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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 1,297 records · Page 72
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

The Effect of Thermomechanical Processing on the Tensile, Fatigue, and Creep Behavior of Magnesium Alloy AM60

Tensile, fatigue, fracture toughness, and creep experiments were performed on a commercially available magnesium-aluminum alloy (AM60) after three processing treatments: (1) as-THIXOMOLDED (as-molded), (2) THIXOMOLDED then thermomechanically processed (TTMP), and (3) THIXOMOLDED then TTMP then annealed (annealed). The TTMP procedure resulted in a significantly reduced grain size and a tensile yield strength greater than twice that of the as-molded material without a debit in elongation to failure ({epsilon}{sub f}). The as-molded material exhibited the lowest strength, while the annealed material exhibited an intermediate strength but the highest {epsilon}{sub f} (>1 pct). The TTMP and annealed materials exhibited fracture toughness values almost twice that of the as-molded material. The as-molded material exhibited the lowest fatigue threshold values and the lowest fatigue resistance. The annealed material exhibited the greatest fatigue resistance, and this was suggested to be related to its balance of tensile strength and ductility. The fatigue lives of each material were similar at both room temperature (RT) and 423 K (150 C). The tensile-creep behavior was evaluated for applied stresses ranging between 20 and 75 MPa and temperatures between 373 and 473 K (100 and 200 C). During both the fatigue and creep experiments, cracking preferentially occurred at grain boundaries. Overall, the results indicate that thermomechanical processing of AM60 dramatically improves the tensile, fracture toughness, and fatigue behavior, making this alloy attractive for structural applications. The reduced creep resistance after thermomechanical processing offers an opportunity for further research and development.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Fabrication of Copper-Rich Cu-Al Alloy Using the Wire-Arc Additive Manufacturing Process

An innovative wire-arc additive manufacturing (WAAM) process is used to fabricate Cu-9 at. pct Al on pure copper plates in situ, through separate feeding of pure Cu and Al wires into a molten pool, which is generated by the gas tungsten arc welding (GTAW) process. After overcoming several processing problems, such as opening the deposition molten pool on the extremely high-thermal conductive copper plate and conducting the Al wire into the molten pool with low feed speed, the copper-rich Cu-Al alloy was successfully produced with constant predesigned Al content above the dilution-affected area. Also, in order to homogenize the as-fabricated material and improve the mechanical properties, two further homogenization heat treatments at 1073 K (800 °C) and 1173 K (900 °C) were applied. The material and mechanical properties of as-fabricated and heat-treated samples were compared and analyzed in detail. With increased annealing temperatures, the content of precipitate phases decreased and the samples showed gradual improvements in both strength and ductility with little variation in microstructures. The present research opened a gate for in-situ fabrication of Cu-Al alloy with target chemical composition and full density using the additive manufacturing process.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effect of Joining Atmosphere in Transient Liquid Phase Bonding of Inconel 617 Superalloy

In the present paper, transient liquid phase (TLP) bonding of Inconel 617 alloy under the different furnace atmospheres of vacuum, argon gas, and air was investigated. The experiments were carried out at 1408 K (1135 °C) for 300 minutes by using BNi-1 filler metals with different thicknesses of 50 and 100 µm. The microstructure, chemical analysis, shear strength, and microhardness of the resultant joints were studied. After 300 minutes, complete isothermal solidification was obtained for both the interlayers having 50 and 100 µm thicknesses under all the furnace atmospheres. The specimens bonded under vacuum condition showed the highest shear strength, failure energy, and ductility. Also, the joint produced under argon gas atmosphere using 100-µm-thick filler metal showed a good strength and may be acceptable in many applications. However, the specimens bonded under air atmosphere were weak, due to their further cavities and non-dissolved carbides. The presence of non-dissolved carbides in the diffusion-affected zone resulted in increasing the hardness of the base metal near the interface. The carbides were dissolved and the hardness of the diffusion-affected zone was reduced under vacuum condition compared to argon and air atmospheres. The best result was obtained for the specimen joined using the 50-µm-thick interlayer under vacuum atmosphere, with about 95.8 pct joint efficiency.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Use of Friction Stir Processing for Improving Heat-Affected Zone Liquation Cracking Resistance of a Cast Magnesium Alloy AZ91D

