Coarsening Rate Research Articles

Effect of long-term aging on microstructure stability of a novel nickel base powder superalloy

The microstructure stability, evolution law and identification of carbide and harmful phase TCP of nickel-base powder superalloy aged at 800 ℃ for 100 h —10000 h were studied by means of field emission scanning electron microscope, atomic force microscope and electron backscatter diffraction. The results show that with the increase of aging time, the grain size changed little, the grain boundary coarsened and the twin boundary Σ3 increased first and then decreased. γ' phase coarsened obviously from 500 h aging, and the coarsening rate increased first and then decreased The coarsening mechanism is mainly the combination of adjacent γ' phase by AFM analysis. Lath-like σ phase CrCo precipitated at the grain boundary first after aging for 1000 h. When the aging time was up to 2000 h, the needle-like μ phase Co7(W, Mo)6 was observed in the crystal. With the further increase of aging time, the total content of TCP phase at grain boundary and in crystal increased obviously. However, at 10000 h, the TCP phase in the crystal and at the grain boundary almost disappeared, mainly because the atoms in the TCP phase diffused into the γ' phase to form a new solid solution and deposited on the grain boundary, which further coarsened the grain boundary.

Effect of Re and Ru additions on morphology and long-term stability of gamma prime particles in new modified superalloys prepared by a vacuum arc melting process

Abstract This research investigated the effects of adding rhenium and ruthenium with various contents to new modified nickel-based superalloys, which were prepared and cast using a vacuum arc melting process. Heat treatment processes were then carried out. The size, circularity and the area fraction of the gamma prime phase were characterized via scanning electron microscopy after solution treatment, precipitation aging and long-term heating processes. The addition of ruthenium seems to provide a stronger effect than adding rhenium by increasing the area fraction of the gamma prime phase after precipitation aging. Furthermore, rhenium and/or ruthenium additions maintain the precipitation of the gamma prime particles in a cubic shape. For phase stability analyses, the alloys were subsequently heated at 1173 K in intervals up to 1080 ks. The study showed that the coarsening rate of γ′ particles can be significantly retarded with the addition of rhenium and/or ruthenium, and topologically close-packed phase can occur with excessive rhenium addition. In addition, the highest addition of ruthenium content in this study strongly exhibited high gamma prime phase stability. Thus, increasing the content of ruthenium and decreasing the content of rhenium can inhibit the formation of the topologically close-packed phase.

The Effect of Solution Treatment on the Si Particles’ Morphology Evolution and the Thermal Conductivity and Tensile Properties of Sb-Modified Al-8Si-0.6Mg Alloys

The effects of solution treatment time on the morphology evolution of Si particles and the thermal conductivity and tensile properties of Sb-modified alloys were studied. The results show that the evolution of Si particles follows four mechanisms: spheroidization, necking and splitting of particles with large aspect ratios, fusion of spherical particles, and coarsening controlled by diffusion. The first three mechanisms mainly occur at the early stage of solution treatment. The addition of Sb does not change the evolution law of the Si particles, but it does change the contribution of various evolution mechanisms, including promoting spheroidization, fusion, and coalescence, as well as significantly reducing the coarsening rate, which makes the thermal modification of Sb-modified alloys more effective. The increase in thermal conductivity during solution treatment is related to the decrease of the anharmonicity of lattice vibration, lattice wave scattering, and electron scattering of Si particles. The 0.4 wt. % Sb-modified alloy exhibits excellent tensile strength and elongation under as-cast T4- and T6-heat-treated conditions, because the modification significantly reduces the stress concentration of the Si particles and delays the germination and propagation of microcracks.

