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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.

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At least 1,783 records · Page 99
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Microstructure and Mechanical Properties of Cast Al-5Zn-2Mg Alloy Subjected to Equal-Channel Angular Pressing

In the present work, cast Al-5Zn-2Mg alloy was processed through equal-channel angular pressing (ECAP) in route B{sub C} up to four number of passes. Microstructure and mechanical properties were investigated on processed and unprocessed materials. In cast condition, the material was composed of dendritic structure. After homogenization treatment, large-sized grains were observed. After ECAP processing, significant grain refinement was observed. After ECAP processing, high-density dislocations and high degree of misorientation between the grains were observed. In cast material, rod-shaped precipitates were observed, while, after ECAP processing, spherical-shaped precipitates were observed. ECAP processing leads to a noticeable improvement in the mechanical properties of the material. After four passes, 122% improvement in the microhardness and 135% improvement in the ultimate tensile strength of the material were observed. After three passes, a slight decrease in the mechanical properties was observed. This is attributed to the dissolution of the metastable η′ phase, annihilation of dislocations, dynamic recrystallization and texturing during ECAP processing. Brittle fracture mode was observed in tensile testing cast and homogenized samples. After ECAP processing, fracture mode was changed into shear fracture mode.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructural Characterization and Electrochemical Behavior of AA2014 Al-Cu-Mg-Si Alloy of Various Tempers

AA2014 Al-Cu-Mg-Si alloy is subjected to aging heat treatment, and microstructural features and electrochemical behavior of the AA2014 Al-Cu-Mg-Si alloy of various tempers have been characterized. Hardness measurement of the alloy of different tempers showed typical age hardening behavior. X-ray diffraction and transmission electron microscopy studies revealed the presence of characteristic phases. Differential scanning calorimetry study displayed the sequence of solid-state precipitation and dissolution reactions in the AA2014 alloy of various tempers. Electrochemical behavior was assessed by measurement of variation of open-circuit voltage with time, potentiodynamic polarization and potentiodynamic cyclic polarization of the AA2014 alloy of various tempers in 3.5 wt.% NaCl solution at near neural (pH ~ 7) and at alkaline (pH 12) conditions. Potentiodynamic polarization studies of different tempers in 3.5 wt.% NaCl solution at pH ~ 7 showed that the corrosion potential (E{sub corr}) value has shifted toward more negative (active) direction with increasing aging time. The cyclic polarization in 3.5 wt.% NaCl solution at pH ~ 7 displayed active–passive behavior and also showed a large hysteresis loop indicating high pit growth damage.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Zinc Interlayer in Microstructure Evolution and Mechanical Properties in Dissimilar Friction Stir Welding of Aluminum to Titanium

The welding of aluminum (Al) and titanium (Ti) is difficult and challenging due to the differences in their chemical and physical properties, and the formation of brittle intermetallic phases. In the present experiment, a zinc (Zn) interlayer was used during friction stir welding of Al to Ti. The weld was characterized in detail to understand the mechanisms associated with microstructural evolution and improvement in mechanical properties of the weld. X-ray computed tomography results reveal three-dimensional distribution of particles and flakes of titanium in the weld nugget. It was also observed that Ti particles are inhomogeneously distributed in the weld and the distribution depends on their morphology. Such a distribution of Ti was informative in understanding material flow. In addition, the consolidation of Zn in the Al matrix reveals the nature of material flow in the weld nugget as well. The importance of the Zn interlayer and mechanism of phase formation was explored in this study. It was characterized that the mechanical mixing of Zn with Al and Ti alters phase evolution and restricts the formation of conventional Al{sub 3}Ti intermetallic phase. The presence of zinc homogenizes elemental distribution and inhibits the formation of brittle intermetallic phases, which leads to a substantial improvement in mechanical properties of the weld.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure, Phase Transformation, Mechanical Behavior, Bio-corrosion and Antibacterial Properties of Ti-Nb-xSn (x = 0, 0.25, 0.5 and 1.5) SMAs

