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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 OSTI2023

Additive manufacturing enabled synergetic strengthening of bimodal reinforcing particles for aluminum matrix composites

An additive manufactured TiB 2 /Al-Cu-Mg-Ni composite with a minor amount of Sc was fabricated by laser powder bed fusion (LPBF). The composite shows a yield strength of ~370 MPa, almost doubling the strength of its wrought matrix counterpart, and an elongation of ~7 %. The superior mechanical properties are attributed to a unique micro-nano hierarchical microstructure, consisting of nanoscale and microscale TiB 2 particles dispersed in a matrix of fine aluminum grains (3 µm) together with intragranular intermetallic nanoparticles and nano-cellular networks (cell size 30 nm). The formation of the TiB 2 nanoparticles is the consequence of partial dissolution of the TiB 2 particulates and the enrichment of Sc in the newly formed nanoparticles. Remarkable strengthening effects are achieved by the bimodal TiB 2 particles, intermetallic nanoparticles and intragranular nano-cellular networks. Finally, this study provides new insights into the role of additive manufacturing in tailoring the microstructure of particulate reinforced metal matrix composites (MMCs) with advanced properties.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effect of excess Mg to control corrosion in molten MgCl 2 and KCl eutectic salt mixture

Structural alloys may experience corrosion when exposed to molten chloride salts due to selective dissolution of active alloying elements. One way to prevent this is to make the molten salt reducing. For the KCl + MgCl 2 eutectic salt mixture, pure Mg can be added to achieve this. However, Mg can form intermetallic compounds with nickel at high temperatures, which may cause alloy embrittlement. This work shows that an optimum level of excess Mg could be added to the molten salt which will prevent corrosion of alloys like 316 H, while not forming any detectable Ni-Mg intermetallic phases on Ni-rich alloy surfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Thin film combinatorial sputtering of Al-Ce alloys: Investigating the phase separation of as-deposited solid solutions and determining the coefficient of thermal expansion

Al x Ce 100–x thin films with a composition range of ~75.0 < x < 99.5 at% (36.5 < x < 97.5 wt%) were synthesized via combinatorial co-sputtering from an Al and an Al 50 Ce 50 target. The crystal structure, phase fraction, film morphology, electrical resistivity, and temperature-dependent coefficients of thermal expansion (CTE) are all correlated to the Al x Ce 100–x composition. The as-deposited films form a metastable solid-solution, and annealing leads to the formation of the thermodynamically stable two-phase system of Al and the α-Al 11 Ce 3 intermetallic. Temperature dependent x-ray diffraction (XRD) reveals that the two phases expand independently of one another, and the thin film Al temperature-dependent CTE is similar to bulk Al. The thin film Al 11 Ce 3 intermetallic phase has a nearly constant CTE of ~1.5 × 10 –5 /°C within the temperature range studied (25–550 °C). To confirm the thin film Al 11 Ce 3 results, bulk stoichiometric Al 11 Ce 3 and +/- 1 wt% Ce samples were prepared and the CTE of each was measured with the same conditions. A Rietveld analysis of the bulk data enabled an estimation of the CTE in each of the 3 orthorhombic lattice parameters, which displayed anisotropic behavior. The thin film and bulk CTE measurements were in very good agreement. Estimations of the temperature dependent CTE of the two-phase alloys are made via the Reuss and Voigt models. Finally, by demonstrating the efficacy of the approach, more complex multi-component rapid materials discovery of low CTE Al-alloys can be pursued via the combinatorial thin film synthesis and XRD measurement.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Heterogeneous phase transformation pathways in additively manufactured Al-Ce-Mn alloys

