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At least 1,531 records · Page 85
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Microstructure and Mechanical Properties of W-ZrC Composites Synthesized by Reactive Melt Infiltration of Zr{sub 2}Cu into Porous Preforms from Partially Carburized W Powders

W-ZrC composites with different W contents from 48 to 73 vol.% have been synthesized by reactive melt infiltration of Zr{sub 2}Cu melt into porous preforms from partially carburized W powders at 1300 °C for 1 h in vacuum. The influences of carbon content and porosity in the preforms on microstructure and mechanical properties of W-ZrC composites are investigated. Cold isostatic pressing followed by pre-sintering process is used to produce porous preforms with suitable porosities of 53.6-47% under a pressure of 100 MPa to allow sufficient penetration of Zr{sub 2}Cu melt into the preforms. Small amounts of Cu-rich phases form in the synthesized W-ZrC composites after a complete reaction of y/2xZr{sub 2}Cu(l) + WC{sub y}(s) = y/xZrC{sub x}(s) + W(s) + y/2xCu(l). These Cu-rich phases are distributed not only at the phase boundaries of W matrix and ZrC grains, but also in the interior of ZrC{sub x} grains. With decreasing W content from 73 to 48 vol.% in the W-ZrC composites, the flexural strength and fracture toughness increase from 519 to 657 MPa and from 9.1 to 10.6 MPa m{sup 1/2}, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of the As-Forged and Heat-Treated Microstructure on the Room Temperature Anisotropic Ductile Fracture of Inconel 718

The purpose of the present work was to investigate the effect of primary carbides and the δ-phase on the anisotropic ductile fracture of Inconel 718 in the forging process. Inconel 718 alloys were prepared by VIM + VAR processes with various carbon contents (0.009 and 0.027 wt.%). Then, the alloys were forged and annealed at temperatures of 980 and 1030 °C. The room temperature mechanical anisotropy of the alloys was evaluated at the longitudinal direction (LD) and transverse direction (TD). Tensile and impact tests were used to characterize the mechanical properties of the specimens. The microstructural characterization and the fractography of the alloys were carried out by FE-SEM. The obtained results showed that the fracture strain and the impact energy in the TD were 30-50% lower than the LD. The fracture was accelerated by the δ-phase, leading to the reduction of impact energy in the longitudinal and the lateral directions up to 50%. The low-carbon alloy indicated similar characteristics in both the LD and the TD. Aligned carbides changed the fracture path from a zigzag path in the LD to a fibrous path in the TD, while the δ-phase created a flat fracture path. The shear lip area ratio in the tensile fracture cross section was decreased by reducing ductility.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructures and Mechanical Properties of Inconel 718 Alloy at Ultralow Temperatures

The microstructures and mechanical properties of powder metallurgy Inconel 718 alloy were investigated in the temperatures range between 25 and − 253 °C. Tensile strength increased with the decrease in temperature, while the ductility first increased and then decreased. There was no significant change in impact toughness. When the temperature was − 253 °C, a zigzag stress–strain curve was observed for the alloy, owing to the interaction of dislocation glide and twinning, which effectively maintained the relatively good ductility.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Mechanical Properties of Stainless Steel/Brass Joints Brazed by Sn-Electroplated Ag Brazing Filler Metals

To develop a high-Sn-content AgCuZnSn brazing filler metal, the BAg50CuZn was used as the base filler metal and a Sn layer was electroplated upon it. Then, the 304 stainless steel and the H62 brass were induction-brazed with the Sn-plated brazing filler metals. The microstructures of the joints were examined with an optical microscope, a scanning electron microscope and an x-ray diffractometer. The corresponding mechanical properties were obtained with a universal tensile testing machine. The results indicated that the induction brazed joints consisted of the Ag phase, the Cu phase and the CuZn phase. When the content of Sn in the Sn-plated Ag brazing filler metal was 6.0 or 7.2 wt.%, the Cu{sub 5}Zn{sub 8}, the Cu{sub 41}Sn{sub 11} and the Ag{sub 3}Sn phases appeared in the brazed joint. The tensile strength of the joints brazed with the Sn-plated filler metal was higher compared to the joints with the base filler metal. When the content of Sn was 6.0 wt.%, the highest tensile strength of the joint reached to 395 MPa. The joint fractures presented a brittle mode, mixed with a low amount of ductile fracture, when the content of Sn exceeded 6.0 wt.%.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The Effect of Prestrain Temperature on Kinetics of Static Recrystallization, Microstructure Evolution, and Mechanical Properties of Low Carbon Steel