Porous Ti-Nb-xSn shape memory alloys (SMAs) are fabricated by microwave sintering technology. The microstructures exhibit needle-like morphologies, β{sub (N)} (normal straight and crossed needles along with needle-like morphology that resembles spaghetti or irregular lines with α-phases in between) as well as plate-like morphologies [normal straight plate-like morphology, α′′ and dendritic plate-like morphology, β{sub (D)}]. Increases in Sn addition significantly induce an increase in the density of the α-phase. XRD patterns exhibited three phases, namely the β-main phase with smaller intensities of α′′ and α. Further, the addition of 0.25% Sn led to more effective improvement in the intensity of the α′′-phase compared with 0.5% and 1.5% Sn addition. Additions of Sn also enhanced the fracture strength and its corresponding strain along with the shape memory effect (SME), where the best enhancement was achieved at 0.25% Sn. The corrosion rate (Ri) was reduced by rising Sn content, while both corrosion resistance and antibacterial zones were increased. The lower elastic modulus, as well as the robust mechanical properties and bioactivity, made these SMAs rather suitable for biomedical application purposes, where the low elastic modulus had value in terms of avoiding the problem of “stress shielding.”.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Fabrication, Analysis of Microstructure and Oxidation Resistance of Cemented C/C Composites by Laser

Cylindrical carbon–carbon composites have been cemented using a CO{sub 2} laser actuated by a raster scanning mirror system in a four-step sequence. Complete and homogenous surface cementation as well as thermal stability of the composites was sought after and achieved. The line of heat created from the laser beam impinged parallel to the axis of the pre-coated cylinder, while suspended from one end and slowly rotated. Four slurry compositions, all containing chloride salts from refractory elements, some combined with silicon, were pre-deposited by air spraying it over each specimen and then laser treated sequentially. Confined heat sublimed the chlorine ions resulting in silicide compounds that reacted with the carbon substrate to form a bonding layer. Microstructure characterization was performed using scanning electron microscopy, energy-dispersive spectroscopy and x-ray diffraction, immediately after the cementation was consolidated and after heat treated under argon. Preliminary oxidation weight loss tests were performed up to 1423 K under argon and air, revealing a 9.8 and 30.6% weight reduction, respectively, after 9 h of exposure. Cemented specimens were then coated with a glass overcoat layer that allowed them to perform moderately better against oxidation achieving up to a 28% weight loss after 20 h at 1273 K in air.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Cold Deformation and Heat Treatment on the Microstructure and Mechanical Behavior of High Entropy Alloy CuCrFeNi{sub 2}Al{sub 0.5}

CuCrFeNi{sub 2}Al{sub 0.5}, a high entropy alloy (HEA) with a single FCC phase, was deformed at room temperature and then heat treated. The small particles precipitated from the deformed FCC solid solution matrix after heating at 700 °C, softening the solid solution matrix. This outcome confirms that the microstructure of HEAs can be controlled by heat treatment in addition to controlling the chemical composition and the rate of cooling from liquid alloys. The hardness of the samples increased due to precipitation hardening, and the specimen heated at 700 °C with 10% predeformation had the highest hardness. As the predeformation increases, the precipitates tend to interweave in the network morphology. Meanwhile, grains in all specimens became more severely coarse after heat treatment at 1100 °C. Predeformation followed by annealing can obviously slow down the creep rate, indicating that annealing at an appropriate temperature can improve the mechanical properties of deformed HEA.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and Mechanical Properties of Powder Thixoforged Amorphous Ni{sub 55}Nb{sub 35}Si{sub 10}-Reinforced Al Matrix Composites

