Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “PRESSING”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,171 records · Page 65
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Influence of neutron irradiation on Al-6061 alloy produced via ultrasonic additive manufacturing

Samples of aluminum alloy 6061 produced via ultrasonic additive manufacturing (UAM) were irradiated in the High Flux Isotope Reactor (HFIR) up to 17.3 dpa at ~70°C while in contact with water using perforated rabbit capsules. The irradiation campaign included as-received (AR) material, specimens subjected to various post-weld heat treatments (PWHTs, including hot isostatic pressing [HIP]), and reference (wrought) alloy samples. Mechanical tensile tests, accompanied by digital image correlation (DIC) analysis, fractography, and metallography, were performed as a part of the post-irradiation evaluation. The X- and Y-specimens (i.e., oriented in the sonotrode moving and vibration directions, respectively) showed pronounced radiation hardening and ductility decrease. Specific serration flow behavior and propagation of deformation bands were observed under various material conditions up to 3.5 dpa but disappeared at 17.3 dpa. In all cases, the fracture mechanism of X- and Y-specimens was ductile; ductile dimples dominated the fracture surface. Irradiated X- and Y-specimens showed good performance, regardless of material conditions (AR or PWHT). The performance of Z-specimens oriented in the build direction was strongly dependent on the PWHT. The AR and aged specimens showed fracture stress decrease with dose, and they experienced fracture under irradiation after 3.5 dpa; specimen cross section analysis revealed specific interface degradation that was likely related to corrosion. Recrystallization significantly improved in-reactor performance. Finally, HIP suppressed interface degradation due to recrystallization and pore removal, which led to good in-reactor performance for Z-specimens.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Crystal structure and high temperature transport properties of Yb-filled p-type skutterudites Yb{sub x}Co{sub 2.5}Fe{sub 1.5}Sb{sub 12}

Partially Yb-filled Fe substituted polycrystalline p-type skutterudites with nominal compositions Yb{sub x}Co{sub 2.5}Fe{sub 1.5}Sb{sub 12}, with varying filler concentrations x, were synthesized by reacting the constituent elements and subsequent solid state annealing, followed by densification by hot-pressing. The compositions and filling fractions were confirmed with a combination of Rietveld refinement and elemental analysis. Their thermoelectric properties were evaluated from 300 to 800 K. The Seebeck coefficients for the specimens increase with increasing temperature and plateau at around 750 K. The thermal conductivity decreases with increasing Yb filling fraction, and bipolar conduction becomes evident and increases at elevated temperatures. A maximum ZT value of 0.8 was obtained at 750 K for Yb{sub 0.47}Co{sub 2.6}Fe{sub 1.4}Sb{sub 12}. The thermoelectric properties and potential for further optimization are discussed in light of our results. - Graphical abstract: Display Omitted - Highlights: • Yb-filled Fe-substituted p-type CoSb{sub 3} skutterudites, Yb{sub x}Co{sub 2.5}Fe{sub 1.5}Sb{sub 12}, were structurally and physically characterized. • Both filling fraction and Co-to-Fe ratio affect the transport properties. • High ZT was obtained for the composition with ∼50% Yb filling and a slightly lower than nominal Fe content.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Phase transitions and hydrogen bonding in deuterated calcium hydroxide: High-pressure and high-temperature neutron diffraction measurements

In situ neutron diffraction measurements combined with the pulsed neutron source at the Japan Proton Accelerator Research Complex (J-PARC) were conducted on high-pressure polymorphs of deuterated portlandite (Ca(OD){sub 2}) using a Paris–Edinburgh cell and a multi-anvil press. The atomic positions including hydrogen for the unquenchable high-pressure phase at room temperature (phase II′) were first clarified. The bent hydrogen bonds under high pressure were consistent with results from Raman spectroscopy. The structure of the high-pressure and high-temperature phase (Phase II) was concordant with that observed previously by another group for a recovered sample. The observations elucidate the phase transition mechanism among the polymorphs, which involves the sliding of CaO polyhedral layers, position modulations of Ca atoms, and recombination of Ca–O bonds accompanied by the reorientation of hydrogen to form more stable hydrogen bonds. - Graphical abstract: Crystal structures of high-pressure polymorphs of Ca(OD){sub 2}, (a) at room temperature (phase II′) and (b) at high temperature (phase II), were obtained from in situ neutron diffraction measurements. - Highlights: • We measured in situ neutron diffraction of high-pressure polymorphs of Ca(OD){sub 2}. • Hydrogen positions of the high-pressure phase are first determined. • The obtained hydrogen bonds reasonably explain Raman peaks of OH stretching modes. • A phase transition mechanism among the polymorphs is proposed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Different site occupancies in substitution variants of Mo{sub 3}Sb{sub 7}

