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At least 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Microstructural refinement in ultrasonically modified A356 aluminum castings

Two A356 aluminum alloys (Al-Si-Mg), one with 0.09 wt.% Fe and one with 0.91 wt.% Fe, were cast in a graphite mold with the simultaneous application of local ultrasonic intensification to refine the as-cast microstructure. Ultrasonication during casting transformed the morphology of primary Al grains from dendritic (~140-290 microns in size) to globular (~33-36 microns in size). The alloy with high Fe exhibited globular grains at distances up to 45 mm away from the ultrasound probe, while the alloy with low Fe exhibited globular grains at distances only up to 6 mm away from the ultrasound probe. Near the location of the ultrasound probe (< 2 mm away), a second non-dendritic microstructural morphology was observed with fine aluminum grains (~9-25 microns in size). This unique fine-grained morphology has not been previously reported, contains a greater concentration of Si relative to the globular microstructure, and may be a large, fully eutectic region. Ultrasonication during casting also transformed the morphology of the ß-Al 5 FeSi phase particles (which are deleterious to the strength and ductility of the alloy) in the high Fe alloy from needle-like to rectangular, which could enable the greater use of secondary Al alloys. Thermodynamic simulations conducted to calculate the solidification paths of the two alloys studied predict that the ß-Al 5 FeSi phase begins to form earlier in the alloy with high Fe. Finally, data suggest that the ß-Al 5 FeSi phase (which is more abundant in alloys with high Fe content) may enhance ultrasonically-induced grain refinement.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Microstructure, Hardness, and Residual Stress of the Dissimilar Metal Weldments of SA508-309L/308L-304L

A dissimilar metal weldment consisting of SA508-309L-308L-304L is widely used in light-water nuclear reactors. These weldments demonstrate dissimilar susceptibility to stress corrosion cracking that are related to the microstructure, properties, and residual stress. In this work, microstructures, hardness, and the residual stress distribution of the dissimilar metal weldments were investigated, with the correlation of increased hardness in the heat-affected zone (HAZ) to the microstructure. 304L HAZ demonstrated similar grain morphology as the base material, and the increase in hardness was primarily attributed to the increased dislocation density. SA508 HAZ demonstrated a change of grain morphology resulting from the different peak temperatures and cooling rates. The increased hardness in the SA508 HAZ was attributed to the refined grain morphology, higher dislocation density, and higher number density of precipitates. A ~ 20–30-μm-wide martensitic zone formed at the fusion boundary of SA508-309L, where Cr-rich carbide precipitates were observed, with the average size and the number density of 44.1 ± 16.9 nm and 1.5 × 10 21 m ₋3 , respectively. In conclusion, residual stress results demonstrated the largest tensile stress at 309L butter, indicating its high cracking susceptibility.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Microstructural Evolution of Tantalum During Deformation and Subsequent Annealing

Microstructure-aware models are necessary to predict the behavior of material based on process knowledge or to extrapolate mechanical properties of materials to environmental conditions which are not easily reproduced in the laboratory, e.g., nuclear reactor environments. Elemental Ta provides a relatively simple BCC system in which to develop a microstructural understanding of deformation processes which can then be applied to more complicated BCC alloys. In situ neutron diffraction during compressive deformation and subsequent heat treatment have been used to monitor the evolution of microstructural features in Ta throughout simulated processing steps. Crystallographic texture and dislocation density are determined as a function of first plastic strain, then temperature. Lattice strains are determined and attributed to stresses at macroscopic, grain and dislocation length scales. The increase of the dislocation density through deformation and subsequent recovery during heat treatment is monitored through the changing diffraction line profile. Also, randomization of the texture is used as a signature of recrystallization. The recovery of dislocations through annihilation is not observed to depend on the initial dislocation density in the range studied here. In contrast, recrystallization is observed to depend strongly on the initially dislocation density.

