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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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Microstructure Evolution and Mechanical Properties of Keyhole Repair Welds in AA 2219-T851 Using Refill Friction Stir Spot Welding

The search for a suitable friction-based keyhole repair technique that fulfills the requirements for high-quality repair welds has become an important research topic, especially for aerospace applications. In order to provide and analyze an universal keyhole repair method for lightweight metals, refill friction stir spot welding is applied to through-hole repairs in 3- and 6-mm-thick sheets of precipitate hardening AA 2219-T851. Keyholes with a diameter of 7.5 mm were repair welded achieving a defect-free microstructure. The correlation between the microstructural evolution imposed by the repair process and the resulting mechanical properties is shown. A comprehensive analysis of the precipitate evolution in peak-aged AA 2219 during RFSSW is presented. Thermal cycle measurements revealed high heating rates and peak temperatures of up to 520 °C in the weld center. The thermal cycle caused mainly dissolution and minor equilibrium phase formation in the stirred zone. In the HAZ, overaging of the strengthening precipitates dominates with minor dissolution and equilibrium phase formation only in the direct proximity of the SZ. Microstructural analysis revealed typical weld zone formation with inhomogeneous grain size distribution in the SZ. The resulting mechanical properties are dominated by an inhomogeneous hardness distribution with lowest hardness in the TMAZ at 5 mm from the center of the weld. During tensile loading main yielding and the final fracture occur in the area of lowest strength. Tensile testing showed yield strength of 40 to 46% and UTS of 28 to 25% below BM values in 3- and 6-mm-thick sheets, respectively. The sheet thickness and post-weld natural aging were found to influence the mechanical properties of the weld significantly.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure evolution and properties of graphene nanoplatelets reinforced aluminum matrix composites

Highlights: • Lamellar structure of GNPs fibers and Al matrix was obtained in the rolling direction. • Stacking faults were observed in Al grains due to greatly increased GNPs/Al interfaces. • Strength of Al-0.2 wt% GNPs increased 36.8% with negligible electrical conductivity loss. - Abstract: Aluminum matrix composites reinforced with graphene nanoplatelets (GNPs) were prepared by continuous casting and subsequent rolling. Al-GNPs master alloy, prepared by ball milling and cold pressing, was added into Al melts to fabricate the composites. Microstructure evolution of the composites were observed using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron back scattered diffraction (EBSD). GNPs distributed uniformly in the as-cast specimen and then transformed into fibers in the rolling direction after deformation and finally a lamellar structure formed in the composites. Raman spectra showed that the structural damage of GNPs mainly came from ball milling. The interface between GNPs and Al was well combined in general while a few microcracks were observed, which reduced the ductility of the composites and no adverse aluminum carbide (Al{sub 4}C{sub 3}) was detected at the interface. Stacking faults were observed interior of the aluminum grain, which may due to the existence of plentiful interface introduced by GNPs. Fracture observation revealed that the load transferred from Al matrix to GNPs. The ultimate tensile strength of Al-0.2 wt% GNPs composites was about 36.8% higher than that of pure Al with the same casting and rolling process, which should result from the lamellar structure and load transfer, while the conductivity of the composites decreased slightly, indicating that interface scattering between Al and GNPs is very limited. The investigation results show that Al-GNPs composite is potential for high strength and high conductivity application.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2017

Effect of Gold on the Microstructural Evolution and Integrity of a Sintered Silver Joint

There is a need for next-generation, high-performance power electronic packages and systems employing wide band gap devices to operate at high temperatures in automotive and electric grid applications. Sintered silver joints are currently being evaluated as an alternative to Pb-free solder joints. Of particular interest is the development of joints based on silver paste consisting of nanoscale or micron scale particles that can be processed without the application of an external pressure. Microstructural evolution at the interface of a pressureless sintered silver joint formed between a SiC die with a Ti/Ni/Au metallization and an Active Metal Brazed substrate with Ag metallization at 250 °C was evaluated using Scanning Electron Microscopy, X-ray microanalysis, and X-ray Photo Electron Spectroscopy. Results from Focused Ion Beam cross-sections show that during sintering, the pores in the sintered region close to the Au layer tend to be smaller and are oriented predominantly with their longer dimension oriented parallel to the interface. With further densification, this results in the alignment of small pores parallel to the interface, creating a path for easy crack propagation. Lastly, X-ray microchemical analyses results confirm interdiffusion between Au and Ag and that a region with poor mechanical strength is formed at the edge of this region of interdiffusion.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Microstructural evolution in doped high entropy alloys NiCoFeCr-3X (X=Pd/Al/Cu) under irradiation

