Showing papers in "JOM in 2014"

Computational Analysis Methods in Atomistic Modeling of Crystals

Abstract: This article discusses computational analysis methods typically used in atomistic modeling of crystalline materials and highlights recent developments that can provide better insights into processes at the atomic scale. Topics include the classification of local atomic structures, the transition from atomistics to mesoscale and continuum-scale descriptions, and the automated identification of dislocations in atomistic simulation data.

Microstructure and Properties of Aluminum-Containing Refractory High-Entropy Alloys

Abstract: A new metallurgical strategy, high-entropy alloying (HEA), was used to explore new composition and phase spaces in the development of new refractory alloys with reduced densities and improved properties. Combining Mo, Ta, and Hf with “low-density” refractory elements (Nb, V, and Zr) and with Ti and Al produced six new refractory HEAs with densities ranging from 6.9 g/cm3 to 9.1 g/cm3. Three alloys have single-phase disordered body-centered cubic (bcc) crystal structures and three other alloys contain two bcc nanophases with very close lattice parameters. The alloys have high hardness, in the range from H v = 4.0 GPa to 5.8 GPa, and compression yield strength, σ 0.2 = 1280 MPa to 2035 MPa, depending on the composition. Some of these refractory HEAs show considerably improved high temperature strengths relative to advanced Ni-based superalloys. Compressive ductility of all the alloys is limited at room temperature, but it improves significantly at 800°C and 1000°C.

High-Entropy Alloys with a Hexagonal Close-Packed Structure Designed by Equi-Atomic Alloy Strategy and Binary Phase Diagrams

Abstract: High-entropy alloys (HEAs) with an atomic arrangement of a hexagonal close-packed (hcp) structure were found in YGdTbDyLu and GdTbDyTmLu alloys as a nearly single hcp phase. The equi-atomic alloy design for HEAs assisted by binary phase diagrams started with selecting constituent elements with the hcp structure at room temperature by permitting allotropic transformation at a high temperature. The binary phase diagrams comprising the elements thus selected were carefully examined for the characteristics of miscibility in both liquid and solid phases as well as in both solids due to allotropic transformation. The miscibility in interest was considerably narrow enough to prevent segregation from taking place during casting around the equi-atomic composition. The alloy design eventually gave candidates of quinary equi-atomic alloys comprising heavy lanthanides principally. The XRD analysis revealed that YGdTbDyLu and GdTbDyTmLu alloys thus designed are formed into the hcp structure in a nearly single phase. It was found that these YGdTbDyLu and GdTbDyTmLu HEAs with an hcp structure have delta parameter (δ) values of 1.4 and 1.6, respectively, and mixing enthalpy (ΔH mix) = 0 kJ/mol for both alloys. These alloys were consistently plotted in zone S for disordered HEAs in a δ-ΔH mix diagram reported by Zhang et al. (Adv Eng Mater 10:534, 2008). The value of valence electron concentration of the alloys was evaluated to be 3 as the first report for HEAs with an hcp structure. The finding of HEAs with the hcp structure is significant in that HEAs have been extended to covering all three simple metallic crystalline structures ultimately followed by the body- and face-centered cubic (bcc and fcc) phases and to all four simple solid solutions that contain the glassy phase from high-entropy bulk metallic glasses.

Additive Manufacturing: Making Imagination the Major Limitation

Abstract: Additive manufacturing (AM) refers to an advanced technology used for the fabrication of three-dimensional near-net-shaped functional components directly from computer models, using unit materials. The fundamentals and working principle of AM offer several advantages, including near-net-shape capabilities, superior design and geometrical flexibility, innovative multi-material fabrication, reduced tooling and fixturing, shorter cycle time for design and manufacturing, instant local production at a global scale, and material, energy, and cost efficiency. Well suiting the requests of modern manufacturing climate, AM is viewed as the new industrial revolution, making its way into a continuously increasing number of industries, such as aerospace, defense, automotive, medical, architecture, art, jewelry, and food. This overview was created to relate the historical evolution of the AM technology to its state-of-the-art developments and emerging applications. Generic thoughts on the microstructural characteristics, properties, and performance of AM-fabricated materials will also be discussed, primarily related to metallic materials. This write-up will introduce the general reader to specifics of the AM field vis-a-vis advantages and common techniques, materials and properties, current applications, and future opportunities.