Heat treatment of additively manufactured Al-Ce based multicomponent alloys leads to complex microstructure evolution. In this research, the ability to extend the phase transformation theories involving nucleation of a product phase from a heterogeneous multi-phase microstructure typical to that of additively manufactured samples is explored. The Al-10Ce-8Mn (wt%) was used as a model alloy system. Under additive manufacturing conditions different solidification microstructures were obtained due to spatial and temporal variations of thermal gradients (G) and liquid-solid interface velocities (R) within a given melt pool. Near the melt pool boundary (high G and low R, referred as MPB region), initially, Al 20 Mn 2 Ce forms from the liquid followed by a eutectic of FCC Al and Al 11 Ce 3 . In the melt pool interiors (low G and high R referred as ES region) a eutectic structure between FCC Al and Al 20 Mn 2 Ce is observed. During subsequent heat treatments, the MPB and ES regions transform into different sets of microstructures. In the MPB region, a fine globular microstructure containing FCC Al, Al 11 Ce 3 , Al 6 Mn, and Al 12 Mn results from the decomposition of Al 20 Mn 2 Ce. In the ES region a faceted Al 51 Mn 7 Ce 4 plate phase results from the decomposition of Al 20 Mn 2 Ce. The formation of the Al 51 Mn 7 Ce 4 phase within the eutectic microstructure at the boundaries of FCC Al and Al 20 Mn 2 Ce has not been reported in the literature. Further, these two distinct phase transformation pathways are rationalized based on the role of driving force on the nucleation of (Al 6 Mn) and/or metastable intermetallic (Al 51 Mn 7 Ce 4 ) phases at the interface of aluminum (FCC) and the non-equilibrium intermetallic (Al 20 Mn 2 Ce) phases.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Investigating the failure behavior of cast Al-11Ce-0.4Mg alloys using in-situ scanning electron microscopy tensile testing

Within the last decade, research on Al-Ce-Mg alloys has reported promising results for use in cast part applications. In this paper, the failure behavior of cast Al-11Ce-0.4Mg (wt%) was investigated experimentally with focus on the effect the matrix and intermetallic phases have on the fracture propagation behavior at failure. For the first time, in-situ SEM tensile testing was used to study the failure behavior of cast Al-Ce alloys, reporting results for uniaxial, DIC, and single edge notch tensile tests. The results of the in-situ SEM tensile testing were compared with the materials characterization experiments, which included serial sectioning, EBSD, EDS, and fractography. Analysis of EBSD and EDS mapping of cast Al-11Ce-0.4Mg showed that the cast microstructure was a hypereutectic two phase Al-Ce alloy with grains encompassing large complex colonies of laminar eutectic Al 11 Ce 3 intermetallic. The uniaxial tensile results reported the effect casting defects have on the strength and ductility of the alloy, and DIC in-situ testing showed that the eutectic colonies plastically deform less than the matrix phase. In-situ SEM single edge notch tensile testing displayed how the strength of an individual phase affected the crack propagation direction in the alloy. The results of both the materials characterization and in-situ tensile testing experiments on the failure of this alloy revealed further directions for future alloy development that can improve both the strength and fracture toughness of Al-Ce-Mg alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Electrical property enhancement of non-heat-treatable wrought aluminum alloys using graphene additives

With growing efforts of electrification, aluminum’s role as a light-weight conductor material has become increasingly prominent. There is a critical need to improve the electrical performance of aluminum at room temperature and high operating temperatures. In this study, the effect of graphene nanoparticle additives on the electrical performance of a non-heat treatable alloy were (AA3003) explored. Graphene’s unusual structure and electronic properties were used to improve AA3003 properties. Here, in this work, the effects of graphene on the evolution of electrical properties and microstructural features have been explored on lab scale hot extruded AA3003-graphene composites. Hot pressing schedules and extrusion temperatures were varied to investigate changes in intermetallic dispersion characteristics in the presence of dispersed graphene. We measured a reduction of 10.3 % in the temperature coefficient of resistance in the AA3003 sample with 0.05 wt% graphene extruded at 400 °C, along with a maximum increase of 1.1 % in electrical conductivity at 20 °C. Increasing the hot-pressing times up to 8 hours was also found to consistently increase the electrical conductivity, due to increased precipitation of intermetallic phases. Despite being a non-heat treatable alloy, AA3003 displays interesting precipitation dynamics and grain recrystallization trends that can be modulated with varying levels of heat treatment, graphene concentrations, and hot extrusion process parameters.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Hardenability and microstructural evolution of a precipitation strengthened Ni 50 Ti 21 Hf 25 Al 4 alloy

