colonies or primary α grains, not GB α. In this work, electron backscattered diffraction is used to identify these facets as GB α. Furthermore, a simple geometric model predicting the effective slip length of GB α is presented.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Spark plasma sintered, MoNbTi-based multi-principal element alloys with Cr, V, and Zr

MoNbTi, MoNbTiZr, CrMoNbTiZr, and MoNbTiVZr multi-principal element alloys (MPEAs) were fabricated via spark plasma sintering (SPS) and investigated for use in high-strength applications. The fabrication method by SPS and powder metallurgy differs from those presented in the prior literature, where most MPEAs are fabricated using arc melting (AM) methods. Here cryogenic milling was used to maximize potential defect sinks (grain boundaries) for radiation resistance and to increase ultimate tensile strength through the Hall-Petch effect. SPS was chosen for consolidation in order to maintain a fine-grained structure during densification. Each alloy was characterized using x-ray diffraction and scanning electron microscopy for phase identification and compositional homogeneity. The base ternary alloy MoNbTi presented a predominantly single BCC system with minor cubic phases. The introduction of additional alloying elements—Zr, V, and Cr—heightened the phase complexity and increased the fractions of a secondary BCC phase and an HCP phase from Zr. The addition of Cr induced a larger fraction of the Laves phase to form. The addition of V caused the precipitation of small Mo inclusions. Thermodynamic analysis was performed to understand the separation of phases in each alloy. Discrepancies among the phase predictions generated by thermodynamic models, phases previously presented in the literature, and the characterization results suggest that MPEA fabrication methods, especially solid-state methods, require significant investigation to ensure that alloys can remain stable throughout their anticipated service lifetimes.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Mechanical properties of ZrB2- and HfB2-based ultra-high temperature ceramics fabricated by spark plasma sintering

Flexural strengths at room temperature, at 1400 C in air and at room temperature after 1 h oxidation at 1400 C were determined for ZrB2- and HfB2-based ultra-high temperature ceramics (UHTCs). Defects caused by electrical discharge machining (EDM) lowered measured strengths significantly and were used to calculate fracture toughness via a fracture mechanics approach. ZrB2 with 20 vol.% SiC had room temperature strength of 700 90 MPa, fracture toughness of 6.4 0.6 MPa, Vickers hardness at 9.8 N load of 21.1 0.6 GPa, 1400 C strength of 400 30 MPa and room temperature strength after 1 h oxidation at 1400 C of 678 15 MPa with an oxide layer thickness of 45 5 m. HfB2 with 20 vol.% SiC showed room temperature strength of 620 50 MPa, fracture toughness of 5.0 0.4 MPa, Vickers hardness at 9.8 N load of 27.0 0.6 GPa, 1400 C strength of 590 150 MPa and room temperature strength after 1 h oxidation at 1400 C of 660 25 MPa with an oxide layer thickness of 12 1 m. 2 wt.% La2O3 addition to UHTCs slightly reduced mechanical performance while increasing tolerance to property degradation after oxidation and effectively aided internal stress relaxation during spark plasma sintering (SPS) cooling, as quantified by X-ray diffraction (XRD). Slow crack growth was suggested as the failure mechanism at high temperatures as a consequence of sharp cracks formation during oxidation.

Zapata-Solvas, E.↗
DOE OSTI2014

Joining of beta-SiC by spark plasma sintering

Spark plasma sintering (SPS) was employed to join monolithic -SiC with or without titanium as intermediate joining material. Both the localizedand rapid heating contributed to the inherent energy saving of electric current assisted joining technique. The effects of uniaxial pressure and surfacepreparation were analyzed independently with respect to the flexural strength and the morphology of the joints. In particular samples polisheddown to 1 m and joined at 1900 C for 5 min achieved the strength of the as received material. The failure occurred outside the joining interface,confirming the optimum quality of the joint. Pressure in combination with surface preparation was necessary to achieve perfect adhesion and porefree direct joining of SiC. The use of Ti foil as a joining material and pressure allowed joining of unpolished SiC.

