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

Effect of Welding Parameters on the Corrosion Behavior of Dissimilar Alloy Welds of T6 AA6061 Al-Galvanized Mild Steel

Partial replacement of steel by Aluminium (Al) alloys is a promising approach adopted by automotive sector to improve fuel efficiency without compromising on the strength. However, this results in the generation of dissimilar welds of Al alloys and steel. Understanding the corrosion behavior of Al alloys–steel welds as a function of welding parameters is critical in the successful application of such alloys. The present study reports the intergranular corrosion behavior of T6 heat-treated AA6061 Al alloy-galvanized mild steel lap joints, welded by metal inert gas welding–brazing technique as a function of different welding parameters, viz. weld speed (4-6 m/min) and wire feed rate (0.8-0.9 m/min), following ASTM G 67-04. Weld characterization was performed by field emission scanning electron microscopy (FESEM), x-ray diffraction and nano-hardness measurement. Results indicate heavy dissolution and metal loss at the interface. A high volume fraction of Al-Fe intermetallics was precipitated at the weld interfaces, resulting in high hardness and higher localized corrosion. The thickness of Al-Fe intermetallic layer increased with increasing wire feed rate and lower weld speed which enhanced the severity of galvanic and intergranular corrosion.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Role of Precipitates in Recrystallization Mechanisms of Nb-Mo Microalloyed Steel

Recrystallization behavior of a Nb-Mo microalloyed steel has been investigated using double-hit compression tests. A suitable reheating temperature and soaking time were established for the complete dissolution of the microalloying precipitates prior to hot deformation. The influence of hot deformation conditions on the flow behavior and the effect of alloying elements, Nb in particular, on austenite recrystallization kinetics are highlighted. The strain-induced precipitation has been found to play an important role in hindering recrystallization kinetics. Post-deformation microstructural analysis indicates that strain-induced grain boundary migration (SIMB) is one of the mechanisms for the formation of recrystallized grains. Retardation of recrystallization has been explained by estimating pinning force of Nb(C, N) precipitates and recrystallization driving force at 1000 °C, which is in good agreement with the experimental observations in the present study as well as other reported data. The study brings out a better understanding on the influence of strain-induced precipitates on recrystallization behavior.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Different Post-treatments on the Microstructure of EBM-Built Alloy 718

Electron beam melting (EBM) of Alloy 718 is of rapidly growing interest as it allows cost-effective production of complex components. However, the inherent flaws in the component in as-built state are of concern in view of the severe working conditions in which Alloy 718 components typically operate. The present work entails an investigation of changes in microstructure that accompany some post-treatments that are being widely considered to address defects in EBM processed Alloy 718. The effect of two different post-treatments, namely hot isostatic pressing (HIP) and a combined HIP + heat treatment (HT) carried out inside the HIP vessel, have been studied and results from as-built and post-treated specimens were compared in terms of porosity/lack-of-fusion, microstructure, phase constitution (NbC content, δ-phase) and micro-hardness. Post-treatment resulted in reduction in defect content by more than an order of magnitude. HIPing led to complete dissolution of δ phase. In comparison to as-built material, HIPed specimens exhibited significant drop in hardness. However, a sharp ‘recovery’ of hardness to yield values higher than in as-built condition was observed after HIP + HT and can be attributed to precipitation of γ′′ phase.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Lithium on the Discharge and Corrosion Behavior of Mg-3 wt.% Al Alloy as the Anode for Seawater Activated Battery

The discharge and corrosion behavior of Mg-3 wt.% Al alloys added with different contents of lithium in 3.5 wt.% NaCl aqueous solution is studied by electrochemical methods and microstructure characterization. The results indicate that adding 4 wt.% lithium severely promotes the self-corrosion and the addition of 12 wt.% lithium is not valuable to the spalling of discharge products. In contrast, doping Mg-3 wt.% Al with 8 wt.% lithium negatively shifts the discharge potentials and inhibits the self-discharge, attributed to the dual-phase structure of α-Mg plus β-Li that removes the discharge products and favors the uniform dissolution in the course of discharge. Moreover, the multi-pass rolling with 40% reduction could tailor the microstructure of Mg-3 wt.% Al-8 wt.% Li alloy and further enhance its discharge activity, even though the corrosion resistance is slightly reduced. This means that the rolled Mg-3 wt.% Al-8 wt.% Li sheet is suitable to serve as the anode for seawater activated battery.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effects of Solution Treatment on Microstructure Transformation, Tensile and Exfoliation Corrosion Properties of 7136 Aluminum Alloy