Commonly studied equatomic single-phase FCC high entropy alloys based on 3d transition metals like NiCoFeCr do not provide adequate strength and radiation resistance at high doses for nuclear structural applications. In the current study, the major alloying effects like lattice distortion, ordering and clustering tendencies were investigated by adding low concentration of Pd, Al, or Cu respectively to study the doping effects on the ion irradiation response of NiCoFeCr alloy. The alloys were irradiated with 3 MeV Ni 2+ ions at 500 °C to a fluence of 1 × 10 17 /cm 2 at a beam flux of approximately 2.8 × 10 12 ions/cm 2 /s. The microstructural evolution upon irradiation i.e., formation of dislocation networks, radiation induced segregation and precipitation, and void formation were studied in detail. Further, post-irradiation characterization results showed that a Pd addition leads to a high void nucleation rate but controlled void growth, which may be attributed to increased lattice distortion. In Al added HEA, our microstructural analysis indicates that radiation induced ordered L1 2 precipitates do not affect void swelling significantly. Cu addition led to Cu precipitation that drastically suppressed dislocation density and void swelling of the alloy. Additionally, a model was developed to qualitatively describe the trend in void swelling of typical FCC alloys under ion irradiation. This model was able to qualitatively explain the suppression and reappearance of void swelling in ion irradiated alloys that generally occurs near the region with peak implanted ion concentration.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Unveiling the microstructural evolution of carbon fibers derived from polyamide-6

Polyacrylonitrile-based carbon fibers have dominated the industry for decades, but the high cost of polyacrylonitrile has prevented the widespread adoption of carbon fiber in high-volume structural applications. As such, a significant amount of research has been dedicated to finding an alternative, low-cost carbon fiber precursor. In this work, carbon fibers were produced from polyamide-6 using metal salt impregnation and a thermo-oxidative stabilization step. To gain further insight into the carbonization process and microstructural transformation, the morphologies, crystallinities, elemental compositions, and thermal stabilities of the fibers were characterized at various stages of processing. The stabilization step resulted in a significant increase in carbon yield, indicating a dramatic increase in thermal stability. This is due to the crosslinking of polyamide-6 chains, which was confirmed by functional group analysis. The crystallinity of the fibers was also significantly altered during processing, as the produced carbon fibers consisted of pseudo-amorphous carbon with two distinct regions of metal salt impregnation. Finally, the findings and microstructural evolution mechanisms provide guidelines for further research into carbon fiber produced from polyamide-6.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Influence of post deposition annealing on the microstructural evolution and tensile behavior of austenitic stainless-steel alloy 709 made by laser powder bed fusion

Structural materials used in the next generation nuclear reactors or other advanced energy systems are often required to withstand higher operating temperatures for a longer service life. Alloy 709 (A709), a Nb-containing and N-stabilized Fe-25Ni-20Cr austenitic stainless steel, has been recommended for high-temperature reactor constructions due to the better performance over the baseline Type 316H stainless steel. Here, in the current manuscript, for the first time, the microstructure evolution and tensile behavior of an A709 made by laser powder bed fusion (LPBF) is discussed. The as deposited and heat treated LPBF alloys are compared with a reference wrought alloy. Owing to the chemical segregation and a higher number of dislocations, the LPBF materials show a significantly higher density of precipitates formed during high temperature treatments when compared to the wrought samples. This is reflected when looking at the tensile properties, both at room and elevated temperatures, where LPBF samples show higher strengths compared to wrought materials.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Microstructure evolution during binder jet additive manufacturing of H13 tool steel

H13 is one of the most used tool steels for both hot and cold work tooling applications. Binder jet additive manufacturing offers the potential to deposit complex tools at scale due to larger powder bed sizes and faster deposition rates. However, to date there is no published literature on the sintering of H13 to full densification. Here, we discuss the pressureless sintering of binder jet AM H13 steel to full densification via supersolidus liquid phase sintering (SLPS) while presenting appropriate process windows (1360 °C – 1380 °C) for densification without distortion. The process windows have been rationalized based on thermodynamic calculations of liquid volume fractions with temperature. We show that higher binder saturation results in higher carbon retention and subsequently early liquid formation that can initiate the sintering process at lower temperatures. We report abnormal grain growth during sintering and found that the solidification phase transformations play a critical role on microstructural evolution and must be considered to accurately model the kinetics of SLPS.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructural evolution and hardness response in the laser beam welded joints of pure titanium during recrystallization and grain growth