Grasso, Salvatore↗
DOE OSTI2021

Phase evolution during reactive flash sintering of Li 6.25 Al 0.25 La 3 Zr 2 O 12 starting from a chemically prepared powder

Here, reactive flash sintering (RFS) of a chemically prepared multiphase precursor powder was performed to fabricate Li 6.25 Al 0.25 La 3 Zr 2 O 12 (Al-LLZO) ceramics. This approach allowed for obtaining single-phase dense samples in a remarkably short processing time of 30 s, at a furnace temperature of 600 °C, with an electric field of 50 V cm -1 and a current limit of 150 mA mm -2 . The ceramics display high bulk conductivity of 0.18 mS cm -1 at room temperature. Furthermore, phase evolution is studied by in-situ X-ray diffraction during: i) conventional heating and ii) RFS under current rate mode. As expected, the intermediate phases progressively dissolved into the Al-LLZO matrix by conventional heating. On the other hand, RFS promoted the growth of the intermediate La 2 Zr 2 O 7 , an effect that was overcome by the thermally driven formation of Al-LLZO at higher temperatures. The observed different reaction pathway suggests that RFS can be used for stabilizing phases that are not thermodynamically favored upon conventional heating.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Irradiation creep of nano-powder sintered silicon carbide at low neutron fluences

The irradiation creep behavior of nano-powder sintered silicon carbide was investigated using the bend stress relaxation method under neutron irradiation up to 1.9 dpa. The creep deformation was observed at all temperatures ranging from 380 to 1180 °C mainly from the irradiation creep but with the increasing contributions from the thermal creep at higher temperatures. Microstructural observation and data analysis were performed.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Microstructural and crystallographic effects of sol-gel synthesized Ti-doped UO 2 sintered under reducing conditions

Titanium (Ti)-doped UO 2 microspheres of three different Ti concentrations (1000, 2000, and 4000 wppm) were synthesized using an internal gelation process. The microspheres were pressed into pellets, and a two-step heat treatment was applied to form monolithic cylindrical pellets with high densities (≥95%TD). Microstructure of these samples consisted of equiaxed grains with >300% increase in average grain size compared to the undoped UO 2 pellets. Secondary Ti-rich chemical phases corresponding to a liquid eutectic formed during sintering were observed at grain boundaries of UO 2 for samples doped with 4000 wppm Ti. Furthermore, these Ti-rich chemical phases were not observed in 1000 or 2000 wppm Ti samples at microscale using electron microscopy investigations. The 0.02–0.04% lower lattice parameter values for the Ti-doped UO 2 samples compared to the undoped UO 2 confirms the incorporation of Ti into the UO 2 lattice.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

High-temperature compressive creep tests of U 3 Si 2 with spark plasma sintering: Experiments and finite element modeling

This paper reports a systematic high-temperature creep behavior of dense U 3 Si 2 pellets using a spark plasma sintering (SPS) apparatus at elevated temperatures and pressures under vacuum conditions. The stress exponent was subsequently derived to be 3.21 at 1173 K and 2.17 at 1223 K, respectively, indicating a grain boundary sliding creep mechanism. The creep activation energy was determined to be 203.6 ± 19.0 kJ/mol, which agrees well with the literature. Finite element modeling was performed using the creep parameters fitted from the strain-time plot. Additionally, the results suggest an excellent match with the experimental data, confirming the validity of the experiments. Microstructure characterizations indicate that the main phase of the specimens after creep tests remains to be U 3 Si 2 , with a 4 μm thick layer of nano-sized particles induced from the diffusion between U 3 Si 2 and alumina disc used to avoid electric current passing through the sample. The successful conduct of creep experiments demonstrates the great potential of SPS to perform high-temperature mechanical testing of nuclear fuels under vacuum conditions. The subsequent finite element modeling exhibits excellent capabilities for accurately predicting material performance in the creep tests and provides a practical tool in evaluating nuclear fuels’ performance for a much-extended time scale.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Experimentally validated multiphysics modeling of fracture induced by thermal shocks in sintered UO 2 pellets

Uranium Dioxide (UO 2 ) fuel powers almost all commercial Nuclear Power Plants (NPPs) worldwide, generating carbon-free energy and contributing to the fight against climate change. UO 2 fuel incurs damage and fractures due to large thermal gradients that develop across the fuel pellet during normal and transient operating conditions. A comprehensive understanding of the underlying mechanisms by which these processes take place is still lacking. A combined experimental and computational approach is utilized here to quantify the behavior of UO 2 fuel fracture induced by thermal shock. Here, this work introduces both (1) an experimental study to understand the fuel fracturing behavior of sintered UO 2 pellets when exposed to thermal shock, and (2) a Multiphysics phase-field fracture model capable of simulating this process. Parametric studies were conducted to evaluate the effects of uncertainties in fracture properties on the fracture behavior of UO 2 due to thermal shocking. A set of energy release rate (or equivalently fracture toughness) and contract area (the part of the fuel pellet in direct contact with the cold bath) were able to capture the overall fracture trends of the corresponding experimental data. Our combined approach presents a new method for accounting for the effects of microstructure and sample size on the energy release rate/fracture toughness. The experimental data were collected from multiple experiments that exposed UO 2 pellets to high-temperature conditions (589–676 °C) followed by a quench in sub-zero water. This work demonstrates that joint experimental and computational efforts are able to advance the understanding of thermal fracture in the primary fuel source for existing and future NPPs.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Lattice parameter evolution in Pt nanoparticles during photo-thermally induced sintering and grain growth

