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Review and construction of interatomic potentials for molecular dynamics studies of hydrogen embrittlement in Fe─C based steels
Abstract Reducing hydrogen embrittlement in the low‐cost Fe─C based steels have the potential to significantly impact the development of hydrogen energy technologies. Molecular dynamics studies of hydrogen interactions with Fe─C steels provide fundamental information about the behavior of hydrogen at microstructural length scales, although such studies have not been performed due to the lack of an Fe─C─H ternary interatomic potential. In this work, the literature on interatomic potentials related to the Fe─C─H systems are reviewed with the aim of constructing an Fe─C─H potential from the published binary potentials. We found that Fe─C, Fe─H, and C─H bond order potentials exist and can be combined to construct an Fe─C─H ternary potential. Therefore, we constructed two such Fe─C─H potentials and demonstrate that these ternary potentials can reasonably capture hydrogen effects on deformation characteristics and deformation mechanisms for a variety of microstructural variations of the Fe─C steels, including martensite that results from γ to α phase transformation, and pearlite that results from the eutectic formation of the Fe 3 C cementite compound.
Microstructural modification of a high-pressure die-cast A380 alloy through friction stir processing and its effect on mechanical properties
Aluminum alloy A380 is one of the major high-pressure die-cast (HPDC) alloys fabricated in the die casting sector. However, microstructural features such as shrinkage and gas porosity, entrapped oxide inclusions, change in the morphology of eutectic Si particles as a function of casting location, and the presence of multiple second phase particulates, especially Fe-bearing phases in HPDC A380 alloy results in limited ductility, thus, affecting its structural application. However, through proper modification of the as-cast microstructure, it is possible to improve the mechanical performance of alloy A380. In this study, we report the effect of friction stir processing (FSP), a well-known severe plastic deformation (SPD) and thermomechanical processing tool for selective microstructural modification, on A380. FSP effectively refines the as-cast microstructure and breaks down the coarse Si particles, creating a homogenized distribution of equiaxed Si particles in the aluminum matrix. Additionally, FSP eliminates porosity that acts as crack initiation sites, leading to improved mechanical strength and ductility.
Casting Characteristics of High Cerium Content Aluminum Alloys
This paper compares the castability of the near eutectic aluminum-cerium alloy system to the aluminum-silicon and aluminum-copper systems. The alloys are compared based on die filling capability, feeding characteristics and tendency to hot tear in both sand cast and permanent mold applications. The castability ranking of the binary Al–Ce systems is as good as the aluminum-silicon system with some deterioration as additional alloying elements are added. In alloy systems that use cerium in combination with common aluminum alloying elements such as silicon, magnesium and/or copper, the casting characteristics are generally better than the aluminum-copper system. In general, production systems for melting, de-gassing and other processing of aluminum-silicon or aluminum-copper alloys can be used without modification for conventional casting of aluminum-cerium alloys.
CONV-3D Code Application for CFD-Modeling of Lead-Bismuth Coolant
The results of testing the three-dimensional CFD-code on problems very close to real processes for lead–bismuth eutectic are presented: flow in a 19-fuel element assembly with three spacing lattices and convection in the ‘tumbler’ model. The results of convergence on a sequence of grids permit drawing the conclusion that for a 19-fuel element assembly the error in the calculation of the friction coefficient on a more finely divided grid does not exceed 10% with two-fold smaller grid step in each direction. Similar arguments are valid for the calculation of the hydraulic resistance coefficient.
