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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 1,639 records · Page 91
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Effect of Annealing on the Impact Resistance and Fracture Mechanism of PNC-60 Sinters After Cold Plastic Deformation

Compaction of sintered metal powders by free upsetting at room temperature dangerously reduces their plastic properties, particularly the fracture toughness. The reason for the occurrence of two unfavorable phenomena accompanying this deformation route is sought. The first phenomenon depends on the initial porosity of the preform and the amount of cold work resulting in the destruction of bonds and violation of continuity between sintered powder particles. The second phenomenon is the deformation of the metal matrix, which reduces the plasticity of the sintered product. To correctly assess the effect of changes occurring in the matrix as a result of deformation, it is necessary to use the properly defined sintered matrix hardening parameter discussed in this study. The adverse effects of cold forming can be eliminated by annealing the deformed sinters at a temperature below the sintering point and above the recrystallization temperature. The annealing temperature causes favorable changes in the structure and increases the impact resistance of the deformed sinters depending on the degree of deformation of the sintered matrix and the initial porosity of the preform. Annealing at a properly selected temperature restores the ductility of the metal matrix and improves the integrity of the sintered material impaired by the effect of deformation. Annealing also causes favorable changes in the porosity morphology, removing defects in the form of gaps and resulting in the spheroidization of voids. With an appropriately selected initial porosity and degree of deformation, subsequent annealing of sinters compacted by free cold upsetting provides products with strength properties comparable to sinters with the same density and subjected to single compaction and sintering but with considerably higher fracture toughness.

36 MATERIALS SCIENCE↗
DOE OSTI2019

On Mechanical Properties of New Railway Wheel Steels for Desert Environments and Sand Caused Wheel Damage Mechanisms

Worn railway wheels after service in the desert were investigated by means of failure analysis, hardness measurements, tensile tests, fracture toughness tests and low-cycle fatigue (LCF) tests. The results complete both previous small-scale laboratory experiments and FEM simulations performed by the authors on widely used steels and new steel grades allowing to better understand the main damage mechanisms due to the sand. These new steel grades were developed for forged-rolled solid wheels in alternative to AAR Class B/C/D steel grades and were designed to ensure high resistance to wear and rolling contact fatigue in the presence of sand, debris or gravel on the rails. This paper covers the in-depth mechanical characterization of wheels made of the new steels. Hardness measurements, tensile tests, fracture toughness tests and LCF tests were carried out. The results are compared with AAR Class B/C/D steels showing an excellent combination of fracture toughness, cyclic yield strength and ductility.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Tailoring Microstructure and Properties of a Superelastic Ti–Ta Alloy by Incorporating Spark Plasma Sintering with Thermomechanical Processing

A powder metallurgy Ti–35 at.% Ta alloy was produced using a spark plasma sintering process. The sintered alloy exhibited Ta-rich regions at a sintering temperature of 1100 °C. Complete diffusion of elemental powders was achieved via homogenization heat treatment at 1500 °C for 12 h, but the ductility declined drastically. The brittle behavior of the homogenized sample was due to the high-oxygen content, continuous grain boundary α-phase and high-angle grain boundaries. Excellent mechanical properties were achieved when hot forging was used along with preheating the homogenized specimen at 1100 °C. The mechanisms underlying this phenomenon were suppression of the solid grain boundary α-phase, high fraction of low-angle grain boundaries and dynamic recrystallization. Tensile strength improved to the maximum value of 868 MPa. The strain recovery ratio increased to 90% in the first cycle during a cyclic loading–unloading test. Incomplete strain recovery of the first cycle was due to the presence of the martensite phase (α″).

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Dual-Phase Heat Treatment on Microstructure and Mechanical Properties of S135 High-Strength Drill Pipe Steel

Effect of dual-phase heat treatment (DPHT) on microstructure and mechanical properties of S135 high-strength drill pipe steel was studied by means of optical microscope, scanning electron microscope and mechanical property testing. The results show that the ferrite–martensite dual-phase microstructures were obtained at 760-800 °C. With increase in DPHT temperature, the volume fraction of martensite increases and the ferrite decreases. When the DPHT temperature is low, martensite is distributed as an island on the ferrite matrix, and ferrite is distributed as an island on the martensite matrix when the DPHT temperature is high. The hardness and strength of the steel increase, the plasticity and toughness decrease, the steel changes from ductile fracture to brittle fracture and the micro-fracture morphology changes from dimple to cleavage with increase in DPHT temperature. The strain hardening exponent of the as-received steel shows one ‘n’ value, but exhibits two ‘n’ values after DPHT. When the DPHT temperature is 750 °C, the relative content of martensite in the duplex structure is 15-20%, and the impact energy of the steel is similar to that of the as-received S135 drill pipe steel, and the strength is increased by about 15%. Micropores and micro-cracks on the subsurface of tensile fracture surface are mainly formed at the martensite/ferrite phase boundary or in the martensite slit.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure Characteristics and Comparative Analysis of Constitutive Models for Flow Stress Prediction of Inconel 718 Alloy