An in-situ optical pump/x-ray probe technique has been used to study the size dependent lattice parameter of Pt nanoparticles subjected to picosecond duration optical laser pulses. The as-prepared Pt nanoparticles exhibited a contracted lattice parameter consistent with the response of an isolated elastic sphere to a compressive surface stress. During photo-thermally induced sintering and grain growth, however, the Pt lattice parameter did not evolve with the inverse particle size dependence predicted by simple surface stress models. Lastly, the observed behavior could be attributed to the combined effects of a compressive surface/interface stress and a tensile stress arising from intergranular material.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

Three-dimensional reconstruction and morphologic characteristics of porous metal fiber sintered sheet

This paper presents an approach to achieve morphological characterizing for complex porous materials based on micro X-ray tomography images, with an example of a novel porous metal fiber sheet produced through solid-state sintering method. The geometrical reconstruction was performed after selection of volume of interest and image processing of anisotropic diffusion smooth. The reconstructed gray level images were then transferred into binary images by adjusting binarization threshold according to the actual porosity. Taking into account the tubular structural feature of the fibers, skeleton extraction algorithm based on the distance transform function was applied and further improved by the scale axis transform method. The skeleton was later pruned and segmented according to the contact points to perform morphological characterizing. Compared with actual manufacturing parameters, the style, length, radius, orientation and tortuosity of fiber segments were discussed. The results show that our proposed method can well describe the actual geometrical and morphological characteristics, which will provide a promising method for the structural description of fibrous networks. - Highlights: • Micro-CT technology was used to achieve the 3D geometrical reconstruction. • Skeleton extraction algorithm was modified to get the medial skeleton. • Skeleton filter operation was adopted to deal with the segmented skeleton. • Useful morphological statistics was obtained through skeleton segments. • Relationship between structure and manufacturing processes was discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Microstructure and mechanical behavior of direct metal laser sintered Inconel alloy 718

In this paper, we investigate microstructure and quasi-static mechanical behavior of the direct metal laser sintered Inconel 718 superalloy as a function of build direction (BD). The printed material was further processed by annealing and double-aging, hot isostatic pressing (HIP), and machining. We characterize porosity fraction and distribution using micro X-ray computed tomography (μXCT), grain structure and crystallographic texture using electron backscattered diffraction (EBSD), and mechanical response in quasi-static tension and compression using standard mechanical testing at room temperature. Analysis of the μXCT imaging shows that majority of porosity develops in the outer layer of the printed material. However, porosity inside the material is also present. The EBSD measurements reveal formation of columnar grains, which favor < 001 > fiber texture components along the BD. These measurements also show evidence of coarse-grained microstructure present in the samples treated by HIP. Finally, analysis of grain boundaries reveal that HIP results in a large number of annealing twins compared to that in samples that underwent annealing and double-aging. The yield strength varies with the testing direction by approximately 7%, which is governed by a combination of grain morphology and crystallographic texture. In particular, we determine tension–compression asymmetry in the yield stress as well as anisotropy of the material flow during compression. We find that HIP lowers yield stress but improves ductility relative to the annealed and aged material. These results are discussed and critically compared with the data reported for wrought material in the same condition. - Highlights: • Microstructure and mechanical properties of DMLS Inconel 718 are studied in function of build direction. • Inhomogeneity of microstructure in the material in several conditions is quantified by μXCT and EBSD. • Anisotropy and asymmetry in the mechanical response are determined by tension and compression testing.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Pulse current auxiliary forging of sintered TiAl alloys based on the investigation of dynamic recrystallization and phase transformation

