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At least 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Microstructure Optimization and Cracking Control of Additive Manufactured Bainite Steel by Gas Metal Arc Welding Technology

In this work, bainite steel thin wall was prepared by additive manufacturing (AM) with an in situ isothermal transformation at 300 °C. The microstructures were analyzed by scanning electron microscopy (SEM), x-ray diffraction (XRD) and transmission electron microscopy (TEM). The mechanical properties of the AM samples were measured by microhardness testing and tensile testing. The results show that the bainite steel thin wall is free from cracks and each layer has a good bond with preceding one. The bainite steel thin wall displays consistent mechanical properties. The formation of the bainite microstructure improves the toughness of the thin wall and reduces the embrittlement caused by dendritic segregation.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Evolution of the Microstructure and Hardness of Fe-8Cr-2.1Mo-Si-V Die Steel at Different Cooling Rates after Hot Deformation

In this study, the effect of microstructure evolution, especially that involving the proeutectoid carbides, on hardness distribution of ledeburite steel containing 8% chromium content was investigated at different cooling rates after hot deformation. The continuous cooling transformation was carried out using a fully automatic transformation measuring apparatus under cooling rates in the range of 0.03-80 °C/s. The transformation temperatures, Ac1, Ac3 and Ms, were measured by dilatometry. A nanoindentation test was employed to evaluate the mechanical properties of the cooled specimens. The evolution of microstructure indicated that the proportion trend of the precipitated carbides was dependent on the cooling rates. In addition, a mixture of pearlite and proeutectoid carbides was observed under slow cooling rates in the range of 0.03-0.2 °C/s, for which only a martensite transformation took place in austenite, and the proeutectoid carbides precipitated negligibly at cooling rates greater than or equal to 10 °C/s. The results also showed that the nanohardness significantly decreased and then slowly increased to a stable value with increasing cooling rates, which was similar to the content trend of the carbides that precipitated during the cooling process.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Heat Treatment on Microstructure and Mechanical Properties of Extruded Mg-4Zn-1.5Al-2Sn Alloy

The microstructure and mechanical properties of extruded Mg-4Zn-1.5Al-2Sn (ZAT422) alloy under different heat treatments were investigated by optical microscopy, scanning electron microscopy, x-ray diffraction, transmission electron microscopy, micro-hardness tests, and tensile tests. The results show that the as-extruded and heat-treated ZAT422 alloys mainly consisted of α-Mg and Mg{sub 2}Sn, and no ternary phase was detected. After aging, the amount of secondary phase precipitates increased significantly and the mechanical properties were improved. Among all the heat treatments, T5 heat treatment at 150 °C for 25 h resulted in the best combination of strength and ductility for ZAT422 alloy. The ultimate tensile strength, yield strength, and elongation (ε{sub f}) were found to be 290, 182 MPa, and 20%, respectively. Based on microstructural characterization, the variations in mechanical properties of the alloy subjected to heat treatment were analyzed in detail.

36 MATERIALS SCIENCE↗
DOE OSTI2019

A Study on the Microstructures and Properties of Selective Laser Melted Babbitt Metals

Babbitt metal cubes were prepared by means of selected laser melting (SLM) process using Sn—11% Sb—6% Cu alloy powders; their microstructures were studied using optical microscope (OM), SEM, EDS, XRD and DSC; and their mechanical properties were tested using Vickers hardness and tensile methods. The results show that fully dense Babbitt metal components can be prepared by using appropriate SLM process parameters and an interlayer staggered laser scanning strategy. Anisotropy characteristics appear in both the microstructure and mechanical properties of SLM-Babbitt cubic specimens. The average hardness was in the range of 32.5 HV0.05 to 35.3 HV0.05. Both tensile strength and elongation were somewhat higher in a direction parallel to the laser scanning speed (Y axis) than in a direction perpendicular to the laser scanning speed (X axis). The strengthening mechanism is suggested to include solid solution strengthening of oversaturated Sb in the Sn matrix, as well as dispersion strengthening of finely dispersed SnSb and Cu{sub 6}Sn{sub 5} particles. The overgrown acicular Cu{sub 6}Sn{sub 5} phases at low laser scanning speeds and void formation at higher laser scanning speeds seriously deteriorate the mechanical properties of the SLM-Babbitt specimens.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Impact of Cryogenic Treatment on Mechanical Behavior and Microstructure of Ti-6Al-4V ELI Biomaterial