The Investigation of the Toughening Mechanism of PHBV/PBAT with a Novel Hyperbranched Ethylenediamine Triazine Polymer Based Modifier: The Formation of the Transition Layer and the Microcrosslinking Structure
A new type of designed hyperbranched ethylenediamine trazine polymer (HBETP) is successfully synthesized and characterized based upon NMR and GPC. The prepared HBETP is used to modify the poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV)/poly(butylene adipate-co-terephthalate) (PBAT) blends. The effect of HBETP on the microstructure, mechanical properties and thermal properties of the blends is studied. The results indicate that upon addition of 1.0 wt% of HBETP, the impact strength of the PHBV/PBAT blends is increased by 47.1%; ∆T{sub g} of the blends decreases from 53.2 to 49.9 °C. These results, together with the morphology analysis of the fractured surface of the blends, conclude the formation of the transition layer between PHBV and PBAT. Also, the XRD result shows that the addition of HBETP can limit the growth of the PHBV crystals and causes the decrease of both the crystallinity and the grain crystalline size. The DSC result demonstrates that the addition of HBETP mainly affects the crystallization of the HB-HV binary eutectic region within PHBV. The mechanism of PHBV/PBAT toughening is due to the formation of the strong physical hydrogen bonding and the chemical micro-crosslinking between HBETP and PHBV/PBAT, which is proposed based on XPS characterization.
Determination of kinetic properties of Sm(III)/Sm(II) reaction in LiCl–KCl molten salt using cyclic voltammetry and electrochemical impedance spectroscopy
Electrochemical and kinetic properties of samarium (Sm) in LiCl-KCl eutectic salt were explored at various concentrations and temperatures. Cyclic voltammetry (CV) were performed to determine diffusion coefficient and the rate constant (k0). In addition, electrochemical impedance spectroscopy (EIS) was conducted for additional understandings on the kinetic behaviors of Sm(III)/Sm(II) couple. The values of the diffusion coefficients were in the range of 3.54 × 10 -6 to 9.58 × 10 -5 cm 2 s -1 and the values of k 0 were estimated to be in the range of 0.63 × 10 -2 to 1.11 × 10 -2 cm s -1 . The values of i 0 for Sm(III)/Sm(II) were computed by obtaining the charge transfer resistance from the curve fitting analysis, ranging from 1.04 ×10 -3 to 2.97×10 -3 A cm -2 .
Exploring Fission–Fusion Synergies to Accelerate Compatibility Understanding
To address the significant commercial interest in fusion energy, it will be necessary to accelerate the compatibility research associated with liquid breeders including Li, eutectic Pb–Li and LiF-BeF 2 (FLiBe) molten salt. Particularly for FLiBe, compatibility understanding is limited especially for fusion relevant materials such as reduced activation ferritic-martensitic steels, SiC and V alloys. The historical knowledge associated with molten salt reactors (MSRs) and recent work to commercialize MSRs can benefit fusion research. Recent experimental and modeling work has improved understanding and this knowledge can be applied to fusion relevant materials. For liquid metals (LMs), the comparisons to Li and Pb–Li are less direct but nevertheless can help guide the pathway toward commercialization. For Pb–Li, Al-rich coatings have been shown to inhibit dissolution and potentially increase operating temperatures. For commercialization, the experience with sensors and on-line cleanup can help guide future developments. Thus, it is worth considering the potential for fission-related research with LMs and molten salts to help accelerate fusion research.
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.
A New Recycled Al–Si–Mg Alloy for Sustainable Structural Die Casting Applications
The use of secondary aluminum for structural components in the automotive industry is limited by the high Fe contents in recycled alloys which often result in the formation of brittle β-Al 5 FeSi phase which reduces the ductility of aluminum castings. In this study, a new secondary Al-Si-Mg alloy with high Fe content (about 0.44 wt.%) was developed for die casting applications. Based on thermodynamic modeling, manganese was added to obtain a designed Fe-to-Mn ratio of 2 which successfully suppressed the formation of β-Al 5 FeSi phase, by forming α-Al 15 (Fe,Mn) 3 Si 2 phase with rounded or hexagonal morphology. Additionally, a fine needle-like π-Al 8 FeMg 3 Si 2 phase was also formed within the eutectic regions. The new recycled alloy showed comparable mechanical properties in as-cast and heat treated (T5 and T6) conditions to three major primary die cast alloys (≤ 0.2 wt.% Fe) with similar composition. Ductility up to 7.4% from tensile elongation was achieved in as-cast recycled alloy due to the modification and refinement of α-Al 15 (Fe,Mn) 3 Si 2 by Mn and Sr additions. Tensile elongation was further improved to 9.1% after T6 treatment as a result of dissolution of π-Al 8 FeMg 3 Si 2 phase, defragmentation of α-Al 15 (Fe,Mn) 3 Si 2 and the spheroidization of Si phase. This new alloy provides a promising path for increasing usage of recycled aluminum with high Fe content in structural die castings for automotive and other applications.