Highlights: • DRX kinetics model of sintered near γ TiAl alloy was established. • DRX process was accompanied by twinning when compressed at 1125 °C/0.1 s{sup − 1}. • Hot deformation has a promotion to the phase transformation of γ → α. - Abstract: Pulse current auxiliary forging is a new and efficient way to fabricate high performance TiAl based alloys. Dynamic recrystallization (DRX) and phase transformation have a pronounced effect on the subsequent deformability of TiAl based alloys. Hence, the dynamic recrystallization kinetics were determined at the temperature range of 1125–1275 °C and strain rate range of 0.1–0.001 s{sup − 1}. In addition, the DRX mechanism and phase transformation of TiAl based alloys were investigated to model the microstructure evolution in order to provide guidance for the pulse current auxiliary forging process. When the specimens were compressed at 1125 °C/0.1 s{sup − 1}, twinning played an important role in the DRX process. With increasing deformation temperature, the microstructure transformed from near gamma into duplex microstructure. In addition, according to the kinetics of dynamic recrystallization and phase transformation, fully DRXed TiAl based alloys were fabricated through pulse current auxiliary forging. The as-forged TiAl-based alloys show good superplastic performance with an elongation over 410%.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Simultaneous enhancement of the coercivity and remanence at high temperatures in a sintered Nd-Fe-B magnet after grain boundary diffusion with Dy60Co40 alloy

Highlights: • Dy and Co enriched shells were formed on the Nd{sub 2}Fe{sub 14}B grain boundaries. • Coercivity and remanence were increased simultaneously at high temperatures. • Remanence stability was improved with little change of the Curie temperature. - Abstract: The grain boundary diffusion process (GBDP) using Dy{sub 60}Co{sub 40} alloy was applied to a sintered Nd-Fe-B magnet. After GBDP the temperature stability of remanence was improved greatly with the remanence temperature coefficient improving from −0.140%/°C to −0.095%/°C while the Curie temperature increased marginally. Both the coercivity and remanence were increased simultaneously at high temperatures. Microstructure and composition analysis revealed that Dy and Co enriched shells were formed on Nd{sub 2}Fe{sub 14}B grain boundaries after GBDP. The Dy enriched shell increased the coercivity while the Co enriched shell decreased the temperature sensitivity of remanence and then increased the remanence at high temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Structural characteristics, sinterability and electrical conductivity of Ba-site non-stoichiometric Ba{sub x}Ce{sub 0.50}Zr{sub 0.40}Y{sub 0.10}O{sub 3-{delta}}

The influence of barium content on the structural characteristics, sinterability and electrical conductivity of proton conductor Ba{sub x}Ce{sub 0.50}Zr{sub 0.40}Y{sub 0.10}O{sub 3-{delta}} (x = 0.95, 0.97, 1.00, 1.03, 1.05) is investigated. Compositions with barium deficiency show the presence of fluorite precipitate detected by powder X-ray diffraction, whilst pure perovskite phase is present for other samples. Barium deficiency promotes the densification process of the samples. The electrical conductivity of Ba{sub x}Ce{sub 0.50}Zr{sub 0.40}Y{sub 0.10}O{sub 3-{delta}} increases with barium content, which is mainly ascribed to the decreased activation energy due to the increasing lattice volume, especially for the case in wet 5% H{sub 2}/Ar. The present results suggest that it is very important to control the stoichiometry of cations to obtain desirable perovskite type high temperature proton conductors.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Structural elucidation, and morphological and magnetic behavior evaluations, of low-temperature sintered, Ce-doped, nanostructured garnet ferrites

Highlights: • Ce{sub x}Gd{sub 3.5-x}Fe{sub 4.5}O{sub 12} were synthesized via sol gel self-combustion route. • Crystallite size was found in the range of 20–38 nm. • Morphology of Ce-Gd doped and undoped garnets were observed systematically. • Switching field distribution was calculated for further applications of the nanoferrites. • These garnets also have potential applications in microwave high frequency devices. - Abstract: Low-temperature sintered Ce{sub x}Gd{sub 3.5-x}Fe{sub 4.5}O{sub 12} (x = 0.0, 0.4, 0.8, 1.2, 1.6, 2.0 and 2.4) garnet ferrites were synthesized via a sol-gel self-combustion route. Phase, structure, morphology and magnetic investigations were done using FTIR, XRD, FESEM, and VSM, respectively. Single-phase structures of all Ce{sup 3+} substituted nano-sized garnet ferrites were confirmed through XRD. The crystallite size was decreased with Ce{sup 3+}incorporation. Relevant structural parameters such as site radii, bond lengths, ionic radii, and shared edges were calculated using XRD. The vibrational spectroscopy stretching band studies were also done using FTIR. The grain size and morphology were also analyzed using FESEM. The magnetic remanance, saturation, and coercivity were found to increase systematically with an increase in Ce{sup 3+} concentration in these garnets. Switching field distribution curves were also plotted using the first derivative of the demagnetization data from MH loops. The better magnetic properties of these nanogarnets suggest their use for miniaturization of the devices in various applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