Ti-6Al-4V with its ELI variant is a potential biomaterial for structural implants. However, its poor resistance to plastic shearing results in poor tribological properties that hinders its widespread use in biomedical implants. Cryogenic treatment being an established technique for improving wear resistance and other mechanical properties of ferrous materials is applied on Ti-6Al-4V ELI alloy. To ensure comprehensive investigations, the widely studied microstructures of Ti-6Al-4V ELI, i.e., lamellar, martensite, two variants of bimodal and mill annealed are considered in this work. Deep cryogenic treatment at the soaking temperature (− 196 °C) for 24 h soaking time followed by tempering has been applied. The impact of cryogenic treatment has been assessed in terms of changes in the mechanical behavior including wear resistance, hardness, tensile properties and impact strength. The alterations in mechanical behavior due to cryogenic treatment are correlated with microstructure evolution. Cryogenic treatment has caused significant improvement in mechanical behavior of Ti-6Al-4V ELI alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Comparative Corrosion Behavior of Five Different Microstructures of Rebar Steels in Simulated Concrete Pore Solution with and Without Chloride Addition

The present work discusses the effect of five different microstructures, coarse, fine and very fine ferrite–pearlite, martensite and tempered martensite, made by furnace cooling, air cooling, forced air-cooling, water quenching and tempering, respectively, of a rebar steel on its corrosion performance in freely aerated with and without chloride-contaminated simulated concrete pore solution using the dynamic polarization and electrochemical impedance spectroscopy. The corrosion performance of the steels with five different microstructures relates to the polarization resistance, protective ability of rusts and the extent of the galvanic attack. The corrosion rate of the steels has been found to be comparable in the simulated concrete pore (SCP) solution. However, in chloride-containing SCP solution, corrosion rate has been found to increase in the following sequence: forced air-cooled–air-cooled–quenched–furnace-cooled–tempered steels.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Tungsten Inert Gas Arc Remelting on Microstructure and Wear Properties of Plasma-Sprayed NiCr-Cr{sub 3}C{sub 2} Coating

In this work, a NiCr-Cr{sub 3}C{sub 2} coating was prepared on an AISI 1045 steel substrate by plasma spraying and then remelted with the tungsten inert gas (TIG) arc method. The purpose was to study the effects of TIG remelting treatment on the microstructure, mechanical properties and wear properties of a plasma-sprayed NiCr-Cr{sub 3}C{sub 2} coating. The results show that after TIG remelting, the density of the coating increased, the internal porosity decreased from 8.1 to 0.2%, and the number of cracks and inclusions greatly reduced. The hardness of the coating also became uniform and increased from 1007 to 1141 HV{sub 0.1}, with an obvious increase of 13.3%, and the fracture toughness increased by 321.3%. The wear volume after TIG remelting at room temperature was reduced by 59.5%, and the wear pattern changed from brittle fracture and abrasive wear to abrasive wear and adhesive wear. The wear loss at 700 °C was also reduced by 43.6%, and the wear pattern was transformed from brittle fracture and fatigue wear caused by oxidation to oxidative wear and adhesive wear. Therefore, TIG remelting treatment can effectively improve the microstructure, mechanical properties and wear resistance of a plasma-sprayed NiCr-Cr{sub 3}C{sub 2} coating.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effects of Postweld Heat Treatment on Microstructure and Properties of Laser-Welded Ti-24Al-15Nb Alloy Joint

