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At least 829 records · Page 46
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Thermodynamics and kinetics of precipitation and austenite reversion during aging of Ti-free grade 300 maraging steel manufactured by laser powder bed fusion (LPBF)

Austenite reversion and intermetallic precipitation are two key phase transformations in maraging steels which determine the strength and ductility. Usually, an optimum aging temperature/time is determined using trial and error approach with inputs from thermodynamic databases and a-posteriori microscopy/hardness evaluations. In this study, a thermo-kinetic model is developed to predict the precipitation and austenite reversion kinetics during various post-fabrication heat treatments in a Ti-free variant of grade 300 maraging steel manufactured using laser powder bed fusion. The thermo-kinetic model was based on the principles of Scheil solidification and the classical nucleation theory. The predictions from the thermo-kinetic model were verified using suitable microscopy techniques, and the results from the thermo-kinetic model agree well with the experimental measurements. Overall, results highlight the fact that an Integrated Computational Materials Engineering approach can be used to predict the precipitation and austenite reversion kinetics, and be used as an input for an efficient design of heat treatments for achieving optimum mechanical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Time-resolved 3D characterisation of early-age microstructural development of Portland cement

Abstract Time-resolved in-situ synchrotron X-ray microtomography reveals new levels of detail about the chemical and physical processes that take place as Portland cement hardens. The conversion of a fluid paste into a hardened product can be monitored on a sub-minute time-scale, and with sample movement/settlement corrections applied to enable individual particles to be tracked as they react, hydrate, and become interconnected into a single strong monolith. The growth of the strength-giving hydrate phases surrounding cement grains, and of the fluid-filled pore network that surrounds them, is able to be directly viewed at the level of individual cement particles through the application of this tracking protocol. When cement is brought into contact with water, a layer which differs in density from the bulk of the cement grains becomes observable on the grain surfaces during the induction period (during which time the heat evolution from the paste is relatively low). As hydration continues, reaction products grow both from particle surfaces into the initially fluid-filled region, and also into the space originally occupied by the cement particles, forming a density gradient within the microstructure. As the reaction accelerates and larger volumes of solid phases precipitate, the newly-formed solid structure percolates via interconnection of agglomerated low-density outer hydrates, which then densify as hydration continues. This eventually leads to solidification of the structure into a hardened porous matrix. Graphical abstract

36 MATERIALS SCIENCE↗
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 OSTI2024

A Gaussian process autoregressive model capturing microstructure evolution paths in a Ni–Mo–Nb alloy

Additive manufacturing is increasingly being employed to produce components of complex geometries in structural alloys because of the expected energy savings associated with the near-net-shape capability and the ability to build in novel internal features that are not possible with many conventional manufacturing approaches. However, because of the extreme thermal conditions encountered, the non-equilibrium microstructures produced during powder bed-based additive manufacturing processes must be subjected to custom post-heat treatment processes to recover the target mechanical properties. Phase-field models and simulation techniques have matured to a state where the microstructure evolution paths, and the morphologies of the resulting precipitate phases can be predicted reasonably accurately, considering alloy-specific thermodynamic and kinetic aspects of the nucleation and growth processes. However, phase-field simulations are computationally intensive, which precludes the ability to apply the simulations directly to the length scale of the entire component. Therefore, it is highly desirable to develop low-computational-cost surrogate models that effectively capture the physics at the microstructural length scale, while facilitating the design of optimized processing conditions resulting in location-specific targeted microstructures at the component scale. The work presented here demonstrates the application of the materials knowledge system framework to develop a surrogate model that effectively captures the microstructural path during annealing of a Ni–Mo–Nb alloy containing different Mo and Nb compositions known to segregate during solidification under additive manufacturing conditions. Specifically, the surrogate model built in this work is based on a Gaussian process autoregressive model informed by statistical representation of simulated microstructures using two-point correlations and dimensionality reduction through principal component analysis. In conclusion, this surrogate model is shown to capture the bifurcation of the microstructural path during precipitation, which yields a microstructure dominated by the $\gamma^{\prime\prime}$ phase at high Nb concentrations and the $\delta$ phase at low Nb concentrations.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Senary refractory high-entropy alloy HfNbTaTiVZr