Highlights: • The LABs and high local misorientations indicate the existence of plastic strains in the as-welded HAZ and FZ. • During heat treatment the recrystallization occurs in the HAZs, decreasing plastic strains, but it doesn't appear in the FZs. • The coarse and unstrained grains gradually replace the fine and strained ones in the HAZs during recrystallization. • The decrease of plastic strains in the HAZs during recrystallization causes the descent of average hardness values based on the strain strengthening. - Abstract: Microstructural evolution and hardness response in the laser beam welded joints of pure titanium during recrystallization and grain growth were investigated by means of electron backscatter diffraction and hardness measurement. The low-angle boundaries (LABs) and high local misorientations, in the as-welded heat affected zone (HAZ) and fusion zone (FZ), indicate the existence of plastic strains caused by the plastic strains due to the thermal stress and the non-equilibrium phase transformations. The recrystallization occurs in the HAZs during heat treatment, but the grain growth after recrystallization does not appear under the heat treatment conditions. Moreover as a result of the slightly high impurity contents in the as-welded FZ, no distinct recrystallization is found in the FZs in this work. During recrystallization in the HAZs, the coarse and unstrained grains with equiaxed shape and smooth boundary gradually consume the fine and strained ones with irregular shape and serrate boundary. Meanwhile, the LABs disappear and the low local misorientations replace the high ones in the modified grains. The recrystallization during heat treatment causes the decrease of plastic strains in the HAZs, which reflects on the descent of average hardness values based on the strain strengthening.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure evolution and mechanical properties of Cu/Sn/Ag TLP-bonded joint during thermal aging

Highlights: • The thermal reliability of the Cu/Sn/Ag TLP-bonded joint was investigated through high-temperature storage. • A diffusion reaction occurred at the interface between Cu{sub 3}Sn and Ag{sub 3}Sn during thermal aging. • The mechanism of the Cu{sub 3}Sn/Ag{sub 3}Sn interfacial reaction was investigated from thermodynamics and kinetics. - Abstract: Low-temperature transient liquid phase (TLP) bonding is a promising joining technology for high-temperature electronic devices packaging. However, the effect of thermal aging on the joint microstructure and performance has been rarely studied so far. In this paper, the relationship between microstructural evolution and mechanical property was developed for the Cu/Sn/Ag TLP-bonded joint during 350 °C aging. Experimental results show that an interfacial reaction occurs between the intermetallic compounds (IMCs) of Cu{sub 3}Sn and Ag{sub 3}Sn, which induces that a great amount of Cu{sub 3}Sn particles or wedges embed into the Ag-Sn phase layer. The reaction process is controlled by a volume diffusion of Cu elements from the Cu substrate, and capable of promoting the transformation from Ag{sub 3}Sn into Ag(Sn) solid solution. Cu{sub 41}Sn{sub 11} phase can be formed at the expense of Cu{sub 3}Sn and Cu, of which transformation process is completely achieved after aging for 480 h, and the Cu(Sn) solid solution intimately contacts with the opposite Ag(Sn) solid solution in many areas after aging for 960 h. The joint mechanical property initially declines due to the formation of Cu{sub 41}Sn{sub 11} phase and Cu-Sn IMCs grain coarsening, and then increases because of the complete depletion of Cu{sub 3}Sn and substantial precipitation of Cu(Sn) solid solution. After aging for nearly 1000 h, the joint still maintains a high shear strength of >40 MPa, indicating an excellent thermal reliability.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Microstructure evolution of Ag–8Au–3Pd alloy wire during electromigration

As the continuous shrinkage of the interconnect line width in microelectronics devices, there is a growing concern about the electromigration (EM) failure of bonding wire. In addition, an innovative Ag–8Au–3Pd alloy wire has shown promise as an economical substitute for gold wire interconnects due to the cost pressure of gold in the last decade. In present study of the Ag–8Au–3Pd alloy wire, the surface diffusion occupied the dominant position during EM failure, and the activation energy was found to be 0.61 eV. In order to reveal the failure mechanism, the cross-sections of the Ag–8Au–3Pd alloy wire during EM were prepared by focused ion beam (FIB) micro-machining for electron backscatter diffraction (EBSD) analysis. The microstructure evolution of the Ag–8Au–3Pd alloy wire was characterized by the grain size and grain boundary. As a result, the EM failure originates in the atom transportation, which causes grain size increasing and atom diffusion on the wire surface. - Highlights: • The activation energy of Ag–8Au–3Pd alloy wire was obtained as 0.61 eV. • During EM, the silver atoms diffused from negative to the positive terminal on the wire surface. • The microstructure (grain size and grain boundary) was characterized by FIB-EBSD. • During EM, the atom transportation was found to cause grain size growth and atom diffusion on the wire surface.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Microstructural evolution and hardness changes in ion irradiated nickel-based Haynes 282 superalloy