Microfluidic Investigation of Foam Coarsening Dynamics in Porous Media at High-Pressure and High-Temperature Conditions

Coarsening or Oswald ripening, induced by interbubble gas diffusion, is considered to dominate foam structure evolution in porous media. We present the first study of trapped foam coarsening dynamics under realistic deep reservoir conditions (up to 3200 psi/22 MPa of pore pressure and 100 °C of temperature) in a high-pressure and high-temperature microfluidic system. The findings are expected to help predict foam structure evolution in applications such as enhanced oil recovery and CO2 geological sequestration. It is shown that, in porous media, larger bubbles grow at the expense of smaller bubbles. The growth rate of the average bubble area (⟨a⟩) over time shows a long-term linear increase when ⟨a⟩ is between 1/5 and 1/2 of the average pore size. The foam coarsening kinetics are determined by the liquid film permeability, gas-liquid interfacial tension, and the molar volume of the dispersed phase. In summary, foams prepared with less water-soluble gases (e.g., N2 and air) and lower foam quality show slower coarsening kinetics due to a lower film permeability. Foam coarsening is more sensitive to surfactant concentration (than surfactant type), as it determines the interfacial tension that controls the mass transfer driving force (capillary pressure difference). The transport properties of the dispersed phase depend strongly on its density, which increases with increasing pore pressure and decreasing temperature. At the same experimental conditions, gas CO2 foam shows a 10-fold faster coarsening rate than N2 foam. However, dense (i.e., liquid and supercritical) CO2 foams show a remarkable 20-500-fold reduction in coarsening kinetics compared with gas N2 and CO2 foams due to the significantly reduced mass transfer driving forces. In a sense, trapped CO2 foam can be stronger than N2 foam at high-pressure and high-temperature conditions.

Microstructure of die-cast alloys Mg–Zn–Al(–Ca): a study by electron microscopy and small-angle neutron scattering

AbstractThe microstructure of a magnesium die-cast alloy with 8 wt.% Zn and 5 wt.% Al (ZA85) and of two alloy modifications with 0.3 and 0.9 wt.% Ca (ZACa8503 and ZACa8509) are compared. Optical, scanning electron and transmission electron microscopy (TEM) were used to study the microstructure in as-cast and annealed conditions. Small-angle neutron scattering (SANS) measurements were carried out to characterize the early stages of the precipitation process. The die-cast microstructure of ZA85, ZACa8503 and ZACa8509 consists of dendritic α-Mg grains and coarse intermetallic grain-boundary phases. In the Ca-free ZA85 alloy, grain boundaries are decorated with a quasi-crystalline phase with icosahedral point-group symmetry (I phase), a metastable modification of the equilibriumτphase (Mg32(Al, Zn)49). At the grain boundaries of the Ca-containing ZA85 alloys, small amounts of the stable cubicτphase Mg32(Al, Zn, Ca)49as well as Al2(Ca, Zn) phase, were detected in addition to the I phase. In the as-cast condition, regions supersaturated with Al and Zn were found to surround the α-Mg grains and to becomeτphase precipitation sites during high-temperature annealing in all alloys investigated. TEM and SANS measurements revealed no influence of Ca on the size distribution and the coarsening rate of the precipitates, which are formed by a continuous precipitation process.

Microstructural/mechanical characterizations of electron beam welded IN738LC joint after post-weld heat treatment

The effect of post-weld heat treatment on microstructure and mechanical properties of electron beam welded IN738LC joint has been investigated. The fusion zone consists of γ-Ni matrix, bimodal-size γʹ particles, granular MC carbides (Ti-rich) and Cr-rich M23C6 carbide chain in IN738LC joint subjected to post-weld heat treatment. The secondary γʹ particle in fusion zone precipitates and grows ceaselessly with the increased aging time. The coarsening of γʹ particle follows the Lifshiitz-Slyozov-Wagner coarsening theory with a constant coarsening rate of 1.003 × 10−27m3/s. The secondary γʹ particle in base metal shows a similar size with that of fusion zone, but a lower volume fraction (52%) than that of fusion zone (54%). M23C6 carbide is easier to precipitate along high angle grain boundaries. As aging time increases, the morphology of M23C6 carbide changes from discrete granular shape to continuous chain. The thickness of M23C6 carbide also increases dramatically. Owing to smaller grain size and higher solute concentration in fusion zone, more M23C6 carbides form in FZ than that of base metal. With the increased aging time, the tensile strength of IN738LC joint at 900°C decreases gradually, while the elongation of joint firstly increases and then decreases, which is attributed to the increased channel width of γ-Ni matrix. Due to the formation of continuous M23C6 carbide chain, the rupture life of IN738LC joint firstly increases and then decreases as aging time increases. Combined with the mechanical properties and size stability of γʹ precipitates in IN738LC joint, 1125°C/2 h + 850°C/24 h is an optimized post-weld heat treatment regime.