Powder thixoforging was used to produce amorphous Ni{sub 55}Nb{sub 35}Si{sub 10}-reinforced Al matrix composites from recycled Al powders obtained from ball-milled Al520 end-milled billets. The amorphous reinforcing material was uniformly dispersed in the Al matrix and bonded adequately with it without undesired interfacial reactions. The compressive yield strength of composites with 45% reinforcement content increased 248% to 794 MPa over as-cast samples. The maximum elongation and hardness of samples were 15%, and 60.4 HRA, respectively. The relative density of the unreinforced thixoforged sample was 99.8%, indicating near-perfect compaction of Al powders. The maximum elongation, hardness, and compressive yield strength of thixoforged monolithic matrix alloys were, respectively, 40%, 56.9 HRA, and 747 MPa (a 228% increase in CYS over the cast alloy). In contrast, cold-forged samples showed only 82% increase in compressive yield strength over the as-cast alloy. Powder thixoforging introduced in this study—even without reinforcement—delivers better mechanical properties compared to cold forging most likely due to the refinement and modification of the microstructure during powder ball milling.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and Corrosion Properties of Laser-Welded SAF 2507 Super Duplex Stainless Steel Joints

SAF 2507 super duplex stainless steel has been welded using laser beam (LB) welding and laser/gas metal arc hybrid (LGH) welding processes. The pitting and intergranular corrosion properties of the welding joints have been tested using electrochemical testing and microstructure observations. The volume fraction of the ferrite phase reaches 70% in the weld zone of the LB welding joint, while the volume fraction of the ferrite phase is 60% for the welding joint of LGH due to the introduction of Ni from welding wire. The pitting corrosion resistance of the welding joint for LB is better than that of LGH. The pitting corrosion is easily conceived in the heat-affected zone (HAZ) because of the formation of Cr{sub 2}N in the HAZ during welding.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effects of Thermal Exposure on the Microstructure and Mechanical Properties of Al-Si-Cu-Ni-Mg-Gd Alloy

The present study focused on the effect of thermal exposure on the phase evolution and properties of the cast-T6 Al-12Si-4Cu-2Ni-0.8Mg-xGd (x = 0, 0.1, and 0.2) alloy used for cast piston.The thermal stability of the alloys was studied in the temperature range of 200-300 °C for exposure duration up to 500 h, thus obtaining the relationship between the microstructural evolution and its impact on the mechanical properties. The fine Al{sub 2}Cu phase grew into the coarse θ-Al{sub 2}Cu phases during the thermal exposure, resulting in a decrease in the mechanical strength of the alloy. The thermal stability and stiffness of the Al-Si-Cu-Ni-Mg alloys were improved with the increase in Gd content from 0 to 0.2%.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Improvement of Microstructure and Mechanical Properties of CoCrCuFeNi High-Entropy Alloys By V Addition

V element had positive effect in improving the strength of many alloys, so it was possible that V had potential to strengthen CoCrCuFeNi high-entropy alloys (HEAs) with face-centered cubic (FCC) crystal structure, which was relatively weak in strength and had outstanding ductility. In this paper, we studied the alloying effect of V on the phase evolution, microstructure and the mechanical properties of the (CoCrCuFeNi){sub 100−x}V{sub x} (x = 0-16, atomic ratio, hereafter in at.%) HEAs systematically. The results showed that V element had capacity to induce sigma phase precipitation. The volume fraction of sigma phase increased from 0 to 12%; the compressive yield stress of (CoCrCuFeNi){sub 100−x}V{sub x} HEAs increased from 300 to 613 MPa with V content increasing from 0 to 16% (atomic ratio, hereafter in at.%). However, the compression fracture strain decreased from 50 to 28%. V addition was beneficial in improving the strength of CoCrCuFeNi HEA, and the increase in sigma phase volume fraction was the key factor for the improvement of the (CoCrCuFeNi){sub 100−x}V{sub x} HEAs in yield stress.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Enhanced Surface Properties and Microstructure Evolution of Cr12MoV Using Ultrasonic Surface Rolling Process Combined with Deep Cryogenic Treatment

In this study, the enhanced surface properties and microstructure evolution of quenched and tempered cold work die steel Cr12MoV, which were induced by ultrasonic surface rolling process combined with deep cryogenic treatment (UDCT), were systematically investigated. The results indicated that UDCT had an advantage over conventional ultrasonic surface rolling process (USRP) in improving surface mechanical properties, including smaller surface roughness and smoother machined surface with less cracks and defects; higher surface microindentation hardness with the value of ~ 890 HV (increased by ~ 6.2% compared with USRP); and lower friction coefficient. Such enhancements in surface properties are mainly attributed to the martensitic transformation of retained austenite, the dispersion strengthening of small secondary carbides and the homogenized carbides distribution during UDCT.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Influence of Rolling Temperature on the Mechanical Properties and Microstructure of Variable-Plane-Rolled Mg-3Al-1Zn Alloy