A series of samples with the initial formula Mo{sub 3–x}Fe{sub x}Sb{sub 7} and Mo{sub 3–x}Ni{sub x}Sb{sub 7} have been prepared by solid state reaction at 873 K, starting from the elements in the stoichiometric ratios. The materials were compressed via hot-pressing, and their thermoelectric properties determined. Single crystal data show that Ni atoms in part occupy the cubic voids of the Mo{sub 3}Sb{sub 7} structure, as well as replace some Mo atoms, in contrast to the Fe analogues, where we found no evidence for Fe in the cubic voids. Supported by electronic structure calculations, replacing Mo with Fe or Ni increases the valence-electron count, thus decreases the numbers of p-type carriers of Mo{sub 3}Sb{sub 7}. On the other hand, filling the holes with Ni atoms appears to have no such effect. As a consequence, the Ni containing samples exhibit higher electrical conductivity than the Fe analogues, and ultimately lower thermoelectric figure-of-merit, as the p-type carrier concentration remains too high. - Graphical abstract: Trying to substitute Ni atoms for Mo in Mo{sub 3}Sb{sub 7} resulted in partial filling of the cubic voids, in contrast to Mo{sub 3–x}Fe{sub x}Sb{sub 7}. Since the Ni atoms in the Sb{sub 8} cubes do not act as electron donors, the Ni-containing materials exhibit very high carrier concentration, and thus high electric conductivity and low Seebeck coefficient. - Highlights: • Various substitution variants of Mo{sub 3}Sb{sub 7} with Fe and Ni were characterized. • While the Fe atoms replace the Mo atoms, the Ni atoms also fill the cubic voids. • Higher Fe/Ni amount on the Mo sites causes a decrease of the charge carriers. • The Ni atoms on the cubic sites do not decrease the charge carriers. • The lower the carrier concentration, the better the thermoelectric properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

High pressure structure studies of 6H-SrIrO{sub 3} and the octahedral tilting in 3C-SrIrO{sub 3} towards a post-perovskite

The high pressure behaviors of the two perovskite structures (hexagonal 6H-SrIrO{sub 3} and orthorhombic 3C-SrIrO{sub 3}) have been studied in diamond anvil cells to 43 and 60 GPa, respectively, using synchrotron powder X-ray diffraction. 6H-SrIrO{sub 3} was first synthesized at ambient pressure and subsequently transformed into 3C-SrIrO{sub 3} in a large volume press at 8.8 GPa and 1000 °C. Both polymorphs were found to retain the initial symmetry up to the highest pressures measured, but in the case of 6H-SrIrO{sub 3}, two anomalies were identified: a change in the axial compressibilities at 24 GPa and a change in both the axial and volume compressibilities at 32 GPa. Fitting a 3rd order Birch-Murnaghan equation of state to the obtained P-V data yielded bulk moduli of K{sub 0}=151.5(12) GPa (fitted range 0<P<30 GPa) for 6H-SrIrO{sub 3} and K{sub 0}=187.1(9) GPa for 3C-SrIrO{sub 3}. Analysis of the structural parameters for 6H-SrIrO{sub 3} aided by F-f plots suggests the anomalies are caused by changes in the compression mechanism. Comparison of the two polymorphs reveals that 6H-SrIrO{sub 3} becomes less compressible than 3C-SrIrO{sub 3} above 32 GPa as a result of the mechanistic change, and a crossing of their P-V curves is avoided. For 3C-SrIrO{sub 3}, analysis of the octahedral tilt angles shows that these increase monotonically from the ambient value of 7.23(6) to 23.0(2)° at 60 GPa suggesting that a transition to a post-perovskite is approached. - Graphical abstract: The structure of 3C-SrIrO{sub 3} synthesized at high pressure has been studied in a diamond anvil cell and is found to maintain its orthorhombic structure up to 60 GPa but distorts drastically due to octahedral tilting, strongly suggesting that a post-perovskite transition is possible. The compression is compared to 6H-SrIrO{sub 3} synthesized at ambient pressure, which is initially found to be more compressible but undergoes changes in its compression mechanism and 3C remains denser throughout compression. Display Omitted - Highlights: • Structures of 6H- and 3C-SrIrO{sub 3} were studied to pressures of 43 and 60 GPa, respectively. • Both structures maintain the initial symmetry up to the highest pressures measured. • 6H-SrIrO{sub 3} is initially more compressible than 3C-SrIrO{sub 3}. • Due to changes in the compression mechanism in 6H-SrIrO{sub 3} the 3C-SrIrO{sub 3} remains denser throughout. • 3C-SrIrO{sub 3} show large octahedral tilting, strongly suggesting a post-perovskite transition is imminent.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Stability and phase evolution of mullite in reducing atmosphere