The effects of solution treatment on microstructure transformation and tensile properties of 7136 aluminum alloys were investigated by means of scanning electron microscopy, electron backscatter diffraction, differential scanning calorimeter, x-ray diffraction and tensile test, electrochemical evaluations, exfoliation corrosion tests, respectively. The results indicate that solution treatment had a significant effect on the microstructure transformation and tensile properties. Soluble constituents of 7136 aluminum alloys were more likely to form the η(MgZn{sub 2}) phase rather than the S(Al{sub 2}CuMg) phase. With increasing the solution temperature, the η phase was re-dissolved into the α(Al) matrix gradually. After solution treatment of 475 °C/1 h, the η phase was completely dissolved into the matrix, with insoluble Fe-rich phases remained. The dissolution of η phase would increase the strength and the corrosion resistance of the alloys. When continuing to increase the solution temperature, the recrystallized fraction increased obviously, resulting in the decrease in the strength and the corrosion resistance. Consequently, the strength and the corrosion resistance of the alloys increased firstly and then decreased with the increase in solution temperature. After solution treatment of 475 °C/1 h, the tensile strength and yield strength of the aged alloy reached their peaks, 738 and 715 MPa, respectively. The elongation of the alloy solution treated at 475 °C/1 h reached 11.2%, and its exfoliation corrosion rating got EB level. Furthermore, after the double-stage solution treatment of 450 °C/1 h + 475 °C/1 h, the recrystallized fraction decreased, resulting in a slight increase to 12.6% of the elongation, a slight increase in the corrosion resistance and a slight decrease in the strength.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effects of Chloride and Carbonate on Corrosion of API-X100 Pipeline Steel

Effects of chloride, introduced with 0.03, 0.05, 0.10, 0.12, 0.22, 0.25 M, on the corrosion behavior of API-X100 pipeline steel in 0.15 M carbonate solutions were studied. The evaluations were carried out with monitoring the open-circuit potentials and electrochemical impedance spectroscopy (EIS). The open-circuit potentials gave an electrochemical evidence on the weakening opposite kinetics of the passive film formation to the kinetics of dissolution with higher chloride concentrations. The open-circuit potentials were less with higher chloride concentrations. The EIS tests indicated that the corrosion resistance decreased, and species diffusion increased with the chloride concentration, in multi-time constant interactions. In all tests, slippery transparent films formed with no presence of pitting on the sample surfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Tempering Temperature on the Precipitation Behavior and Mechanical Properties of a Martensite Ferritic Steel

In this investigation, the precipitation of boron-containing M{sub 23}C{sub 6} in the CB2 steel during tempering was investigated. Scanning electron microscopy, energy-dispersive x-ray spectroscopy, transmission electron microscopy and differential scanning calorimetry were used to analyze the influence of temperature on the microstructure and properties of CB2-tempered martensite ferritic steel. The steel was normalized at 1100 °C for 2 h and then tempered at six different temperatures between 690 and 790 °C for 2 h. During air-cooling following austenitization, the precipitation of M{sub 3}C particles occurred in flakes forming with their diameter and thickness of 100 and 20 nm, respectively. The boron-containing M{sub 23}C{sub 6} was formed on the lath boundary after the dissolution of M{sub 3}C within the boundary. When the tempering temperature increased from 690 to 790 °C, the average diameter of boron-containing M{sub 23}C{sub 6} increased from 132 to 198 nm. Owing to the rapid growth of the precipitates, the phenomenon of lath widening as well as the recovery of microstructure would take place at high temperature. With an increase in the temperature, the tensile and yield strengths decreased gradually, while the plasticity increased and the boron-containing M{sub 23}C{sub 6} grew rapidly. Furthermore, when the tempering temperature exceeded 750 °C, the plasticity of CB2 steel was significantly improved.

36 MATERIALS SCIENCE↗
DOE OSTI2019

High-Temperature Interaction of Liquid Gd with Y{sub 2}O{sub 3}

The sessile drop method combined with contact heating procedure was applied for the investigation of high-temperature interaction between liquid Gd and Y{sub 2}O{sub 3} substrate. Real-time behavior of Gd sample in flowing inert gas (Ar) atmosphere upon heating to and at temperature of 1362 °C was recorded using high-speed high-resolution CCD camera. The results evidenced that molten Gd wets Y{sub 2}O{sub 3} substrate (the contact angle θ < 90°) immediately after melting of metal sample observed at T = 1324 °C (T{sub m} = 1312 °C). During the first 3 min of the sessile drop test, the contact angle dropped from θ = 52° to θ = 24° and then stabilized at the final value of θ{sub f}{sup *} = 33°. The solidified Gd/Y{sub 2}O{sub 3} couple was subjected to structural characterization using optical microscopy, scanning electron microscopy coupled with x-ray energy-dispersive spectroscopy. The results evidenced that the wettability in the Gd/Y{sub 2}O{sub 3} system has a reactive nature and the leading mechanism of the interaction between liquid Gd and Y{sub 2}O{sub 3} is the dissolution of the ceramic in the liquid metal responsible for the formation of a deep crater in the substrate under the drop. Therefore, the final contact angle θ{sub f}*, estimated from the side-view drop image, should be considered as an apparent value, compared to the more reliable value of θ{sub f} = 70° measured on the cross section of the solidified couple.