Strontium Effects on the Formation of Iron-Intermetallic Phases in Secondary Al–9Si–0.6Fe Alloys
The presence and morphology of Fe-containing intermetallic phases affect the mechanical properties of aluminum alloys, especially in secondary Al–Si-based cast alloys. Although strontium (Sr) addition of 50 to 500 ppm is known to refine the needle-type eutectic silicon structure, the influence of Sr on the formation of Fe-intermetallic phases remains unclear. The present work investigates the combined additions of Sr and Mn to Al–9Si–0.6Fe–0.35Mg (All compositions are in wt pct except otherwise stated.) alloys on the formation of Fe-intermetallic phases at different solidification rates from ~ 1.5 to ~ 60 °C/s. Long and branched-type AlFeSi phase with size ranging from 50 to 120 µm are more common when solidified at the rate of 1.5 °C/s regardless of Sr and Mn additions. However, at the fast solidification rate of 60 °C/s, a 60 ppm Sr addition significantly reduced the average length of needle-shaped AlFeSi phase to less than 3 to 5 µm. Thermodynamic simulations have been performed using CALculation of PHAse Diagrams (CALPHAD) models to predict the formation of various phases and their possible interactions during solidification. The results indicated that the combination of a high solidification rate and about 60 ppm of Sr is beneficial to refining the δ-Al 3 FeSi 2 phase in Al–Si–Mg alloys containing 0.6 pctFe. As a result, this unexpected finding of Fe-intermetallic refinement by low Sr addition (~60 ppm) provides an important guide in designing secondary alloys for sustainable casting applications.
The Beneficial Effect of Iron in Aluminum-Cerium-Based Cast Alloys
Iron (Fe) has been considered a major impurity since it is detrimental to the mechanical properties of many cast aluminum alloys due to the formation of Fe-containing brittle intermetallic phases. Fe is found naturally as an impurity in bauxite ore, resulting in Fe contamination of aluminum alloys with increasing contamination from current recycling practices. The Al–Ce–Fe system was investigated using CALPHAD (CALculation of PHAse Diagrams) modeling and experimental casting techniques. It was found that additions of Fe to the Al–Ce system are beneficial to the strength (slightly) and ductility (significantly) of the ternary alloys, which is attributed to the formation of fine metastable Al 8 CeFe 2 phase with aggregate morphology and equilibrium Al 10 CeFe 2 phase, suppressing coarse proeutectic Al 11 Ce 3 phase in near-eutectic Al–Ce alloys. Heat treatment study showed that the metastable Al 8 CeFe 2 phase transforms to predicted equilibrium Al 10 CeFe 2 phase at 500 °C, with essentially no intermetallic or grain coarsening; thus, the alloy displayed excellent property retention. The Al–Ce–Fe alloy system offers opportunities for sustainable, recyclable alloy development using low-cost Fe and low-cost cerium (a byproduct of rare-earth extraction).