In this work, a cast magnesium alloy AZ91D was friction stir processed. Detailed microstructural studies and Gleeble hot ductility tests were conducted on the as-cast and the FSPed samples to comparatively assess their heat-affected zone liquation cracking behavior. The results show that the use of FSP as a pretreatment to fusion welding can strikingly improve the heat-affected zone liquation cracking resistance of alloy AZ91D by reducing the amount and size of the low-melting eutectic β (Mg{sub 17}Al{sub 12}) as well as by refining the matrix grain size.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Mechanical Properties, Microstructural Evolution, and the Effect of Friction on the Plastic Flow of the AISI 321 Austenitic Stainless Steel Tube During Cold Pilgering: An Experimental and Simulation Analysis

The mechanical properties, microstructural evolution, and the effect of friction on the plastic flow of the AISI 321 austenitic stainless steel (ASS) tube were investigated during the cold pilger process. The elastic–plastic behavior of as-received tube was simulated by the Johnson-cook model. The model parameters were obtained by the compression and tensile tests. The mechanical properties of the material were examined by the tensile and microhardness tests. Based on the ring compression test results, three different situations for friction conditions were considered. The stress and strain states at these conditions were examined for the outer and inner surfaces of the tube. Numerical evaluation of plastic shearing due to the cold pilgering of the AISI321 tube was performed. The X-ray diffraction, ferrite scope tests, and optical microscope were also used for the microstructure evaluation and verification of the simulation results, respectively. The Latham-Cockcraft damage was calculated for different conditions by introducing the new subroutine, showing that it was strongly dependent on the friction conditions and the shear strain, ε{sub zr}, respectively. It was shown that the friction coefficient of 0.3 resulted in the minimum damage of the tube and the different friction conditions between the two surfaces of the tube enhanced the damage function. Also, the strain induced ά-martensite increased the work-hardening capacity and affected the ductility of the tube.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Processing Techniques for ODS Stainless Steels

Oxide dispersion-strengthened (ODS) alloys have been studied extensively, in part as a response to the need to develop the high-temperature materials that are required for energy efficient systems. ODS stainless steels, in particular, are of technological interest because of their high ductility and high strength at elevated temperatures, which when combined with their resistance to corrosion, render them ideal for many applications that require extreme performance quality. Survey of the published literature reveals that efforts have been devoted to incorporate specific strengthening mechanisms in order to meet demanding performance criteria for ODS alloys. As a result, a number of novel processing approaches have been developed in an effort to design distinct microstructures that can enhance the performance of ODS alloys. In the current paper, various techniques used to manufacture ODS stainless steel alloys are compared and summarized on the basis of a literature search, and results are contrasted with selected experimental data obtained by the authors. In particular, cryomilling, water-atomization, and melt-spinning processes are described in detail as three novel approaches that can be effectively used to distribute nanometric oxides in 316L stainless steel matrix. The microstructural features, mechanical properties, and sintering behavior of the fabricated materials are described and discussed. The critical processing parameters for each processing technique are discussed with reference to the properties of the material. The versatility and potential challenges of the individual techniques are also described and contrasted.

36 MATERIALS SCIENCE↗
DOE OSTI2019

A Novel Shim-Assisted Resistance Spot Welding Process to Improve Weldability of Medium-Mn Transformation-Induced Plasticity Steel

Medium-Mn transformation-induced plasticity steels have great potential to significantly reduce vehicle weight and improve fuel economy due to their outstanding combination of high strength and excellent ductility. One bottleneck to the application is their poor weldability resulting from their high Mn contents. In this paper, three resistance spot welding set-ups, including no shim, an interstitial-free steel shim at the faying interface (shim-in) and shims against the electrodes (shim-out), were incorporated to investigate the weldability of Fe-7Mn-0.14C medium-Mn steel. Tensile-shear, cross-tension, and microhardness tests were used to evaluate the mechanical properties of the welds. Experimental results demonstrated that the failure mode of the welds transitioned from the interfacial fracture in the case of no shim to the desired nugget pull-out fracture in the shim-out set-up, resulting in dramatical improvements in both peak loads and their corresponding extensions during the tensile-shear and cross-tension tests. In contrast, the shim-in set-up made no improvement. What can contribute to such improvement was then discussed on the basis of observed morphologies and microstructures of welds.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Aluminothermic Reduction Process Under Nitrogen Gas Pressure for Preparing High Nitrogen Austenitic Steels