36 MATERIALS SCIENCE↗
DOE OSTI2019

The Microstructure, and Mechanical and Corrosion Properties of As-Cast and As-Extruded Mg-2%Zn-x%Cu Alloys After Solution and Aging Heat Treatments

The effects of different Cu concentrations and heat treatment on the microstructure, mechanical and corrosion properties of the as-cast and hot-extruded Mg-2%Zn alloys were studied. The results showed that T6 heat treatment led to the dissolution of intermetallics and breaking the continuous networks of intermetallics. Also, the grain size of the base alloy decreased with further Cu additions after solution and aging heat treatments. Microstructural characterization indicated that Mg(Zn,Cu) and Mg(Zn,Cu){sub 2} intermetallics were formed which their volume fractions increased with exceeding Cu additions. Moreover, the addition of 3 wt.% Cu and T6 heat treatment increased the Brinell hardness of both as-cast and hot-extruded specimens reached to 94 HBN and 116 HBN, respectively. The optimum amounts of the ultimate tensile strength and elongation values obtained from 0.1 wt.% Cu addition for both conditions after T6 treatment which were 253 MPa, 11.5% and 263 MPa, 14.7%, respectively. However, more Cu addition (> 0.5 wt.%) deteriorated the tensile properties of the alloy due to the high volume fraction of intermetallics. The yield tensile strength increased continuously with different Cu additions for the cast and extruded alloys after solution and aging heat treatments. After T6 heat treatment, extruded alloys showed better anti-corrosion properties than those of the cast alloys, while the extruded Mg-2%Zn-0.1%Cu alloy exhibited the best anti-corrosion property. Further Cu addition increased the volume fraction of intermetallics and enhanced the corrosion rate which was due to the galvanic couple effect.

36 MATERIALS SCIENCE↗
DOE OSTI2019

The Anodic Role of Ni-Containing LPSO Phases During the Microgalvanic Corrosion of Mg{sub 98}Gd{sub 1.5}Ni{sub 0.5} Alloy

In the present study, the corrosion characteristics of cast Mg{sub 98}Gd{sub 1.5}Ni{sub 0.5} alloy with Ni-containing long-period stacking ordered (LPSO) structures were investigated. The results reveal that the Ni-containing LPSO phases were more active than the Mg matrix and preferentially eroded during the immersion process. This behavior indicates that the Ni-containing LPSO phases could have acted as microanodes, in contrast to the cathodic role of the Ni-free LPSO phases in other Mg alloys. Furthermore, the potential difference of approximately 600 mV between the phases accelerated the dissolution rate of the LPSO phase. Corrosion of the Mg matrix also occurred due to the inhomogeneous microstructure of the matrix. The synergetic corrosion between the Ni-containing LPSO phases and the enrichment of Cl{sup −} in the thick corrosion product films dominated the propagation of corrosion, which substantially deteriorated the corrosion resistance and accelerated the corrosion process of the Mg{sub 98}Gd{sub 1.5}Ni{sub 0.5} alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Stress Corrosion Cracking of AZ91 + xCe Alloy Using Proof Ring Test in ASTM D1384 and NaCl-K{sub 2}CrO{sub 4} Solutions

The effect of cerium addition on AZ91 magnesium alloy and its influence on stress corrosion cracking (SCC) were studied using constant load proof ring testing NaCl-K{sub 2}CrO{sub 4} and ASTM D1384 solutions at 70 and 80% of yield stress, respectively. The investigation revealed that the optimum grain refinement of desired phases occurred with 0.5% addition of cerium. The addition levels of cerium above 1.5 wt.% to AZ91 alloy resulted in premature failures due to poor SCC resistance in both solutions (K{sub 2}CrO{sub 4} and ASTM D1384 solutions). The microstructural studies of the experimental alloys were characterized by using optical and scanning electron microscopy. The elemental compositions of cerium-added AZ91 magnesium alloys were studied by energy-dispersive spectroscopy. The results indicate that the mode of SCC failure was found to be transgranular in nature along with secondary cracks containing cleavages which resulted from the hydrogen embrittlement and anodic dissolution phenomena. The AZ91 alloy with 0.5 wt.% cerium exhibits better mechanical properties and high stress corrosion resistance.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Studying the Wettability and Reactivity of Liquid Si-Ti Eutectic Alloy on Glassy Carbon