In this research, the samples of a low carbon steel sheet were rolled up to a thickness prestrain of 67% at three different temperatures consisted of room, blue brittleness, and subzero temperature. Microhardness, SEM, and tensile tests were carried out to evaluate the static recrystallization kinetics defined by the Avrami equation, microstructural evolution, and mechanical properties. It was found that the Avrami exponent is altered with change in prestrain temperature and it achieves the value of 1 to 1. 5. Moreover, it was indicated that prestraining at subzero temperature followed by annealing at 600 °C leads to considerable enhancement in tensile properties and kinetics of static recrystallization compared to room and blue brittleness temperatures. The prestraining at blue brittleness temperature followed by annealing treatment caused, however, a higher strength and faster kinetics compared with that at room temperature. It was concluded that although from the steel ductility point of view, the blue brittleness temperature is called an unsuitable temperature, but it can be used as prestraining temperature to develop noticeable combination of strength and ductility in low carbon steel.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Residual Stress Distribution and Microstructure of a Multiple Laser-Peened Near-Alpha Titanium Alloy

Laser peening without coating (LPwC) was performed on a Ti-2.5 Cu alloy with multiple passes (1, 3 and 5), using a Nd:YAG laser (1064 nm) at a constant overlap rate of 70% and power density of 6.7 GW cm{sup −2}. Hardness and residual stress profiles indicated thermal softening near the surface (< 100 μm) and bulk softening due to adiabatic heating. Maximum hardness (235 HV at 500 μm) and maximum residual stress (− 890 MPa at 100 μm) were observed for LPwC with 1 pass. Surface roughness and surface 3-D topography imaging showed that the surface roughness increased with the increase in the number of passes. XRD results indicated no significant β phases. However, peak shifts, broadening and asymmetry were observed and interpreted based on dislocation activity. Microstructures indicated no melting or resolidification or refinement of grains at the surface. Twin density was found to increase with the increase in the number of passes.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Analysis of Microstructure and Sliding Wear Behavior of Co{sub 1.5}CrFeNi{sub 1.5}Ti{sub 0.5} High-Entropy Alloy

Α Co{sub 1.5}CrFeNi{sub 1.5}Ti{sub 0.5} high-entropy alloy (HEA) of the well-known family of CoCrFeNiTi has been designed using empirical parameters. The aim of this design was the production of a HEA with fcc structure that gives ductile behavior and also high strength because of the solid solution effect. The VEC calculations (8.1) supported the fcc structure while the δ factor calculations (4.97) not being out of the limit values, advised a significant lattice distortion. From the other hand, the ΔΗ{sub mix} calculations (− 9.64 kJ/mol) gave strong indications that no intermetallic would be formed. In order to investigate its potential application, the Co{sub 1.5}CrFeNi{sub 1.5}Ti{sub 0.5} HEA was prepared by vacuum arc melting and a primary assessment of its surface degradation response was conducted by means of sliding wear testing using different counterbody systems for a total sliding distance of 1000 m. An effort to correlate the alloy’s wear response with the microstructural characteristics was attempted. Finally, the wear behavior of the Co{sub 1.5}CrFeNi{sub 1.5}Ti{sub 0.5} HEA was compared with that of two commercially used wear-resistant alloys. The results obtained provided some first signs of the high-entropy alloys’ better wear performance when tested under sliding conditions against a steel ball.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Interfacial Microstructure and Mechanical Properties of Friction Stir Welded Joints of Commercially Pure Aluminum and 304 Stainless Steel