NiTi-based quaternary alloys are used in a variety of mechanical components, such as bearings, actuators, and dampers, owing to their good hardenability, wear resistance, and corrosion resistance. Additionally, one of the most notable characteristics of NiTi-based alloys is their shape memory effect and pseudoelastic properties. Connecting the macroscopic processing parameters employed in the design of new intermetallic alloys to the nanoscale structural characteristics dictating their behavior is crucial for improving their mechanical properties and expanding the spectrum of potential applications. Here, in this work, an arc melted Ni 50 Ti 21 Hf 25 Al 4 (at%) alloy was solution treated at 1050 °C followed by quenching and aging at 600 °C to investigate the effect of aging time on the microstructure and mechanical properties. Two types of nano-sized precipitates were observed and determined as face-centered orthorhombic H-phase (TiHf)Ni and L2 1 Heusler precipitates Ni 2 TiAl. The morphology and orientation of the H-phase were investigated using scanning and transmission electron microscopy (SEM and TEM), elucidating the coarsening kinetics and strengthening contribution of that phase to the intermetallic mechanical behavior. Following coarsening, the presence of Heusler nanoprecipitates was detected under overaged conditions through TEM imaging and nanobeam electron diffraction patterns. A peak hardness condition of 756 HV was achieved after 70 h of aging, indicating that the co-precipitation of H-phase and Heusler precipitates through a well-designed aging treatment can lead to optimal mechanical performance, thus elevating the alloy’s potential as a viable material for industrial applications.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Assessment of binary eutectic Ni alloys for high-temperature applications via laser remelting

It has been recently demonstrated that eutectic alloys processed by additive manufacturing have excellent high-temperature mechanical properties. We suggest that nickel-base eutectic alloys may enable new combinations of structural and functional properties. To this end, we investigate the processability, microstructure, and thermal stability of five, binary near-eutectic Ni-X (X = B, Ce, La, Y, and Zr) alloys processed via surface laser-remelting. The microstructure of all alloys contain a two-phase lamellar eutectic microstructure consisting of γ-Ni and intermetallic phases; this microstructure is significantly finer (100–200 nm lamellar spacing) in the laser-remelted alloys than in the cast substrate (0.5–1.0 µm lamellar spacing). The microhardness of the laser-remelted alloys (550–770 HV) is 35–50% higher than that of the cast alloys (370–570 HV) due to this finer eutectic spacing. An anomalous eutectic microstructure appears at the meltpool boundaries, containing globular and lamellar γ-Ni phases. The alloys contain a high volume fraction (>50 vol%) of intermetallic phase which forms a continuous network, causing brittleness. Following laser-remelting trials, the alloys showed a high density of solid-state cracks, except for the Ni-Zr alloy which processed well. During thermal exposure at 700 and 900°C for up to 500 h, the eutectic microstructure coarsens. Coarsening occurs heterogeneously and initiates at the meltpool boundaries. This process occurs more slowly in the Ni-Zr and Ni-Y alloys, and more rapidly in the remaining alloys, resulting in greater microhardness retention in the Ni-Zr and Ni-Y alloys following thermal exposure at 700°C. Thus, among the five alloys, the Ni-Zr system exhibits a good combination of high-temperature mechanical properties and processability. Here, we conclude with recommendations for future work on designing additively manufactured alloys based on these eutectic Ni systems.

Additive manufacturing↗
DOE OSTI2023

A novel design of transitional layer structure between reduced activation ferritic martensitic steels and tungsten for plasma facing materials

Plasma-facing components (PFCs) are among the most critical gaps for fusion energy to establish technical and economic feasibility. Tungsten as a first wall/blanket material in PFCs requires to be integrating with reduced activation ferritic martensitic (RAFM) steels as a structural component. Currently, major drawbacks are the requirement of brazing, the formation of a brittle interface, and a large difference between the coefficients of thermal expansion of tungsten and steel. Here, a novel transitional multilayer structure was designed and investigated to join tungsten and RAFM steels using three interlayers. The composition of each interlayer was selected based on computational thermodynamics and diffusion kinetics to ensure a body-centered cubic (bcc) single-phase structure and prevent the formation of a brittle intermetallic phase region in the temperature range of 600–1150 °C. Although the transitional layer structure was designed for additive manufacturing, spark plasma sintering (SPS) as proof of concept was used to bond the individual layers. Interfaces were investigated using scanning and transmission electron microscopy methods but no layered intermetallic phase was observed. Nanoindentation maps across the interface suggest major hardness differences at the interface between tungsten and the vanadium interlayer, as well as the interface between RAFM steel and the FeCrAl interlayer.