AZ31 magnesium alloy was produced via variable-plane rolling (VPR) under different temperatures. The effects of rolling temperature on the microstructure, texture and mechanical properties were investigated. A double-peak basal texture was formed during VPR treatment. Dynamic recrystallization (DRX) was observed during the VPR treatment, and it can reduce the texture intensity owing to the coalescence of sub-grains. Four types of twins were observed in the VPR treatment: {10-12} extension twins, {10-13}, {10-11} contraction twins and {10-11}-{10-12} double twins. The suitable temperature of AZ31 magnesium alloy for VPR treatment is determined as 623 K through experiments with tensile yield strength of 240 MPa, compressive yield strength of 162 MPa and elongation of 13%. Owing to the important impact of temperature on mechanical properties, the alloy was further VPRed with a decrease in temperature pass-by-pass. The high temperature of 623 K at the initial passes randomized the texture, which led to notably high ductility.

36 MATERIALS SCIENCE↗
DOE OSTI2019

The Effect of Brazing on Microstructure of Honeycomb Liner Material Hastelloy X

Labyrinth seals with abradable metallic honeycomb structures are common in gas turbines to ensure a tight sealing between rotor and stator. These seals have a big impact on the turbine efficiency. The honeycombs made from nickel-based superalloys are brazed onto backing segments in order to form liners. In this work, the microstructure of Hastelloy X honeycombs after brazing with a nickel-chromium-silicon braze filler alloy is investigated. It was found that the braze filler can wet the honeycomb foils over their entire height up to the contact surface with the rotor. After solidification, the braze filler alloy forms γ and silicides (NiSi and Ni{sub y}(Mo, Cr){sub x}Si). The intermetallic silicides give high hardness to the honeycomb liners. Between the braze filler alloy and the honeycomb foils, interdiffusion zones are observed. For the rub-in behavior of the honeycomb liners, not only the foil material but also the comportment of the composite made up of the foil material, the braze filler material and the interdiffusion zones has to be considered.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Microstructure and Properties of Ni-WC Composite Coatings on Their Solid Particle Erosion Behavior

Metal matrix composites (MMCs) of WC in Co or Ni matrices are widely used due to their high wear resistance. Previous studies showed that tailoring the mechanical properties through optimum WC content can significantly extend the life cycle of MMCs. In this study, composite coatings of Ni with various WC content ranging from 10 to 55 vol.% were cold sprayed using agglomerated WCNi powders and two different sizes of Ni powders, − 10 + 4 (d{sub 50} = 7 µm) referred to as Ni(7) and − 35 + 15 (d{sub 50} = 25 µm) to as Ni(25). Microstructural characterization, including the deformed structure of Ni splats and retention and distribution of WC, was performed by scanning electron microscopy (SEM). Composite coatings were subjected to solid particle erosion following ASTM G76-13 standard. At low WC content coatings, erosion occurred primarily by plastic deformation of Ni matrices and knocking out of WC particles. At high WC concentrations, network distribution of compacted WCNi particles in the matrices led to a significant drop in erosion rates of coatings, due to a change in erosion mechanism from severe cutting and plowing of Ni and WC dislodging to WCNi splats spalling. Composite coatings sprayed using finer size Ni powder were usually more erosion resistant.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and Mechanical Characterization of Incoloy 925 Welds in the As-Welded and Direct Aged Conditions