An accurate constitutive model is essential for analyzing deformation behavior of material and reliable numerical simulations in metal forming processes. In this study, hot tensile tests of Inconel 718 alloy have been conducted over a wide range of temperatures (300-973 K at an interval of 100 K), strains (0.01-0.3 at an interval of 0.01) and quasi-static strain rates (0.0001, 0.001, 0.01 s{sup −1}). Flow stress behavior is significantly affected by test temperatures and strain rates. Microstructure characteristics of deformed test specimens have been examined using scanning electron microscope and electron backscatter diffraction (EBSD). The fractography study revealed that fracture is mix-mode type, i.e., ductile and brittle. Subsequently, EBSD analysis shown that dynamic recrystallization mechanism is more pronounced at a higher temperature. Furthermore, four constitutive models, namely modified Cowper–Symonds, modified Johnson Cook, modified Zerillie-Armstrong and integrated Johnson Cook–Zerillie-Armstrong (JC-ZA) models have been investigated for flow stress prediction. Capability of models has been evaluated based on the correlation coefficient (R), average absolute error (Δ) and its standard deviation (δ). Accurate prediction of flow stress behavior is found by integrated JC-ZA model with R = 0.9873, Δ = 2.44 and δ = 4.08%.

36 MATERIALS SCIENCE↗
DOE OSTI2019

The Application of Canned Deformation to a Hard-Deformed Spray-Deposited Al-Zn-Mg-Cu Alloy

A wide variety of industrial applications require Al-Zn-Mg-Cu alloys with high strength and ductility. However, Zn-rich Al-Zn-Mg-Cu alloys prepared by rapid solidification methods usually show excellent strength but poor formability, which restricts their wide application in complex forming. In this study, we developed a new deformation method for the spray-deposited Zn-rich Al-Zn-Mg-Cu alloy to avoid crack formation and achieve isotropic properties. The microstructure evolution and crystallographic textures were analyzed and compared with the results of a normal deformation process. Crack-free forgings with the ultimate tensile strength (UTS), yield strength (YS) and elongation of 727 ± 5 MPa, 690 ± 3 MPa, 5.0 ± 0.7% along the radial direction and 735 ± 2 MPa, 693 ± 3 MPa, 6.3 ± 0.2% along the tangential direction were finally obtained. The improvement in the mechanical properties can be attributed to the larger amount of subgrains and substructures generated in the canned deformed alloy. The results of this study show excellent advantages and potential application prospects of the canned deformation to hard-deformed Al-Zn-Mg-Cu alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Failure Analysis of Three-Sheet Stackup Structure Made of Dissimilar High-Strength Steel

A three-sheet stackup structure consisting of B1500HS and B250P1 dissimilar high-strength steel was produced by resistance spot welding. The failure mode and energy absorption ability of two typical samples were investigated in typical loading state such as shear and cross-tension test. Moreover, the fracture characteristics were observed by scanning electron microscopy and the fracture process was analyzed based on the loading state. Results show two samples fracture in interfacial mode or pullout mode in shear test and fracture only in pullout mode in cross-tension test. Despite the different fracture modes, fractured sample shows ductile characteristics during mechanical tests. Compared with the sample fractured in interfacial mode, sample fractured in pullout mode has higher ultimate load and energy absorption ability in shear test. Moreover, sample absorbs more failure energy in cross-tension test than that in shear test because of the deformation of structure in cross-tension test. Both carbon equivalent and welding conditions affect the hardness of the fusion zone—a lower CE value leads to a softer HAZ which helps to promote pullout failure in the cross-tension test.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Creep Deformation and Rupture Behavior of P92 Steel Weld Joint Fabricated by NG-TIG Welding Process