In the present investigation, friction stir welding of commercially pure aluminum and 304 stainless steel was carried out at varying tool rotational speeds from 200 to 1000 rpm in steps of 200 rpm using 60 mm/min traverse speed at 2 (degree) tool tilt angle. Microstructural characterization of the interfacial zone was carried out using optical microscope and scanning electron microscope. Energy-dispersive spectroscopy indicated the presence of FeAl{sub 3} intermetallic phase. Thickness of the intermetallic layer increased with the increase in tool rotational speed. X-ray diffraction studies indicated the formation of intermetallic phases like FeAl{sub 2}, Fe{sub 4}Al{sub 13}, Fe{sub 2}Al{sub 5}, and FeAl{sub 3}. A maximum tensile strength of ~ 90% that of aluminum along with ~ 4.5% elongation was achieved with the welded sample at tool rotational speed of 400 rpm. The stir zone showed higher hardness as compared to base metals, heat affected zone, and thermo-mechanically affected zone due to the presence of intermetallics. The maximum hardness value at the stir zone was achieved at 1000 rpm tool rotational speed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Mechanical Properties of Three-Layer TIG-Welded 2219 Aluminum Alloys with Dissimilar Heat Treatments

2219-C10S and 2219-CYS aluminum alloys are 2219 aluminum alloys with different heat treatment processes, and they have been widely used in the aerospace industry. In the present study, 2219-C10S and 2219-CYS aluminum alloys were butt-welded by three-layer tungsten inert gas arc welding (with the welding center of the third layer shifted toward the CYS side), and the microstructure characteristics and mechanical properties of the welded joint were investigated. The lamellar θ′ phases, the bulk or rod θ phases, and the coarse rod-shaped or pancake-shaped Al-Cu-Fe-Mn phases coexisted in the two aluminum alloys. The Cu content of the α-Al matrix and the distribution of eutectic structures of different welding layers in the weld zone (WZ) were varied, implying that the segregation degrees of the Cu element were different due to the different welding thermal cycles in different welding layers. The microhardness values of the CYS side were much higher than those of the C10S side in each region on both sides of the joint. The tensile test deformation was concentrated mainly in the regions of WZ and the over aged zone (OAZ), where the microhardness values were relatively low. The main deformation concentrated region was transferred from the CYS side to the C10S side with the increase in the tensile load during the tensile test. The fracture behavior of the tensile test showed that the macroscopic crack initiated near the front weld toe had gone through the crack blunt region, the shear fracture region of the partially melted zone (PMZ), and the shear fracture region of OAZ. Meanwhile, the fracture characteristics gradually evolved from brittle to ductile. The concentrated stress and the dense eutectic structure in the region near the front weld toe of the C10S side contributed to the fracture of the joint. The shift of the welding center of the third layer to the CYS side resulted in two effects: (i) the microhardness values from the middle layer to the top layer in the PMZ of the CYS side were the most significantly increased and (ii) the distance between the front weld toe and the fusion line of the CYS side was significantly larger.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Mechanical Properties of Low-Activation Fe-Cr-V Multicomponent Single-Phase Alloys

A ternary equiatomic and nearly equiatomic alloy composed of the low-activation elements Fe, Cr, and V was designed for potential application as low-activation structural materials for the first wall of fusion reactors. The optimal composition of the multicomponent alloy was determined by minimizing the Gibbs free energy for solid-solution formation. The microstructure, phase stability, and mechanical properties of the equiatomic and optimized Fe-Cr-V alloys were studied. The equiatomic alloy consisted of a body-centered-cubic (BCC) solid-solution phase with a small amount of a face-centered-cubic (FCC) solid-solution phase, whereas the optimized alloy consisted of a single BCC solid-solution phase in the as-cast state. The crystal grains of both alloys were equiaxial. No phase transition was observed in the optimized alloy after homogenization; however, for the equiatomic alloy, the FCC solid-solution phase disappeared and surface relief was observed. The Vickers microhardness and yield strength of the Fe-Cr-V alloys increased after homogenization compared with those of the as-cast alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure Evolution and Mechanical Properties of High-Speed Friction Stir Welded Aluminum Alloy Thin Plate Joints