The aim of this study is to investigate the stability of mullite and its phase evolution when heated at 1600-1650 deg. C in reducing atmosphere created by a carbon bed. The stoichiometric mullite (3Al{sub 2}O{sub 3}.2SiO{sub 2}) was synthesized by reacting the proper precursors at 1400 deg. C in air atmosphere. Samples containing the primary mullite and graphite or carbon were prepared by pressing the mix and heated at 1600-1650 deg. C. Products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the stoichiometric mullite could decompose to corundum and alumina-rich mullite. During firing under reducing atmosphere, the stoichiometric mullite became richer in Al{sup 3+} cations. The gaseous SiO compound was found to be released from the sample and reacting with C to form SiC phase. Comparison of general mullite solid solution (Al{sub 4+2x}Si{sub 2-2x}O{sub 10-x}) with alumina-rich mullite obtained from decomposition revealed that x would take different values depending on temperature and atmosphere. The ultimate decomposition product was found to be corundum. - Research Highlights: {yields} This method is a new route for investigation of thermochemical stability of stoichimetric mullite under reducing condition which have done by the authors. {yields} This research has got very good results for stability of mullite at different conditions. {yields} This work has also studied the mechanism of stability of mullite under reducing atmosphere.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Characterisation of titanium-titanium boride composites processed by powder metallurgy techniques

In this work, a detailed characterisation of titanium-titanium boride composites processed by three powder metallurgy techniques, namely, hot isostatic pressing, spark plasma sintering and vacuum sintering, was conducted. Two composites with different volume percents of titanium boride reinforcement were used for the investigation. One was titanium with 20% titanium boride, and the other was titanium with 40% titanium boride (by volume). Characterisation was performed using X-ray diffraction, electron probe micro analysis - energy dispersive spectroscopy and wavelength dispersive spectroscopy, image analysis and scanning electron microscopy. The characterisation results confirm the completion of the titanium boride reaction. The results reveal the presence of titanium boride reinforcement in different morphologies such as needle-shaped whiskers, short agglomerated whiskers and fine plates. The paper also discusses how mechanical properties such as microhardness, elastic modulus and Poisson's ratio are influenced by the processing techniques as well as the volume fraction of the titanium boride reinforcement. - Highlights: Black-Right-Pointing-Pointer Ti-TiB composites were processed by HIP, SPS and vacuum sintering. Black-Right-Pointing-Pointer The completion of Ti-TiB{sub 2} reaction was confirmed by XRD, SEM and EPMA studies. Black-Right-Pointing-Pointer Hardness and elastic properties of Ti-TiB composites were discussed. Black-Right-Pointing-Pointer Processing techniques were compared with respect to their microstructure.

