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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 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Investigation of the Effect of Residual Stress Gradient on the Wear Behavior of PVD Thin Films

The control of residual stresses has been seldom investigated in multilayer coatings dedicated to improvement of wear behavior. Here, we report the preparation and characterization of superposed structures composed of Cr, CrN and CrAlN layers. Nano-multilayers CrN/CrAlN and Cr/CrN/CrAlN were deposited by Physical Vapor Deposition (PVD) onto Si (100) and AISI4140 steel substrates. The Cr, CrN and CrAlN monolayers were developed with an innovative approach in PVD coatings technologies corresponding to deposition with different residual stresses levels. Composition and wear tracks morphologies of the coatings were characterized by scanning electron microscopy, high-resolution transmission electron microscopy, atomic force microscopy, x-ray photoelectron spectroscopy, energy-dispersive x-ray spectroscopy, x-ray diffraction and 3D-surface analyzer. The mechanical properties (hardness, residual stresses and wear) were investigated by nanoindentation, interferometry and micro-tribometry (fretting-wear tests). Observations suggest that multilayer coatings are composed mostly of nanocrystalline. The residual stresses level in the films has practically affected all the physicochemical and mechanical properties as well as the wear behavior. Consequently, it is demonstrated that the coating containing moderate stresses has a better wear behavior compared to the coating developed with higher residual stresses. The friction contact between coated samples and alumina balls shows also a large variety of wear mechanisms. In particular, the abrasive wear of the coatings was a combination of plastic deformation, fine microcracking and microspallation. The application of these multilayers will be wood machining of green wood.

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

The Sliding Wear and Friction Behavior of M50-Graphene Self-Lubricating Composites Prepared by Laser Additive Manufacturing at Elevated Temperature

M50 steel is widely applied to manufacture aircraft bearings where service lives are mainly determined by the friction and wear behaviors. The main purpose of this study is to investigate the tribological behaviors and wear mechanisms of M50-1.5 wt.% graphene composites (MGC) prepared by laser additive manufacturing (LAM) (MGC-LAM) sliding against Si{sub 3}N{sub 4} ball from 25 to 550 °C at 18 N–0.2 m/s. XRD, EPMA, FESEM, and EDS mapping were conducted to understand the major mechanisms leading to the improvement in the sliding behavior of MGC-LAM. The results indicated that MGC-LAM showed the excellent friction and wear performance at 25-550 °C for the lower friction coefficient of 0.16-0.52 and less wear rate of 6.1-9.5 × 10{sup −7} mm{sup 3} N{sup −1} m{sup −1}. Especially at 350 °C, MGC-LAM obtained the best tribological performance (0.16, 6.1 × 10{sup −7}mm{sup 3} N{sup −1} m{sup −1}). It was attributed to the dense coral-like microstructure, as well as the formed surface lubricating structure which is composed of the upper uniform lubricating film with massive graphene and the underneath compacted layer.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Plasma Electrolytic Oxidation Coatings on Pure Ti Substrate: Effects of Na{sub 3}PO{sub 4} Concentration on Morphology and Corrosion Behavior of Coatings in Ringer’s Physiological Solution

Plasma electrolytic oxidation has been used as a relatively new method for applying ceramic coatings having different features. In the present study, commercially pure titanium is used as substrate, and effects of trisodium phosphate electrolyte concentration on the microstructure, as well as corrosion behavior of the coating in Ringer’s physiological solution are investigated. The morphology and phase compositions of coatings were analyzed by using scanning electron microscopy (SEM) and x-ray diffraction patterns. The study on the corrosion behavior of samples in a Ringer’s physiological solution was carried out using open-circuit potential potentiodynamic polarization and electrochemical impedance spectroscopy. The results of electrochemical analysis proved that higher concentration of phosphate electrolyte leads to increase in the corrosion resistance of applied coatings. Accordingly, obtained results revealed that the optimum electrolyte concentration for the best corrosion behavior was 20 g L{sup −1}. Furthermore, SEM images and reduction in the dielectric breakdown potential indicated that increase in the electrolyte concentration leads to morphological improvement and smoothening of the surface.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Study on the Microstructure, Mechanical Properties and Corrosion Behavior of Mg-Zn-Ca Alloy Wire for Biomaterial Application