Sound friction stir welded (FSW) joints of 6061-T6 aluminum alloy sheets with an 0.8 mm thickness were obtained at conventional speed (2000 rpm, 300 mm/min) and high speed (11,000 rpm, 1500 mm/min). The recrystallization mechanism, precipitate evolution, mechanical properties and fracture behavior were investigated in detail. Microstructure analyses revealed that the grain structure evolution in the nugget zone (NZ) was dominated by continuous dynamic recrystallization. In the process of FSW, high speed facilitates the formation of finer equiaxed recrystallized grains, higher density of dislocations and substructures, and a larger number of precipitates in the NZ compared to the conventional speed, which further significantly improves the hardness and tensile strength of the joints. The maximum tensile strength was obtained with 292.6 MPa, 83.2% for the 6061-T6 aluminum alloy and 122.6% for the conventional-speed FSW joints. This work provides an effective method for preparing FSW aluminum alloy thin plate joints with excellent mechanical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Mechanical Properties and Microstructural Evolution of Yb-Modified Al-20%Si Alloy

In the study, the mechanical properties and microstructural evolution of hypereutectic Al-20%Si-xYb alloys (x = 0, 0.1, 0.3, 0.5 and 0.7 wt.%) were studied. The as-prepared samples were characterized by scanning electron microscopy, optical microscope, x-ray diffraction (XRD) and electron probe microanalysis. The characterization results showed with the addition amount of Yb increasing to 0.5%, the eutectic Si structure was gradually modified from coarse needle-like/platelet-like structure into fine coral-like fibrous structure and the primary Si phases were gradually converted from coarse platelet-like/star-like/polygonal shape into fine blocky shape. In particular, according to the results of WDS and XRD analyses, the Yb-rich intermetallic phase in the Al-20%Si alloy is Al{sub 3}Yb. Tensile test was carried out to study the mechanical properties of Al-20%Si-x%Yb alloys, and results showed that the elongation and ultimate tensile strength increased by 73.6% and 64.5%, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Wear Resistance of FeCrBSi Plasma-Sprayed Coating Remelted by Gas Tungsten Arc Welding Process

Herein, an FeCrBSi coating was fabricated via plasma spray on AISI1045 steel, and subsequently, a gas tungsten arc welding (GTA) process was employed to remelt the coating. The microstructure, microhardness, fracture toughness and surface roughness of the coating before and after remelting were investigated, as well as the wear resistance was tested by a UMT-3-type sliding wear apparatus. The results showed that, upon remelting, most defects in the as-sprayed coating were effectively eliminated, the surface roughness decreased by 43%, and the coating–substrate interface bonding changed from mechanical to metallurgical. The phase composition of the as-sprayed coating was primarily α-Fe and a small amount of hard Fe{sub 3}B phase, while the remelted coating consisted of α-Fe and (Fe,Cr){sub 23}C{sub 6} and a small quantity of CrB. In addition, remelting the coating was found to induce a 287.6% increase in the fracture toughness, a 33.4% increase in the average microhardness, and a 47.5% decrease in the wear volume, while the failure mechanism changed from abrasive wear to fatigue wear upon remelting. Therefore, GTA remelting of plasma-sprayed coating was found to be a feasible method to obtain a coating with good wear resistance.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Mechanical Properties of Rapidly Cooled Friction Stir Welded Ti-6Al-4V Alloys

Two mm-thick Ti-6Al-4V alloy plates were friction stir butt welded under natural- and rapid-cooling conditions. The microstructures and mechanical properties of joints were investigated and compared. The rapid cooling via spraying cooling liquid nitrogen on the plate top surface was beneficial to obtaining a narrower cross section and finer and more uniform grains in the stir zone than those obtained by natural cooling. Joint tensile strength of 1021 MPa and elongation of 1.88% under the rapid-cooling condition respectively reached 102.7 and 150.4% of those under the natural-cooling condition, respectively. The uniform-and-compact equiaxed grains led to equiaxed dimples on the fracture surface, which demonstrated a ductile fracture of the welding joints.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Deep Cryogenic Treatment on Mechanical Properties and Microstructure of the Tool Steel CR7V for Hot Stamping