Applications of Polymer Matrix Syntactic Foams

Abstract: A collection of applications of polymer matrix syntactic foams is presented in this article. Syntactic foams are lightweight porous composites that found their early applications in marine structures due to their naturally buoyant behavior and low moisture absorption. Their light weight has been beneficial in weight sensitive aerospace structures. Syntactic foams have pushed the performance boundaries for composites and have enabled the development of vehicles for traveling to the deepest parts of the ocean and to other planets. The high volume fraction of porosity in syntactic foams also enabled their applications in thermal insulation of pipelines in oil and gas industry. The possibility of tailoring the mechanical and thermal properties of syntactic foams through a combination of material selection, hollow particle volume fraction, and hollow particle wall thickness has helped in rapidly growing these applications. The low coefficient of thermal expansion and dimensional stability at high temperatures are now leading their use in electronic packaging, composite tooling, and thermoforming plug assists. Methods have been developed to tailor the mechanical and thermal properties of syntactic foams independent of each other over a wide range, which is a significant advantage over other traditional particulate and fibrous composites.

Functional Metal Matrix Composites: Self-lubricating, Self-healing, and Nanocomposites-An Outlook

Abstract: Many different types of advanced metal matrix composites are now available, some of which possess functional properties. Recent work on particle-reinforced, self-lubricating and self-healing metals and metal matrix nanocomposites (MMNCs) synthesized by solidification synthesis is reviewed. Particle-based MMNCs have been developed by several modern processing tools based on either solid- or liquid-phase synthesis techniques that are claimed to exhibit exciting mechanical properties including improvements of modulus, yield strength, and ultimate tensile strength. This article presents a brief and objective review of the work done over the last decade to identify the challenges and future opportunities in the area of functional nanocomposites. Increasing interest in lightweight materials has resulted in studies on hollow particle-filled metal matrix syntactic foams. Syntactic foams seem especially suitable for development with functional properties such as self-healing and self-lubrication. The metal matrix micro and nanocomposites, and syntactic foams having combinations of ultrahigh strength and wear resistance, self-lubricating, and/or self-healing properties can lead to increased energy efficiency, reliability, comfort of operation, reparability, and safety of vehicles. The focus of the present review is aluminum and magnesium matrix functional materials.

Alloy Design, Combinatorial Synthesis, and Microstructure–Property Relations for Low-Density Fe-Mn-Al-C Austenitic Steels

Abstract: We present recent developments in the field of austenitic steels with up to 18% reduced mass density. The alloys are based on the Fe-Mn-Al-C system. Here, two steel types are addressed. The first one is a class of low-density twinning-induced plasticity or single phase austenitic TWIP (SIMPLEX) steels with 25–30 wt.% Mn and 10 wt.% Al. Three topics are addressed in more detail, namely, the combinatorial bulk high-throughput design of a wide range of corresponding alloy variants, the development of microstructure–property relations for such steels, and their susceptibility to hydrogen embrittlement.

Recent Progress in Developing and Qualifying Nanostructured Ferritic Alloys for Advanced Fission and Fusion Applications

Abstract: This article summarizes the recent progress on developing a class of potentially transformational structural materials called nanostructured ferritic alloys, which are leading candidates for advanced fission and fusion energy applications. Here, we focus on Fe-Cr-based ferritic stainless steels containing a very high concentration of Y-Ti-O nano-oxide features that enable a host of outstanding high-temperature properties, along with unique irradiation tolerance and thermal stability. Perhaps most notably, these alloys have an unprecedented capability to manage very high helium concentrations, pertinent to fusion service, in a way that transforms this element from a severe liability to a potential asset. In addition to providing some necessary background, we update progress on: (I) the character of the nanofeatures; (II) some unifying insights on key mechanical properties; (III) a quantitative model for nanofeature coarsening; (IV) recent irradiation experiments of the effects of helium on cavity evolution and void swelling; and (V) a powerful new mechanism controlling the transport, fate, and consequences of helium.