The contact heating sessile drop method was applied for a fundamental study concerning the wettability and reactivity of liquid Si-16.2 at.%Ti (eutectic composition) in contact with glassy carbon. The evolution of the contact angle behaviors and spreading kinetics were evaluated at three different temperatures. A strong temperature dependence was found by analyzing the spreading kinetics in determining the achievement of the equilibrium at the interface. Different spreading stages with different slopes depend on whether the diffusion of reacting phases (liquid Si and C) through the growing reaction layer (SiC) starts to be the rate-limiting factor. On the contrary, the final contact angle value seems not affected by the temperature. The solidified Si-Ti eutectics/GC samples were examined both at the top of the drop and at the cross section by SEM/EDS. The presence of C pockets of dissolution and a SiC layer at the interface confirmed the reactive nature of the interaction mechanisms. In addition, a thicker layer of SiC at the triple line was detected and most probably responsible for pinning the alloy drop at a measured final value of contact angle of 43° ± 2.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effects of Process Control Agents on Characteristics of Cu-Ta Nanocomposite during Milling and Subsequent Sintering

Effects of type and concentration of process control agent on characteristics of the nanocrystalline Cu-1Ta nanocomposite during milling and subsequent sintering were investigated. Microstructural analyses showed that addition of stearic acid (SA) into Cu-1Ta mixture was more effective compared to toluene. The results revealed that application of 2% SA provided a significant dissolution of Ta in Cu lattice after milling for 32 h; however, the extended milling encouraged Ta rejection and increased the crystallite size. Reduction of SA to 1% resulted in a narrower crystallite size distribution with mean size of 18.5 nm. The mechanical and physical tests also indicated that addition of 1% SA rather than 2% toluene caused an increase of 33% in electrical conductivity with nearly no change in micro-hardness.

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

The Evolution of Deformation-Induced Carbides during Divorced Eutectoid Transformation in GCr15 Steels

A series of rolling experiments were carried out using GCr15 steel to investigate the effect of initial microstructure on the spheroidizing process, and especially the effect of deformation-induced carbides (DIC) on the divorced eutectoid transformation (DET). The results indicated that the existence of DIC was beneficial to the spheroidizing process, the optimal initial microstructure for DET was lamellar pearlite, and DIC precipitated at grain boundaries. The average thickness of the cementite lamella and the size of DIC decreased with a decrease in rolling temperature in the critical temperature region. When the specimen was rolled at 1053 K, a small amount of DIC of size ~ 1 μm precipitated at grain boundaries. In contrast, the amount of DIC was increased when the rolling temperature was decreased to 1023 K, whereas the size of DIC was reduced to ~ 0.5 μm. Interrupted quenching experiments indicated that DIC (M{sub 7}C{sub 3}) were transformed to M{sub 3}C during the process of partial austenitization, and the evolution of spheroidization was influenced by the variation in the initial microstructure. For the specimens rolled at 1023 K, only ~ 60 min was needed to achieve the dissolution of DIC, which was the minimum, and the total time needed to achieve spheroidization was reduced to ~ 90 min.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Corrosion Fatigue Behaviors of Type 316 LN Cold-Worked Stainless Steels in Acidified Chloride Environment

The effect of cold work (CW) on the microstructure and corrosion fatigue (CF) resistance of 316 LN stainless steel (SS) using microscopic, electrochemical and surface analytical methods was investigated. The CF resistance increases with CW. The fractographs revealed that the crack growth zone was minimum for the base metal as compared to CW SS. Fatigue-tested sample revealed a slip offset was high for as-received, while not much significant difference for 5 and 20% CW SS at low mean stress levels. The presence of NH{sub 4} improved passivity by the formation of nitrate ions that inhibits anodic dissolution.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Optimizing the TiO{sub 2} Content to Obtain the Highest Corrosion Resistance in Ir-Ru-Ta-Based Mixed Metal Oxide Coating in Oxygen Evolution Reaction Application

In this article, the role of TiO{sub 2} on deactivation and corrosion mechanism of a MMO coating was assessed in 1 M Na{sub 2}SO{sub 4} solution. The Ti/40IrO{sub 2} + 30RuO{sub 2} + (30 − x)Ta{sub 2}O{sub 5} + (x)TiO{sub 2} anodes were prepared by thermal decomposition method. Surface morphology and microstructure of the coatings were characterized by scanning electron microscopy, energy-dispersive spectroscopy and grazing incidence x-ray diffraction analysis. The electrochemical properties and corrosion behavior of these coatings were performed by accelerated lifetime test, investigation of active surface area (q*) and electrochemical impedance spectroscopy. The results showed that the uniform dissolution of the active surface layers was the main reason behind the anode deactivation. Furthermore, addition of titanium oxide up to 15% to the above-mentioned composition improved the performance of the anode and increased its working life up to 200 h.

36 MATERIALS SCIENCE↗