The microstructure evolution and mechanical properties of laser-welded joints of Ti-24Al-15Nb alloy under different postweld heat treatment (PWHT) conditions were systematically investigated. The results show that the microstructure is very sensitive to PWHT temperatures. The weld zone consists of B2 and O phases after PWHT. With the PWHT temperatures increasing from 850 to 1000 °C, the amount of O phase decreases gradually, while the grains of O precipitates become coarser. After PWHT, there are some thin acicular O precipitates in heat-affected zone (HAZ), and the decomposition of α{sub 2}-phase caused by niobium diffusion can be observed in the HAZ. The PWHT can significantly increase the microhardness of joints, resulting from O phase precipitation hardening effect. The tensile strength and elongation of joints can be remarkably improved after PWHT, which was closely related to the strengthening effect of O precipitates and slip transmission between O and B2 phases. In addition, the results indicate that the best mechanical properties can be achieved only when the number and size of O phase and B2 phase are the best match.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Chromium Content on Microstructure, Hardness, and Wear Resistance of As-Cast Fe-Cr-B Alloy

The effects of chromium content on microstructure, hardness and wear resistance of as-cast Fe-xCr-1.0B (x = 0, 4, 8, 12, 16, 20) alloys were investigated by means of optical microscope (OM), scanning electron microscope (SEM), x-ray diffractometer (XRD), hardness tester and ring block wear tester. The results showed that the microstructures of chromium-free alloy were mainly composed of pearlite, ferrite, and eutectic borocarbides, and the hardness was very low, only 18.8 HRC. With the increase in chromium content, the matrix of as-cast Fe-Cr-B alloy gradually began to transform from pearlite and ferrite to lath martensite, and the type of borocarbides changed from Fe{sub 2}(B, C) to M{sub 2}(B, C), M{sub 7}(C, B){sub 3} and M{sub 23}(C, B){sub 6} (M = Fe, Cr). When the chromium content was 12 wt.%, the hardness of alloy reached the highest value of 61.1 HRC, and the wear resistance of alloy was the best. The results of worn surface analysis showed that the wear failure of as-cast Fe-Cr-B alloy was mainly caused by multiple plastic deformation and micro-cutting.

36 MATERIALS SCIENCE↗
DOE OSTI2019

On the Microstructure and Solidification Behavior of N-Bearing Ti-46Al-8Ta (at.%) Intermetallic Alloys

Ti-46Al-8Ta (at.%) intermetallic alloys exhibit acceptable strength at low and high temperatures with good oxidation resistance. Different alloying elements are usually added to TiAl-based alloys to develop required properties. In this work, effects of N addition up to 2 at.% on microstructural features and solidification behavior of the Ti-46Al-8Ta alloy is investigated. The achieved results showed that alloying with N causes dendrite refinement and reduction of the retained metastable B2 phase in the as-cast microstructure. The dendrite morphology is also changed from fourfold to fivefold and then sixfold symmetry by increasing N content. The effect of N on interlamellar spacing, amount of constituent phases, and lattice parameters was not uniform. Increasing N content resulted in an increase in both small punch maximum load and hardness up to 1.05 at.%, whereas further N alloying, despite increasing of hardness, led to a drop in the maximum load.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Multiscale Modeling of Microstructural Evolution in Fused-Coating Additive Manufacturing

A quantitative phase field model with adaptive-computational-domain-expansion (ACDE) method was developed to simulate the microstructural evolution in fused-coating additive manufacturing process. A heat transfer and fluid flow model was established to investigate the solidification behavior of the molten pool in a single deposited layer. The solidification parameters such as temperature gradient and solidification rate were calculated from the macroscopic model and then fed into the phase field model to get the microstructure at different positions within a single deposited layer. Columnar dendrites were formed at the bottom of the deposited layer, and equiaxed dendrites appeared in the upper part of the layer. The ACDE method is used to improve the simulation efficiency. The metallographic experiments and energy-dispersive spectrometer analysis for composition were conducted on the fused-coating specimen, and the results were in good accordance with the simulations.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Solutionizing Time on Improving the Microstructure and Mechanical Properties of Aged AZ80 Mg Alloy