Due to their excellent biocompatibility and biodegradability, magnesium alloy wires have attracted much attention for biomaterial applications including orthopedic K-wires and sutures in wound closure. In this study, Mg-3Zn-0.2Ca alloy wires were prepared by cold drawing combined with proper intermediate annealing process. Microstructures, texture, mechanical properties and corrosion behavior of Mg-3Zn-0.2Ca alloy wire in a simulated body fluid were investigated. The results showed that the secondary phase and average grain size of the Mg-3Zn-0.2Ca alloy were refined in comparison with the as-extruded alloy and a strong (0002)<10-10>//DD basal fiber texture system was formed after multi-pass cold drawing. After the annealing, most of the basal planes were tilted to the drawing direction (DD) by about 35°, presenting the characteristics of random texture, and the texture intensity decreased. The as-annealed wire shows good mechanical properties with the ultimate tensile strength (UTS), yield strength (YS) and elongation of 253 ± 8.5 MPa, 212 ± 11.3 MPa and 9.2 ± 0.9%, respectively. Electrochemical and hydrogen evolution measurements showed that the corrosion resistance of the Mg-3Zn-0.2Ca alloy wire was improved after the annealing. The immersion test indicated that the Mg-3Zn-0.2Ca wire exhibited uniform corrosion behavior during the initial period of immersion, but then exhibited local corrosion behavior.

36 MATERIALS SCIENCE↗
DOE OSTI2018

X-ray Diffraction Investigation of Annealing Behavior of Peened Surface Deformation Layer on Precipitation Hardening Stainless Steel

In order to investigate the recrystallization behavior of peened surface deformation layer of precipitation hardening stainless steel, a classic x-ray diffraction line profile analysis, Voigt method, was carried out on peened 17-4PH with different isothermal annealing temperatures. The activation energy of domain boundary migration (Q{sub a}) and the activation energy of microstrain relaxation (Q{sub b}) were calculated by regression analysis in different annealing temperature conditions. The results show that the value of Q{sub a} decreases with annealing temperature increasing, which is due to the influence of precipitation (ε-Cu) size on the movements of grain and subgrain boundaries. The maximum growth rate of ε-Cu particles occurs during 400 to 500 °C interval. Compared with growth behavior of domain size, microstrain relaxation behavior is less sensitive to precipitation particle size. The effects of annealing temperature and time on dislocation density are both significant when annealing temperature is lower than 500 °C. However, the effect of annealing temperature on dislocation density becomes insignificant when annealing temperature is higher than 500 °C. 300 °C annealing temperature only leads to the microstrain relaxation but nearly cannot lead to the domain size growth even if prolonging annealing time. Microstructure enhancement effect still exists in plastic deformation layer when 300 °C annealing temperature lasts for 60 min but nearly disappears when 600 °C annealing temperature lasts for 20 min.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Prediction of the Stress–Strain Behavior of Open-Cell Aluminum Foam under Compressive Loading and the Effects of Various RVE Boundary Conditions

The prediction of the mechanical behavior of metallic foams with realistic microstructure and the effects of various boundary conditions on the mechanical behavior is an important and challenging issue in modeling representative volume elements (RVEs). A numerical investigation is conducted to determine the effects of various boundary conditions and cell wall cross sections on the compressive mechanical properties of aluminum foam, including the stiffness, plateau stress and onset strain of densification. The open-cell AA6101-T6 aluminum foam Duocel is used in the analyses in this study. Geometrical characteristics including the cell size, foam relative density, and cross-sectional shape and thickness of the cell walls are extracted from images of the foam. Then, the obtained foam microstructure is analyzed as a 2D model. The ligaments are modeled as shear deformable beams with elastic-plastic material behavior. To prevent interpenetration of the nodes and walls inside the cells with large deformations, self-contact-type frictionless interaction is stipulated between the internal surfaces. Sensitivity analyses are performed using several boundary conditions and cells wall cross-sectional shapes. The predicted results from the finite element analyses are compared with the experimental results. Finally, the most appropriate boundary conditions, leading to more consistent results with the experimental data, are introduced.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Electrochemical Behavior Assessment of As-Cast Mg-Y-RE-Zr Alloy in Phosphate Buffer Solutions (X Na{sub 3}PO{sub 4} + Y Na{sub 2}HPO{sub 4}) Using Electrochemical Impedance Spectroscopy and Mott–Schottky Techniques