This study investigated the crucial aspects of thermal and irradiation stability in precipitation hardened Haynes 282 Ni-based superalloy. The Haynes 282 Ni-based alloy was irradiated by 8 MeV Ni 3+ ions to assess its resistance to phase instabilities and mechanical property alterations. The mid-range doses (at a depth of ∼1 µm) were 1 and 10 displacements per atom (dpa) at temperatures of 600°C and 750°C. Nanoindentation tests provided insights into bulk equivalent hardness of the irradiated and pristine regions, while scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS) were used to examine the microstructural evolution of irradiation-induced defects and γ′ -Ni 3 (Al, Ti) precipitates and defect structure under irradiation. These precipitates, with an average diameter of 29 nm and a number density of 5 x 10 21 /m 3 , acted as robust dispersion strengthening agents with high radiation point defect sink strength. Remarkably, irradiation did not significantly alter the size or number density of the γ′ precipitates, indicating exceptional thermal stability and radiation resistance of these precipitates at the examined conditions. Radiation-induced dislocation loops were observed at 600°C, albeit without a substantial impact on mechanical properties due to the dominance of γ′ precipitates on the overall alloy strength. The superior stability of γ′ precipitates observed in this study contrasts with several previous research findings on Ni-based alloys that reported poor precipitate stability. Plausible reasons for this difference are discussed. Moreover, this work explicitly outlines a physically grounded approach to ensure accurate microstructure-hardness correlations and clarifies the hardening model by addressing common misapplications of superposition in prior studies.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Microstructural evolution in nickel alloy C-276 after Ar-ion irradiation at elevated temperature

In present work, the irradiation damage in nickel-base alloy C-276 irradiated with Ar-ions was studied. Specimens of C-276 alloy were subjected to an irradiation of Ar-ions (with 120 keV) to dose levels of 6 and 10 dpa at 300 and 550 Degree-Sign C, respectively. The size distributions and densities of dislocation loops caused by irradiation were investigated with transmission electron microscopy. Irradiation hardening due to the formation of the loops was calculated using the dispersed barrier-hardening model, showing that irradiation hardening was greatest at 300 Degree-Sign C/6 dpa. The microstructure evolution induced by Ar-ion irradiation (0-10 dpa) in nickel-base alloy C-276 has been studied using a multi-scale modeling code Radieff constructed based on rate theory, and the size of dislocation loops simulated by Radieff was in good agreement with the experiment. - Highlights: Black-Right-Pointing-Pointer High density of dislocation loops appeared after Ar ions irradiation. Black-Right-Pointing-Pointer Irradiation hardening due to the formation of loops was calculated by the DBH model. Black-Right-Pointing-Pointer Size of loops simulated by Radieff was in good agreement with the experiment.

36 MATERIALS SCIENCE↗
DOE OSTI2022

In-Situ Study of Microstructure Evolution of Spinodal Decomposition in an Al-Rich High-Entropy Alloy

High-entropy alloys (HEAs) are materials which leverage the entropy of mixing to motivate the formation of single-phase solid solutions, even of immiscible elements. While these materials are well-recognized for their application to structural engineering, there is increasing interest in the use of HEAs for functional applications such as memory storage and energy devices. The current work investigates the HEA Al 1.3 CoCrCuFeNi, which has been previously shown to be single-phase at high temperatures, but undergoes phase separation at lower temperatures, transforming the structural and the functional properties. This phase separation is investigated at high temperatures with in-situ small angle neutron scattering (SANS) and scanning transmission electron microscopy (EDS). These techniques show that increasing the temperature up to 800°C, the microstructure of the HEA adiabatically disorders and abruptly homogenizes near 700°C, which is consistent with spinodal decomposition. Overall, the microstructural evolution proceeds mainly by the atomistic redistribution of the constituent elements within simple crystal lattices, producing coherent phase mixtures.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Microstructure evolution and mechanical characterization of Nd:YAG laser beam welded titanium tubes