Design of cuboidal<i> γ</i>/<i>γ′</i> coherent microstructure and its stability in low-density Co-Ni-Al-Mo-Cr-Ti/ Nb/Ta superalloys

Co-base superalloys generally have high strengths, good oxidation- and corrosion-resistances, as well as excellent creep-resistant properties at high temperatures (HTs), which are ascribed to the coherent precipitation of cuboidal<i> γ′</i> phase into face-centered-cubic (FCC) <i>γ</i> matrix induced by co-alloying of multiple elements. However, the cuboidal <i>γ</i>/<i>γ′</i> coherent microstructure is liable to be destabilized after a long-time aging at HTs in Co-base superalloys. In the present work, the cluster formula is used to design a series of low-density Co-base superalloys with the composition of [Al-(Co<sub>8</sub>Ni<sub>4</sub>)]((Al<sub>0.5</sub>(Ti/Nb/Ta)<sub>0.5</sub>Mo<sub>0.5</sub>)(Mo<sub>0.5</sub>Cr<sub>0.5</sub>Co<sub>0.5</sub>)) (=Co<sub>8.5</sub>Ni<sub>4</sub>Al<sub>1.5</sub>Mo<sub>1.0</sub>Cr<sub>0.5</sub>(Ti/Nb/Ta)<sub>0.5</sub>). Alloy ingots are prepared by arc melting under an argon atmosphere, and are solid-solutionized at 1300 ℃ for 15 h and then aged at 900 ℃ for up to 500 h. Microstructural characterizations and mechanical properties of these alloys in different aged states are obtained by using XRD, SEM, EPMA, TEM, and HV. It is found that all these alloys with Ti/Nb/Ta, Ti/Nb, and Ti/Ta in an equi-molar mixing have a special coherent microstructure with cuboidal <i>γ′</i> phase uniformly-precipitated into the γ matrix, which is contributed to the moderate lattice misfit of <i>γ</i>/<i>γ′</i> (0.27%–0.34%). Moreover, these cuboidal<i> γ′</i> phase are coarsened slowly during aging, in which the microhardness does not vary obviously with aging time (275 HV–296 HV). Especially, the alloy with (Ti/Ta)<sub>0.5</sub> exhibits the highest <i>γ</i>/<i>γ′</i> microstructural stability with a slow coarsening rate after aging 500 h, and no other second phases appear near the grain boundaries. While needle and bulk particles would precipitate on grain boundaries in other alloys after 500 h-aging.

Review of foam stability in porous media: The effect of coarsening

Foam stability in porous media is crucial to industrial applications such as enhanced oil recovery (EOR) and CO 2 geological sequestration. Though many studies of foam stability in porous media have been carried out, the results from these studies often interpreted the combined effects of drainage, coalescence, and coarsening. This convoluted interpretation does not add to the understanding and contribution of the isolated foam instability mechanisms. Coarsening or Ostwald ripening dominates the evolution of foam structure in porous media due to the scale effect. To bridge the knowledge gap, a comprehensive review of the mass transfer fundamentals and the experimental methods for observing the foam coarsening process in porous media was conducted. Unlike an open system where bubbles can grow freely, the restriction from the geometric confinement in the rock (or rock-like) matrix decreases the foam coarsening rate. The confinement can even reverse the mass transport direction to go from larger bubbles to smaller bubbles (anti-coarsening). Various foam coarsening models based on Fick's diffusion law from the bubble-scale to the bulk-scale are reviewed to better understand the foam coarsening mechanisms. From the reviewed literature, foam coarsening dynamics are dominated by the average bubble size, the liquid film thickness between bubbles, and the gas-liquid interfacial properties. The proper selection of the foaming agents (surfactants and nanoparticles) and the gas phase (N 2 and CO 2 ) is critical to control the foam coarsening kinetics. The review of the experimental methods shows that both optical and Atomic Force Microscopy (AFM) are commonly used in studying the foam coarsening process at the bubble scale. X-ray microtomography (μCT) provides a robust tool to investigate the temporal evolution of trapped gases at the pore scale in real rocks. Microfluidics allow seeing the foam kinetics in 2D and 2.5D feature representations of the porous media. The fabrication of 3D micromodels with controlled wettability remains a challenge. Additionally, the challenge of capturing the dynamics of the foam coarsening process with moving bubbles remains understudied and ill-resolved. • Compared with drainage and coalescence, coarsening is the most significant process impacting foam stability in porous media. • Fick's diffusion law-based coarsening models are introduced from bubble-scale to bulk-scale. • The selection of the foaming agents and the gas phase (N 2 and CO 2 , etc.) is critical to the coarsening kinetics of foams. • A micromodel is a robust visualization tool of the foam structure evolutionin porous media.