In the present study, electrochemical behavior of as-cast Mg-Y-RE-Zr alloy (RE: rare-earth alloying elements) was investigated using electrochemical tests in phosphate buffer solutions (X Na{sub 3}PO{sub 4} + Y Na{sub 2}HPO{sub 4}). X-ray diffraction techniques and Scanning electron microscopy equipped with energy dispersive x-ray spectroscopy were used to investigate the microstructure and phases of the experimental alloy. Different electrochemical tests such as potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS) and Mott–Schottky (M–S) analysis were carried out in order to study the electrochemical behavior of the experimental alloy in phosphate buffer solutions. The PDP curves and EIS measurements indicated that the passive behavior of the as-cast Mg-Y-RE-Zr alloy in phosphate buffer solutions was weakened by an increase in the pH, which is related to formation of an imperfect and less protective passive layer on the alloy surface. The presence of the insoluble zirconium particles along with high number of intermetallic phases of RE elements mainly Mg{sub 24}Y{sub 5} in the magnesium matrix can deteriorate the corrosion performance of the alloy by disrupting the protective passive layer that is formed at pH values over 11. These insoluble zirconium particles embedded in the matrix can detrimentally influence the passivation. The M–S analysis revealed that the formed passive layers on Mg-Y-RE-Zr alloy behaved as an n-type semiconductor. An increase in donor concentration accompanying solutions of higher alkalinity is thought to result in the formation of a less resistive passive layer.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Corrosion Behavior of Wire Electrical Discharge Machined Surfaces of P91 Steel

The extensive usage of P91 steel in certain reactor components requires its precision fabrication by wire electrical discharge machining (WEDM). The WEDM cut surfaces consist of primarily re-melted and solidified layer (recast layer) of 2-8 µm. The recast layer is a result of fast removal of heat by unaffected metallic substrate and dielectric medium leading to rapid cooling. The WEDM surface is expected to affect the component performance in terms of its corrosion properties. The microstructural changes taking place due to EDM cutting of P91 steel bring about a drastic change in the electrochemical response of surface as compared to that for bulk P91 steel. The surface microstructure is shown to alter the passivation behavior and resistance to localized corrosion. Electrochemical polarization studies were done to evaluate passivation, pitting and intergranular corrosion (IGC) behavior and field emission gun—scanning electron microscopy was used for microstructural characterization of P91 steel surfaces. The passivation behavior for P91 steel WEDM cut surfaces was better than that for diamond polished surface. But pitting corrosion resistance decreased for the P91 steel WEDM surface which is a result of inhomogeneities in the microstructure of recast layer. The IGC resistance though increased due to the decreased grain size in the surface layer.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Ratcheting Behavior of Additive Manufactured 4043 Aluminum Alloy

In this paper, 4043 aluminum alloy was printed using additive manufacturing (AM) with robot cold metal transfer (CMT) technology. The microstructure and mechanical properties of different parts, including the bottom, middle and upper portions of the printed AM 4043 aluminum alloy, were analyzed. Ratcheting tests were performed using an asymmetric load, and the tensile properties of the pre-ratcheting and un-ratcheted specimens were tested. The results indicate that the dendrites in the middle region are larger than those at the top and bottom regions and that the coarse microstructure associated with the interlayer junction resulted in the decline of the mechanical properties. The tensile strength of the AM specimens is 165 MPa, and the yield strength is 50 MPa. The printed 4043 aluminum alloy exhibits mechanical properties similar to those for conventional cast and welded aluminum alloys. The ratcheting behavior indicates that an increase in the mean stress or stress amplitude leads to an increase in the ratcheting strain, and the accumulation of ratcheting strain causes plastic damage to the materials. Pre-ratcheting behavior causes the tensile and yield strengths to increase compared to the behavior of the un-ratcheted specimens.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Variation of the Corrosion Behavior Prior to Crack Initiation of E690 Steel Fatigued in Simulated Seawater with Various Cyclic Stress Levels

The electrochemical process and corrosion behavior prior to crack initiation of E690 steel fatigued in simulated seawater are investigated under different cyclic stress levels by methods of electrochemical impedance spectroscopy, cyclic current response, electrochemical noise and morphology observation. The results show that the electrochemical behavior changes with the peak stress of cyclic loading and the corrosion behavior transforms as the peak stress increases over the proof strength. The charge transfer resistance (R{sub ct}) decreases with elevated peak stress. Continuous decreasing R{sub ct} with cycling number under peak stress of 1.02 σ{sub p0.2} indicates that continuous fresh metal induced by work-softening is exposed to the seawater, which is also proved by cyclic current response induced by cyclic stress and contact angle test. As a result, the corrosion type of E690 steel changes from general corrosion to pitting corrosion with the peak stress close to and above the proof strength. That induces the corrosion fatigue crack initiation mechanism to change correspondingly.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Tempering and Rolling on Fatigue Crack Growth Behavior of Modified 9Cr-1Mo Steel