36 MATERIALS SCIENCE↗
DOE OSTI2013

In situ TEM investigation of microstructural behavior of superplastic Al–Mg–Sc alloy

Dynamic changes in microstructure of the superplastic ultrafine-grained Al–3Mg–0.2Sc (wt.%) alloy refined by equal-channel angular pressing (ECAP). were observed by in situ transmission electron microscopy at temperatures up to 300 °C (annealing and tensile deformation) in order to simulate the initial stages of superplastic testing. It was found that the microstructure changes significantly during the preheating before the superplastic deformation, which was accompanied by decreased microhardness. During the deformation at 300 °C, high dislocation activity as well as motion of low-angle grain boundaries was observed while high-angle grain boundaries did not move due to the presence of scandium in the alloy. - Highlights: ► We performed in situ TEM annealing and straining on superplastic Al–Mg–Sc alloy. ► We simulated the conditions of early stages of superplastic testing. ► Significant changes in microstructure occur during preheating before deformation.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Fabrication of ultra-fine grained aluminium tubes by RTES technique

Recently, repetitive tube expansion and shrinking have been exploited as a means for producing ultra-fine grained and nano-crystalline microstructures for magnesium alloy tubes. This method includes two different half-cycles and was based on pressing a tubular part through an angular channel die with two shear zones. Since the aluminium alloys are the most widely used materials in industries, in this study, repetitive tube expansion and shrinking as a new severe plastic deformation technique was applied to commercially pure aluminium for fabricating ultra-fine grained aluminium tubes for the first time and the ability of this process in significant grain refinement is determined even after single cycle. Transmission electron microscopy and X-ray diffraction were used to evaluate the microstructure of the repetitive tube expansion and shrinking processed materials and the examinations showed ultra-fine grains with the average grain size of 320 nm after one cycle of repetitive tube expansion and shrinking. The yield strength, ultimate tensile strength increased notably by the factor of 2.17 and 1.27 respectively, after one cycle of repetitive tube expansion and shrinking, whereas the elongation to failure as well as the uniform elongation decreased. Furthermore, micro-hardness distribution through the part's section proposed the hardness increasing to ~ 55 HV from the initial value of ~ 28 HV after one cycle of repetitive tube expansion and shrinking. - Highlights: • RTES was introduced for fabricating the UFGed AA1050 tubes for the first time. • Nano-grained AA1050 tube was obtained by RTES process. • Grain size of ~ 320 nm was obtained after two half-cycles of RTES process. • Yield and ultimate strength increased by the factor of 2.17 and 1.27 respectively. • The microhardness increased to ~ 55 HV from the initial value of ~ 28 HV.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Thermal stability of bimodal microstructure in magnesium alloy AZ91 processed by ECAP

The changes in microstructure of equal channel angular pressing (ECAP) processed magnesium alloy AZ91 during thermal exposure were studied in this paper. The microstructure stability was investigated by means of electron backscatter diffraction (EBSD), which allowed to measure the changes in grain size, mutual ratio of low-angle boundaries (LABs) to high-angle ones (HABs) and local lattice distortion evaluated by the kernel average misorientation (KAM) parameter. It was found experimentally that the threshold temperature at which significant grain coarsening takes place is 350 °C. No modification to mean grain diameter occurs below this temperature, nonetheless, some changes in LAB and HAB fraction, as well as in local lattice distortion, can be observed. - Highlights: • Thermal stability of bimodal UFG AZ91 alloy was assessed by means of EBSD. • Threshold temperature for pronounced grain coarsening was found at 350 °C. • Below 350 °C increase in LAB fraction and local lattice distortion takes place. • Local lattice distortion (LLD) can be well described using KAM approach. • LLD is influenced by coarsening and precipitation of Mg{sub 17}Al{sub 12} particles.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Infiltrated W–Cu composites with combined architecture of hierarchical particulate tungsten and tungsten fibers