The creep deformation and rupture behavior of P92 steel weld joint fabricated by narrow-gap TIG (NG-TIG) welding process have been investigated at 923 K over a stress range of 80-140 MPa. The prior-austenite grain size, M{sub 23}C{sub 6} precipitate size and hardness have been found to vary significantly in the weld joint. The reduction in hardness from weld metal to base metal with a trough at the outer edge of heat-affected zone (HAZ) has been observed. Coarsening of M{sub 23}C{sub 6} precipitate, recovery of martensite lath dislocation structure and formation of subgrain structure led to lower hardness in the intercritical HAZ. The creep rupture life of NG-TIG weld joint was lower than the base metal, the difference in creep rupture life between base metal and weld joint has been increased significantly with decrease in applied stress. The fracture location in the joint changed from base metal at high-stress regime to the fine grain (FG) HAZ (Type IV cracking) under lower stress level. Fracture in the FGHAZ evidenced the significant reduction in ductility, localized deformation and extensive localized cavitation. Extensive Laves phase formation with significant loss of solution strengthening contribution from tungsten and coarsening of M{sub 23}C{sub 6} precipitate with prolonged creep exposure led to reduction in hardness and more extensive cavitation in the FGHAZ resulting in premature Type IV failure of the weld joint. Weld strength reduction factor about 0.59 has been evaluated for 10{sup 5} h at 923 K.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Strain Rate on the Tensile Behavior of CoCrFeNi and CoCrFeMnNi High-Entropy Alloys

High-entropy alloys (HEAs), a novel class of metal alloys, have been receiving increasing attention from the scientific community. HEAs have the potential to be used in critical load-bearing applications in replacement of conventional alloys such as stainless steel and nickel-base superalloys. Tensile experiments at quasi-static to dynamic strain rates (10{sup −4}-10{sup 3} s{sup −1}) were performed on two single-phase face-centered cubic HEAs, CoCrFeNi and CoCrFeMnNi. Electron backscatter diffraction was used to study the microstructure of the samples before the experiments, and transmission electron microscopy was performed postmortem. The dominant deformation mechanisms were dislocation slip at quasi-static strain rates with the addition of deformation nano-twins at dynamic strain rates. Ultimate dynamic tensile strength and ductility improved with the increase in strain rate, which can be attributed to the activation of deformation nano-twins in HEAs. CoCrFeNi and CoCrFeMnNi both have low stacking fault energies, which could promote twinning at high strain rates to accommodate plastic deformation. The strain rate sensitivity of the flow stress increased with increasing strain rate, beginning with negligible strain rate sensitivity in the quasi-static range to high strain rate sensitivity in the dynamic range. CoCrFeMnNi showed greater strain rate sensitivity of flow stress. CoCrFeNi, with less configurational entropy, had higher mechanical properties and strain-hardening rates compared to CoCrFeMnNi, which was attributed to the weakening effect of the addition of Mn on the solid solution hardening.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Preheating and Water Cooling on the Performance of Friction-Stir-Welded Aviation-Grade Aluminum Alloy Joints

This paper examined the effect of preheating and water cooling on the thermal, mechanical, metallurgical, and texture properties of friction-stir-welded AA6082. Joints are fabricated under different preheating (FSW-P) and water cooling (FSW-C) conditions. The experimental outcomes indicate that the preheating and water cooling enhance the mechanical properties of the joint. The maximum tensile strength of 295 MPa is obtained for FSW-P1. It is also observed that preheating has more impact on microhardness. Microstructural analysis using optical microscope and electron backscatter diffraction analysis indicated that FSW produced fine equiaxed grains in the nugget zone due to dynamic recrystallization. Texture analysis shows that finer grain grains are obtained under preheating and water cooling conditions. The minimum grain size of 5.3 µm is obtained under FSW-P1. X-ray diffraction analysis shows that Mg{sub 2}Si phase in nugget zone that is an indication of dissolution and formation of strengthening precipitates. Fracture analysis indicates that preheating also improves the ductility of the joint.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Investigation on Microstructure and Mechanical Properties of TA10–Q245R Composite Plate Formed by Explosive Welding

The titanium alloy TA10 and the carbon steel Q245R are welded by explosive welding, and post-weld heat treatment (PWHT) is carried out for the TA10–Q245R composite plate. Microstructure and mechanical properties of the composite plate are investigated both in as-welded (AW) condition and after PWHT, respectively. Results show that, in AW condition, the bonding interface of composite plate presents wavy morphology, and at both sides of the interface plastic deformation is generated to a certain degree. Near the interface interdiffusion zone of alloying elements exists, and no Fe-Ti intermetallic compound is produced at the interface. SEM images indicate that the tensile fracture of composite plate presents the characteristic of ductile fracture, and no defects such as gas pores or impurities are found on the surface of shear fracture. After PWHT, the interfacial microstructure of the composite plate is obviously changed. Compared with the mechanical properties of the composite plate in AW condition, both tensile strength and shear strength of the composite plate are decreased to a certain extent after PWHT. With various factors into consideration, the suitable heat treatment procedure of TA10–Q245R composite plate is determined as follows: 550 °C × 1.5 h.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Quasi-static and Dynamic Compression of Aluminum Foam at Different Temperatures