Phase Stability of Low-Density, Multiprincipal Component Alloys Containing Aluminum, Magnesium, and Lithium

Abstract: A series of low-density, multiprincipal component alloys containing high concentrations of Al, Mg, Li, Zn, Cu and/or Sn was designed using a strategy based on high-entropy alloys (HEAs). The alloys were prepared by induction melting under high-purity argon atmosphere, and the resulting microstructures were characterized in the as-cast condition. The resulting microstructures are multiphase and complex and contain significant volume fractions of disordered solutions and intermetallic compounds. By analyzing the atomic size difference, enthalpy of mixing, entropy of mixing, electronegativity difference, and valence electron concentration among the constituent elements, modified phase formation rules are developed for low-density multiprincipal component alloys that are more restrictive than previously established limits based on more frequently studied HEAs comprising mostly transition metals. It is concluded that disordered solid solution phases are generally less stable than competing ordered compounds when formulated from low-density elements including Al, Mg, and Li.

Phase Selection in High-Entropy Alloys: From Nonequilibrium to Equilibrium

Abstract: High-entropy alloys (HEAs) have been investigated considerably in the last decade. The phase selection in HEAs has attracted much attention recently, especially on forming of the solid solutions. Up to now, phase diagrams of most HEAs are still not well developed, and the empirical phase selection rules play an important role in HEAs area. In this brief review, the physical factors controlling the phase stability in HEAs are discussed, and the phase selection rules are identified. Different from previous results, the rules on equilibrium phase selection within a certain temperature range are carefully reviewed and presented in this article.

Composition Dependence of Phase Stability, Deformation Mechanisms, and Mechanical Properties of the CoCrFeMnNi High-Entropy Alloy System

Abstract: The proposal of configurational entropy maximization to produce massive solid-solution (SS)-strengthened, single-phase high-entropy alloy (HEA) systems has gained much scientific interest. Although most of this interest focuses on the basic role of configurational entropy in SS formability, setting future research directions also requires the overall property benefits of massive SS strengthening to be carefully investigated. To this end, taking the most promising CoCrFeMnNi HEA system as the starting point, we investigate SS formability, deformation mechanisms, and the achievable mechanical property ranges of different compositions and microstructural states. A comparative assessment of the results with respect to room temperature behavior of binary Fe-Mn alloys reveals only limited benefits of massive SS formation. Nevertheless, the results also clarify that the compositional requirements in this alloy system to stabilize the face-centered cubic (fcc) SS are sufficiently relaxed to allow considering nonequiatomic compositions and exploring improved strength–ductility combinations at reduced alloying costs.

High-Entropy Metallic Glasses

Abstract: The high-entropy alloys are defined as solid-solution alloys containing five or more than five principal elements in equal or near-equal atomic percent. The concept of high mixing entropy introduces a new way for developing advanced metallic materials with unique physical and mechanical properties that cannot be achieved by the conventional microalloying approach based on only a single base element. The metallic glass (MG) is the metallic alloy rapidly quenched from the liquid state, and at room temperature it still shows an amorphous liquid-like structure. Bulk MGs represent a particular class of amorphous alloys usually with three or more than three components but based on a single principal element such as Zr, Cu, Ce, and Fe. These materials are very attractive for applications because of their excellent mechanical properties such as ultrahigh (near theoretical) strength, wear resistance, and hardness, and physical properties such as soft magnetic properties. In this article, we review the formation and properties of a series of high-mixing-entropy bulk MGs based on multiple major elements. It is found that the strategy and route for development of the high-entropy alloys can be applied to the development of the MGs with excellent glass-forming ability. The high-mixing-entropy bulk MGs are then loosely defined as metallic glassy alloys containing five or more than five elements in equal or near-equal atomic percent, which have relatively high mixing entropy compared with the conventional MGs based on a single principal element. The formation mechanism, especially the role of the mixing entropy in the formation of the high-entropy MGs, is discussed. The unique physical, mechanical, chemical, and biomedical properties of the high-entropy MGs in comparison with the conventional metallic alloys are introduced. We show that the high-mixing-entropy MGs, along the formation idea and strategy of the high-entropy alloys and based on multiple major elements, might provide a novel approach in search for new MG-forming systems with significances in scientific studies and potential applications.

