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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Improving Joint Formation and Tensile Properties of Dissimilar Friction Stir Welding of Aluminum and Magnesium Alloys by Solving the Pin Adhesion Problem

Friction stir welding (FSW), as a solid-state welding technology invented by TWI in 1991, has potential to join dissimilar Al/Mg alloys. In this study, the pin adhesion phenomenon affecting joint quality during FSW of 6061-T6 aluminum and AZ31B magnesium alloys was investigated. The adhesion phenomenon induced by higher heat input easily transformed the tapered-and-screwed pin into a tapered pin, which greatly reduced the tool’s ability to drive the plasticized materials and further deteriorated joint formation. Under the condition without the pin adhesion, the complex intercalated interlayer at the bottom of stir zone was beneficial to mechanical interlocking of Al/Mg alloys, improving tensile properties. However, the formation of intermetallic compounds was still the main reason of the joint fracture, significantly deteriorating tensile properties. Under the welding speed of 60 mm/min without the pin adhesion phenomenon, the maximum tensile strength of 107 MPa and elongation of 1.2% were achieved.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Constitutive Behavior and Forming Limit of Mg-Al-Ca-Gd Alloy Sheets at Room and Elevated Temperatures

The constitutive behavior and forming limit of a newly developed magnesium alloy sheet containing rare earth elements (Mg-Al-Ca-Gd) were investigated. Anisotropic constitutive constants were obtained through tensile tests at various temperatures and strain rates. A modified Grosman equation was adopted to describe the constitutive behavior of the magnesium alloy, taking into account the effects of temperature and strain rate. Results revealed that the proper forming temperature and the strain rate for the newly developed magnesium alloy were 210 °C and 0.001 s{sup −1}, respectively. Forming limit curves at room temperature and 210 °C were, respectively, established by the combination of hemispherical punch tests and theoretical predictions using a modified Marciniak–Kuczynski model. The accuracy of the theoretical prediction of FLD was discussed. The forming limit of Mg-Al-Ca-Gd alloy sheet increased when heated from room temperature to 210 °C.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Phase equilibria in the Tb-Mg-Co system at 500 °C, crystal structure and hydrogenation properties of selected compounds

The isothermal section of the Tb–Mg–Co phase diagram at 500 °C has been built on the basis of XRD analysis of forty samples prepared by powder metallurgy. The existence of two ternary compounds Tb{sub 4}Mg{sub 3}Co{sub 2} and Tb{sub 4}MgCo was confirmed. The formation of two solid solutions, Tb{sub 1−x}Mg{sub x}Co{sub 3} (0≤x≤0.4) and Tb{sub 1-−x}Mg{sub x}Co{sub 2} (0≤x≤0.6), was found for the first time. It is shown that Tb{sub 5}Mg{sub 24} also dissolves a small amount of Co. Other binary compounds do not dissolve the third component. The Tb{sub 4}MgCo and TbMgCo{sub 4} compounds form hydrides (12.7 and 5.3 at.H/f.u. capacity, respectively) that retain the original structure of metallic matrices. Upon thermal desorption the Tb{sub 4}MgCoH{sub 12.7} hydride was stable up to 300 °C and disproportionated at higher temperature. Two other hydrides, Tb{sub 4}Mg{sub 3}Co{sub 2}H{sub ∼4} and Tb{sub 2}MgCo{sub 9}H{sub 12}, are unstable in air and decompose into the initial compounds. - Highlights: • The phase equilibria at 500 °°C in the Tb–Mg–Co system has been studied. • The existence of two ternary compounds, Tb{sub 4}Mg{sub 3}Co{sub 2} and Tb{sub 4}MgCo, was confirmed. • The formation of two solid solutions, Tb{sub 1−x}Mg{sub x}Co{sub 3} and Tb{sub 1−x}Mg{sub x}Co{sub 2}, was found. • Hydrogen sorption–desorption properties have been studied for the selected alloys.