To solve the failure problem of hot stamping tool, the CR7V steel was deep cryogenically treated at different dwelling times with multiple tempering. The hardness, impact toughness and wear resistance of the CR7V steel were examined, and the effects of deep cryogenic treatment on the microstructure of the CR7V steel were studied by means of optical microscope, scanning electron microscope and energy-dispersive spectroscopy. The results show that deep cryogenic treatment has little effect on the hardness of the tempered CR7V steel, but improves its impact toughness and wear resistance. The improvement in mechanical properties is a result of the combination of dissolution of large carbide particles, formation of small-sized and uniformly distributed carbide particles and fine tempered martensite structure. The cryogenic treatment is optimized as follows: vacuum gas quenching, deep cryogenic treatment at − 196 °C for 6 h and triple tempering at 560 °C for 2 h, which improves the wear resistance by 68% and the impact toughness by 58% compared to the conventional heat treatment.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Friction-Wear Behavior of Multi-arc Ion Plating TiAlNC Ceramic Coating on WC-6%Co Substrate

Nanocomposite coating could be used for machining tools, which can be used for cutting, drilling, milling, whetting, profiling, honing and superfinishing of various materials with high productivity. In this work, the TiAlNC ceramic coating was prepared by multi-arc ion plating method on WC-6%Co substrate. The microstructure and morphology of the coating were observed by scanning electron microscopy and transmission electron microscopy. The phase and chemical composition of the coating were determined by x-ray diffraction and x-ray energy-dispersive spectroscopy, respectively. The adhesive force of the coating was measured by a scratch tester, at the same time the friction-wear behavior of the coating was studied. The results show that there were TiN and (Ti,Al)(N,C) two phases, and there was the coexistence of amorphous and nanocrystalline in the coating. The chemical composition of the coating was composed of 20 ± 9 at.% Ti, 16 ± 5 at.% Al, 39 ± 5 at.% N and 25 ± 10 at.% C. The thickness of the coating was about 4.5 μm. The average root mean square value of the surface roughness was 225 ± 16 nm. A critical force for coating failure in scratch testing was about 25 N. The friction coefficients of the substrate and the coating were 0.55 and 0.35, respectively. The ceramic coating could improve the wear resistance of the substrate.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

High-Temperature Deformation Behavior of Ti-6Al-2Sn-4Zr-2Mo Alloy with Lamellar Microstructure Under Plane-Strain Compression

High-temperature plane-strain compression of a Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) alloy with a lamellar structure was applied by a thermal simulation machine Gleeble 3800 at a temperature of 1223-1370 K and a strain rate of 0.01-10 s{sup −1}. Constitutive relations between the flow stress and strain at different temperatures and strain rates were constructed based on the Arrhenius equation. Processing maps based on the dynamic material model at strains of 0.5, 1.0, and 1.25 were also constructed to analyze the mechanism and instability of high-temperature deformation. Dynamic recrystallization was found to occur at 1223-1313 K and 0.01-0.1 s{sup −1}, with a peak efficiency of power dissipation of 70%. Observations of the microstructure demonstrated that α platelets and dynamic globularization were responsible for dynamic softening in the α + β phase field. Flow instability behaviors occurred at 1263-1343 K/1.78-10 s{sup −1}, 1233-1283 K/0.1-3.16 s{sup −1}, and 1353-1373 K/0.56-1.78 s{sup −1}, and adiabatic shear bands and microcracks were observed in these domains. Thus, for the lamellar Ti-6242 alloy, the recommended hot rolling parameters were 1243-1333 K and 0.01-0.1 s{sup −1}. Electron backscattered diffraction results revealed that the dynamic restoration mechanism in the β phase field was dynamic recovery when the strain rate was as high as 10 s{sup −1}.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructures and Mechanical Properties of Resistance Spot-Welded Steel/Aluminum Alloy Joints with Process Tapes

The microstructure and mechanical property of resistance spot-welded joints of steel and aluminum were studied by using process tapes. The steel/aluminum joint consists mainly of aluminum nugget and interface zone. The liquid nugget solidification has an epitaxial growth characteristic, and the interface zone displays a double-layer structure, Fe{sub 2}Al{sub 5} layer at steel side and Fe{sub 4}Al{sub 13} layer at aluminum nugget side. Compared with joints without process tape, the joint nugget diameter and tensile shear load increased by 35.45 and 10.98%, respectively, and its indentation ratio decreased by 56.25% due to the process tapes increasing heat input, producing larger nugget diameter and continuous interface layer, and protecting against deep indentation. The joining area of intermetallic layer and aluminum nugget is weak link in the joint. It is favorable to use the process tapes for improving welding quality, raising joint strength, and energy saving.

36 MATERIALS SCIENCE↗