High-Strength Low-Alloy (HSLA) Mg–Zn–Ca Alloys with Excellent Biodegradation Performance

Abstract: This article deals with the development of fine-grained high-strength low-alloy (HSLA) magnesium alloys intended for use as biodegradable implant material. The alloys contain solely low amounts of Zn and Ca as alloying elements. We illustrate the development path starting from the high-Zn-containing ZX50 (MgZn5Ca0.25) alloy with conventional purity, to an ultrahigh-purity ZX50 modification, and further to the ultrahigh-purity Zn-lean alloy ZX10 (MgZn1Ca0.3). It is shown that alloys with high Zn-content are prone to biocorrosion in various environments, most probably because of the presence of the intermetallic phase Mg6Zn3Ca2. A reduction of the Zn content results in (Mg,Zn)2Ca phase formation. This phase is less noble than the Mg-matrix and therefore, in contrast to Mg6Zn3Ca2, does not act as cathodic site. A fine-grained microstructure is achieved by the controlled formation of fine and homogeneously distributed (Mg,Zn)2Ca precipitates, which influence dynamic recrystallization and grain growth during hot forming. Such design scheme is comparable to that of HSLA steels, where low amounts of alloying elements are intended to produce a very fine dispersion of particles to increase the material’s strength by refining the grain size. Consequently our new, ultrapure ZX10 alloy exhibits high strength (yield strength R p = 240 MPa, ultimate tensile strength R m = 255 MPa) and simultaneously high ductility (elongation to fracture A = 27%), as well as low mechanical anisotropy. Because of the anodic nature of the (Mg,Zn)2Ca particles used in the HSLA concept, the in vivo degradation in a rat femur implantation study is very slow and homogeneous without clinically observable hydrogen evolution, making the ZX10 alloy a promising material for biodegradable implants.

Recent Progress in Discrete Dislocation Dynamics and Its Applications to Micro Plasticity

Abstract: We present a self-contained review of the discrete dislocation dynamics (DDD) method for the numerical investigation of plasticity in crystals, focusing on recent development and implementation progress. The review covers the theoretical foundations of DDD within the framework of incompatible elasticity, its numerical implementation via the nodal method, the extension of the method to finite domains and several implementation details. Applications of the method to current topics in micro-plasticity are presented, including the size effects in nano-indentation, the evolution of the dislocation microstructure in persistent slip bands, and the phenomenon of dislocation avalanches in micro-pillar compression.

Graphene: The Thinnest Known Coating for Corrosion Protection

Abstract: Graphene, a two-dimensional (2-D) atomically thin film of carbon atoms, has unique characteristics that have triggered unprecedented research excitement. Besides several other unique properties, graphene possesses remarkable chemical inertness. Graphene coatings either a single or a few atomic layers thick on metals has been shown to improve their corrosion resistance by up to one-and-half orders of magnitude. Although the studies on this topic are limited to just a few carried out in 2011–2013, great variability is found in the extent of corrosion resistance from graphene reported in these studies. This article presents a brief review of graphene as a corrosion-protecting coating, identifies potential and presents some new data to suggest graphene as durable corrosion-resistant coating.

Bulk Nanocrystalline Metals: Review of the Current State of the Art and Future Opportunities for Copper and Copper Alloys

Abstract: It is a new beginning for innovative fundamental and applied science in nanocrystalline materials. Many of the processing and consolidation challenges that have haunted nanocrystalline materials are now more fully understood, opening the doors for bulk nanocrystalline materials and parts to be produced. While challenges remain, recent advances in experimental, computational, and theoretical capability have allowed for bulk specimens that have heretofore been pursued only on a limited basis. This article discusses the methodology for synthesis and consolidation of bulk nanocrystalline materials using mechanical alloying, the alloy development and synthesis process for stabilizing these materials at elevated temperatures, and the physical and mechanical properties of nanocrystalline materials with a focus throughout on nanocrystalline copper and a nanocrystalline Cu-Ta system, consolidated via equal channel angular extrusion, with properties rivaling that of nanocrystalline pure Ta. Moreover, modeling and simulation approaches as well as experimental results for grain growth, grain boundary processes, and deformation mechanisms in nanocrystalline copper are briefly reviewed and discussed. Integrating experiments and computational materials science for synthesizing bulk nanocrystalline materials can bring about the next generation of ultrahigh strength materials for defense and energy applications.

Low-Density Steels: Complex Metallurgy for Automotive Applications

Abstract: The current aim in the development of third-generation steels for lightweighting automotive applications is to increase strength keeping at least the same formability as current steel concepts. In this philosophy, an optimal concept would be one that brings, in addition, a lower density. For this purpose, low-density steels have been designed with important aluminum additions obtaining density reductions of 8–10% or higher in comparison with low-carbon steels. At the levels required for lightweighting, aluminum introduces complex phenomena in steels. Here, some of the effects of aluminum in phase stability, CALPHAD-type modeling, and microstructure development are described, the latter in relation with mechanical properties. Finally, the potential of two families of lightweight steels for automotive applications is assessed by comparison with a steel currently present in automotive structures.