36 MATERIALS SCIENCE↗
DOE OSTI2023

A Technique for the Quantitative Characterization of Weld Microstructure and Application to Mo Welds

The choice of weld parameters determines the size, shape, and curvature of grains in the fusion zone (FZ) and heat-affected zone (HAZ) of welds while the mechanical properties of the welds are correlated to this microstructure. Here, a new technique to quantitatively evaluate these microstructural characteristics in both zones of welds has been applied to molybdenum gas tungsten arc welds fabricated using different weld parameters. Trends in microstructural characteristics in the FZ and HAZ were evaluated and correlated with changes to heat input, weld speed, and weld technique. The use of this approach showed that a 20 pct decrease in heat input caused a 20 pct decrease in the number of FZ grains with aspect ratios ≥ 4. The orientations of the FZ grain segments as a function of distance from the FZ centerline were significantly affected by the weld speed and its effect on weld pool shape. A 50 pct increase in weld speed caused a 20 pct decrease in grain segments orientated 60 to 90 deg from the normal to the direction of welding. This technique also captured differences in grain sizes and grain size anisotropy in the FZ between welds made with a constant current, pulsed current, and use of a 4-pole-magnetic oscillator.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Hydrogen-Aided Microstructural Engineering of Additively Manufactured Ti–6Al–4V

Electron beam melting (EBM) additive manufacturing of Ti–6Al–4V subjects the material to complex thermal cycles, resulting in a columnar morphology of the prior β grains (PBGs). While the columnar PBGs of EBM-processed Ti–6Al–4V can be transformed to an equiaxed morphology through a super-transus (i.e., above the β -transus temperature) heat treatment, this also leads to the formation of a coarse lamellar two-phase microstructure. Such a microstructure is prone to strain localization and premature fracture. In this study, we present a thermohydrogen post-process treatment that achieves equiaxed PBG morphology in EBM-processed Ti–6Al–4V without sacrificing mechanical properties. Our results show that a three-step thermohydrogen post-process treatment can transform the columnar PBG morphology to an equiaxed morphology with fine microstructure, and strength and ductility levels comparable to those of the most optimum as-fabricated samples. This three-step thermohydrogen post-process treatment involves hydrogenation and phase transformation treatment in a hydrogen atmosphere, and subsequent dehydrogenation treatment in vacuum. Notably, all these treatments are carried out at temperatures well below the β -transus temperature of hydrogen-free Ti–6Al–4V.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Grain Refinement, Microstructure and Mechanical Properties Homogeneity of Mg-Gd-Y-Nd-Zr Alloy During Multidirectional Forging

Multidirectional forging (MDF) of Mg-Gd-Y-Nd-Zr alloy was performed under decreasing temperature condition. The grain refinement, microstructure and mechanical properties homogeneity of Mg-Gd-Y-Nd-Zr alloy during MDF process were investigated. The results show that the grain refinement mechanism of Mg-Gd-Y-Nd-Zr alloy during MDF process was dynamic recrystallization. The dynamic precipitations were observed after MDF process, which were mainly located at the grain boundaries and had a significant influence on the grain refinement, and the amount of dynamic precipitations increased as the MDF passes increase. The microstructure of the billet was not homogeneous due to the deformation and temperature inhomogeneity during MDF process. Optimization MDF process has been proposed, which benefited the microstructure homogeneity and reduced the amount of the dynamic precipitations. Based on the optimization MDF process, the elongation to fracture significantly increased.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Mechanical Properties of Friction Stir Welded Q235 Low-Carbon Steel

Three-millimeter-thick Q235 steel plates were friction stir welded using a tool made of tungsten-rhenium alloy in the present study. The microstructure and mechanical properties of welded joints at various traverse speeds from 50 to 150 mm/min were analyzed. It was demonstrated that traverse speed had great impacts on microstructure and mechanical properties of the joints. Phase transformation accompanied with recrystallization occurred during FSW process which resulted in different microstructures in the WNZ, and the grain size in other zones was significantly affected by traverse speed. The highest hardness was located in the nugget zone, while the lowest hardness was found in the heat-affected zone. The transverse tensile test suggested that all the joints showed slightly lower tensile strength than that of the base material, and the tensile strength increased with increasing traverse speed. All the joints fractured in the heat-affected zone with ductile fracture mode. The impact test showed that the impact toughness decreased with the increasing traverse speed and the impact fracture surface could be divided into shear lip zone and fibrous zone which showed a typical ductile fracture.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure Evolution and Stress Corrosion Cracking Susceptibility of 12Cr Martensitic Steel Upon Long-Term Service in Power Plants