Phase Selection During Solidification and Solid-State Phase Transformations in an Al-10Ce-8Mn (wt pct) Alloy
In multicomponent Al-Ce alloys, and especially after additive manufacturing (AM), complex and metastable solidification microstructures are frequently observed. Here, in this research, the relationship between solidification conditions and phase selection are explored for an Al-10Ce-8Mn (wt pct) alloy using a systematic study of laser melting conditions. Three solidification modes were observed: primary Al 10 Mn 2 Ce; primary Al 20 Mn 2 Ce; and eutectic FCC Al + Al 20 Mn 2 Ce. These solidification modes were correlated to specific liquid-solid interface velocities using a simple thermal model, showing the change in primary solidification phase for low (< 6.8 × 10 −4 m/s), moderate (between 8.2 × 10 −4 and 5.9 × 10 −2 m/s) and high solidification velocities (> 6.2 x 10 −2 m/s) for the above three solidification microstructures, respectively. These results were rationalized by using interface response function (IRF) theory to describe the solidification undercooling for the possible primary intermetallic phases. The implication of the local phase selection from differing solidification conditions is summarized by a comparison of hardness which demonstrates the potential variance of Vickers hardness from 101 to 242 (VHV) by changing the laser velocity from 1 to 83 mm/s. Interestingly, on heat treatment at 400°C, the decomposition pathways of the solidification microstructure and hardness were also found to be different, thereby opening multiple pathways for spatial microstructure and property control within AM components.
Microstructural Characterization, Mechanical Properties, and Corrosion Resistance of Dental Co-Cr-Mo-W Alloys Manufactured by Selective Laser Melting
In this paper, the microstructure, mechanical properties, and corrosion resistance of the dental Co-Cr-Mo-W alloys fabricated by selective laser melting (SLM) were systematically investigated and correlated. The microstructure of the SLM group (SG) specimens is composed of extremely fine columnar crystals, exhibiting supersaturated and homogeneous character. The matrix mainly consists of face-centered-cubic (FCC) cobalt solid solutions, and minimal hexagonal-close-packed (HCP) cobalt solid solutions. Mechanical test results showed that the 0.2% yield strength of SG was 775 ± 7 MPa, and the ultimate tensile strength was 1118 ± 25 MPa. The elongation of SG was 8.28 ± 0.4%, and the microhardness of SG was 476 ± 10 HV. All of these values largely exceeded the cast group (CG) specimens. The high strength is attributed to their fine columnar crystals and supersaturated matrices. The fewer content from the HCP phase and a lack of intermetallic compounds or eutectic contribute to the high ductility. Furthermore, a higher E{sub corr}, lower I{sub corr} were acquired for better corrosion resistance compared to cast specimens, due to even less composition segregation and more microstructural homogeneity.
A Low-Cost Lightweight Entropic Alloy with High Strength
In this study, a series of lightweight Al–Mg system entropic alloys containing Zn, Cu, and Si were designed based on the order/disorder or entropy, and eutectic concepts. The alloys of Al{sub 58.5}Mg{sub 31.5}Zn{sub 4.5}Cu{sub 4.5}Si{sub 1}; Al{sub 63}Mg{sub 27}Zn{sub 4.5}Cu{sub 4.5}Si{sub 1}; Al{sub 66.7}Mg{sub 23.3}Zn{sub 4.5}Cu{sub 4.5}Si{sub 1}; Al{sub 80}Mg{sub 14}Zn{sub 2.7}Cu{sub 2.7}Si{sub 0.6}; Al{sub 85}Mg{sub 10.5}Zn{sub 2.025}Cu{sub 2.025}Si{sub 0.45}; and Al{sub 90}Mg{sub 7}Zn{sub 1.35}Cu{sub 1.35}Si{sub 0.3} were prepared by induction melting under a high-purity argon atmosphere and then casted into stainless steel molds. The microstructures which were tested in the as-cast state exhibited multiphases and contained apparent volume fractions of intermetallic compounds and solid solutions. Then, the compressive mechanical properties of the alloys were measured, and high fracture strengths of 577, 677, 590, 498, 814, and 794 MPa, respectively, were determined. Strong hardening phenomena were observed in the Al{sub 80}Mg{sub 14}Zn{sub 2.7}Cu{sub 2.7}Si{sub 0.6}, Al{sub 85}Mg{sub 10.5}Zn{sub 2.025}Cu{sub 2.025}Si{sub 0.45}, and Al{sub 90}Mg{sub 7}Zn{sub 1.35}Cu{sub 1.35}Si{sub 0.3} alloys at room temperature, with amazing plasticity percentages of 13.8, 24.8, and 32.7%, respectively. The property differences in the lightweight alloys were analyzed using the following parameters: the critical values of the enthalpy of mixing; atomic size differences; ratio of entropy to enthalpy; valence electron concentration (VEC); and Pauling electronegativity difference. Finally, three of the aforementioned parameters (atomic size difference, enthalpy of mixing, and Pauling electronegativity difference) were regarded in this study as the crucial rules for the lightweight multicomponent alloys.