Temperature Effects on Deformation and Serration Behavior of High-Entropy Alloys (HEAs)

Abstract: Many materials are known to deform under shear in an intermittent way with slip avalanches detected as acoustic emission and serrations in the stress–strain curves Similar serrations have recently been observed in a new class of materials, called high-entropy alloys (HEAs) Here, we discuss the serration behaviors of several HEAs from cryogenic to elevated temperatures The experimental results of slow compression and tension tests are compared with the predictions of a slip-avalanche model for the deformation of a broad range of solids The results shed light on the deformation processes in HEAs Temperature effects on the distributions of stress drops and the decrease of the cutoff (ie, of the largest observed slip size) for increasing temperature qualitatively agree with the model predictions The model is used to quantify the serration characteristics of HEAs, and pertinent implications are discussed

Self-Healing Metals and Metal Matrix Composites

Abstract: Self-healing in inorganic materials is a relatively new area in materials science and engineering that draws inspiration from biological systems that can self-repair damage. This article reviews the preliminary attempts to impart self-healing behavior to metals. Several challenges yet exist in the development of metallic alloys that can self-repair damage, including surface bonding issues, such as liquid/solid contact angle (wetting) and oxidation, and practical issues, such as capillary pressure for delivery of a liquid metal to a damaged area or crack, and the overall mechanical properties of a composite system. Although the applied research approaches reviewed have obtained marginal success, the development of self-healing metallic systems has the potential to benefit a wide range of industrial applications and thus deserves greater investment in fundamental research.

Grain Boundary Segregation of Interstitial and Substitutional Impurity Atoms in Alpha-Iron

Abstract: The macroscopic behavior of polycrystalline materials is influenced by the local variation of properties caused by the presence of impurities and defects. The effect of these impurities at the atomic scale can either embrittle or strengthen grain boundaries (GBs) within. Thus, it is imperative to understand the energetics associated with segregation to design materials with desirable properties. In this study, molecular statics simulations were employed to analyze the energetics associated with the segregation of various elements (helium, hydrogen, carbon, phosphorous, and vanadium) to four 〈100〉 (Σ5 and Σ13 GBs) and six 〈110〉 (Σ3, Σ9, and Σ11 GBs) symmetric tilt grain boundaries in α-Fe. This knowledge is important for designing stable interfaces in harsh environments. Simulation results show that the local atomic arrangements within the GB region and the resulting structural units have a significant influence on the magnitude of binding energies of the impurity (interstitial and substitutional) atoms. These data also suggest that the site-to-site variation of energies within a boundary is substantial. Comparing the binding energies of all 10 boundaries shows that the Σ3(112) boundary possesses a much smaller binding energy for all interstitial and substitutional impurity atoms among the boundaries examined in this study. Additionally, based on the Rice–Wang model, our total energy calculations show that V has a significant beneficial effect on the Fe grain boundary cohesion, while P has a detrimental effect on grain boundary cohesion, much weaker than H and He. This is significant for applications where extreme environmental damage generates lattice defects and grain boundaries act as sinks for both interstitial and substitutional impurity atoms. This methodology provides us with a tool to effectively identify the local as well as the global segregation behavior that can influence the GB cohesion.

Modification Mechanism and Microstructural Characteristics of Eutectic Si in Casting Al-Si Alloys: A Review on Experimental and Numerical Studies

Abstract: After more than 80 years of practical experience and despite many noted research efforts, theories that rigorously explained the formation of the silicon eutectic phases and the modification of the morphology of those phases by specific chemical additives remained elusive. Almost all papers related to the growth and modification of silicon in casting Al-Si alloys refer to the importance of twinning and a mechanism called a twin-plane reentrant edge. However, a review paper containing detailed information on how the parallel twins are formed in a crystal during melt growth, why the twins are generated parallel to each other, what is the prerequisite for growing a facetted dendrite, and how effective are various rare earth elements is missing in the literature. A comprehensive review is conducted on the models proposed for the flaky silicon growth including twin-plane reentrant edge and the models proposed for eutectic modification: impurity induced twinning and restricted growth theory. Furthermore, the papers with focus on modifying efficiency of the rare-earth metals have been reviewed.