The aluminothermic reduction casting process under nitrogen gas pressure to make austenitic Cr-Mn-N (Ni-free) and Cr-N (Ni/Mn-free) high nitrogen stainless steels was investigated. Thermodynamic simulation of the redox reaction depending on process parameters was performed. As a result, the optimal ratio of aluminum to oxygen in the initial powder mixture to obtain the highest yield of metal product with minimal aluminum nitride contamination was predicted to be slightly greater than the stoichiometric ratio of 1.125. Microstructures of aluminothermic 26Cr1N and 23Cr9Mn1N steels, prepared taking into account the results of thermodynamic simulation, were investigated by X-ray diffraction, metallography, and transmission electron microscopy. The as-cast microstructure was a pseudo-pearlite (layered ferrite-nitride mixture) in 26Cr1N steel and a ferrite-austenite with signs of discontinuous austenite decomposition in 23Cr9Mn1N steel. After hot forging and subsequent water quenching from 1200 °C, the microstructure was fully austenitic in both steels. Tensile tests of quenched 23Cr9Mn1N steel showed a combination of high strength (ultimate strength of 1324 MPa) and ductility (elongation of 27 pct). The results illustrate that the aluminothermic casting process for producing high nitrogen steel is competitive with the commonly used methods, such as pressure electroslag remelting, both in terms of cost and mechanical properties of manufactured steel.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Fabrication of AZ31/MWCNTs Surface Metal Matrix Composites by Friction Stir Processing: Investigation of Microstructure and Mechanical Properties

The surface metal matrix composites of AZ31 Mg alloy reinforced with multiwall carbon nanotubes (MWCNTs) have been fabricated through the friction stir processing by a conventional and two stepped tools. The microstructure and mechanical properties of fabricated composites were studied by optical and electron microscopy, microhardness and tensile tests, respectively. The processing has developed a fine-grain structure along with good distribution of reinforcements. The hardness and tensile strength of fabricated MWCNT/AZ31 composites are generally higher than as-received and FSPed samples. The accumulative effect of grain refinement and reinforcing nanotubes is assumed to be the reason for increasing the ductility after friction stir processing. The hardness is nearly doubled for FSPed samples and some more for nanocomposites compared with the as-received sample. The elongation of nanocomposites is about two times greater than that of the as-rolled sample. The speed ratio, pass number and CNT amount are three important factors influencing the resulting microstructure and mechanical properties. The stepped tools also give a more uniform distribution of reinforcement and higher grain refinement.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Tempering Temperature on the Microstructure and Properties of Fe-2Cr-Mo-0.12C Pressure Vessel Steel

To obtain the high-temperature strength and toughness of the medium–high-temperature–pressure steel, the microstructure evolution and mechanical properties of Fe-2Cr-Mo-0.12C steel subjected to three different tempering temperatures after being normalized were investigated. The results show that the microstructure of the sample, tempered in the range 675-725 °C for 50 min, did not change dramatically, yet the martensite/austenite constituents decomposed, and the bainite lath merged together and transformed into polygonal ferrite. At the same time, the precipitate size increased with an increase in tempering temperature. With the increase in the tempering temperature from 675 to 725 °C, the impact absorbed energy of the Fe-2Cr-Mo-0.12C steel at −40 °C increased from 257 to 325 J, and the high-temperature yield strength decreased; however, the high-temperature ultimate tensile strength tempered at 700 °C was outstanding (422-571 MPa) at different tested temperatures. The variations of the properties were attributed to the decomposition of M/A constituents and the coarsening of the precipitates. Fe-2Cr-Mo-0.12C steel normalized at 930 °C and tempered at 700 °C was found to have the best combination of ductility and strength.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Holding Pressure on Microstructure and Mechanical Properties of A356 Aluminum Alloy

In this study, the effect of holding pressure on microstructure and mechanical properties of low-pressure die cast A356 aluminum alloy was investigated. The results showed that the application of high holding pressure (300 kPa) generated castings with denser structure and superior mechanical properties. By increasing the holding pressure up to 300 kPa, the size of secondary dendrite arm spacing greatly reduced by 22.7% at the cooling rate of 1°C/s and decreased by 12.8% at 10°C/s. The Feret’s diameter and aspect ratio of eutectic silicon particles decreased by 8.4 and 5.1% at the cooling rate of 1°C/s and decreased by 9.3 and 6.4% at 10°C/s, respectively. Meanwhile, the density of A356 aluminum alloy increased to 2.678 g/cm{sup 3} and the area fraction of porosity decreased to 0.035%. Thus, tensile properties of A356 aluminum alloy obtained at high holding pressure were enhanced, especially the ductility. All these could be associated with the better filling capability and faster cooling rate caused by high holding pressure. In the analytical range of experimental conditions, the correlation of mechanical properties with process parameters was established by statistical models to predict the ultimate tensile strength and elongation of low-pressure die cast A356 aluminum alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Characterization and Strain-Hardening Behavior of Friction Stir-Welded Ferritic Stainless Steel