The joints of Incoloy 925 obtained from gas tungsten arc welding (GTAW) with pulsating current utilizing ERNiCrMo-3 and ERNiCrMo-10 were investigated under as-welded and direct aged conditions. The microstructure showed the microsegregation of Nb-Mo-, Ti-rich phases in the weld zone of ERNiCrMo-3 in the as-welded condition. Post-weld heat treatment (PWHT) involving direct aging was carried out on the weldments at 732 °C for 4 h followed by air cooling. On direct aging, the tensile strength of the base metal increases by 40% when compared to the as-received ones. The failures were experienced in the parent metal of Incoloy 925 upon tension testing in as-welded conditions. Although the tensile strength was improved considerably, the failures occurred at the fusion zones, for both the fillers in the direct aged conditions. The joint efficiencies of Incoloy 925 welds employing ERNiCrMo-3 and ERNiCrMo-10 were reported as 95 and 86%, respectively. The impact toughness of the weldments in both the as-welded and PWHT conditions was considerably greater compared to that of the candidate metal.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and Mechanical Properties of As-Cast γ-TiAl Alloys with Different Cooling Rates

γ-TiAl alloys composed of Ti-45Al-5Nb-1W-1B (at.%) solidified at three different cooling rates by vacuum electromagnetic induction melting under a controlled degree of vacuum of under 2 Pa. The influence of the cooling rate on the microstructure and mechanical properties of the γ-TiAl alloy samples was investigated. A relatively large temperature gradient at the beginning of solidification led to a thin fine-grained layer around the graphite mold, and undercooling had two main effects: (a) promoting nucleation in the grain-refining surface layer and (b) promoting the growth of columnar crystals beneath the surface. Moreover, all prepared ingots of this alloy could be clearly divided into Regions S, M and C from the surface to the core. The colony size of Region M was larger than those of the other two regions due to composition segregation and the growth of columnar grains. The existence of tungsten led to micro-inhomogeneity, and the mechanical properties of the alloy changed according to concentration fluctuations. The degrees of homogeneity were expressed as H{sub 0.1} and H{sub 0.2}. The alloy with the highest values, which possesses the best mechanical properties, solidified at a relatively low cooling rate. The deformation mechanism is also discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructural Analysis of Powder Metallurgy Tool Steels in the Context of Abrasive Wear Behavior: A New Computerized Approach to Stereology

The present work describes a new methodology designed to characterize the microstructures of tool steels containing carbide hard phases, with the focus set on their abrasive wear resistance. A series of algorithms were designed and implemented in MATLAB® to (i) recognize each of the features of interest, (ii) measure relevant quantities and (iii) characterize each of the phases and the alloy in function of attributes usually neglected in wear description applications: size distribution, shape and contiguity of the hard phases. The new framework incorporates new parameters to describe each one of these attributes, as observed in SEM micrographs. All three aforementioned stages contain novel contributions that can be potentially beneficial to the field of materials design in general and to the field of alloy design for severely abrasive environments in particular. Models of known geometry and micrographs of different powder metallurgy steels were analyzed, and the obtained results were compared with the obtained by the linear intercept method. The relation between the new parameters and the ones available in the scientific literature is also discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effects of Bi-Sb Addition and Solution Treatment on Microstructures and Mechanical Properties of Al-20 wt.% Mg{sub 2}Si Composites

In this study, the effects of compound modification with Bi and Sb and solution treatment on the microstructures evolution and the mechanical properties of Al-20 wt.% Mg{sub 2}Si composites were investigated. The specimens were observed by optical microscope, scanning electron microscope equipped with an energy-dispersive spectrometer and x-ray diffraction. The characterization results showed that 2.0 wt.% Sb and 0.2 wt.% Bi addition refined the primary Mg{sub 2}Si from 95 to 10 μm, accompanied by a change in morphology from coarse dendrites to dispersive fine polygons. After solution treatment at 540 °C for 15 h, the sharp polygonal primary Mg{sub 2}Si particles were blunted and the flake-like eutectic Mg{sub 2}Si phases were changed to small spherical particles, resulting in the increase in the Brinell hardness of the composites from HB 94.53 to 134.67. Mechanical tests revealed that both tensile strength and hardness of the composite were improved by compound modification with Bi and Sb, and the ultimate tensile strength (UTS) was increased from 216 MPa unmodified to 257 MPa with optimum modification of 2 wt.% Sb and 0.2 wt.% Bi. A propounded change in hardness from HB 94.53 to 130.67 was also investigated.

36 MATERIALS SCIENCE↗