The effects of solutionizing soak time and aging on the microstructure and mechanical properties of cast AZ80 Mg alloy were investigated. Optical microscopy, scanning electron microscopy and x-ray diffraction revealed the dissolution of β-Mg{sub 17}Al{sub 12} phase in the solution-treated samples which was later detected in the aged condition. Solution treatments at 420 °C for 4-16 h with water quenching followed by aging at 200 °C for 10 h were carried out, causing discontinuous precipitates to be formed with different precipitate morphologies at the various soak times. The discontinuous precipitates were observed to transform from the interdendritic segregation to parallel lamellae with increasing soak time to 8-10 h. As the soak time increased to 16 h, parallel lamellae transformed into tree-like precipitate morphology. The results showed significant changes in the interlamellar spacing at different soak times, and such precipitate morphology could influence the microstructure and mechanical properties after the aging process. A good combination of strength and ductility was obtained when the cast AZ80 Mg alloy was solution-treated at 420 °C for 10 h and aged at 200 °C for 10 h. Compared to the as-cast condition, a significant increase in ultimate tensile strength (50%), yield strength (47%) and elongation (43%) were observed.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and Mechanical Properties of Ni-based Superalloy K418 Produced by the Continuous Unidirectional Solidification Process

Adopting effective strategies to control the solidification structure of Ni-based superalloys is a very interesting subject for metallurgists. Despite the achievement of developing and applying the continuous unidirectional solidification process in simple alloys, the utilization of this process for K418 alloys has been ignored. The microstructure and mechanical properties of a K418 alloy ingot produced by the continuous unidirectional solidification process were investigated. We found that the γ dendrites were typically cross-shaped in the transverse section. The orientation of surviving grains along the casting direction was <001> during competitive growth. The secondary dendrite arm spacing was 32.3 ± 2.5 μm at a cooling rate of 1.75 ± 0.35 °C/s from the surface to the center of the K418 alloy ingot. Due to the higher cooling rate than that of the conventional casting process, a more uniform microstructure and finer γ′ precipitation were obtained in the ingot. Thus, compared with the conventional casting, the tensile strength and the elongation are increased by 8.4 and 21.3%, respectively, at 25 °C. The tensile strength and elongation increased by 15.2 and 49.3%, respectively, at 800 °C. In addition, the fracture surfaces exhibited numerous typical dimples and dendritic fracture characteristics.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Microstructure of X-120 Steel on CO{sub 2} Corrosion Inhibition by 2-Mercaptobenzimidazole

The use of 2-mercaptobenzimidazole (2MBI) as corrosion inhibitor for API X-120 pipeline with two different microstructures in a CO{sub 2}-saturated 3.5% NaCl solution has been investigated. One microstructure was predominantly martensitic and bainitic (steel A), whereas the second one was mainly ferritic (steel B). It was found that 2MBI behaved as a mixed type of corrosion inhibitor and it changed the corrosion-controlled mechanism. In the uninhibited solution, steel A had a dissolution rate higher than steel B. However, in the presence of inhibitor, dissolution rate for steel A was lower than that for B because the film-formed inhibitor is adsorbed in a better way for steel A which has smaller precipitates distributed in a more homogeneous way than that for steel B.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Solution Treatment on the Microstructure, Micromechanical Properties, and Kinetic Parameters of the β → α Phase Transformation during Continuous Cooling of Ti-6Al-4V Titanium Alloy