In this study, friction stir-welded joint of 3-mm-thick plates of 409 ferritic stainless steel (FSS) was characterized in light of microstructure, x-ray diffraction analysis, hardness, tensile strength, ductility, corrosion and work hardening properties. The FSW joint made of ferritic stainless steel comprises of three distinct regions including the base metal. In stir zone highly refined ferrite grains with martensite and some carbide precipitates at the grain boundaries were observed. X-ray diffraction analysis also revealed precipitation of Cr{sub 23}C{sub 6} and martensite formation in heat-affected zone and stir zone. In tensile testing of the transverse weld samples, the failure eventuated within the gauge length of the specimen from the base metal region having tensile properties overmatched to the as-received base metal. The tensile strength and elongation of the longitudinal (all weld) sample were found to be 1014 MPa and 9.47%, respectively. However, in potentiodynamic polarization test, the corrosion current density of the stir zone was highest among all the three zones. The strain-hardening exponent for base metal, transverse and longitudinal (all weld) weld samples was calculated using various equations. Both the transverse and longitudinal weld samples exhibited higher strain-hardening exponents as compared to the as-received base metal. In Kocks–Mecking plots for the base metal and weld samples at least two stages of strain hardening were observed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructural Evolution and Mechanical Properties of Ti-22Al-25Nb (At.%) Orthorhombic Alloy with Three Typical Microstructures

Microstructural evolution, tensile and creep behavior of Ti-22Al-25Nb (at.%) orthorhombic alloy with three typical microstructures were investigated. The three typical microstructures were obtained by different solution and age treatment temperatures and analyzed by the BSE technique. The tensile strengths of the alloy at room temperature and 650 °C were investigated. The creep behaviors of the three typical microstructures were also studied at 650 °C/150 MPa for 100 h in air. The phase transformation mechanisms in creep deformation were also found. The experimental results showed that the formations of the three typical microstructures were decided by the isothermal forging and heat treatment. It was supposed that the high-temperature solution treatment might be dominant for the volume fraction and diameter of the equiaxed particles. While the double age treatment would lead to lamellar O phases. Due to grain refinement strengthening, the equiaxed microstructure presented the best tensile strength and ductility. The fully lamellar microstructure had the best creep resistance than that of other microstructures. In this paper, the phenomenon of creep-induced α{sub 2} phase decomposition was occurred during creep deformation of the equiaxed microstructure.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effect of Welding Processes on the Microstructure, Mechanical Properties and Residual Stresses of Plain 9Cr-1Mo Steel Weld Joints

9Cr-1Mo steel designated as P9 is widely used in the construction of power plants and high-temperature applications. It is chosen for fabricating hexcan fuel subassembly wrapper components of fast breeder reactors. Arc welding processes are generally used for fabricating 9Cr-1Mo steel weld joints. A-TIG welding process is increasingly being adopted by the industries. In the present study, shielded metal arc (SMA), tungsten inert gas (TIG) and A-TIG welding processes are used for fabricating the 9Cr-1Mo steel weld joints of 10 mm thickness. Effect of the above welding processes on the microstructure evolution, mechanical properties and residual stresses of the weld joints has been studied in detail. All the three weld joints exhibited comparable strength and ductility values. 9Cr-1Mo steel weld joint fabricated by SMAW process exhibited lower impact toughness values caused by coarser grain size and inclusions. 9Cr-1Mo steel weld joint fabricated by TIG welding exhibited higher toughness due to finer grain size, while the weld joint fabricated by A-TIG welding process exhibited adequate toughness values. SMA steel weld joint exhibited compressive residual stresses in the weld metal and HAZ, while TIG and A-TIG weld joint exhibited tensile residual stresses in the weld metal and HAZ.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of Tungsten Addition on the Microstructure and Mechanical Properties of Near-Eutectic AlCoCrFeNi{sub 2} High-Entropy Alloy