Discovery of new single-phase high-entropy alloys (HEAs) is important to understand HEA formation mechanisms. The present study reports computational design and experimental validation of a senary HEA, HfNbTaTiVZr, in a body-centered cubic structure. The phase diagrams and thermodynamic properties of this senary system were modeled using the CALPHAD method. Its atomic structure and diffusion constants were studied using ab initio molecular dynamics simulations. Here, the microstructure of the as-cast HfNbTaTiVZr alloy was studied using X-ray diffraction and scanning electron microscopy, and the microsegregation in the as-cast state was found to qualitatively agree with the solidification predictions from CALPHAD. Supported by both simulation and experimental results, the HEA formation rules are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Design of Novel Precipitate-Strengthened Al-Co-Cr-Fe-Nb-Ni High-Entropy Superalloys

Here, a series of non-equiatomic Al-Co-Cr-Fe-Nb-Ni high-entropy alloys, with varying levels of Co, Nb and Fe, were investigated in an effort to obtain microstructures similar to conventional Ni-based superalloys. Elevated levels of Co were observed to significantly decrease the solvus temperature of the γ' precipitates. Both Nb and Co in excessive concentrations promoted the formation of Laves and NiAl phases that formed either during solidification and remained undissolved during homogenization or upon high-temperature aging. Lowering the content of Nb, Co, or Fe prevented the formation of the eutectic type Laves. In addition, lowering the Co content resulted in a higher number density and volume fraction of the γ' precipitates, while increasing the Fe content led to the destabilization of the γ' precipitates. Various aging treatments were performed which led to different size distributions of the strengthening phase. Results from the microstructural characterization and hardness property assessments of these high-entropy alloys were compared to a commercial, high-strength Ni-based superalloy RR1000. Potentially, precipitation-strengthened high-entropy alloys could find applications replacing Ni-based superalloys as structural materials in power generation applications.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Asymmetric Cracking in Mar-M247 Alloy Builds During Electron Beam Powder Bed Fusion Additive Manufacturing

In the electron beam powder bed fusion (EB-PBF) process, a substantial number of high-gamma prime Ni-based superalloys are considered as non-printable due to a high propensity to form cracks. In this research, we focused on computational modeling framework to predict solidification-related cracking phenomena in EB-PBF processes. The cracking analysis was performed on cylindrical overhang structures where the cracks are observed only on one side of the part. Comprehensive microstructural characterization correlated the cracking tendency to low-melting point liquid-film formation along columnar grain boundaries with high misorientation angles due to partitioning of alloying elements. Uncoupled numerical thermal and mechanical models were used to rationalize the relationship between process parameters, build geometry, and cracking. The simulations showed asymmetric temperature distributions and associated asymmetric tensile thermal stresses over a cross section due to differences in section modulus and periodic changes in beam scanning directions. Here, the results provide a potential pathway based on spatially varying beam scanning strategies to reduce the cracking tendency during additive manufacturing of complex geometries on the overhang structure in high-gamma prime nickel-based superalloys.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Process-Defect-Structure-Property Correlations During Laser Powder Bed Fusion of Alloy 718: Role of In Situ and Ex Situ Characterizations

Components made by laser powder bed fusion (L-PBF) additive processes require extensive trial and error optimization to minimize defects and arrive at targeted microstructure and properties. In this paper, in situ infrared thermography and ex situ surface roughness measurements were explored as methodologies to ensure Inconel ® 718-part quality. For a given laser energy of 200 Watts, prismatic samples were produced with different exposure times (80 to 110 µs) and point spacings (80 to 110 µm). The infrared intensities from laser–material interaction zones were measured spatially and temporally. The conditions leading to higher IR intensity and lowest surface roughness values correlated well with less porosity and coarse solidification grain structure. The transition from highly columnar to misoriented growth is attributed to changes in thermal gradients and liquid–solid interface velocities. Hardness measurements and electron microscopy of the as-processed and post-processed heat-treated samples show complex transitions in microstructural states including the heavily dislocated FCC matrix, reduction of dislocation density, and copious precipitation, respectively. These results show that the geometry-process-structure-property correlations are dynamic, and they cascade depending on the transitions of phase states from powder to liquid to solid, as well as phase decompositions and deformations within the solid FCC phase. Finally, validity of using analytical weld process models to describe the above phenomena is also highlighted.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Manipulating Retained Austenite Fraction and Stability With Controlled Chemical Heterogeneities in Q&P Steels

