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At least 847 records · Page 47
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Process Evaluation of AISI 4340 Steel Manufactured by Laser Powder Bed Fusion

Laser powder bed fusion (L-PBF) involves the consolidation of metal powder, layer by layer, through laser melting and solidification. In this study, process parameters are optimized for AISI 4340 steel to produce dense and homogeneous structures. The optimized process parameters produce mechanical properties at the center of the build plate that are comparable to wrought in the vertical and horizontal orientations after heat treatment and machining. Four subsequent builds are filled with specimens to evaluate the mechanical behavior as a function of location and orientation. Variations in the mechanical properties are likely due to recoater blade interactions with the powder and uneven gas flow. The results obtained in this study are analyzed to assess the reliability and reproducibility of the process. A different build evaluates the performance of near-net-shaped tensile specimens angled 35°-90° from the build plate surface (horizontal). Ductility measurements and surface roughness vary significantly as a function of the build angle. In the stress-relieved and as-built conditions, the mechanical behavior of vertically oriented specimens exhibits somewhat lower and more variable ductility than horizontally oriented specimens. Therefore, several process variables affect the mechanical properties of parts produced by the L-PBF process.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Titanium Alloys Thin Sheet Welding with the Use of Concentrated Solar Energy

The present study deals with the welding of titanium alloys thin sheets 1.3 mm thick, with the use of concentrated solar energy. The experimental part of the work took place at a medium size solar furnace at the installation of the Centre National de la Recherche Scientifique, at Odeillo, in Southern France, where similar and dissimilar defect-free welds of titanium Grades 4 and 6 were achieved, in the butt joint configuration. After the determination of the appropriate welding conditions, the optimum welded structures were examined and characterized microstructurally, by means of light optical microscopy, scanning electron microscopy, and microhardness testing. In addition, test pieces extracted from the weldments were tested under uniaxial tensile loading aiming to the estimation of the strength and the ductility of the joint. The analysis of the experimental results and the recorded data led to the basic concluding remarks which demonstrate increased hardness distribution inside the fusion area and severe loss of ductility, but adequate yield and tensile strength of the welds.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of Strain Rate on Tensile Deformation and Fracture Behavior of Directionally Solidified Superalloy DZ125L

The tensile deformation and fracture behavior of directionally solidified superalloy DZ125L were studied with a strain rate ranging from 10{sup −3} to 7 × 10{sup 2} s{sup −1} at room temperature. Results show that the strength of DZ125L alloy presents positive strain rate sensitivity in tested strain rate range which is mainly resulted from the increasing accumulation of dislocations in the matrix. The ductility of the alloy first increases and then it remains insensitive with the increase in strain rates. The enhancement of ductility is primarily due to the increasing amount of broken MC carbides and plastic deformation in the matrix at higher strain rates. The strain rates influence the amount of broken MC carbides and then affect the deformation behavior of the alloy. Based on observations and discussions of the lateral microstructures and dislocations, it is concluded that the MC carbides with sizes larger than 10 μm are easy broken in the early deformation stage and act as crack sources during the proceeding plastic deformation. The growth and connection of microcracks and the propagation of main crack lead to the fracture of the matrix.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Micromechanism of High-Temperature Tensile Deformation Behavior of a Directionally Solidified Nickel Base Superalloy

High-temperature tensile deformation behavior of directionally solidified nickel base superalloy CM 247 DS is studied by conducting tensile tests in temperature range RT-955 °C employing a constant strain rate of 10{sup −3} s{sup −1} and carrying out extensive electron microscopic examinations to understand the concomitant substructural evolution. The alloy exhibits yield strength anomaly (YSA) behavior like many other superalloys, and the yield strength maxima occur at 750 °C. However, unlike in most of the superalloys, ductility continuously increases with temperature. The deformation behavior of the alloy changes significantly with temperature. Transmission electron microscopic examination confirmed that at lower temperature (≤ 750 °C), γ′ shearing is the dominant deformation mechanism; whereas at temperatures above 750 °C, thermally activated dislocation looping around γ′ precipitate is dominant. Substructures evolved during deformation at 750 °C consists mainly of superlattice stacking faults (SSFs) inside γ′ precipitates, whereas at 850 °C uniform dislocation tangles are observed in γ matrix. Superlattice stacking faults result from shearing of γ′ precipitate by a/3〈112〉 dislocations, which arise from the decomposition of a/2〈110〉 matrix dislocations. YSA in this alloy is attributed to dislocation interactions inside γ′; however, the enhanced ductility even at 750 °C is due to formation of SSFs.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Yttrium Addition on the Microstructure and Mechanical Properties of Cu-Rich Nano-phase Strengthened Ferritic Steel