A New Polycrystalline Co-Ni Superalloy

Abstract: In 2006, a new-ordered L12 phase, Co3(Al,W), was discovered that can form coherently in a face-centered cubic (fcc) A1 Co matrix. Since then, a community has developed that is attempting to take these alloys forward into practical applications in gas turbines. A new candidate polycrystalline Co-Ni γ/γ′ superalloy, V208C, is presented that has the nominal composition 36Co-35Ni-15Cr-10Al-3W-1Ta (at.%). The alloy was produced by conventional powder metallurgy superalloy methods. After forging, a γ′ fraction of ~56% and a secondary γ′ size of 88 nm were obtained, with a grain size of 2.5 μm. The solvus temperature was 1000°C. The density was found to be 8.52 g cm−3, which is similar to existing Ni alloys with this level of γ′. The alloy showed the flow stress anomaly and a yield strength of 920 MPa at room temperature and 820 MPa at 800°C, similar to that of Mar-M247. These values are significantly higher than those found for either conventional solution and carbide-strengthened Co alloys or the γ/γ′ Co superalloys presented in the literature thus far. The oxidation resistance, with a mass gain of 0.08 mg cm−2 in 100 h at 800°C, is also comparable with that of existing high-temperature Ni superalloys. These results suggest that Co-based and Co-Ni superalloys may hold some promise for the future in gas turbine applications.

Trends in Grain Boundary Mobility: Survey of Motion Mechanisms

Abstract: Grain boundary (GB) motion in polycrystalline materials is expected and observed to be dominated by thermally activated processes. This has important implications for properties influenced by the presence of GBs. Here, the GB motions of a catalog of 388 simulated nickel boundaries reveal a rich set of behaviors, which demonstrate that the temperature dependencies of GB mobility are far more complex than originally believed. In the set of 388 boundaries, four different general classes were observed with the following percentages: (I) ~57% exhibited traditional thermally activated mobility; (II) ~20% exhibited non-thermally activated mobility, where mobility was either independent of temperature or mobility decreased with increasing temperature (i.e., not thermally activated); (III) ~14% exhibited mixed modes of mobility, where different trends were exhibited over different temperature regimes (e.g., thermally activated at low temperature and non-thermally activated at high temperature); and (IV) ~9% exhibited unclassifiable mobility trends or were immobile over the studied temperature range. Thus, although the studied set of boundaries is not statistically representative of all GBs, it indicates that we must expand our preconceived notions to include new and interesting phenomena.

Overview of Lightweight Ferrous Materials: Strategies and Promises

Abstract: Reducing the density of steels is a novel approach for weight reduction of automobiles to improve fuel efficiency. In this overview article, strategies for the development of lightweight steels are presented with a focus on bulk ferrous alloys. The metallurgical principles of these steels and their mechanical properties of relevance to automotive applications are discussed. Some of the engineering aspects highlighting the possible problems related to mass production of these steels are also considered. Application prospects of these steels vis-a-vis standard automotive steels are shown.

Development of ODS FeCrAl for Compatibility in Fusion and Fission Energy Applications

Abstract: Oxide dispersion strengthened (ODS) FeCrAl alloys with 12–15% Cr are being evaluated for improved compatibility with Pb-Li for a fusion energy application and with high temperature steam for a more accident-tolerant light water reactor fuel cladding application. A 12% Cr content alloy showed low mass losses in static Pb-Li at 700°C, where a LiAlO2 surface oxide formed and inhibited dissolution into the liquid metal. All the evaluated compositions formed a protective scale in steam at 1200°C, which is not possible with ODS FeCr alloys. However, most of the compositions were not protective at 1400°C, which is a general and somewhat surprising problem with ODS FeCrAl alloys that is still being studied. More work is needed to optimize the alloy composition, microstructure and oxide dispersion, but initial promising tensile and creep results have been obtained with mixed oxide additions, i.e. Y2O3 with ZrO2, HfO2 or TiO2.

Design of Novel Bainitic Steels: Moving from UltraFine to Nanoscale Structures

Abstract: The concepts of phase transformation theory can be exploited to design nanostructured steels that transform to bainite at temperatures as low as 150°C. The microstructure obtained is so refined that it is possible to achieve strength in excess of 2.5 GPa in a material that has considerable toughness (40 MPam1/2). Such a combination of properties has never been achieved before with bainite. A description of the characteristics and significance of this remarkable microstructure in the context of the mechanism of transformation is provided.