After 230,000-h long-term service at 550 °C/13.7 MPa, the microstructure of a kind of 12Cr martensitic steel was investigated using scanning electron microscopy, backscattering electron microscope and x-ray diffraction analysis. The results show that the precipitation and coarsening of carbides at grain/lath boundaries are the main cause of microstructure degradation. The static immersion tests and the slow strain rate test coupled with electrochemical impedance spectroscopy were conducted on the served steel in 1.0% NaCl solution, and it turns out that the pitting corrosion resistance and repassivation ability of the steel are significantly reduced as a result of microstructure degradation. The stress corrosion cracking susceptibility of the steel was also studied. Fracture morphology analysis shows that the secondary crack in conjunction with slip lines is a result of the coalescence of micro-cracks nucleated from the pits.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Understanding Microstructural Evolution During Rapid Heat Treatment of Microalloyed Steels Through Computational Modeling, Advanced Physical Simulation, and Multiscale Characterization Techniques

An AISI 1045 steel modified with vanadium (V) and niobium (Nb) was studied to evaluate microstructural conditioning prior to and throughout a rapid heat treat process. In order to accomplish this, both computational and physical simulation techniques have been employed with the goal of assessing the microstructural evolution in a medium-carbon bar steel during the rapid austenitization and quenching procedures involved in an induction hardening process. The appropriate thermal profiles for induction hardening were obtained through finite element modeling using Flux 2D software. Physical simulations of the induction hardening process were carried out using a Gleeble{sup ®} 3500. Analysis of prior austenite grain size is complemented by observation of nanoscale carbonitride precipitation via transmission electron microscopy, scanning transmission electron microscopy, and high-energy synchrotron small-angle x-ray scattering. Through a combination of characterization techniques, this study presents a deeper understanding of nano- and microstructural changes occurring in a microalloyed steel during an induction hardening process.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and Mechanical Property Inhomogeneities of a Modified AZ80 Magnesium Alloy Wide Plate Under Horizontal Flat Die Extrusion

In the present paper, inhomogeneities in the microstructure and mechanical properties of a modified AZ80 plate under industrial production conditions were investigated. The results indicated that the microstructure of the edge and middle region shows more homogeneous and fine recrystallized grains. Additionally, more coarse recrystallized grains existed between the edge and the middle region. The ultimate tensile strength and yield strength of the edge region of the plate are maximal (287 and 162 MPa), and those near the 1/8 W region are minimal (259 and 152 MPa). The grain size increased from 19.5 to 27.2 μm along the center layer to the surface layer. The ultimate tensile stress (yield stress) increases from 258 (121 MPa) to 284 MPa (176 MPa) in the direction from the surface layer to the middle layer. The microhardness decreased from the surface layer (72HV) to the center layer (64HV). The metal strain state and temperature distribution lead to different degrees of dynamic recrystallization, which causes the microstructure and mechanical properties of the plate.

36 MATERIALS SCIENCE↗
DOE OSTI2019

The Effects of Sc Addition on the Microstructure and Mechanical Properties of Be-Al Alloy Fabricated by Induction Melting

This study investigated the alloying effects of 0.4 wt.% Sc on the microstructure, mechanical properties, and fracture behavior of a cast Be-Al alloy prepared by vacuum induction melting. The micromechanics of Sc-containing secondary-phase particles (SPs) were characterized by nanoindentation, and the fracture behavior of the alloy was investigated via tensile fractography and optical metallography. The addition of 0.4 wt.% Sc significantly modified the alloy microstructure by decreasing the secondary dendritic arm spacing (SDAS), weakening the dendritic structure of Be, and producing an equiaxed/cellular alloy microstructure with refined Be grains. Sc alloying increased the Vickers hardness by 36%, elastic modulus by 28%, yield strength by 35%, ultimate strength by 52%, but decreased the elongation from 3.6 to 2.9%. Sc-containing SPs had a higher elastic modulus, yield strength, microhardness, and a lower plasticity index than the Be and Al matrices, resulting in interface separation during the fracture process. SPs located at the Be/Al interfaces did not act as microcrack sources for the matrices, whereas highly faceted SPs within the Be grains induced microcrack formation and decreased the elongation. The failure mode of the cast Be-Al-0.4Sc alloy still involved the brittle cleavage failure of Be regions and ductile dimple failure of Al regions, which was not the case with the cast Be-Al alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure, Texture, and Mechanical Properties of AM60 Magnesium Alloy Processed by Extrusion and Multidirectional Forging

