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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 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
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 OSTI2019

Selective laser melting of 304L stainless steel: Role of volumetric energy density on the microstructure, texture and mechanical properties

The role of volumetric energy density on the microstructural evolution, texture and mechanical properties of 304L stainless steel parts additively manufactured via selective laser melting process is investigated. 304L is chosen because it is a potential candidate to be used as a matrix in a metal matrix composite with nanoparticles dispersion for energy and high temperature applications. The highest relative density of 99 %±0.5 was achieved using a volumetric energy density of 1400 J/mm 3 . Both XRD analysis and Scheil simulation revealed the presence of a small trace of the delta ferrite phase, due to rapid solidification within the austenitic matrix of 304L. Here, a fine cellular substructure ranged between 0.4–1.8 μm, was detected across different energy density values. At the highest energy density value, a strong texture in the direction of [100] was identified. At lower energy density values, multicomponent texture was found due to high nucleation rate and the existing defects. Yield strength, ultimate tensile strength, and microhardness of samples with a relative density of 99 % were measured to be 540 ± 15 MPa, 660 ± 20 MPa and 254 ± 7 HV, respectively and higher than mechanical properties of conventionally manufactured 304L stainless steel. Heat treatment of the laser melted 304L at 1200 °C for 2 h, resulted in the nucleation of recrystallized equiaxed grains followed by a decrease in microhardness value from 233 ± 3 HV to 208 ± 8 HV due to disappearance of cellular substructure.

304L stainless steel↗
DOE OSTI2018

Microstructure and mechanical properties of intragranular W-Cu/TiC-ZrC composite prepared by reactive melt infiltration at 1300 °C

Highlights: • W-Cu/TiC-ZrC composites with nano W dispersoids were prepared by reactive melt infiltration. • The nano-sized dispersoids were analyzed by TEM in detail. • The intragranular nano W dispersoids and ribbon like precipitates have toughening effects. - Abstract: Newly intragranular W-Cu/TiC-ZrC composites with nano W dispersoids in TiC matrix have been prepared by reactive melt infiltration at 1300 °C for 1 h in vacuum using molten Zr-Cu binary alloys and a (Ti{sub 0.75},W{sub 0.25})C solid solution powder as raw materials. The as-synthesized composites consist of major phases of W, Cu, TiC and ZrC. With the decrease of Zr content from 66 to 21 at.% in the Zr-Cu infiltrator, the reaction formed W phase in the composite has changed from micron-sized intergranular grains to nano-sized rounded inclusions in TiC{sub y} grains. The crystallographic orientation relationship at W and TiC interfaces are detected as:(0 1 1-bar ){sub W}//(1 1 1-bar ){sub TiC}&[0 1 1]{sub W}//[0 1 1]{sub TiC} and (4 1 1){sub W}//(4 2 4){sub TiC}&[0 1 1-bar ]{sub W}//[4 2-bar 3-bar ]{sub TiC}. Nano-sized lamellar structure with both W and Cu rich is detected to have super-lattice structure based on the TiC lattice. The elastic modulus, flexural strength, fracture toughness of the intragranular W-Cu/TiC-ZrC composite infiltrated with Zr{sub 14}Cu{sub 51} melt are 207.5 GPa, 457 MPa and 10.21 MPa∙m{sup 1/2}, respectively. The intragranular nano W dispersoids and ribbon like precipitates hinder crack propagation via crack deflection and bridging.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

PROCESSING, MICROSTRUCTURE AND MECHANICAL PROPERTY CORRELATION IN Al-B4C SURFACE COMPOSITE PRODUCED VIA FRICTION STIR PROCESSING

Friction stir processing (FSP) was employed to prepare surface composites (SC) composed of B4C particles in 5024 Al matrix. The processing parameters, such as hole pattern and geometry,and the number of FSP passes, were optimized to obtain uniform powder distribution. The micrographs revealed a homogeneous distribution of the particles with good interfacial bonding. The hardness of the composite was uniform across the processed region which again indicates the uniformity of powder distribution. The modulus of the surface composite was measured using strain gage and showed a significant improvement. This improvement in modulus lies in the load sharing capability from the soft matrix to the hard particles.