In the present work, fatigue crack growth (FCG) behavior of modified 9Cr-1Mo steel has been investigated near fatigue threshold and Paris regime at room temperature (RT) and 650 °C. The modified 9Cr-1Mo steel was subjected to normalizing at 1050 °C followed by tempering (650-750 °C) and warm rolling at 550 °C followed by tempering in order to improve its precipitation state and refine microstructure. The microstructural parameters such as grain size, block size, lath width and precipitate size were examined and correlated with FCG and tensile properties of investigated samples. The results showed that decreasing tempering temperature significantly reduces the fatigue threshold at RT while increases the yield strength at elevated temperature (i.e., 650 °C). Increase in fatigue threshold is found to increase with the increase in block size, whereas high density of fine precipitates contributed the high temperature strength. The FCG rate in Paris regime was less affected by heat treatment and rolling process at RT. However, rolled sample tested at 650 °C shows a decrease in FCG rate as compared to heat-treated samples. In terms of higher FCG resistance and yield strength, warm rolling followed by tempering at low temperature (i.e., 700 °C) can be considered optimum. For the numerical simulation of FCG behavior, extended finite element method (XFEM) along with Paris law has been implemented in ABAQUS using python script. The simulated FCG behavior is found in good agreement with the experimental results.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Constitutive Modeling for Hot Deformation Behavior of Al-5083 + SiC Composite

In the present research, isothermal hot deformation behavior of aluminum 5083 alloy + 15 (wt.%) SiC composite was obtained through compression test on INSTRON 8801 universal tensile testing machine (UTM) under a wide temperature range of 473-773 K and strain rate range of 0.01-10 s{sup −1}. The experimental true stress–strain data were employed to establish constitutive equations based on modified Johnson–Cook (JC) model and modified Zerilli–Armstrong (ZA) model to predict the hot flow behavior of the composite. The flow stress values obtained from these two models were plotted against the experimental flow curves to check the accuracy of these models. Suitability of the models was evaluated by comparing correlation coefficient (R), average absolute relative error and relative errors of prediction. The results show that the hot flow stresses of the present material depend on temperature and strain rate significantly. Both the models give good description of the hot deformation behavior of the composite. The prediction accuracy is found to be higher for modified ZA model compared to modified JC model, though the number of materials constants involved and time needed to evaluate them to establish the model are lower for modified JC model.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Dry Sliding Wear Behavior and Mechanism of a Hot-Dip Aluminized Steel as a Function of Sliding Velocity

Fe-Al intermetallic coatings were prepared on AISI 1045 steel by hot-dip aluminizing and subsequent high-temperature diffusion. The dry sliding wear tests were performed for the aluminized steel with various substrate hardnesses in the velocity range of 0.75-4 m/s on a pin-on-disk configuration. The dry sliding wear behavior and mechanisms of the aluminized steel under various velocities were explored. It was found that the wear rate decreased substantially with an increase in sliding velocity from 0.75 to 1.5 and then slightly varied in the velocity range of 1.5-4 m/s. The wear behavior and mechanisms of the aluminized steel were noticed to be closely related with tribo-layers. As tribo-layers contained more oxides with Fe{sub 2}O{sub 3}, the wear resistance of the aluminized steel was markedly improved because of their protective function. The substrate hardness also changed the wear behavior of the aluminized steel by affecting the stability of tribo-layers because the substrate hardness was inversely proportional to the thermal stability of the substrate structure. Under a low velocity of 0.75 m/s, adhesive wear and abrasive wear prevailed owing to the metal-on-metal contact, resulting in a higher wear rate. At 1.5-4 m/s, mild oxidative wear prevailed due to the stable existence of tribo-oxides layers. However, at 1.5 and 4 m/s for the substrate hardness of 45 HRC, the wear rate was slightly increased because the protection of tribo-layers was weakened by a double-layered structure or more cracks in tribo-layers and coatings.