In this article, novel W–Cu composites reinforced with topologically-inserted tungsten fibers (W{sub f}) have been fabricated by hot-press sintering and infiltration method. By pre-sputtering of ~ 100 nm thick chromium layer onto the surface of W{sub f}, the contiguity or connectivity between W{sub f} and neighboring tungsten particles (W{sub p}) or Cu after sintering and infiltration was enhanced. Combined SEM, TEM and STEM techniques confirmed that the intact interfaces of W{sub f}/W{sub p} and W{sub f}/Cu free from precipitates, impurities and porosities would provide desirable strength and ductility. Further mechanical tests also validated its superior compressive strength and plasticity at various temperatures, together with significantly improved tensile strength (by 23.6%) and hardness (by 9.3%) for the W–Cu composite after reinforcement with Cr-coated W{sub f}, which promotes the engineering application of the composite greatly. - Highlights: • W-fibers reinforced W–Cu composites were fabricated by sintering and infiltration. • The sputtered Cr onto W{sub f} has dissolved into adjacent W{sub f} and W{sub p} during fabrication. • The intact interfaces of W{sub f}/W{sub p} and W{sub f}/Cu confer enhanced strength and ductility. • Tensile strength and hardness improve by 23.6% and 9.3% after interface tuning.

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 OSTI2016

Evolution of mechanical properties of ultrafine grained 1050 alloy annealing with electric current

The tensile properties and microstructures of 1050 aluminum alloy prepared by equal channel angular pressing at cryogenic temperature (cryoECAP) after electric current annealing at 90–210 °C for 3 h were investigated by tensile test, electron back scattering diffraction (EBSD) and transmission electron microscopy (TEM). An unexpected annealing-induced strengthening phenomenon occurs at 90–210 °C, due to a significant decrease in the density of mobile dislocations after annealing, and thus a higher yield stress is required to nucleate alternative dislocation sources during tensile test. The electric current can enhance the motion of dislocations, lead to a lower dislocation density at 90–150 °C, and thus shift the peak annealing temperature from 150 °C to 120 °C. Moreover, the electric current can promote the migration of grain boundaries at 150–210 °C, result in a larger grain size at 150 °C and 210 °C, and thus causes a lower yield stress. The sample annealed with electric current has a lower uniform elongation at 90–120 °C, and the deviation in the uniform elongation between samples annealed without and with electric current becomes smaller at 150–210 °C. - Highlights: • An unexpected annealing-induced strengthening phenomenon occurs at 90–210 °C. • The d. c. current can enhance the motion of dislocations at 90–150 °C, and thus shift the peak annealing temperature from 150 °C to 120 °C. • The d. c. current can promote the grain growth at 150–210 °C, and thus cause a lower yield stress. • The DC annealed sample has a lower uniform elongation at 90–120 °C.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Microstructure and mechanical properties of aluminium matrix composites reinforced by Al{sub 62}Cu{sub 25.5}Fe{sub 12.5} melt spun ribbon

Aluminium matrix composites containing 15, 30 and 50 vol.% of pulverized Al{sub 62}Cu{sub 25.5}Fe{sub 12.5} (in at.%) melt spun ribbons have been prepared by a vacuum hot pressing (T = 673 K, P = 600 MPa). The microstructure of the initial ribbon and the composites was investigated using X-ray, scanning and transmission electron microscopy. In the as-spun ribbon the quasicrystalline icosahedral phase (i-phase) coexisted with the cubic copper rich β-Al(Cu, Fe) intermetallic compound. The phase composition of Al-Cu-Fe particles changed after consolidation process and the i-phase transformed partially to the ω-Al{sub 70}Cu{sub 20}Fe{sub 10} phase. Additionally, the Θ-Al{sub 2}Cu phase formed at the α(Al)/Al-Cu-Fe particle interfaces. With an increase in volume fraction of the reinforcement the hardness of the composites increased up to HV = 180 for the highest amount of added particles. The ultimate compression strength of the same sample reached the value of 545 MPa. - Highlights: • Al and 15, 30, 50% of pulverized Al{sub 62}Cu{sub 25.5}Fe{sub 12.5} melt spun ribbon were consolidated. • The initial ribbon consisted of the icosahedral i-phase and copper rich β-Al(Cu, Fe). • The i-phase partially transforms to ω-Al{sub 7}Cu{sub 2}Fe phase in all composites. • Increase of microhardness and compressive strength with content of reinforcement • Ultimate compression strength 545 MPa for 50% of added particles.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Influence of severe plastic deformation on intermetallic particles in Mg-12 wt.%Zn alloy investigated using transmission electron microscopy