Aluminum MMC,↗
DOE OSTI2016

Effect of solvent/polymer infiltration and irradiation on microstructure and tensile properties of carbon nanotube yarns

Recently carbon nanotube (CNT) yarns have been gaining importance as an approach to harvest the excellent properties of the CNTs. However, the properties of CNT yarns at this stage are well below the expected value. Investigation of the structure of CNT yarns and possible approaches to enhance the strength and modulus are reported. Scanning electron microscopy and focused ion beam imaging reveal the inherently porous structure and poor orientation, emphasizing the need to enhance packing of CNT bundles in the yarns for increased strength and modulus. Densification of CNT yarn by toluene or polystyrene increases the strength by 140 or 172 % and modulus by 79 or 218 %, respectively, as compared to that of the pristine yarn. E-beam irradiation was investigated as a means to introduce crosslinking and enhanced internanotubes bonding to increase strength and modulus. However, the irradiation resulted in generation of defects and damages to the yarn contributing to reduction in strength and modulus. Raman spectroscopy studies on the irradiated samples reveal the change in bonding characteristics resulting in poor mechanical properties. As a result, denser packing of nanotubes and increased interaction without any damage is the key to improve the properties of CNT yarns.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Advanced Process Possibilities in Friction Crush Welding of Aluminum, Steel, and Copper by Using an Additional Wire

Joining sheet metal can be problematic using traditional friction welding techniques. Friction crush welding (FCW) offers a high speed process which requires a simple edge preparation and can be applied to out-of-plane geometries. In this work, an implementation of FCW was employed using an additional wire to weld sheets of EN AW5754 H22, DC01, and Cu-DHP. The joint is formed by bringing together two sheet metal parts, introducing a wire into the weld zone and employing a rotating disk which is subject to an external force. The requirements of the welding preparation and the fundamental process variables are shown. Thermal measurements were taken which give evidence about the maximum temperature in the welding center and the temperature in the periphery of the sheet metals being joined. The high welding speed along with a relatively low heat input results in a minimal distortion of the sheet metal and marginal metallurgical changes in the parent material. In the steel specimens, this FCW implementation produces a fine grain microstructure, enhancing mechanical properties in the region of the weld. Aluminum and copper produced mean bond strengths of 77 and 69 pct to that of the parent material, respectively, whilst the steel demonstrated a strength of 98 pct. Using a wire offers the opportunity to use a higher-alloyed additional material and to precisely adjust the additional material volume appropriate for a given material alignment and thickness.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Microstructures and Microhardness Properties of CMSX-4{sup ®} Additively Fabricated Through Scanning Laser Epitaxy (SLE)

Epitaxial CMSX-4{sup ®} deposition is achieved on CMSX-4{sup ®} substrates through the scanning laser epitaxy (SLE) process. A thorough analysis is performed using various advanced material characterization techniques, namely high-resolution optical microscopy, scanning electron microscopy, energy-dispersive x-ray spectroscopy, x-ray diffraction, and Vickers microhardness measurements, to characterize and compare the quality of the SLE-fabricated CMSX-4{sup ®} deposits to the CMSX-4{sup ®} substrates. The results show that the CMSX-4{sup ®} deposits have smaller primary dendritic arm spacing, finer γ/γ′ size, weaker elemental segregation, and higher microhardness compared to the investment cast CMSX-4{sup ®} substrates. The results presented here demonstrate that CMSX-4{sup ®} is an attractive material for laser-based AM processing and, therefore, can be used in the fabrication of gas turbine hot-section components through AM processing.

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 OSTI2018

Stress Wave Attenuation in Aluminum Alloy and Mild Steel Specimens Under SHPB Tensile Testing

Investigations on the effect of intensity of incident pressure wave applied through the striker bar on the specimen force histories and stress wave attenuation during split Hopkinson pressure bar (SHPB) tensile testing are presented. Details of the tensile SHPB along with Lagrangian x–t diagram of the setup are included. Studies were carried out on aluminum alloy 7075 T651 and IS 2062 mild steel. While testing specimens using the tensile SHPB setup, it was observed that the force calculated from the transmitter bar strain gauge was smaller than the force obtained from the incident bar strain gauge. This mismatch between the forces in the incident bar and the transmitter bar is explained on the basis of stress wave attenuation in the specimens. A methodology to obtain force histories using the strain gauges on the specimen during SHPB tensile testing is also presented. Further, scanning electron microscope images and photomicrographs are given. Correlation between the microstructure and mechanical properties is explained. Further, uncertainty analysis was conducted to ascertain the accuracy of the results.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Fiber Laser Welding–Brazing Characteristics of Dissimilar Metals AZ31B Mg Alloys to Copper with Mg-Based Filler

Fiber laser welding–brazing of 1-mm-thick AZ31B Mg alloys to 1.5-mm-thick copper (T2) with Mg-based filler was performed in a lap configuration. The weld appearance, interfacial microstructure and mechanical properties were investigated with different heat inputs. The results indicated that processing windows for optimizing appropriate welding parameters were relatively narrow in this case. Visually acceptable joints with certain strength were achieved at appropriate welding parameters. The maximum tensile-shear fracture load of laser-welded-brazed Mg/Cu joint could reach 1730 N at the laser power of 1200 W, representing 64.1% joint efficiency relative to AZ31Mg base metal. The eutectic structure (α-Mg + Mg{sub 2}Cu) and Mg-Cu intermetallic compound was observed at the Mg/Cu interface, and Mg-Al-Cu ternary intermetallic compound were identified between intermetallics and eutectic structure at high heat input. All the joints fractured at the Mg-Cu interface. However, the fracture mode was found to differ. For laser power of 1200 W, the surface was characterized by tearing edge, while that with poor joint strength was almost dominated by smooth surface or flat tear pattern.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Tensile Properties of the Fe-32%Ni-4%Co Alloy During Cryorolling and Subsequent Annealing