In the present paper, multidirectional forging (MDF), as a severe plastic deformation process, was carried out on an extruded AM60 magnesium alloy. The microstructure, texture, and mechanical properties of the MDFed samples were examined. Microstructural findings indicated that the grain size of the alloy decreased from 10.2 to 3.7 µm after six MDF passes, carried out at a constant temperature of 220 °C. Textural studies revealed that after six MDF passes, the fiber texture of the extruded material was transformed to another texture, in which the basal planes tend to rotate toward 45{sup o} to the transverse direction. Shear punch testing (SPT) was employed to assess the shear yield stress (SYS) and ultimate shear strength (USS) at room temperature. Both SYS and USS were enhanced after two MDF passes, but decreased in the next pressing stages, despite developing finer grains at higher levels of imposed strain. The observed drop in strength is ascribed to texture softening that counterbalances the grain refinement strengthening effects. The variation in average microhardness values after different passes of MDF showed the trend that was in concurrence with the rise and fall of the SPT results. Finally, the changes in mechanical properties were discussed, based on the microstructural and textural findings.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure Evolution and Mechanical Properties of Large-Scale AZ80 Magnesium Alloy Billets Produced by Multitemperature Multidirectional Forging

Grain refinement is an effective method to improve the mechanical properties of magnesium (Mg) alloys. In the present research, the combination of multidirectional forging (MDF) and forward extrusion for sizing with decreasing temperature pass by pass was employed to deform as-cast AZ80 magnesium alloy billet with large size to obtain fine grains and homogeneous microstructure. It was found that with the increasing number of MDF passes, the average grain size decreases; the homogeneity of microstructures and the percentages of dynamic recrystallised grains increase simultaneously. After six-pass MDF, fine and homogeneous microstructure with an average grain size of 15 μm was achieved; however, grain refinement became very difficult with a further increase in the MDF pass number. After eight-pass MDF plus forward extrusion for sizing, excellent mechanical properties of large-scale AZ80 magnesium alloy billet were obtained. The tensile strength, the yield strength and the elongation of billet are 410 MPa, 292 MPa and 14%, respectively. Furthermore, during multitemperature MDF, the inverse of the square root of grain size and tensile yield strength fit well with the Hall–Petch relationship.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure, Texture, and Mechanical Properties of Continuously Extruded and Rolled AZ31 Magnesium Alloy Sheets

It is difficult to fabricate magnesium alloy sheets at room temperature due to their hexagonal close-packed structure. In this paper, we proposed a new process that can be used to fabricate AZ31 magnesium alloy sheets with fine-grained microstructure and good mechanical properties. AZ31 magnesium alloy sheets with dimensions of 160 mm × 8 mm were obtained successfully by a continuous extrusion technique. The extruded sheets were then rolled by two different rolling routes, Route 1: the extruded sheet was heated at 350 °C and then rolled without heating of rolls and Route 2: the sheet was rolled without any previous heating by heating rolls at 280 °C. The microstructure and texture evolution as well as mechanical properties of the continuously extruded and rolled sheets were examined experimentally. The magnesium alloy sheets with relatively fine grains of 4.5-8.3 µm were obtained by continuous extrusion followed by rolling processes. The tensile strength of continuously extruded AZ31 Mg alloy sheets can be effectively increased by rolling using heating rolls (Route 2), while the ductility was enhanced by rolling heated sheets (Route 1). The mixed microstructure, consisting of refined DRX grains and deformed grains, contributes to enhancement in tensile strength of the sheet samples rolled by Route 2 due to grain-boundary strengthening and work hardening. Lower temperature and larger rolling reduction are two main factors for improving the tensile strength of AZ31magnesium alloy sheets.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructural Evolution and Toughness of the Various HAZs in 1300-MPa-Grade Ultrahigh-Strength Structural Steel