The in-depth microstructural characterization of intermetallic particles in an Mg-12 wt.%Zn binary alloy subjected to a severe plastic deformation is presented. The alloy was processed by four passes via equal channel angular pressing with an applied back pressure at a gradually decreasing temperature and analyzed using transmission electron microscopy techniques to observe the influence of processing on intermetallic particles. The results are compared with the initial state of the material prior to severe plastic deformation. The microstructural evolution of the α-Mg matrix and the Mg{sub 21}Zn{sub 25}, Mg{sub 51}Zn{sub 20} and MgZn{sub 2} was analyzed using bright field imaging, selected area electron diffraction, high-resolution transmission electron microscopy and high-angle annular dark field imaging in scanning mode. The plastic deformation process influenced the α-Mg matrix and each type of intermetallic particle. The α-Mg matrix consisted of two types of areas. The first type of area had a highly deformed structure, and the second type of area had a partially recrystallized structure with an average grain size of approximately 250 nm. The Mg{sub 21}Zn{sub 25} microparticles exhibited distinct forms in the α-Mg matrix that were characterized as a single-crystalline form, a nano-crystalline form and a broken up form. No evidence of Mg{sub 51}Zn{sub 20} nanoparticles within the α-Mg matrix was found in the microstructure, which indicates their dissolution or phase transformation during the deformation process. MgZn{sub 2} nanoparticles exhibited different behavior in both types of α-Mg matrix. Two orientation relationships toward the highly deformed α-Mg matrix were observed; however, there was no relationship toward the partially recrystallized α-Mg matrix. Additionally, the growth of the MgZn{sub 2} nanoparticles was different in the two types of α-Mg matrix. The Mg{sub 51}Zn{sub 20} nanoparticles inside Mg{sub 21}Zn{sub 25} microparticles exhibited a distinct behavior within the single-crystalline or nano-crystalline form of the parent Mg{sub 21}Zn{sub 25} microparticles. The Mg{sub 21}Zn{sub 25} + Mg{sub 51}Zn{sub 20} eutectic compound was confirmed for both forms of the parent Mg{sub 21}Zn{sub 25} microparticles, and the growth of Mg{sub 51}Zn{sub 20} nanoparticles is discussed. - Highlights: •Comprehensive microstructure characterization of Mg-12 wt.%Zn alloy after ECAP-BP •TEM analysis of changes of intermetallic phases caused by SPD •Mg{sub 21}Zn{sub 25} particles exhibit nano-crystalline regions with grain size below 100 nm. •MgZn{sub 2} preserved its orientation relationship toward highly deformed α-Mg after SPD. •Existence of Mg{sub 21}Zn{sub 25} + Mg{sub 51}Zn{sub 20} eutectic remained for each Mg{sub 21}Zn{sub 25} particle form.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure evolution and properties of graphene nanoplatelets reinforced aluminum matrix composites

Highlights: • Lamellar structure of GNPs fibers and Al matrix was obtained in the rolling direction. • Stacking faults were observed in Al grains due to greatly increased GNPs/Al interfaces. • Strength of Al-0.2 wt% GNPs increased 36.8% with negligible electrical conductivity loss. - Abstract: Aluminum matrix composites reinforced with graphene nanoplatelets (GNPs) were prepared by continuous casting and subsequent rolling. Al-GNPs master alloy, prepared by ball milling and cold pressing, was added into Al melts to fabricate the composites. Microstructure evolution of the composites were observed using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron back scattered diffraction (EBSD). GNPs distributed uniformly in the as-cast specimen and then transformed into fibers in the rolling direction after deformation and finally a lamellar structure formed in the composites. Raman spectra showed that the structural damage of GNPs mainly came from ball milling. The interface between GNPs and Al was well combined in general while a few microcracks were observed, which reduced the ductility of the composites and no adverse aluminum carbide (Al{sub 4}C{sub 3}) was detected at the interface. Stacking faults were observed interior of the aluminum grain, which may due to the existence of plentiful interface introduced by GNPs. Fracture observation revealed that the load transferred from Al matrix to GNPs. The ultimate tensile strength of Al-0.2 wt% GNPs composites was about 36.8% higher than that of pure Al with the same casting and rolling process, which should result from the lamellar structure and load transfer, while the conductivity of the composites decreased slightly, indicating that interface scattering between Al and GNPs is very limited. The investigation results show that Al-GNPs composite is potential for high strength and high conductivity application.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Research on the interfacial structure and property improvement of the Cfiber/2Si-B-3C-N ceramic matrix composite