36 MATERIALS SCIENCE↗
DOE OSTI2019

An Integrated Numerical Method to Predict the Strain Sensing Behavior of Flexible Conductive Fibers under Electric–Thermal Conditions

Flexible conductive textiles have attracted an increasing amount of attention due to being flexible and stretchable with multi-functions, in addition to the strain sensing function. In this paper, an integrated numerical method is established to predict the strain sensing behavior of PPy-coated flexible conductive fibers. Based on our previous investigation on the strain sensing behavior of the PPy-coated elastic conductive fibers, it has been known that the main strain sensing mechanisms of the conductive fibers are micro-cracks opening under stretching and closing under unloading. Therefore, in the numerical approach, extended finite element method is used to simulate the multi-crack initiation and propagation. Then, by establishing a representative element model including a single crack, the resistance variation with the increase in crack width/depth can be obtained using the electric–thermal package in ABAQUS software. Combining the relationship between crack width and strain, the relationship between fractional increment in resistance (ΔR/R{sub 0}) and the strain can be obtained. The prediction of the fractional increment in resistance (ΔR/R{sub 0}) versus strain is consistent with the experimental results of PPy-coated Lycra fibers. The new integrated method is further verified by experimental results of PPy-coated XLA fibers, showing that the integrated numerical approach can capture the complicated strain sensing mechanisms of PPy-coated elastic fibers and predict the strain sensing behavior of flexible conductive fibers. This approach will make it possible to shorten the period, and reduce the cost in R&D of the PPy-coated electrically conductive fibers, comparing with the corresponding experimental work.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Nanoindentation Creep Behavior of CoCrFeNiMn High-Entropy Alloy under Different High-Pressure Torsion Deformations

CoCrFeNiMn high-entropy alloy (HEA) has gained increasing attention in recent years. In this work, CoCrFeNiMn HEA was prepared by arc melting and casting method; and high-pressure torsion (HPT) technology was used to refine the grains. A series of nanoindentation experiments was used to investigate the effect of different loading rates and deformations on the nanoindentation creep behavior of the CoCrFeNiMn HEA. The results show that the grain size of the CoCrFeNiMn HEA refines to the nanoscale after HPT. However, grain refinement is not obvious with further increasing HPT rotation. The loading rate and HPT rotations have an obvious effect on the nanoindentation creep behaviors of the CoCrFeNiMn HEA. The creep behaviors of the as-cast and HPT-treated CoCrFeNiMn HEA may be controlled by dislocations and the collective effect of GB sliding and dislocations, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Characterization and Modeling of Hot Deformation Behavior of a Copper-Bearing High-Strength Low-Carbon Steel Microalloyed with Nb

This study investigates hot deformation behavior of a newly developed Cu-bearing high-strength low-carbon steel microalloyed with Nb (Nb-HSLC). A computational method based on experimental data was employed to design the chemical composition of the alloy. Compression tests were carried out in the temperature range of 850-1100 °C as well as strain rates of 0.001-10 s{sup −1} using BAHR Dil 805 A/D thermo-analyzer equipment. The Arrhenius-type constitutive equations were used to model the hot working behavior of the designed steel. Effects of friction and temperature rise during deformation were corrected to obtain the actual stresses. The results showed that the peak flow stress was increased with increasing Zener–Hollomon parameter. The obtained flow curves at strain rates lower than 0.1 s{sup −1} and temperatures above 950 °C represented the typical dynamic recrystallization (DRX) behavior, while the flow curves at temperatures lower than 950 °C at all strain rates were associated with continuous strain hardening. This feature is in good agreement with the precipitation temperature range of Nb(C, N) particles, i.e., 800-1000 °C. Moreover, the flow curves showed the serrations during hot deformation at strain rates of 0.001 and 0.01 s{sup −1}, indicating that the dynamic strain aging (DSA) phenomenon occurred at low strain rates. The best fit between “peak stress” and “deformation conditions” was obtained by a hyperbolic sine-type equation (R{sup 2} = 0.993). Therefore, the average activation energy was determined as 348 kJ mol{sup −1}. The agreement between the achieved model and experimental flow data was verified using the results of additional tests at a strain rate of 5 s{sup −1}. The maximum difference between the measured and predicted “peak stresses” was calculated as 5 Mpa.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Corrosion and Tribocorrosion Behavior of Ti-B{sub 4}C Composites Joined with TiCuNi Brazing Alloy

Corrosion behavior of joined Ti-B{sub 4}C composites was evaluated in 9 g/L NaCl solution using cyclic polarization and electrochemical impedance spectroscopy. Tribocorrosion behavior was investigated by sliding against an alumina ball under open-circuit potential and potentiodynamic polarization. The results showed that joining did not negatively affect the corrosion behavior of the composites. Regarding tribocorrosion, while joining did not significantly influence the coefficient of friction and total wear volume loss, it resulted in slightly increased electrochemical activity under sliding. These results showed that brazing may be considered as a simple and low-cost technique for joining Ti-B{sub 4}C composites to be operated in tribocorrosive environments.

36 MATERIALS SCIENCE↗