Due to the brittle problem of Cu-rich nano-phase strengthened ferritic steel (CNSFS) after air aging, the effect of Y addition in CNSFS was systemically investigated in the present work. The microstructure, tensile fracture morphology and oxide layer of the steels were surveyed by optical microscope and scanning electron microscope. Transmission electron microscope with the combination of energy-dispersive x-ray spectroscopy and selected area electron diffraction was used to analyze the morphology, size, number density, chemical compositions and crystal structure for nano-crystalline precipitates. Microstructural examinations of the nano-crystalline precipitates show that Cu-rich precipitates and Y compounds in the range of 2-10 and 50-100 nm, respectively, form in the Y-containing steel; meanwhile, the average size of nano-crystalline precipitates in Y-containing steel is larger, but the number density is lower, and the ferritic grains are refined. Furthermore, the tensile strength and ductility of Y-containing steel after air aging are improved, whereas the tensile strength is enhanced and the ductility decreased after vacuum aging. The drag effect of Y makes the oxide layer thinner and be compacted. Tensile properties of CNSFS after air aging are improved due to the refined grains, antioxidation and purification by the addition of Y.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Nanoindentation Stress–Strain for Fracture Analysis and Computational Modeling for Hardness and Modulus

Stress–Strain plots based on nanoindentation load–depth curves were obtained to study phenomena like internal fracture and ductile to brittle transitions. Fracture phenomena during the indentation process were analyzed based on the stress–strain plots. A transition from ductile to brittle fracture was observed on increasing the depth or load of indenter penetration. A new approach with shape factors in the fracture studies based on radial crack branching and micro-cracking was done. Hardness and modulus plots were fitted with polynomials. The fitting parameters were varied to obtain different hardness and modulus responses.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Strain Rate-Dependent Tensile Deformation Behavior of Fiber Laser-Welded Butt Joints of Dual-Phase Steels

The microstructure and tensile deformation behavior of the fiber laser-welded similar and dissimilar dual-phase (DP) steel joints over a wide range of strain rates from 10{sup −3} to 10{sup 3} s{sup −1} were investigated for the further applications on the lightweight design of vehicles. The high strain rate dynamic tensile deformation process and full-field strain distribution of the base metals and welded joints were examined using the digital image correlation method and high-speed photography. The strain rate effects on the stress–strain responses, tensile properties, deformation, and fracture behavior of the investigated materials were analyzed. The yield stress (YS) and ultimate tensile strength (UTS) of the dissimilar DP780/DP980 welded joints were lying in-between those of the DP780 and DP980 base metals, and all materials exhibited positive strain rate dependence on the YS and UTS. Owing to the microstructure heterogeneity, the welded joints showed relatively lower ductility in terms of total elongation (TE) than those of the corresponding base metals. The strain localization started before the maximum load was reached, and the strain localization occurred earlier during the whole deformation process with increasing strain rate. As for the dissimilar welded joint, the strain localization tended to occur in the vicinity of the lowest hardness value across the welded joint, which was in the subcritical HAZ at the DP780 side. As the strain rate increased, the typical ductile failure characteristic of the investigated materials did not change.

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

Effect of Hot Rolling on the Microstructure and Mechanical Properties of Nitrogen Alloyed Austenitic Stainless Steel

In the present investigation, the effect of multi-pass hot rolling in the temperature range of 700–1000 °C on the microstructure and mechanical properties of nitrogen alloyed austenitic stainless steel was studied with the aid of optical microscopy, tensile testing and x-ray diffraction measurements. The microstructural changes that occurred in the hot rolled specimens were elongation of grains in rolling direction, nucleation of new grains at the grain boundaries of elongated grains and growth of nucleated grains to form fully recrystallized grains. Elongated grains formed at lower rolling temperature (700–800 °C) due to inadequate strain/temperature for the initiation of dynamic recrystallization. At higher rolling temperature (900–1000 °C), fine grains formed due to dynamic recrystallization. Tensile properties showed strong dependency on the rolling temperature. Tensile strength increased with the decrease in the rolling temperature at the cost of ductility. Maximum strength was observed in samples hot rolled at 700 °C with yield strength of 917 MPa and ductility of 25%. This variation in the tensile properties with the rolling temperature is attributed to changes in the dislocation density and grain structure. The estimated yield strength from the dislocation density, solid solution and grain boundary strengthening closely matched with experimentally determined yield strength confirming the role of dislocation density and grain size in the strengthening.