This paper focused on the quasi-static (strain rate of ~ 10{sup −3} s{sup −1}) and dynamic compression (strain rate of ~ 10{sup 2} s{sup −1}) behavior of closed-cell AlSi alloy foam at 25, 300, 420 and 560 °C. Quasi-static and dynamic stress–strain relations as well as deformation modes of the foam at different test temperatures were investigated. The results showed that as the test temperature increased, the compressive strength decreased both under quasi-static and dynamic loading. Compressive strength of the foam showed strong strain-rate sensitivity, exhibiting a maximum increase from 20.60 MPa at ~ 10{sup −3} s{sup −1} to 37.27 MPa at ~ 500 s{sup −1} at 25 °C, from 2.99 MPa at ~ 10{sup −3} s{sup −1} to 10.49 MPa at ~ 500 s{sup −1} at 560 °C. At similar strain rate of ~ 500 s{sup −1} as an example, DIF (dynamic increase factor) showed a maximum increase from 1.81 at 25 °C to 3.50 at 560 °C, indicating that the strain-rate sensitivity of compressive strength increased with increase in temperature. Deformation modes of the foam under quasi-static compression revealed a variation from brittle to ductile as temperature increased, cell wall/edges showed brittle fracture at 25 and 300 °C and a combination of brittle fracture and plastic bending at 420 and 560 °C. The foam under dynamic compression showed evident wedge-shaped shear failure at all the temperatures tested.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Welding Parameters on Junction Structure and Fracture Behavior of Friction-Stir-Welded Joints of 2024 Aluminum Alloys

In this paper, the effect of welding speed and tool rotation speed on the junction structure and fracture behavior of friction-stir-welded joints was investigated with use of 2024 aluminum alloys of 3.2 mm thick. Results show that the formation of junction structure is more affected by tool rotation speed than by welding speed due to the differences of heat input levels. When welding speed is at 100 mm/min and tool rotation speed is in the range of 400-600 r/min, the joint junction structure causes complete bonding with no welding defects. While under improper welding parameters, the junction structure causes the precipitate aggregation on the bonding line and the partial bonding with the appearance of both tunnel defects and kissing bond defects. Meanwhile, the results of tensile tests show that the tensile fracture of all FSW joints occur at the weld center. The failure mode of joints with defects-free junction structure characterized by shear fracture is ductile. While the failure mode of joints with defects-existent junction structure characterized by layered fracture surface is brittle. Furthermore, that heterogeneous microstructure, precipitate aggregation and welding defects converge inside the junction structure is the main factor affecting the tensile properties and fracture behavior of the friction-stir-welded joint.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Evaluation of the Microstructure and Performance of Fe-21Cr-15Ni-6Mn-Nb Nonmagnetic Stainless Steel Welded Joints

In the present study, double-sided gas shield welding (GSW), in a single pass, was applied to Fe-21Cr-15Ni-6Mn-Nb nonmagnetic stainless steel plate. The microstructure and performance of the welded joint were investigated via microstructural and phase analysis, mechanical testing and magnetic analysis. The results showed a satisfactory micrographic appearance with full penetration, and no solidification defects observed in the welded joint, which showed typical epitaxial growth characteristics in the welding solidification. Slight grain growth, with an average size of ~ 18.9 μm, occurred in the heat-affected zone (HAZ). The primary dendrite arms, on both sides of welding joint, grew along the direction of the temperature gradient and met at the centerline of the weld with no parting microstructure in evidence. The secondary dendrite arm spacing was stable in all parts of the weld zone (WZ). The hardness of the weld zone was higher than that of the HAZ and the base metal (BM), with the lowest hardness occurring in the HAZ. The results of tensile testing showed a higher strength of the welded metal (WM) compared to the BM, and a good strength-ductility balance of the welded joint. The impact energy of the HAZ was higher than that of the WZ, corresponding to 60.5 J at room temperature and 52.7 J at − 196 °C. Microvoid coalescence, concurrent with crack propagation along the direction of the long axis of the dendritic sub-grains, was observed during plastic deformation in the WZ. Additionally, the excellent paramagnetic characteristics of the welded specimens were revealed.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Forced Cooling Friction Stir Welding of 2060-T8 Al-Li Alloy