Advanced high-strength steels go through complex heat treatment processes with the goal of retaining austenite to enhance the mechanical properties. Martensite start temperature predictions are generally based on bulk composition when designing heat treatments, so compositional heterogeneities are not considered. Here, the effects of compositional heterogeneities on retained austenite fractions in a quenching and partitioning steel are investigated. In conclusion, the results show that Mn banding from solidification can be used to change retained austenite fractions.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Full-Strength Additive Manufacturing of Pure Aluminum Alloy 7075 Using Liquid Metal Jet Printing

Additive manufacturing (AM) of high-strength aluminum alloys can produce high-performance aerospace and biomedical parts with unique geometries and functions. However, many high-performance alloy systems such as AA 7075 pose challenges during AM due to thermal cycles that cause hot cracking, limiting AM to casting-friendly alloys. Stock AA 7075 alloy poses additional challenges due to the excessive growth of insoluble precipitates during casting and needs to be thermomechanically processed to obtain acceptable mechanical properties. Here, this work uses liquid metal jet printing to produce high-resolution pure AA 7075 parts at high strength (593 MPa) and ductility (10 pct), surpassing the specific strength of any type of steel. The parts are printed by depositing fine droplets on a high-temperature substrate that reduces cooling rates, preventing hot cracking. The results show that fully dense, crack-free parts can only be printed at substrate temperatures of 500 °C with a cooling rate threshold of < 2000 K/s to prevent cracking and < 500 K/s to eliminate porosity. The tensile strength, microhardness, and ductility of the parts matched or were beyond the wrought alloy specifications after heat treatment, possibly due to high solidification rates that produce finer insoluble precipitates. Our results indicate that many challenging high-performance alloy systems can now be 3D printed based on the critical thresholds for cooling rates using any AM method that can satisfy these conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Duplex Heterogeneous Nucleation Behavior of Precipitates in C-Mn Steel Containing Sn

The two successive heterogeneous nucleation behaviors of FeSn{sub 2}-MnS-Al{sub 2}O{sub 3} complex precipitates in ultrahigh Sn-bearing steel were investigated. First, Al{sub 2}O{sub 3} was the nucleation site of the MnS at the end of solidification. Then, FeSn{sub 2} nucleated heterogeneously on the MnS particles that nucleated on the Al{sub 2}O{sub 3} particles. The formation sequence of the precipitated phase caused the duplex heterogeneous nucleation to occur consecutively at most twice.

36 MATERIALS SCIENCE↗
DOE OSTI2015

The Influence of Constitutional Supercooling on the Distribution of Te-particles in Melt-Grown CZT

A section of a vertical gradient freeze Cd 0.9 Zn 0.1 Te boule approximately 2100-mm 3 with a planar area of 300-mm 2 was prepared and examined using transmitted IR microscopy at various magnifications to determine the three-dimensional spatial and size distributions of Te-particles over large longitudinal and radial length scales. Te-particle density distributions were determined as a function of longitudinal and radial positions in these strips and exhibited a multi-modal lognormal size density distribution that indicated a slight preference for increasing size with longitudinal growth time, while showing a pronounced cellular network structure. Higher magnification images revealed a typical Rayleigh-instability pearl string morphology with large and small satellite droplets. This study includes solidification experiments in small crucibles of 30:70 mixtures of Cd:Te performed over a wide range of cooling rates that clearly demonstrated a growth instability with Te-particle capture that is suggested to be responsible for one of the peaks in the size distribution using size discrimination visualization. The results are discussed with regard to a manifold Te-particle genesis history as Te-particle direct capture from melt-solid growth instabilities due to constitutional supercooling and as Te-particle formation from the breakup of Te-ribbons via a Rayleigh-Plateau instability.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Microstructure and Mechanical Behavior of High-Entropy Alloys