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

Optimizing diffusion bonding parameters to maximize the strength of AA6061 aluminum and AZ61A magnesium alloy joints

The main difficulty when joining magnesium (Mg) and aluminum (Al) alloys by fusion welding process lies in the existence of oxide films and formation of brittle intermetallic in the weld region. However, solid-state welding processes such as friction welding and diffusion bonding are suitable processes to join these two materials. The diffusion bonding process parameters such as bonding temperature, bonding pressure, holding time, and surface roughness of the bond specimen play a major role to determine the joint strength. In this investigation, an attempt was made to develop empirical relationships to predict the lap shear strength and bonding strength of diffusion bonded dissimilar joints of AZ61A magnesium and AA6061 aluminum alloys, incorporating the above-mentioned parameters. Response surface methodology (RSM) was applied to optimize the diffusion bonding process parameters to attain the maximum shear strength and bonding strength of the joint. From this investigation, it is found that the bonds fabricated with the bonding temperature of 420.43°C, bonding pressure of 7.70 MPa, holding time of 27.15 min, and surface roughness of 0.10 μm exhibited maximum shear strength and bonding strength of 51.24 and 72.10 MPa, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Polarization reversal and domain kinetics in magnesium doped stoichiometric lithium tantalate

The polarization reversal process has been studied in 1 mol. % MgO doped stoichiometric lithium tantalate (LT) single crystal. The revealed stages of domain structure evolution represent (1) continuous nucleation and growth of isolated hexagonal domains with walls oriented along Y directions and (2) continuous motion of the plane domain walls stimulated by merging with isolated domains. The activation field dependence of the switching time has been revealed. The coercive field for quasi-static switching is about 150 V/mm. The bulk screening process has been analyzed. The main parameters of the switching process have been compared with other representatives of LT family.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Friction stir welding joint of dissimilar materials between AZ31B magnesium and 6061 aluminum alloys: Microstructure studies and mechanical characterizations

Friction stir welding is an efficient manufacturing method for joining dissimilar alloys, which can dramatically reduce grain sizes and offer high mechanical joint efficiency. Lap FSW joints between dissimilar AZ31B and Al 6061 alloy sheets were made at various tool rotation and travel speeds. Rotation and travel speeds varied between 560–1400 r/min and 16–40 mm/min respectively, where the ratio between these parameters was such that nearly constant pitch distances were applied during welding. X-ray diffraction pattern (XRD), optical microscopy images (OM), electron probe microanalysis (EPMA) and scanning electron microscopy equipped with an energy-dispersive X-ray spectroscopy (SEM-EDS) were used to investigate the microstructures of the joints welded. Intermetallic phases including Al{sub 12}Mg{sub 17} (γ) and Al{sub 3}Mg{sub 2} (β) were detected in the weld zone (WZ). For different tool rotation speeds, the morphology of the microstructure in the stir zone changed significantly with travel speed. Lap shear tensile test results indicated that by simultaneously increasing the tool rotation and travel speeds to 1400 r/min and 40 mm/min, the joint tensile strength and ductility reached a maximum. Microhardness measurements and tensile stress–strain curves indicated that mechanical properties were affected by FSW parameters and mainly depended on the formation of intermetallic compounds in the weld zone. In addition, a debonding failure mode in the Al/Mg dissimilar weld nugget was investigated by SEM and surface fracture studies indicated that the presence of intermetallic compounds in the weld zone controlled the failure mode. XRD analysis of the fracture surface indicated the presence of brittle intermetallic compounds including Al{sub 12}Mg{sub 17} (γ) and Al{sub 3}Mg{sub 2} (β). - Highlights: • Dissimilar Al/Mg joint was obtained by lap friction stir welding technique. • Effect of rotation and travel speeds on the formation of intermetallic compounds • Microstructure and chemical studies including metallography, XRD, SEM-EDS, and EPMA • Mechanical property tests such as stress–strain curves, failure load and hardness • IMCs as Al{sub 3}Mg{sub 2} and Al{sub 12}Mg{sub 17} were identified in weld nugget and at Al/Mg interface.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Electrochemical properties of magnesium doped LiFePO{sub 4} cathode material prepared by sol–gel method

Magnesium doped Li{sub 1−2x}Mg{sub x}FePO{sub 4}/C (x = 0.00, 0.01, 0.03, 0.05) cathode materials were synthesized by sol–gel method, and the effect of magnesium doping as well as its content on the electrochemical properties for lithium batteries was also investigated{sub .} Their morphology was studied with field emission scanning electron microscope and Li{sub 1−2x}Mg{sub x}FePO{sub 4} materials showed the olivine phase without impurities. The thin carbon layer of Li{sub 1−2x}Mg{sub x}FePO{sub 4}/C was confirmed by high resolution transmission electron microscopy. The magnesium doped Li{sub 1−2x}Mg{sub x}FePO{sub 4}/C particles were smaller than those undoped. The Li{sub 1−2x}Mg{sub x}FePO{sub 4}/C materials showed better cycling behavior than undoped LiFePO{sub 4}, especially at high C-rate in which Li{sub 0.94}Mg{sub 0.03}FePO{sub 4}/C composition exhibited the best electrochemical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2018