The Phase Competition and Stability of High-Entropy Alloys

Abstract: Phase competition and stability of several typical high-entropy alloys (HEAs) were studied, and the effects of alloying additions and processing conditions on phase formation in these alloys were discussed. Alloying with chemically incompatible elements having a large difference in either the atomic size or enthalpy of mixing with constituting components in HEAs, e.g., Cu and Al in the FeCoNiCr alloy system, inevitably induced phase separation and stimulated formation of duplex solid-solution phases and even intermetallic compounds. The solid-solution phase in the as-cast FeCoNiCrMn HEA is extremely stable due to the good chemical compatibility among constituent components, but in the FeCoNiCrAl and (FeCoNiCrAl)99Si1 HEAs with the incompatible elements Al and Si, pretreatment and annealing processes could induce phase transitions and the formation of new phases, indicating that the as-cast solid-solution phases were destabilized by quenched-in chemical segregation, resulting from additions of the dissimilar elements.

Review of Innovative Energy Savings Technology for the Electric Arc Furnace

Abstract: A review of the energy innovations for the electric arc furnace (EAF) steelmaking route is discussed. Preheating of scrap using vertical and horizontal shafts that have been commercially successful in lowering the energy consumption to as much as 90 kWh/t reaching almost the operational limit to heating input scrap materials into the EAF is discussed. Bucket-type and twin-shell preheaters have also shown to be effective in lowering the overall power consumption by 60 kWh/t, but these have been less effective than the vertical shaft-type preheaters. Beyond the scrap preheating technologies, the utilization of waste heat of the slags from the laboratory scale to the pilot scale has shown possible implementation of a granulation and subsequent heat exchange with forced air for energy recovery from the hot slags. Novel techniques to increase metal recovery have shown that laboratory-scale testing of localized Fe concentration into the primary spinel crystals was possible allowing the separation of an Fe-rich crystal from an Fe-depleted amorphous phase. A possible future process for converting the thermal energy of the CO/CO2 off-gases from the EAF into chemical energy was introduced.

Improved High-Temperature Microstructural Stability and Creep Property of Novel Co-Base Single-Crystal Alloys Containing Ta and Ti

Abstract: The influence of Ta and Ti additions on microstructural stability and creep behavior in novel Co-Al-W base single-crystal alloys has been investigated. Compared to the ternary alloy, the γ′ solvus temperature and γ′ volume fraction were raised by individual additions of Ta and Ti, and increased further in the quinary alloy containing both alloying additions. In contrast to ternary and quaternary alloys, an improved microstructural stability with the stable γ–γ′ two-phase microstructure and more than 60% γ′ volume fraction existed in the quinary alloy after prolonged aging treatment at 1050°C for 1000 h. The creep behavior at 900°C revealed lower creep rates and longer rupture lives in the quaternary alloys compared to the ternary alloy, whereas the quinary alloy exhibited even better creep resistance. When the creep temperature was elevated to about 1000°C, the creep resistance of the quinary alloy exceeded the previously reported Co-Al-W-base alloys and first-generation Ni-base single-crystal superalloys. The improved creep resistance at approximately 1000°C was considered to be associated with high γ′ volume fraction, γ′ directional coarsening, and dislocation substructure, which included γ–γ′ interfacial dislocation networks and the sheared γ′ precipitates containing stacking faults and anti-phase boundaries.

Progress Toward an Integration of Process–Structure–Property–Performance Models for “Three-Dimensional (3-D) Printing” of Titanium Alloys

Abstract: Electron beam direct manufacturing, synonymously known as electron beam additive manufacturing, along with other additive “3-D printing” manufacturing processes, are receiving widespread attention as a means of producing net-shape (or near-net-shape) components, owing to potential manufacturing benefits. Yet, materials scientists know that differences in manufacturing processes often significantly influence the microstructure of even widely accepted materials and, thus, impact the properties and performance of a material in service. It is important to accelerate the understanding of the processing–structure–property relationship of materials being produced via these novel approaches in a framework that considers the performance in a statistically rigorous way. This article describes the development of a process model, the assessment of key microstructural features to be incorporated into a microstructure simulation model, a novel approach to extract a constitutive equation to predict tensile properties in Ti-6Al-4V (Ti-64), and a probabilistic approach to measure the fidelity of the property model against real data. This integrated approach will provide designers a tool to vary process parameters and understand the influence on performance, enabling design and optimization for these highly visible manufacturing approaches.