High entropy alloys (HEA) have generated interest in recent years due to their unique positioning within the alloy world. By incorporating a number of elements in high proportion, usually of equal atomic percent, they have high configurational entropy, and thus, they hold the promise of interesting and useful properties such as enhanced strength and alloy stability. The present study investigates the mechanical behavior, fracture characteristics and microstructure of two singlephase FCC high-entropy alloys (HEA's) CoCrFeNi and CoCrFeNiMn with some detailed attention given to melting, homogenization and thermo-mechanical processing. Ingots approaching 8 kg in mass were made by vacuum induction melting (VIM) to avoid the extrinsic factors inherent to small scale laboratory button samples. A computationally based homogenization heat treatment was given to both alloys in order to eliminate any solidification segregation. The alloys were then fabricated in the usual way (forging, followed by hot rolling) with typical thermo-mechanical processing parameters employed. Transmission electron microscopy (TEM) was subsequently used to assess the single phase nature of the alloys prior to mechanical testing. Tensile specimens (ASTM E8) were prepared with tensile mechanical properties obtained from room temperature through 800°C. Material from the gage section of selected tensile specimens was extracted to document room and elevated temperature deformation within the HEA's. Fracture surfaces were also examined to note fracture failure modes. The tensile behavior and selected tensile properties were compared with results in the literature for similar alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Welding Heat Input Influence on UNS S82441 Lean Duplex Stainless Steel Corrosion Resistance Assessed by Scanning Vibrating Electrode Technique (SVET)

The precipitation of the intermetallic phases can impair mechanical and corrosion properties. In the case of lean duplex stainless steels, they present solidification mode as primary ferrite, with austenite precipitation in the solid state, during cooling. This means the heat-affected zone balanced microstructure can be changed during the welding by the cooling rate, which is associated to the welding heat input, the thickness of the plate and the physical properties of the material. This work intends to present the influence of welding heat input on the corrosion resistance of UNS S82441 lean duplex stainless steel. The gas metal arc welding process was used to make the weld beads, using ER2209 as filler metal, which characterizes a dissimilar welded joint. The corrosion resistance of these specimens was analyzed through scanning vibrating electrode technique. It was observed that welding heat input variation distinctly influences corrosion in different zones of the weld beads over the specimen exposure time. The weld bead under intermediate welding heat input showed the highest corrosion rate and the one under the highest welding heat input, the lowest corrosion rate intensity.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Evaluation of the Role of Metallic Matrix on Environmentally Assisted Embrittlement of Austempered Ductile Iron (ADI) Using High Si Steel

The objective of this work stems from the environmentally assisted embrittlement suffered by austempered ductile iron (ADI) when its surface is submerged in water. This phenomenon leads to a drop in elongation and ultimate tensile strength of about 75% and 10%, respectively, while the 0.2% offset strain stress remains unaffected. Heat-treating a high Si steel, an ausferritic matrix similar to that present in the ADI under study is obtained, yielding giving a very good combination of tensile strength and elongation. In this manner, it was possible to examine the influence of the metallic matrix such as that present in ADI, though without nodules and cell boundaries, also called last to freeze zones (LTF). Tensile test evaluations on ADI and steel samples under dry and wet conditions were carried out. Additionally, a very specific test, introducing surface cracks, was conducted with the purpose of expanding the current knowledge on this particular kind of fracture. Earlier investigations on ADI have suggested that the process of embrittlement starts in a surface crack generated at the early stages of plastic deformation in LTF zones, which results from the solidification process, where pores, inclusions and unreacted austenite are present. In this manner, water penetrates and weakens the material strength, making the crack grow very fast and leading to an instantaneous samples failure. The main results obtained in this work include the confirmation of the deleterious effect of the LTF zones in the embrittlement process and the fracture surface analysis showing characteristic features of this kind of failure.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Tribological Behavior of Solid-State Processed Al-1100/GNP Surface Nanocomposites