An Analysis of Characteristics of Magnetostatic Waves Propagating in Nonhomogeneous Fields Across the Ferrospinel Film Thickness

The amplitude-frequency characteristics (AFCs) of magnetostatic waves in the films of magnesium-manganese ferrospinels with nanostructured inhomogeneities are discussed. A common effect, observed in the film AFCs under different process conditions, is the ‘oscillations of propagation’ of magnetostatic waves as a function of the frequency. The oscillation pattern is thought to depend on the inhomogeneous exchange parameters and the surface anisotropy constants. The wave instability is characterized by the resonant interaction of the dipole magnetostatic waves with the surface spin waves. It is shown that the ferrospinel films with periodic nanostructured inhomogeneities of 30–40 nm could be treated as magnon crystals. An inclusion of the inhomogeneity into consideration allows one to provide reasoning for the formation of the rejection bands within the range 9–12 GHz, whose frequencies correspond to Bragg frequencies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Field induced modification of defect complexes in magnesium-doped lithium niobate

Dielectric constant, thermally stimulated depolarization currents (TSDC), and conductivity of undoped and 5% Mg-doped LiNbO{sub 3} single crystals between −100 °C and 200 °C have been investigated. A Debye-like dielectric relaxation with an activation energy of 135 meV is observed in the Mg-doped material, but not in undoped crystals. On heating this relaxation disappears near 140 °C and does not reappear after cooling. Anomalies observed in TSDC around this temperature are attributed to the motion of lithium vacancies, in agreement with conductivity measurements. It is proposed that in thermal equilibrium the electrons from the Mg{sub Li}{sup •} donors are trapped in (4Mg{sub Li}{sup •}+4V{sub Li}{sup ′}) defect complexes. High-temperature poling breaks these defect complexes. The transition of the liberated electrons between the Mg{sub Li}{sup •} donor centers and the Nb{sub Nb} forming the conduction band gives rise to the observed dielectric relaxation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Hydrothermal synthesis of hexagonal magnesium hydroxide nanoflakes

Graphical abstract: Hexagonal Mg(OH){sub 2} nanoflakes were synthesized via hydrothermal method in the presence of PEG-20,000. Results show that PEG-20,000 plays an important role in the formation of this kind of nanostructure. The SAED patterns taken from the different positions on a single hexagonal Mg(OH){sub 2} nanoflake yielded different crystalline structures. The structure of the nanoflakes are polycrystalline and the probable formation mechanism of Mg(OH){sub 2} nanoflakes is discussed. - Highlights: • Hexagonal Mg(OH){sub 2} nanoflakes were synthesized via hydrothermal method. • PEG-20,000 plays an important role in the formation of hexagonal nanostructure. • Mg(OH){sub 2} nanoflakes show different crystalline structures at different positions. • The probable formation mechanism of hexagonal Mg(OH){sub 2} nanoflakes was reported. - Abstract: Hexagonal magnesium hydroxide (Mg(OH){sub 2}) nanoflakes were successfully synthesized via hydrothermal method in the presence of the surfactant polyethylene glycol 20,000 (PEG-20,000). Results show that PEG-20,000 plays an important role in the formation of this kind of nanostructure. The composition, morphologies and structure of the Mg(OH){sub 2} nanoflakes were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). The SAED patterns taken from the different positions on a single hexagonal Mg(OH){sub 2} nanoflake show different crystalline structures. The structure of the nanoflakes are polycrystalline and the probable formation mechanism of Mg(OH){sub 2} nanoflakes is discussed. Brunauer–Emmett–Teller (BET) analysis were performed to investigate the porous structure and surface area of the as-obtained nanoflakes.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure Evolution and Mechanical Properties of the AZ91 Magnesium Alloy with Sr and Ti Additions in the As-Cast and As-Aged Conditions