Highlights: • Joining of thin titanium tubes using laser beam welding. • Fusion zone exhibited coarse granular like grains with serrated and irregular boundaries. • Large spherical and irregular shape pores were observed at lower welding speeds. • Misorientation angle distribution indicated substructure boundaries along with single β grain formation from double α grains. • Fusion zone strengthened by substructure boundaries, solid solution strengthening and dislocation density. - Abstract: Grade 2 titanium tubes of outer diameter 60 mm and wall thickness 3.9 mm were effectively joined by Nd:YAG laser. The welding speed was varied in five levels (2.50, 2.75, 3.00, 3.25 and 3.50 m/min). The microstructure evolution was studied using optical microscopy and advanced characterization techniques. The bead geometry changed from parallel shape to wedge shape with increased welding speed due to changes in surface tension. The macrographs showed three zones namely fusion zone, heat affected zone (HAZ) and base metal. Coarse granular structure with irregular and serrated contours was observed in the fusion zone. Acicular α grains were observed at the extreme outer edge of the fusion zone due to rapid cooling. Spherical and irregular shaped pores were observed near the centerline of the fusion zone at lower welding speeds due to large amount of entrapped gases and downward sweeping solidification front. The grain size decreased with increased welding speed due to higher cooling rate. The misorientation angle distribution map showed three distinct regions. The increase in welding speed caused an increase in low angle boundaries and a reduction in very high angle boundaries. High density of dislocations was observed in the fusion zone due to thermal stresses caused by solidification. The microhardness of the fusion zone was recorded to be higher than base metal due to substructure boundaries, solute elements and dislocations. The increase in welding speed shifted the fracture location from fusion zone to base metal and fracture mode from brittle to ductile.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure evolution, grain morphology variation and mechanical property change of Cu-Sn intermetallic joints subjected to high-temperature aging

Highlights: • The δ-Cu{sub 41}Sn{sub 11} could be formed by reaction of 11Cu{sub 3}Sn + 9Cu → Cu{sub 41}Sn{sub 11} during aging above 350 °C. • The temperature effects on the growth rate and grain morphology of Cu{sub 41}Sn{sub 11} phase were found. • The Cu{sub 41}Sn{sub 11} joints with refined grains exhibited higher hardness and shear strength. • The Cu{sub 41}Sn{sub 11} joints exhibited a combination of transgranular and intergranular fracture during shear test. - Abstract: In this work, the aging treatment was carried out on full Cu{sub 3}Sn joints at different temperatures for various durations, the microstructure evolution, grain morphology variation and mechanical property change of joints were investigated. The temperature effects on the growth rate and grain morphology of Cu{sub 41}Sn{sub 11} phase were found: The complete transformation from Cu{sub 3}Sn phase to Cu{sub 41}Sn{sub 11} phase in joints needed 300 h at 400 °C, but 4 h at 500 °C. The Cu{sub 41}Sn{sub 11} phase formed at 400 °C consisted of columnar grains with an average width of 23.5 μm, while the Cu{sub 41}Sn{sub 11} phase formed at 500 °C consisted columnar grains with an average width of 8.5 μm. The average hardness and shear strength values of the Cu{sub 41}Sn{sub 11} joints formed at 400 °C were 432.5 HV and 62 MPa, while the joints formed at 500 °C exhibited higher hardness and shear strength (451.5 HV and 65 MPa). Besides, the Cu{sub 41}Sn{sub 11} joint was found to exhibit a combination of transgranular and intergranular fracture during shear test.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Characteristics of kinetic sprayed Ta in terms of the deposition behavior, microstructural evolution and mechanical properties: Effect of strain-rate-dependent response of Ta at high temperature