Graphene nanoplatelets (GNPs) were infused onto aluminum alloy (Al-1100) surface by a solid-state processing technique to form surface nanocomposites which enrich the mechanical and tribological properties of the aluminum (Al) surface. Electrical resistance heating which locally softens the Al surface, followed by hot pressing, mechanically impregnates and embeds GNPs onto the Al substrate. It was observed that the GNPs mainly occupy the grain boundaries in the matrix during subsequent solidification which leads to strengthening of the matrix through various mechanisms. A detailed tribological study was carried out, and it was found that the wear rate and coefficient of friction decreases by > 85%. Least wear depth in nanoscratch testing was obtained at processing parameters 2200 A and 0.5 s, which could be identified as the optimum current/time combination. The microhardness and Young’s modulus results also show an improvement > 200%, corroborating the tribological studies.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Ni Addition to Sn0.7Cu Solder Alloy on Thermal Behavior, Microstructure, and Mechanical Properties

The addition of Ni on lead-free solder alloys Sn0.7Cu was tested in this study. A small amount of Ni was added to the solder alloys to evaluate the thermal behavior, microstructure, and mechanical properties of the composite solders after sintering and isothermal aging for 3 days and then compared with the results of the binary Sn0.7Cu solder. The results indicated that the ultimate tensile strength and yield tensile strength changed when adding 0.25 wt.% of Ni. The tested results of the differential scanning calorimeter showed that the addition of Ni (such as 0.5 and 1 wt.%) could obviously increase the solidification temperature of Sn0.7Cu alloys in the cooling process. When the Ni content was increased to 0.25 wt.% in the ternary condition, which includes the Sn0.7Cu-xNi (x = 0, 0.1, 0.25, 0.5, and 1 wt.%) solder alloys for the indentation creep test, the minimum creep rate reached the maximum; however, the trend was reversed as the Ni content was higher than this level. In addition, the microstructure of Sn-0.7Cu-xNi solder alloys was obviously different with the eutectic Sn-0.7Cu solder, such that the Ni gradually accumulated in the (Cu, Ni){sub 6}Sn{sub 5} IMCs within the Ni-containing solder alloys. Additionally, this process refined the microstructure of the Sn-0.7Cu solder. The fracture surface of the eutectic Sn-0.7Cu solder revealed ductile fracture modes; however, there was no mixed ductile–brittle fracture mode occurred when the Ni content was in the range of 0-1 wt.%.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Processing, As-Cast Microstructure and Wear Characteristics of a Monotectic Al-Bi-Cu Alloy

Ternary Al-based monotectic alloys have a good combination of wear resistance and mechanical strength. While self-lubricating soft elements guarantee an adequate wear resistance, the modification with third elements can increase the ability to support load. In the present investigation, a collection of microstructures is generated through transient directional solidification of the Al-3.2wt.%Bi-3.0wt.%Cu alloy. Samples with different Bi spacing have been subjected to micro-adhesive wear ball tests. A relationship linking the wear volume, V, the microstructural spacing and the test time is proposed for Bi spacing higher than 16 μm, according to which V decreases with the decrease in Bi spacing. It is observed that wider and deeper grooves emerged on the surface of the samples related to more refined Bi and Al{sub 2}Cu phases, that is, associated with Bi spacing and Bi diameter lower than 16 and 2.4 μm, respectively. A reverse trend is noted for these finer microstructures, for which V increases with further decrease in Bi spacing. This can be caused by the detachment of the very fine and less cohesive Al{sub 2}Cu lamellas as the Al{sub 2}O{sub 3} oxide breaks up forming debris, with the presence of these lamellas as loose debris at the interface acting as third-body abrasives.

36 MATERIALS SCIENCE↗