36 MATERIALS SCIENCE↗
DOE OSTI2026

The effect of varying powder feedstock chemistry and printing atmosphere on the microstructure of additively manufactured nickel-based ODS alloys: Role on stabilization of cellular structures vs. oxide dispersion formation

Nickel-based alloys have a wide variety of structural applications due to their high corrosion resistance and mechanical strength which depend on solid solution strengthening, or the formation of oxides and/or intermetallic precipitation for their properties. In this study, the microstructure of six Ni–Cr–Y–Ti–Al powder batches designed for the production of oxide dispersion strengthened nickel were compared. These batches varied in chemistry and atomization technique used which included Gas Atomization Reactive Synthesis (GARS). The batches of powder were then consolidated via Additive Manufacturing (AM) Powder Bed Fusion using Laser Beam (PBF-LB) and characterized via transmission electron microscopy to elucidate the influence of powder feedstock (i.e. synthesis methodology and chemistry) on the PBF-LB microstructure. The study investigates (i) how the amount of yttrium and titanium additions in the powder feedstock and the addition of oxygen during the processing (through GARS) affect the microstructure of the powder itself and the AM printed microstructure, and (ii) how the control of oxygen addition in the printing atmosphere during the PBF-LB printing process itself is another important parameter for achieving the formation of the wanted oxide dispersion versus the stabilization of the cellular structure (often observed in AM processed alloys). Microstructural characterization of both powder particles and additively manufactured nickel alloys in this study provide important insights into the movement of yttrium within the material upon solidification, particularly along cell boundaries, and how yttrium behaves depending on alloy chemistry. When a threshold of yttrium content is reached within the system, yttrium consistently reacts to form an intermetallic along cell boundaries instead of forming oxide nanoparticles.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Effect of Ni content in Cu 1-x Ni x coating on microstructure evolution and mechanical properties of W/Mo joint via low-temperature diffusion bonding

The 93W and Mo1 refractory metals were bonded with different Cu 1- x Ni x coating interlayers of various Ni content using plasma-activated sintering at 700 °C. The effects of the Ni content in the Cu 1- x Ni x coating interlayer on the interfacial microstructure evolution and mechanical properties of the W/Mo joints were studied. The maximum average shear strength of the W/Mo joint was 316.5 MPa when the Ni content of the Cu 1- x Ni x coating interlayer was 25 %. When the Ni content of the Cu 1- x Ni x coating interlayer was below 50 %, the atomic diffusion at the W/Mo joint interface was adequate without the formation of intermetallic compounds, as demonstrated by the High Resolution Transmission Electron Microscope analyses of the joints. The presence of Ni in Cu 1- x Ni x promoted diffusion bonding at the interface, which contributed to the high mechanical properties of the W/Mo joint. With an increase in the Ni content of the Cu 1- x Ni x coating interlayer, the MoNi intermetallic compound (IMC) nucleated and grew at the Cu 1- x Ni x coating/Mo1 interface. When the Ni content of the Cu 1- x Ni x coating interlayer was above 50 %, the generation of a brittle MoNi IMC weakened the shear strength of the W/Mo joint dramatically.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Interactions and immobilization of lanthanides with dopants in uranium-based metallic fuels

In uranium-based fuels, production and migration of lanthanide fission products and subsequent chemical interactions with cladding constituents lead to fuel–cladding chemical interaction (FCCI), limiting fuel performance. In general, dopant addition to the fuel matrix to arrest lanthanides within the fuel by forming intermetallics is found to be effective in mitigating FCCI. Recently, we proposed ab-initio based alloy-design principles, which can be useful in identifying dopant(s) that can bind a lanthanide (Nd) inside the fuel matrix. Here, we demonstrate the robustness of such principles by performing a systematic study to choose dopants to form compounds with a range of experimentally-observed lanthanide fission products. Results reveal that for a given dopant, lanthanide Ce and La exhibit similar behavior in their compound-forming tendencies compared to Nd and Pr, which is linked to lanthanide intrinsic characteristics such as the electronic configurations. Our predicted lanthanide-dopant intermetallic formation is verified experimentally in selective cases. Finally, we showed alloy design principles that accurately identify previously known dopants like Pd, and also predict new dopants As and Se, that can be effective in binding all lanthanides within the uranium matrix. Altogether, this research helps to develop generic alloy-design principles for complex multi-component systems based on lanthanide and dopant intrinsic characteristics.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2025

Directed energy deposition of functionally graded V-4Cr-4Ti to Fe-9Cr transition for fusion power systems