The Fe-32%Ni-4%Co alloy was cryorolled to produce thickness reductions of 10, 40, 60 and 90%; then, the strips with a final thickness of 0.52 mm were annealed at 893, 923 and 943 K for different soaking times. The x-ray diffraction measurements indicated that both thermally induced α′-martensite (α′-M) and deformation-induced α′-M were present in the tested alloy. The thermally induced α′-M was mainly lath-shaped, while the morphology of the deformation-induced α′-M gradually changed from lath-shaped to irregular-shaped according to transmission electron microscopy observations. After the 90% reduction due to cryorolling, the total volume fraction of α′-M reached 75.6%, and the average ultimate tensile strength (UTS{sub av.}) was significantly improved from 430.3 to 1306.8 MPa. Upon subsequent annealing at 923 K for 5 min, the tested alloy mainly consisted of a large number of fine grains and a few coarse grains (CGs). In addition, several deformed structures were also observed. As the annealing time increased to 10 and 30 min, the fraction of CGs increased, and a bimodal grain size distribution was obtained, while the deformed structures were still present. The temperature of the reverse phase transformation of α′-M → γ was lower than the recrystallization temperature, which may be the main reason for the bimodal grain distribution. With this bimodal grain structure, the alloy exhibited a combination tensile strength of 543.3 MPa and an average elongation of 34.1%.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Tool Pin Eccentricity on the Microstructure and Mechanical Properties of Friction Stir Processed Al-6061 Alloy

Three different stir tools with the pin eccentricity of 0, 0.4 and 0.8 mm were applied to the friction stir processing (FSP) of Al-6061 alloy. Results show that the pin-affected zone (PAZ) enlarges and the grain size in the stir zone (SZ) decreases as the pin eccentricity increases. Moreover, the low peak FSP temperature induced by the pin eccentricity gives rise to the coarsening of strengthening precipitates in the SZ. Compared with the pin-eccentric FSP samples, the SZ produced without pin eccentricity exhibits the highest hardness and yield strength due to the high amount of fine precipitates. However, the application of pin eccentricity elevates the overall tensile properties of FSP samples through changing the failure location from heat-affected zone (HAZ) to SZ.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Cooling Environment Effect on the Microstructure and Mechanical Properties of Friction Stir-Welded Joints

The heat input in friction stir welding (FSW) influences the localized properties in welded joint. The external cooling during FSW reduces the localized heating and post-weld cooling time by extracting heat comparatively at a faster rate. In the present study, we have investigated the effect of different cooling environments on the performance of friction stir-welded (FSWed) AA5052 strain-hardenable aluminum alloy. Welding was performed under three different cooling environments such as natural air, water and liquid nitrogen in order to understand their effects on grain structure, metallurgical changes and mechanical properties of the joints. It has been noted that the width of weld nugget zone, thermo-mechanically affected zone and heat-affected zone has been decreased with the respective change in cooling environments. Welding speed could also influence the width of different zones in weldment alike the cooling environments. Finer grain size has been noticed with water cooling against the liquid nitrogen cooling. Comparatively, water was observed to be more favorable cooling environment against liquid nitrogen because of its better weld quality (97%) compared with base metal. The favorable mechanical properties resulting from the different weldment regions for different cooling environments have been elaborated in the present study.

36 MATERIALS SCIENCE↗
DOE OSTI2019

High-Carbon Ferrochrome Effects on Microstructure and Mechanical Properties of Powder Metallurgy Titanium Alloys

In this paper, the powder metallurgy method was employed to prepare titanium alloys with high-carbon ferrochrome (HCFeCr). With the HCFeCr addition, the size of the β grains decreased and the lamellae of the α phase became thinner. The TiC strengthening phase precipitated at the grain boundaries, resulting in the inhibition of the β grain growth and hardness enhancement. In addition, Fe and Cr as β-stabilizing elements existed in the enriched β-Ti phase, which increased the β phase amount. The strength improvement was attributed to grain size optimization and strengthening phase formation. With the 9 wt.% HCFeCr addition, the corresponding Vickers hardness increased to 480 HV, which is 60% higher than the Ti6Al4V alloy. The tensile and yield strength was 1228 and 1140 MPa, respectively. The addition of HCFeCr can effectively enhance the strength and hardness while reducing the cost. This is a promising additive to obtain low-cost Ti alloys with high mechanical properties.

36 MATERIALS SCIENCE↗