Highlights: • The effect of high-temperature rate-dependent response of BCC Ta on the deposition state of kinetic sprayed Ta particles with synergistic effect of powder oxidation • Abnormal increase of dislocation density above certain powder preheating temperature under the lower condition (N{sub 2} carrier gas), whereas considerable occurrence of grain refinement under the higher condition (He carrier gas). • Most significant dislocation density with the smallest volume-averaged domain size under the condition of 600 {sup o}C powder pre-heating with N{sub 2} carrier gas due to characteristic response of Ta. - Abstract: Numerous efforts have been focused on the fundamental research regarding kinetic spraying processes, but previous studies were biased to only few kinds of metallic materials. In particular, for Ta with body-centered cubic crystal structure, systematic studies are currently lacking in the literature. In this study, the characteristic deposition behaviors, microstructures, and mechanical properties of kinetic sprayed Ta under various impact conditions were systematically investigated. Generally, unusual features were usually related to strain-rate-dependent response (i.e. Peierls-Nabarro short range barriers) of Ta at high-temperature. This made thermal softening effect to be saturated above a certain degree of temperature, contributing to inferior deposition efficiency for the pre-heating at 600 °C than those of 400 and 500 °C by creating synergy with powder oxidation. In terms of microstructural evolutions, the dislocation density was abnormally increased at a certain powder preheating temperature (i.e., 600 °C), owing to the strain-rate-dependent response of Ta at high-temperature. On the other hand, grain refinement generated by rotational dynamic recrystallization was mainly observed under the higher kinetic energy condition (He carrier gas) at RT. Especially, the dislocation density and volume-average domain size evaluated via high-resolution X-ray diffraction peak profile analysis were corresponding to the results of the microstructural observations. Additionally, the nanomechanical properties (i.e., nanohardness and elastic modulus) evaluated via nanoindentation tests were in good agreement with the results from transmission electron microscopy investigations.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Powder bed binder jet 3D printing of Inconel 718: Densification, microstructural evolution and challenges

Traditional manufacturing of Inconel 718 components from castings and thermomechanical processing routes involve extensive post processing and machining to attain the desired geometry. Additive manufacturing (AM) technologies including direct energy deposition (DED), selective laser melting (SLM), electron beam melting (EBM) and binder jet 3D printing (BJ3DP) can minimize scrap generation and reduce lead times. While there is extensive literature on the use of melting and solidification based AM technologies, there has been limited research on the use of binder jet 3D printing. In this paper, a brief review on binder jet additive manufacturing of Inconel 718 is presented. In addition, existing knowledge on sintering of Inconel 718 has been extended to binder jet 3D printing. We found that supersolidus liquid phase sintering (SLPS) is necessary to achieve full densification of Inconel 718. SLPS is sensitive to the feedstock chemistry that has a strong influence on the liquid volume fraction at the processing temperature. Based on these results, we discuss an empirical framework to determine the role of powder particle size and liquid volume fraction on sintering kinetics. In conclusion, the role of powder packing factor and binder saturation on microstructural evolution is discussed. The current challenges in the use of BJ3DP for fabrication of Inconel 718, as well as, extension to other metal systems, are presented.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Analyzing the microstructural evolution and hardening response of an Al-Si-Mg casting alloy with Cr addition

Highlights: • The formation of β-Al{sub 5}FeSi intermetallics was suppressed since impurity Fe has been consumed by Al{sub 13}Cr{sub 4}Si{sub 4} intermetallics; • A large number of Al{sub 13}Cr{sub 4}Si{sub 4} dispersoids were observed in Al dendrites in the as-quenched and aged alloy; • The precipitation of Cr-bearing dispersoids harden the as-quenched alloy by over 10HV; • The current thermodynamic database underestimate the solubility of Fe in Al{sub 13}Cr{sub 4}Si{sub 4} intermetallics during solidification simulation. - Abstract: Chromium is added to Al-Si based casting alloys for the mitigation of die soldering. In the present work, EBSD, EDS and TEM characterizations are combined with thermodynamic simulations to study the microstructural evolution and hardening response of an Al-Si-Mg-Cr casting alloy during heat treatments. It is shown that the Fe-containing intermetallic compounds formed in the inter-dendritic region were acicular π-Al{sub 8}(Fe, Cr)Mg{sub 3}Si{sub 6} phase and polyhedral Al-(Cr, Fe)-Si phase which is structurally similar to face-centered cubic Al{sub 13}Cr{sub 4}Si{sub 4}. The polyhedral Al-(Cr, Fe)-Si intermetallics that crystallized prior to β-Al{sub 5}FeSi consumed impurity Fe, suppressing the formation of β phase during gravity die casting process. Also, Cr-addition enables the precipitation of nano-scale Cr-bearing dispersoids in Al dendrite during solution treatment at 813 K. Hardness increment of over 10 HV can be induced by the Cr-bearing dispersoids for as-quenched alloy. Though the dispersoids have a moderate hardening effect on the as-quenched alloy, they may impair the ageing hardening response by acting as heterogeneous nucleation sites for coarse Mg-Si rods and Si particles when quenching into water at 338 K.

36 MATERIALS SCIENCE↗