Solidification Characteristics of Silicon-Alloyed Ductile Cast Irons
Solidification pattern of three ductile iron compositions [I-2.5%Si; II-4%Si, and III-4%Si-1.6%Mo] is studied, to evaluate the effects of high Si and Mo contents. Inoculation compensates for the negative effect of Mo on eutectic undercooling. The deviation using a sphere as reference of graphite particles is noticeably increased by silicon alloying, when a characteristic of the graphite particles appears have a larger perimeter, resulting in a large category [IV, V and VI forms, ISO 945]. The sphericity shape factor (SSF) considering the real perimeter of particle is recommended in the nodularity evaluation in high-Si ductile iron, instead of the roundness shape factor, involving maximum ferret, presently incorporated in the ISO 945 standard. The higher the minimum acceptable SSF is, the greater difference in nodularity, between conventional and Si-alloyed ductile irons. For more than 4%Si, ductile irons are characterized by a medium-quality graphite phase, with prevalent form V-ISO 945 graphite. A specific inoculant, with a powerful action on the graphite particle compactness, especially to promote VI graphite form, is necessary.
Brazing of Martensitic Stainless Steel to Copper Using Electroplated Copper and Silver Coatings
Brazing of 3 mm thick sheets of low-carbon martensitic stainless steel and Cu-Cr-Zr-Ti alloy was attempted using electroplated coatings of Cu and Ag. Ni coating of 5 µm thick was provided on the MSS base metal to enhance the wettability. Brazing experiments were carried out at 985 °C for a holding of 15 min. Detailed microstructural studies, hardness, lap-shear, tensile, and pressure tests were carried out on the brazed joints. The reaction between the Cu and Ag coatings lead to the formation of in situ braze metal. The microstructure of the braze metal was found to consist of Cu-rich primary alpha (α{sub P}) and Ag-rich eutectic mixture (β + α{sub E}). In lap-shear tests, the brazed joints produced with an overlap of 9 and 6 mm were found to fail in copper base metal, while the joints produced with an overlap of 3 mm failed in the braze metal exhibiting a lap-shear strength of 147 MPa. In tensile tests, the butt brazed joints were found to fail in copper base metal as well with an average tensile strength of 213 MPa. The braze metal hardness was found to be higher than the hardness of the copper base metal. In pressure tests, the brazed panels were found to survive a pressure of 750 bar without undergoing any damage. These results show that electroplated coatings of Cu and Ag can be advantageously utilized for brazing of large, complex-shaped parts such as thrust chambers in space industry.
Influence of Melt-Pouring Temperature and Composition of Primary Coating of Shell Mold on Tensile Strength and Creep Resistance of Ni-Based Superalloy
Eight investment castings of Inconel 713C superalloy were fabricated, varying in the melt-pouring temperature, from 1400 to 1520 °C, and CoAl{sub 2}O{sub 4} inoculant content, 0 or 5 wt.%, in the primary coat. Their influence on grain size on tensile and creep properties was investigated. The best combination of yield stress (815 MPa) and elongation (A{sub 4} = 7.65%) at ambient temperature was obtained in surface-modified castings poured from 1520 °C. The longest time to rupture was for the unmodified castings (76.8 h) poured from 1520 °C, while for the modified variant, the time to rupture was lower around, 7.4 h, which was still a satisfactory value. During tensile testing, microcracks were formed in large eutectic γ′, carbides and borides, due to the accumulation of stress at interfaces with the matrix. During creep, N-type rafting of γ′ precipitates and phase transformation MC + γ → γ′ + M{sub 23}C{sub 6} took place.