Effect of different amounts of Sr and Ti additions with T6 heat treatment on microstructure and mechanical properties of the AZ91 alloy was investigated. The microstructure of the AZ91 alloy is consisted of the α-Mg phase surrounded by the semicontinuous network of β-Mg{sub 17}Al{sub 12} intermetallics. The average grain size and volume fraction of β phase decreased up to 1 wt.% Sr and 0.1 wt.% Ti additions. Exceeding Ti additions led to increase in both grain size and volume fraction. It was found that the grain refinement mechanism of AZ91 alloys with Sr additions was grain restriction factor. Also, 0.1 wt.% Ti addition could refine the grain structure of the AZ91 alloy by the formation of Al{sub 3}Ti particles that act as nucleation sites for the α-Mg phase. Furthermore, adding 1 wt.% Sr and 0.1 wt.% Ti enhanced the hardness values for both cast and aged AZ91 alloy. Moreover, ultimate tensile strength and tensile elongation values reached their maximum amounts with the 0.5 wt.% Sr and 0.1 wt.% Ti additions and more Sr and Ti additions deteriorated the mechanical properties of cast and aged AZ91 alloys. The fracture surface characterizations had good relations with the tensile properties. The fracture modes were cleavage and quasi-cleavage in the cast and aged AZ91 alloys. Furthermore, the optimum levels of Sr and Ti additions altered the fracture modes to more quasi-cleavage fractures.

36 MATERIALS SCIENCE↗
DOE OSTI2019

A Comparative Study on Microstructure, Mechanical and Tribological Properties of A4, AE41, AS41 and AJ41 Magnesium Alloys

Microstructure, tensile and wear properties of as-cast A4 (Mg-4Al), AE41 (Mg-4Al-0.5Ce-0.5La), AS41 (Mg-4Al-1Si) and AJ41 (Mg-4Al-1Sr) alloys were investigated, and the results were compared with each other in this study. Microstructures were investigated by XRD, optical and scanning electron microscopes. Tensile tests were conducted at both room and elevated temperatures. Tribological properties were examined by pin-on-disk wear tests under different applied loads. Microstructure characterizations revealed that the volume fraction of second phases considerably increased by alloying additions of 1 wt.% Ce/La, Si and Sr. The microstructure of A4 alloy consisted of α-Mg grains and divorced β-Mg{sub 17}Al{sub 12} phases. After individual alloying additions of 1 wt.% Ce/La, Si and Sr, the secondary phases were primarily replaced by needle-shaped and massive blocky-shaped Al{sub 11}(Ce,La){sub 3} phases in AE41 alloy, Chinese-script-type Mg{sub 2}Si phases in AS41 alloy and divorced globular-like and massive blocky-shaped Al{sub 4}Sr and (Mg,Al){sub 17}Sr{sub 2} phases in AJ41 alloy. The tensile tests showed that at both room and elevated temperatures alloying additions of 1 wt.% Ce/La, Si and Sr resulted in an increase in the strength but a decrease in the ductility. Among the studied alloys, AS41 alloy exhibited the best strength. Wear test results showed that AE41 and AJ41 alloys similarly exhibited the best wear resistance owing to the presence of hard and dense intermetallics. Abrasion was the main wear mechanism under low applied loads while delamination, adhesion and oxidation mechanisms were majorly observed under high applied loads.

36 MATERIALS SCIENCE↗
DOE OSTI2017

AC Electrical Properties of MgFe{sub 2−x}Nd{sub x}O{sub 4} (x = 0 to 0.04) for High Frequency Applications

Magnesium ferrite was doped with Nd to get MgFe{sub 2−x}Nd{sub x}O{sub 4} (x = 0 to 0.04). The samples were synthesized by a without water and surfactant (WOWS) sol-gel technique, and pellets were sintered at different temperatures. An inverse spinel structure was confirmed by the X-ray diffraction (XRD) pattern. Lattice constants increased with the increase in Nd doping. Microstructure analysis by SEM shows that particles are spherical in shape. AC electrical properties were studied at room temperature in the frequency range of 20 Hz–3 MHz. A minimum dielectric loss tangent value was achieved with x = 0.02 concentration of Nd, and it is suitable for high-frequency applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Vibrational, Elastic Properties and Sound Velocities of MgTi{sub 2}O{sub 4} Spinel

Zone-centre phonon frequencies, elastic properties and sound velocities along high-symmetry directions and Debye temperature of MgTi{sub 2}O{sub 4} spinel is calculated using rigid ion model. The interatomic interactions up to third nearest neighbours are also calculated. The main results of the present paper reveals that the tetrahedral bonding is stronger than the octahedral bonding. Calculated results are compared with previously available experimental and theoretical data. It is observed that the obtained results are in good agreement with the available results in literature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Synthesis and Characterisation of Superparamagnetic MgFe{sub 2}O{sub 4} Nanoferrite