This study proposes a graded structure via additive manufacturing for divertor and first wall blanket applications in fusion reactors. Materials were selected based on thermodynamic calculations to operate from 1100 °C at the plasma-facing level to 550 °C at the structural steel level. Conventional joining methods often lead to failures due to discrete reaction layers with significant mechanical property differences. Using laser beam-directed energy deposition (LB-DED), this study demonstrates the fabrication of a VCrTi-Gr91 steel functionally graded component through a novel process parameter optimization framework. A systematic approach included powder characterization, single-track depositions, and construction of printability maps. Near full-density specimens of each interlayer were additively manufactured, and a transition from V-based alloys to reduced activation ferritic martensitic steels was achieved. Computational material selection of interlayer alloys and thermodynamic/diffusion kinetics simulations prevented most interface incompatibilities. A brittle intermetallic formed at one interface, causing cracking, which was not predicted by current thermodynamic models. Transition alloy design approach was updated with a more recent database and a mitigation strategy has been proposed to eliminate the formation of deleterious intermetallic phases. Ultimately, LB-DED has proven effective for producing multi-material graded systems for fusion applications, with the demonstrated process parameter optimization framework applicable to various materials.

Additive manufacturing↗
DOE OSTI2025

Bonding of vanadium- and Iron-based alloys as interlayers for plasma-facing and structural materials in fusion systems

Vanadium alloys and FeCrAl were investigated as interlayers between tungsten and reduced activation ferritic martensitic steel for fusion system components to avoid formation of intermetallic phase at operating temperatures between 550 and 1100 °C, while maintaining a body centered cubic phase throughout the interface. Physical and mechanical properties need to be graded between tungsten and steel, but recent results showed a significant hardness increase at the FeCrAl to vanadium alloy interface. Here, a sintered sample of these alloys was annealed for extended time, and the microstructure was investigated to provide a better understanding of the phenomena. A comparison with an additively manufactured interface of the same material is provided. An unexpected L2 1 intermetallic phase formation has been revealed using microscopy and synchrotron techniques and will inform future additive manufacturing approaches of the interface. A Cr layer interface as a preliminary solution was proposed between the Vanadium alloy and FeCrAl alloy interface.

Additive manufacturing↗
DOE OSTI2020

Effect of copper content on the tensile elongation of Al–Cu–Mn–Zr alloys: Experiments and finite element simulations

Microstructures of cast aluminum alloys used in automotive engine applications often consist of intermetallic particles that can impact the tensile elongation of these alloys. Here in this paper, we investigate the effect of intermetallic grain boundary particles on tensile elongation by fabricating a series of alloys with Cu content varying between 6.0 - 9.0 wt% in cast Al–Cu–Mn–Zr (ACMZ) type compositions. The tensile elongation of as-aged ACMZ alloys decreases monotonically with increase in Cu content. While the microstructure within the grains and yield stress of the alloy remains invariant with Cu content, the decrease in tensile elongation correlates well with increase in the size and volume fraction of grain boundary particles. Microstructural observations are combined with finite element simulations to explain the trend in tensile elongation with changing Cu content. Crack initiation is found to occur by brittle fracture of the grain boundary particles. Increase in particle size promotes crack initiation by reduction in size dependent particle fracture strength. Lower inter-particle spacing at higher particle volume fraction further facilitates crack initiation by increasing stress within the particles caused by the interaction between stress fields of neighboring particles. Increase in particle volume fraction also accelerates crack propagation through the formation of macro shear zones in the microstructure. The increase in Cu content of cast ACMZ alloys, therefore, decreases tensile elongation by promoting both crack initiation and crack propagation.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Repurposing the θ (Al 2 Cu) phase to simultaneously increase the strength and ductility of an additively manufactured Al–Cu alloy