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

Development of Graphene Nanoplatelet-Reinforced AZ91 Magnesium Alloy by Solidification Processing

It is a challenging task to effectively incorporate graphene nanoplatelets (GNPs) which have recently emerged as potential reinforcement for strengthening metals into magnesium-based matrices by conventional solidification processes due to their large surface areas and poor wettability. A solidification processing which combines mechanical stirring and ultrasonic dispersion of reinforcements in liquid matrix was employed to develop AZ91 magnesium alloy matrix composites reinforced with 0.25 and 0.5 wt.% GNPs. The microstructural studies conducted with scanning and transmission electron microscopes revealed that fairly uniform distribution and dispersion of GNPs through the matrix were achieved due to effective combination of mechanical and ultrasonic stirring. The GNPs embedded into the magnesium matrix led to significant enhancement in the hardness, tensile strength and ductility of the composites compared to those of unreinforced AZ91 alloy. The strength enhancement was predominantly attributed to the grain refinement by the GNP addition and dislocation generation strengthening due to the coefficient of thermal expansion mismatch between the matrix and reinforcement. The improved ductility was attributed to the refinement of β eutectics by transforming from lamellar to the divorced eutectics due to the GNP additions. In addition, the strengthening efficiency of the composite with 0.25 wt.% GNP was found to be higher than those of the composite with 0.5 wt.% GNP as the agglomeration tendency of GNPs is increased with increasing GNP content. These results were compared with those of the GNP-reinforced magnesium composites reported in the literature, indicating the potential of the process introduced in this study in terms of fabricating light and high-performance metal matrix composites.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Processing Parameter Effects on Residual Stress and Mechanical Properties of Selective Laser Melted Ti6Al4V

Selective laser melting (SLM) process is characterized by large temperature gradients resulting in high levels of residual stress within the additively manufactured metallic structure. SLM-processed Ti6Al4V yields a martensitic microstructure due to the rapid solidification and results in a ductility generally lower than a hot working equivalent. Post-process heat treatments can be applied to SLM components to remove in-built residual stress and improve ductility. Residual stress buildup and the mechanical properties of SLM parts can be controlled by varying the SLM process parameters. This investigation studies the effect of layer thickness on residual stress and mechanical properties of SLM Ti6Al4V parts. This is the first-of-its kind study on the effect of varying power and exposure in conjunction with keeping the energy density constant on residual stress and mechanical properties of SLM Ti6Al4V components. It was found that decreasing power and increasing exposure for the same energy density lowered the residual stress and improved the % elongation of SLM Ti6Al4V parts. Increasing layer thickness resulted in lowering the residual stress at the detriment of mechanical properties. The study is based on detailed experimental analysis along with finite element simulation of the process using ABAQUS to understand the underlying physics of the process.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of ECAP Die Angles on Microstructure Mechanical Properties and Corrosion Behavior of AZ80 Mg Alloy

In this article, the effect of channel angles on material properties was investigated during equal channel angular pressing of AZ80 magnesium alloy using processing route R at 325 °C processing temperature. Channel angles of 90° and 110° and common corner angle 30° have been considered for this study. It has been revealed that the channel angle has a significant influence on deformation homogeneity, microhardness, ultimate tensile strength, ductility and corrosion behavior of Mg alloys. Investigation with reference to as-received AZ80 Mg alloy indicates 18.47% improvement in UTS and 76.07% enhancement in ductility after processing through 3P-90° and 2P-110° ECAP, respectively. Also, the corrosion rate reduces to 89.47% after processing the sample with 3P-110° ECAP die.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Forging on Microstructure, Mechanical Properties and Acoustic Emission Characteristics of Al Alloy (2014): 10 wt.% SiC{sub p} Composite

In this paper a systematic study has been undertaken to study the effect of forging on microstructure, mechanical properties and acoustic emission (AE) characteristics of Al alloy (2014)—10 wt.% SiC metal-matrix composites. Hammer forging was carried out at a temperature of 470 °C in an open die, with a deformation ratio of 3.5:1. Microstructural analyses were carried out at three different positions of forged billet. Significant material flow was observed in the sample taken from the sides of the forged billet, but no appreciable material flow was observed at the center of the forged billet. The microstructural characterization showed that forging resulted in cracking of SiC particles in Al alloy (2014)—10 wt.% SiC composite. The AE results during forging showed an increase in AE signal intensity during the initial yielding period and a subsequent decrease in the intensity of AE signals with sudden burst at the end of the forging. The plastic deformation at room temperature was less ductile in nature but at high temperature, appreciable ductility was observed. The FESEM micrographs of fracture surfaces of the tensile specimens showed appreciable clustering of SiC particles during plastic deformation both at room and high temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Elucidating the Effect of TiB{sub 2} Volume Percentage on the Mechanical Properties and Corrosion Behavior of Al5083-TiB{sub 2} Composites