The aim of this study is to examine the effect of solution treatment temperature (STT) on the microstructure, the micromechanical properties, and the kinetic parameters of the β → α phase transformation during continuous cooling of the dual phase titanium alloy Ti-6Al-4V. Increasing the STT from 1050 to 1200 °C delays the formation of the α phase during cooling and increases the value of its activation energy. The microstructural analysis reveals the emergence of α{sub W} platelets from protuberances on the α{sub GB}/α{sub W} interface. The investigation of the morphology of the α{sub W} platelets reveals the presence of ledges on their longest side showing a sharp extremity. The micromechanical properties determined by nanoindentation and microhardness tests are almost insensitive to the cooling rate but are strongly affected by the STT; the higher the STT, the lower the overall microhardness of the Ti-6Al-4V alloy. In addition, the STT affects the microhardness and the Young’s modulus of both α and β phases differently; when the STT increases, the microhardness and the Young’s modulus of the α phase decrease, whereas those of the β phase increases.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Influence of the Composition on the Solidification Path, Microstructure Evolution and Mechanical Properties of Al-Cu-Mg Alloys

The influence of the initial composition on the solidification path, microstructure evolution, thermophysical properties and mechanical properties of Al-Cu-Mg alloys was investigated. The solidification paths of the investigated alloys were determined by analyzing the solidification structure, and then the experimentally determined paths were compared with those from the calculated results. Three of the investigated alloys experienced typical ternary eutectic solidification, and the other four alloys experienced quasiperitectic reactions. Due to differences in the initial compositions, different eutectic morphologies were formed during the solidification process. The binary eutectic phases, (α-Al + Al{sub 2}Cu) and (α-Al + Al{sub 6}CuMg{sub 4}), and the (α-Al + Al{sub 6}CuMg{sub 4} + Al{sub 8}Mg{sub 5}) ternary eutectic phase tended to exhibit divorced growth, and the (α-Al + Al{sub 2}CuMg), (α-Al + Al{sub 6}CuMg{sub 4}) and (α-Al + Al{sub 2}Cu + Al{sub 2}CuMg) eutectic phases tended to exhibit coupled growth. The results indicated that the initial composition, solidification path, microstructure and morphology could greatly affect the melting enthalpy, microhardness and compressive strength of the investigated Al-Cu-Mg alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2019

A Comparative Analysis on Microstructure and Fracture Mechanism of X100 Pipeline Steel CGHAZ Between Laser Welding and Arc Welding

The objective of this study is to compare and contrast the differences in microstructure and fracture mechanism between CGHAZ of laser welding and submerged arc welding in X100 pipeline steel. The single-pass welding thermal simulation test of laser welding and submerged arc welding was carried out in MMS-200 thermal simulation test machine using self-made fixtures. The results indicated that the microstructure of laser welding CGHAZ and submerged arc welding CGHAZ was lath martensite and granular bainite, respectively. The impact energy of laser welding CGHAZ was 27 J, and the fracture mode was ductile fracture; the impact energy of submerged arc welding CGHAZ was 7.7 J, and the fracture mode was brittle fracture. The nucleation and propagation mechanism of cracks in different welding modes was established. Microcracks nucleated at the position of dislocation accumulation and remained austenite suppressed crack nucleation in laser welding CGHAZ. The nucleation site was at the M–A constituents in submerged arc welding CGHAZ. The main crack of laser welding CGHAZ was formed by one or a small number of microcracks. The main crack in submerged arc welding CGHAZ was formed by a large number of microcracks. The difference between crack nucleation and propagation mechanism is the reason why the impact energy of laser welding CGHAZ is higher than of submerged arc welding CGHAZ.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Characterization of Subsurface Microstructural Alterations Induced by Hard Turning of Inconel 718

Conventional machining especially hard turning is commonly used in aerospace industry to produce the final shape of cylindrical parts. However, the process generates damages and microstructural alterations on the surface and at the subsurface layer, in particular the cracking of hard carbides particles. The present paper focuses on investigating the probable mechanisms responsible for carbide cracking and the formation of a softened layer at a depth between 10 and 30 µm below the machined surface. Advanced techniques, such as laser confocal microscopy and field emission gun scanning electron microscope equipped with in situ picoindenter and electron backscatter diffraction, were used to characterize and analyze the evolution of the microstructure in the affected layer and propose the possible governing mechanisms for the observations.

36 MATERIALS SCIENCE↗