Nanocrystalline MgFe{sub 2}O{sub 4} with crystallite size of 10.4 nm was synthesised using combustion method and characterised for structural, electrical, and magnetic properties. The powder X-ray diffraction analysis reveals the single-phase nature of MgFe{sub 2}O{sub 4}. The field emission scanning electron microscopy image shows the nanosized grains. The magnetic study reveals the superparamagnetic behaviour of MgFe{sub 2}O{sub 4} with saturation magnetisation value of 14.57 emu/g. The blocking temperature is determined using low-temperature magnetic measurement which is equal to 139 K. Electrical study shows that the value of AC conductivity at 1 Hz is 1.12 × 10{sup − 8} S/cm at 303 K and increases with frequency as well as temperature. The complex impedance analysis shows the grain and grain boundary resistance equal to 6.23 × 10{sup 6} Ω and 1.27 × 10{sup 7} Ω, respectively. The complex electric modulus analysis confirms the presence of two relaxation processes in nanocrystalline MgFe{sub 2}O{sub 4}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Structure and Magnetic Properties of Graded (001)-Oriented FePt Films Prepared by Magnetron Sputtering and Rapid Thermal Annealing

L1{sub 0}-FePt/[Fe/Pt]{sub 27} multilayer films were prepared on (100)-oriented single-crystal MgO substrates. Structural analysis indicates epitaxial growth of the L1{sub 0}-FePt films during high-temperature deposition. The room-temperature-deposited [Fe/Pt]{sub 27} multilayer exhibits the fcc-FePt structure grown epitaxially on the L1{sub 0}-FePt base layer. The degree of chemical order and thereby the anisotropy of the top layer has been controlled by varying the temperature of a post-deposition annealing process. Compared with the as-deposited films, coercivities of the films annealed below 400 {sup ∘}C are substantially decreased by introducing the formation of a graded anisotropy in the partially ordered top layer. Coercivities decrease for annealing temperatures that are greater than 400 {sup ∘}C due to improved ordering of the top layer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Effects of Co Substitution on the Microstructural, Infrared, and Electrical Properties of Mg{sub 0.6 − x}Co{sub x}Zn{sub 0.4}Fe{sub 2}O{sub 4} Ferrites

Spinel ferrites having Mg{sub 0.6 − x}Co{sub x}Zn{sub 0.4}Fe{sub 2}O{sub 4} (0 ≤ x ≤ 0.6) compositions were prepared using the sol–gel method. XRD patterns show that samples have the cubic phase of the spinel ferrite with the Fd3̄m space group. The obtained values of the lattice constant and porosity increase with increasing cobalt concentration. FTIR spectra show only the tetrahedral stretching peaks that are shifting towards higher frequencies with increasing Co content. The substitution of Mg by Co leads to the decrease of conductivity of the samples. The behaviors of the real part of permittivity and loss tangent have been investigated using Maxwell theory. Electrical modulus and impedance results show the presence of the electrical relaxation phenomenon for the samples with non-Debye type. The Nyquist representations have been analyzed using a proposed electrical circuit, and the results show that the resistances of the prepared samples resulted mostly from the grain boundary effect.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Thermal Decomposition Synthesis of MgFe{sub 2}O{sub 4} Nanoparticles for Magnetic Hyperthermia

In this work, the effects of the thermal decomposition temperature and time on the structure, cation distributions, magnetic properties, and AC magnetically induced heating characteristics of MgFe{sub 2}O{sub 4} nanoparticles were investigated. The structure and cation distributions between the tetrahedral and octahedral sites determined by X-ray diffraction method showed single-phase MgFe{sub 2}O{sub 4} nanoparticles had a partially inverse structure. The inversion coefficient increased using high reaction temperature and time. The square-like shape and small magnesium ferrite nanoparticles with narrow particle size distribution were synthesized at high reaction temperatures, as observed by transmission electron microscopy. Magnetic properties of MgFe{sub 2}O{sub 4} nanoparticles studied by vibrating sample magnetometry showed the ferrimagnetic characteristics with the highest saturation magnetization of 24 emu/g at a reaction temperature of 300 °C for 60 min. Furthermore, the AC magnetically induced heating characteristics of MgFe{sub 2}O{sub 4} nanoparticles were correlated to the saturation magnetization and coercivity.

77 NANOSCIENCE AND NANOTECHNOLOGY↗