Al5083-TiB{sub 2} in situ composites with 1, 3, and 5 volume percentages of TiB{sub 2} were fabricated. The effect of different volume percentages of TiB{sub 2} reinforcement particles on the mechanical properties and corrosion resistance of these composites was investigated. The hot extrusion process resulted in fragmentation and more homogeneous distribution of TiB{sub 2} particles throughout the Al5083 matrix. Also, the grain size reduced as the amount of TiB{sub 2} reinforcement particles increased which caused the improvement of both the strength and ductility of the material. It was shown that the hardness, the ultimate tensile strength (UTS), and the yield strength (YS) of both the as-cast and extruded materials increased with increasing TiB{sub 2} content. Also, TiB{sub 2} addition combined with the extrusion process improved both UTS and elongation (El.%) of the extruded structures which can be attributed to the presence of TiB{sub 2} particles and obtained microstructural refinement by hot extrusion process. However, the extruded Al5083-5 vol.% TiB{sub 2} composite showed lower ductility in comparison with the extruded Al5083-1 vol.% TiB{sub 2} and Al5083-3 vol.% TiB{sub 2} composites. Moreover, it could be concluded that the corrosion resistance and also the pitting corrosion behavior of the Al5083 alloy do not significantly change in the presence of TiB{sub 2} reinforcement particles.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Relationship between Grain Size and Sample Thickness on the Creep-Rupture Performance of Thin Metallic Sheets of INCONEL Alloy 740H

A study was conducted on INCONEL® alloy 740H® to examine the role of grain size and sheet thickness on the alloy’s creep-rupture behavior. Three different starting sheet thicknesses were utilized and multiple heat-treatment conditions anticipated for compact heat exchanger (CHX) manufacturing were applied to produce a range of grain sizes. Creep-rupture testing was conducted at 750 °C for times up to about 6000 h and the results were compared to wrought databases. The data show that both creep strength and ductility were important factors in the overall creep performance of the sheets. Reductions in performance were observed due either to accelerated creep when grain size was fine or loss of rupture ductility when grain sizes approached the sheet thickness. Some combinations of heat-treatment and thickness were able to produce typical expected wrought creep properties. Historically a ‘rule-of-thumb’ requirement for creep testing suggests 3-5 grains per sample minimum dimension to ensure homogeneous behavior. This research shows that to ensure representative wrought creep performance (i.e. no effect of sample size), the sample minimum cross-section should be 10 times the average ASTM grain size. Statistical analysis of the microstructures suggests the population of larger grains as a controlling feature in creep failure.

14 SOLAR ENERGY↗
DOE OSTI2023

Micromechanical properties of spherical and facetted He bubble loaded copper

Exploring new irradiation resistant materials requires understanding their mechanical responses to irradiation. Resistance to helium bubble formation and understanding bubble effects on the mechanical response of candidate materials are crucial factors to qualify materials as irradiation resistant. Here, in this paper, we explore the effect of spherical and facetted helium bubbles on the mechanical response of copper via in-situ micromechanical tensile testing at room temperature. Bubble formation and shape effects on strength and ductility, and their behavior on grain boundaries are discussed and compared to literature. Loading Cu with helium bubbles is shown here to increase strength but decrease ductility.

42 ENGINEERING↗
DOE OSTI2011

Chromium-free nickel alloys for hot sulfuric and sulfur environments

There are few adequate materials available for severe corrosion conditions, like those of the S–I thermochemical cycle. High Si, Ni-alloys have excellent corrosion resistance, especially in mineral acids, but have typically been limited by poor mechanical properties or difficult fabrication issues. The ductility of nickel silicide, Ni3Si, can be improved through a combination of micro- and macro-alloying. Nb and other minor alloying elements yield a cast alloy with excellent corrosion resistance to sulfuric acid and good mechanical properties. In this paper, efforts to optimize the alloys performance are presented along with progress toward the development of a wrought version of the material. It was found that an appropriate heat treatment provides the largest improvement in the cast Ni–Si alloy microstructure. Trials have resulted in more than a 50% reduction by the cold rolling process. This process not only increases homogenization but also results in a more uniform distribution of G-phase particles, which is beneficial for the improvements in ductility and corrosion resistance. These alloys have great potential for use in future hydrogen production as well as fossil energy combustion.

99 GENERAL AND MISCELLANEOUS↗