The Al–Cu–Mn–Zr (ACMZ) cast family of alloys offers unique properties and value propositions for higher strength, higher temperature lightweight components of future vehicles. Earlier research has demonstrated trade-offs in the selection of the alloy chemistry in which an increase in Cu content from 6 up to 9 wt% improves hot tear resistance but lowers ductility. However, a recent study has demonstrated that higher-Cu (9Cu) ACMZ fabricated with laser powder bed fusion additive manufacturing (AM) results in an increase in both ductility and strength when compared to as-aged microstructure of cast 9Cu alloys. The mechanisms of differing mechanical performance of the cast and AM ACMZ alloys are elucidated in the current paper through the utilization of in situ high energy x-ray diffraction (HEXRD) tensile testing wherein lattice strains of different phases are calculated and correlated to their stresses. The ACMZ alloys consisted of theta (θ) and theta prime (θ') phases (both Al 2 Cu in nominal composition) within an aluminum matrix. The larger micron-size θ phase which decorated the grain boundaries in 9Cu ACMZ cast alloys recorded small lattice strains, while the submicron, homogeneously distributed θ phase in the 9Cu ACMZ AM alloy recorded considerably higher lattice strains. The maximum stress reached in the θ phase for the cast 9Cu alloy was found to be ~280 MPa, which was lower than the AM 9Cu alloy which registered a maximum stress of ~1.4 GPa. These measurements indicate that delayed fracture of the finer intermetallic phases simultaneously improves the ductility and strength of AM 9Cu alloy relative to the cast 9Cu alloy, which exhibits early fracture of the larger intermetallic particles.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Three-dimensional microstructure of a friction stir welded magnesium/steel interface characterized via high-energy synchrotron X-rays

Here, the three-dimensional (3D) microstructure of a friction stir assisted scribe technique (FaST) weld interface consisting of AZ31 magnesium (Mg) alloy and zinc (Zn)-coated DP590 steel was analyzed via synchrotron micro-computed tomography (μ-CT), correlated to synchrotron x-ray diffraction, and electron microscopy. Diffraction assessment of the AZ31/Zn-coated DP590 steel interface revealed the presence of several complex Mg–Zn intermetallic phases and differences in crystalline orientation of the Mg phase at the weld interface. The 3D characterization of the synchrotron μ-CT images of the weld interface revealed: (1) the presence of AZ31 weld nugget material with iron (Fe)-rich particles; (2) the presence of a weld layer at the joining interface with Fe-rich particles; (3) the formation of a thin non-uniform layer between the weld layer and the interface of the scribed DP590 material; and (4) the presence of an Fe–Al-rich intermetallic layer and Fe-rich particles at the interface of the scribed DP590 material. This study also performed a detailed analysis of lump-like features seen in the Zn coating layer at the weld interface. A detailed 3D particle analysis of the Fe-rich particles in the AZ31 weld nugget and the weld layer at the weld interface revealed the presence of several types of particle morphologies (i.e., spherical and irregular).

36 MATERIALS SCIENCE↗
DOE OSTI2023

Microstructure and mechanical properties of hypoeutectic Al–6Ce–3Ni-0.7Fe (wt.%) alloy

Here, a hypoeutectic, Fe-modified Al–Ce–Ni alloy (Al–6Ce–3Ni-0.7Fe, wt.%) is studied in terms of microstructure, thermal stability, ambient temperature strengthening, and creep resistance. The as-cast microstructure consists of primary Al dendrites and interdendritic binary eutectic regions (Al–Al11Ce3 and/or Al–Al9(Fe,Ni)2), with micron/submicron lamellar spacing, depending on the location along the height of the ingot. The cast alloy exhibits excellent coarsening resistance at 400 °C, with mostly unchanged microstructure and microhardness after 6 weeks of aging, indicating good thermal stability of Al11Ce3 and Al9(Fe,Ni)2. Orowan strengthening and load transfer are identified as strengthening mechanisms at ambient and elevated temperature. A high volume fraction of the intermetallic phases (providing load transfer) and relatively coarse eutectic spacing (for modest Orowan strengthening) result in a moderate as-cast microhardness of 566 ± 32 MPa. Creep resistance at 300 and 350 °C is similar to a binary Al-12.5Ce eutectic alloy (with twice the Ce content) because of two countering effects: Al–6Ce–3Ni-0.7Fe shows a higher volume fraction of strengthening intermetallic phases, but it also exhibits a large fraction of primary Al dendrites which weaken the alloy. By contrast, the alloy, when laser-remelted at the surface, has a fully eutectic microstructure without primary aluminum dendrites achieved by high undercooling on solidification, with a refined network of eutectic phases that doubles the microhardness as compared to the cast alloy. Whereas coarsening is faster due to the shorter diffusion distances between the eutectic phases, hardness remains ~30% higher than the as-cast alloy after ~6 weeks aging at 400 °C.

36 MATERIALS SCIENCE↗