The effect of heat input on the microstructure and toughness of simulated subregions was investigated by welding thermal simulations and Charpy impact tests. The results indicate that the microstructures of the simulated coarse and fine grain heat affected zone (CGHAZ and FGHAZ) gradually changed from lath martensite to a mixture of lath martensite/lath bainite and finally to granular bainite with the increase in heat input. The microstructure of the simulated intercritical heat affected zone (ICHAZ) was mainly composed of granular bainite and blocky martensite regardless of heat input. When the heat input increased, the toughness of the simulated CGHAZ and ICHAZ continuously decreased. Nevertheless, the simulated FGHAZ still displayed good toughness (53.16 J) due to its fine structure. The occurrence of martensite-austenite (M-A) constituents was the main reason for the decrease in crack initiation energy of the simulated CGHAZ and ICHAZ at high values of heat input, and the toughness deteriorated as the size of M-A constituents increased. It should be noted that high-misorientation packet and block boundaries can effectively deviate or arrest the propagation of microcracks. When the heat input was in the approximate range of 8.63-14.95 kJ cm{sup −1}, all of the simulated subregions exhibited good toughness.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Composition-Dependent Microstructure-Property Relationships of Fe and Al Modified Ti-12Cr (wt.%)

b-Titanium (Ti) alloys have applications in several industries (e.g. aerospace, automotive, and biomedical) where material performance requirements vary widely. To tailor the microstructure and mechanical properties of b-Ti alloys for various applications, it is critical to understand the influence of individual alloying elements. Toward this goal, we investigated the effect of individual alloying additions on the microstructure and resultant mechanical properties of four model b-Ti alloys: Ti-12Cr, Ti-12Cr-3Al, Ti-12Cr-1Fe, and Ti-12Cr-1Fe3Al (wt.%). The microstructures of these alloys were studied using x-ray diffraction, electron microscopy, and atom probe tomography. The mechanical properties were analyzed via Vickers and Rockwell hardness measurements and tensile testing. The addition of 1 wt.% Fe resulted in an approximate 5% increase in elongation-to-failure (ef), while the addition of 3 wt.% Al did not appear to significantly affect ef. The addition of Fe and Al decreased the yield and ultimate tensile strengths.

Ballor, Joann P.↗
DOE OSTI2020

Evolution of Microstructure, Residual Stress, and Tensile Properties of Additively Manufactured Stainless Steel Under Heat Treatments

This study investigated the microstructure, residual stress and tensile properties of the directed energy deposited stainless steel 316L (SS316L) under thermal annealing. Microstructure characterization shows the as-printed sample has laser-generated patterns where dendritic structure is observed at the edge of the patterns and cellular structure dominates the interior region. Here, the thermal annealing at 983 and 1093°C effectively removes the dendritic/cellular structures. Synchrotron X-ray diffraction reveals that the as-fabricated SS316L exhibits compressive residual stress of -197.4 MPa. The stress is greatly relieved to -53.8 MPa after annealing at 1093°C. The room temperature tensile testing indicates that the yield strength and ultimate tensile strength drop from 378 and 502 MPa in the as-printed sample to 258 and 446 MPa in the annealed samples (1093°C), respectively. Our study provides insights into the relationship between microstructure, residual stress, and tensile properties of laser additive manufactured SS316L.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Microstructure and Residual Stress in Functionally Graded 316L Stainless Steel/Inconel 625 Alloys Fabricated by Direct Energy Deposition

Functionally graded materials (FGMs) provide a unique solution to combine distinct properties within a single component to enhance its overall performance. Understanding the microstructure and residual stress distribution is particularly important as the material dissimilarity in FGMs can result in high residual stress that affects the mechanical integrity of structural components. Here, this work aims to study the microstructure and residual stress of additively manufactured FGMs and the effects of isothermal annealing. In this study, 316L stainless steel/Inconel 625 FGMs were built by direct energy deposition, and characterized by optical and electron microscopy as well as synchrotron-based X-ray diffraction. Our study reveals that thermal annealing at 500°C for 3 h has minimal effects on the microstructure and chemistry of the graded materials, but effectively relieves the residual stress and leads to a uniform distribution of residual stress.

36 MATERIALS SCIENCE↗