Highlights: • The C{sub fiber}/2Si-B-3C-N composite with nice properties was successfully prepared. • ZrO{sub 2} promotes the densification process, homogeneous nucleation and grain growth. • The interface reaction in the composite was well controlled by the addition of ZrO{sub 2}. • The damage to carbon fiber was largely avoided by the addition of ZrO{sub 2}. - Abstract: A mixture of the mechanically alloyed 2Si-B-3C-N powder and short carbon fiber was hot pressed at 1800 °C for 30 mins in nitrogen. Careful investigation to the prepared C{sub fiber}/2Si-B-3C-N ceramic matrix composite revealed that the composite was very difficult to be densified. Heterogeneous nucleation, abnormal grain growth and severe interface reaction occurred on the fiber surface or between the fiber and the ceramic matrix, leading to a small density and weak mechanical properties of the composite. For the prepared composite without any additive, the relative density, flexural strength, elastic modulus and fracture toughness is 71.7%, 64.6 MPa, 14.7 GPa and 1.47 MPa·m{sup 1/2}, respectively. The average size of the SiC grains close to and far away from the carbon fiber is 249.10 nm and 50.45 nm, respectively. However, with the addition of ZrO{sub 2} (7.5 mol%), the above mentioned phenomenons are changed noticeably, and those properties are significantly increased to 88.5%, 112.4 MPa, 111.1 GPa and 2.94 MPa·m{sup 1/2}, respectively. At the same time, the SiC grains close to or far away from the carbon fiber almost have the same average size of about 448.21 nm, and almost no interface reaction occurs. It shows that the addition of ZrO{sub 2} not only facilitates the sintering and densification process, but also depresses the interface reaction, promotes the homogeneous nucleation and grain growth, and hence significantly improves the mechanical properties of the C{sub fiber}/2Si-B-3C-N composite. This interesting finding is favorable for the preparation of carbon fiber reinforced ceramic matrix composite, which requires both a large relative density and well controlled interface reaction.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Synthesis of porous Ti–50Ta alloy by powder metallurgy

Highlights: • The proposed preview milling with the next sintering enables the homogeneous porous Ti50Ta (wt).% alloy to be obtained • The α and β nanocrystalline phases were prepared by ball milling. • For the longest sintering time there is complete diffusion of preview milled initial powders • The press and sinter route is proved to be a promising alternative for melted Ti-based biomaterials. - Abstract: The aim of the present study was to assess the possibility of producing porous Ti50Ta (wt%) alloys using powder metallurgy by characterizing the material properties. The influence of the sintering time on the microstructure, mechanical properties, and corrosion behaviour of the produced alloy was investigated. The samples were examined using scanning electron microscopy (SEM), transmission electron microscopy (TEM), optic microscopy, X-ray diffraction (XRD) and nanoindentation methods. The XRD results confirmed that nanocrystalline α and β phases were formed during high-energy ball milling. A longer sintering time of 72 h allowed complete interdiffusion of the elements in the pre-milled initial powders. The sintering time significantly influenced the microstructure and porosity. Additionally, the corrosion resistance of the Ti-Ta samples in Ringer's solution was characterized and differences were observed between the samples sintered at different temperatures. The open-circuit potential (E{sub OCP}) and the corrosion potential (E{sub CORR}) of all samples were similar. The sintered Ti50Ta (wt%) alloy showed better corrosion resistance than equivalent Ti or Ta reference samples. We concluded that these alloys had suitable properties for medical implant applications in the future.

77 NANOSCIENCE AND NANOTECHNOLOGY↗