Friction stir welding of 2060-T8 Al-Li alloy was experimentally investigated. Forced cooling process was realized by placing two copper pressure plates with a waterway on both sides of weld top surface. The relations between joint formation, microstructures and mechanical properties were mainly discussed. Compared with conventional joint by natural cooling, the welding joint with smaller flashes by forced cooling has the smaller well-distributed grains and higher microhardness in the stir zone (SZ). The tensile strength and elongation of forced cooling joint, respectively, reach 445.7 MPa and 6.7%, which are obviously higher than those of the conventional joint. The fracture locations of the conventional and forced cooling joints are located in the SZ, and the fracture surface morphologies show a typical ductile fracture mode.

36 MATERIALS SCIENCE↗
DOE OSTI2019

A Biomedical Ti-35Nb-5Ta-7Zr Alloy Fabricated by Powder Metallurgy

Ti-35Nb-5Ta-7Zr alloy for biomedical applications with a reduced Young’s modulus of less than 10 GPa, being comparable to that of human bones, was successfully fabricated by powder metallurgy method. The microstructure, compressive and tensile behavior were studied. Results indicate that Ti-rich and Ta-rich phases co-exist in β-matrix. Ti-35Nb-5Ta-7Zr alloys yield at about 10% in compression, with increasing in the content of process control agent, the resultant compressive yield strength and Young’s modulus increase from 650 ± 46 to 1055 ± 90 MPa and from 6.3 ± 0.03 to 8.24 ± 0.04 GPa, respectively. The tensile fractography of alloy with 9.7 GPa tensile Young’s modulus, 433 MPa UTS at the 5.5% elongation consists of cleavage fracture and ductile fracture with dimples. The failure mechanism has been discussed taking the intrinsic microstructural features into consideration.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Fatigue Failure Characteristics of Single-Crystal Fe-3%Al Alloy

To obtain a better understanding of the mechanical properties and failure characteristics of the single-crystal Fe-3%Al alloy (SC), tensile and fatigue properties of the SC and polycrystalline Fe-3%Al alloy (PC) have been investigated. The tensile strength and fracture strain for the SC samples were different depending on the loading direction, i.e., 〈111〉, 〈110〉 and 〈100〉, which could be explained by the Schmid factor. Fatigue strength for the SC samples was basically correlated with their tensile strength. Different fatigue properties were obtained for the PC sample, e.g., the high fatigue strength was obtained in the early fatigue stage, and significant reduction in the fatigue strength was appeared in the later fatigue stage, e.g., low endurance limit at 10{sup 7} cycles. The reason for this was caused by the material brittleness due to the accumulated dislocation at the grain boundaries. Because the high ductile SC samples made severe plastic deformation ahead of the crack tip, the crack growth rate did not enhance even if the high stress intensity factor range was applied in Region II (da/dN versus ΔK). The crack growth rate dropped just before the final fracture. Effective stress intensity factor range was estimated using the crack closure model, but the crack growth rate is found to be affected strongly by the slip system of SC sample.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Synergetic Strengthening of Grain Refinement and Texture in Gradient Zircaloy-4 by Surface Mechanical Rolling Treatment

A Zircaloy-4 rod was subjected to surface mechanical rolling treatment (SMRT) to form a gradient structure, and the evolution of the resulting sub-grain-boundary property and texture across the gradient structure was characterized using electron backscattered diffraction. Dual-gradient structures in grain size and orientation were formed; the grain size was refined from several microns at the center to approximately 400 nm at the topmost surface. Texture analysis revealed that the c-axis gradually tilted from a random orientation toward the parallel-to-radial direction on the radial–tangential plane. The SMRT-induced formation of the dual-gradient microstructure is attributed to the formation of gradient distributions of stress and strain, which resulted in various deformation mechanisms (twinning and dislocation) being active at different depths. During the SMRT process, twinning and dislocations were activated to refine the grains. When the dual-gradient microstructure formed, twinning was mainly activated at the subsurface near the matrix, whereas dislocations were activated across the entire gradient. The geometrically necessary dislocation density increased with decreasing depth and then slightly decreased near the surface. The synergetic strengthening of the dual-gradient microstructure resulted in a gradient distribution of the microhardness near the surface. Thus, the Zircaloy-4 rod exhibited a good combination of strength and ductility.

36 MATERIALS SCIENCE↗