The effects of tungsten addition on the microstructure and mechanical properties of near-eutectic AlCoCrFeNi{sub 2} high-entropy alloy were investigated in this paper. The AlCoCrFeNi{sub 2}W{sub x} alloys comprised the primary BCC phase plus eutectic FCC/BCC phases. It was found that W element can both promote the formation of the primary BCC phase and act as a solid solution strengthening element. The hardness of the AlCoCrFeNi{sub 2}W{sub x} alloys increased from HV 293 to HV 356.2 with the increase in W content. The addition of W element improved the strength of alloys but reduced ductility. Thereinto, the AlCoCrFeNi{sub 2}W{sub 0.2} alloy showed the most excellent compressive properties with the fracture strength of 2785.9 MPa and the plastic strain of 0.42, respectively, which implied the potential industrial application values.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Tensile Behavior of Electron Beam-Welded and Post-Weld Vacuum-Annealed Nb-10% Hf-1% Ti Refractory Alloy Weldments

Nb-10% Hf-1% Ti refractory alloy is a high performance material extensively used for high temperature applications. Electron beam welding is one of the most widely used techniques to join refractory and reactive alloys. Bigger sizes of nozzles for rocket propulsion applications can be either made through deep drawing and flow turning route or by roll bending and welding route both using sheets/plates as input material for fabrication. The latter is a more economical option for mass production of the hardware using such exotic and expensive alloys. In view of this, both as-welded (AW) coupon and weld plus post-weld vacuum-annealed (AW + VA) coupon have been prepared to study their mechanical behavior. It has been observed that tensile strength and ductility have not been reduced in both these conditions vis-à-vis the base metal, confirming weld efficiency of the alloy to be ~ 100%. Microhardness is found to be 150-160 VHN in the base metal and 200-225 VHN in the weld fusion zone in AW condition, which became uniform (145-155 VHN) throughout the weldment in AW + VA condition. Microstructure of the weldment in AW condition is found to be consisting of grains solidified by epitaxial mode from base metal toward the weld centre. In AW + VA condition, improvement in tensile elongation is observed, which is found to be due to the presence of homogenized grains/more uniform microstructure near the heat-affected zone as compared to the steep gradient in grain size in different zones in the weld in AW condition.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Mechanical Properties of Friction Stir Process Derived Al-TiO{sub 2} Nanocomposite

Aluminum-based composites have many advantages over their conventional counterparts. A major problem in such composites is the clustering of particles in the matrix. Friction stir processing (FSP) can homogenize particle distribution in aluminum-based composites. In this study, unannealed TiO{sub 2} particles were used to prepare Al-TiO{sub 2} nanocomposite using FSP. The TiO{sub 2} particles, about 1 µm, were dispersed into an aluminum matrix by 6 passes of FSP. The TiO{sub 2} particles were fractured by multiple FSP passes, leading to a nano-size particle distribution in the matrix. Nanoscale dispersion was confirmed by scanning electron microscopy and transmission electron microscopy. The fractured TiO{sub 2} particles reacted with the aluminum matrix to form Al{sub 3}Ti intermetallic and Al{sub 2}O{sub 3} ceramic. The progression of the Al-TiO{sub 2} reaction from the fourth to the sixth pass of FSP was revealed by x-ray diffraction. Due to the nanoscale dispersion, the yield and ultimate tensile strength of the composite increased to 97 and 145 MPa, respectively. Ductility of the composite decreased marginally compared to the as-received aluminum. As the dispersed particles pin dislocations, the strain-hardening rate of the composite was considerably increased and the same was seen in the Kocks-Mecking plot. The TiO{sub 2} particles are mechanically activated due to their fracture during FSP, hence leading to reaction with the matrix. The particle refinement and dispersion lead to a homogeneous matrix with higher strength.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructures, Martensitic Transformation, and Mechanical Behavior of Rapidly Solidified Ti-Ni-Hf and Ti-Ni-Si Shape Memory Alloys

In this work, the microstructure and mechanical properties of rapidly solidified Ti{sub 50−x/2}Ni{sub 50−x/2}Hf{sub x} (x = 0, 2, 4, 6, 8, 10, and 12 at.%) and Ti{sub 50−y/2}Ni{sub 50−y/2}Si{sub y} (y = 1, 2, 3, 5, 7, and 10 at.%) shape memory alloys (SMAs) were investigated. The sequence of the phase formation and transformations in dependence on the chemical composition is established. Rapidly solidified Ti-Ni-Hf or Ti-Ni-Si SMAs are found to show relatively high yield strength and large ductility for specific Hf or Si concentrations, which is due to the gradual disappearance of the phase transformation from austenite to twinned martensite and the predominance of the phase transformation from twinned martensite to detwinned martensite during deformation as well as to the refinement of dendrites and the precipitation of brittle intermetallic compounds.

36 MATERIALS SCIENCE↗