Pathway-Dependent Grain Coarsening of Block Copolymer Patterns under Controlled Solvent Evaporation.

Solvent evaporation annealing (SEA) is a straightforward, single-step casting and annealing method of block copolymers (BCP) processing yielding large-grained morphologies in a very short time. Here, we present a quantitative analysis of BCP grain-coarsening in thin films under controlled evaporation of the solvent. Our study is aimed at understanding time and BCP concentration influence on the rate of the lateral growth of BCP grains. By systematically investigating the coarsening kinetics at various BCP concentrations, we observed a steeply decreasing exponential dependence of the kinetics power-law time exponent on polymer concentration. We used this dependence to formulate a mathematical model of BCP ordering under nonstationary conditions and a 2D, time- and concentration-dependent coarsening rate diagram, which can be used as an aid in engineering the BCP processing pathway in SEA and also in other directed self-assembly methods that utilize BCP–solvent interactions such as solvent vapor annealing.

Microstructural stability of low-cost Ni-base superalloys with a high volume fraction of cuboidal γ′ nanoprecipitates

The present work designed a series of low-cost Ni-base superalloys with a high volume fraction (f > 70%) of γ′ precipitates via the composition formula of [Al-(Ni,Co)12](Al1(Ti,Nb,Ta)0.5(Cr,Mo,W)1.5) and then investigated the microstructural evolution of coherent γ/γ′ in these alloys during aging at 1173 K. It is found that the basic Co-free alloy with the addition of (Ti,Nb,Ta)0.5 and (Cr,Mo,W)1.5 in an equi-molar mixing has a special coherent microstructure of cuboidal γ′ nanoparticles precipitated into the γ matrix, showing a higher microstructural stability with a slow coarsening rate of γ′ precipitates after 500 h-aging. The addition of Co or the increase of Cr content in the basic alloy can affect the thermal stability of γ/γ′ coherent microstructure, which is closely related to the lattice misfit of γ/γ′ and a moderate lattice misfit (ε = 0.44–0.53%) would contribute to the high stability of cuboidal γ/γ′ microstructure. Also, the alloy with (Cr1.0Mo0.25W0.25) and (Ni8Co4) shows a comparable microstructural stability to the basic alloy. These alloys exhibit prominent mechanical properties, as evidenced by the fact that the yield strength of the basic alloy is 892 MPa at room temperature and 835 MPa at 1123 K, respectively. Moreover, the microhardness keeps almost constant with the aging time, indicating the better microstructural stability.

Stabilization of CO2-foams in brine by reducing drainage and coarsening using alkyldimethylamine oxides as surfactants

Producing CO2-foams in saline environments can be a great challenge in many industrial applications, since fast gas diffusion and reduction of electrical double layer promote fast coarsening and coalescence of bubbles. Amine oxides are zwitterionic surfactants that have been shown to improve foam stability by intermolecular hydrogen bonding, but have not been investigated for CO2-foams yet. This paper reports a study on the use of commercial alkyldimethylamine oxides (CxDAO) as foaming agents in brine for CO2-foams, based on the interactions present at the low pH imposed by the dissolution of the CO2 and the presence of salts. Despite the presence of salt and CO2 increased the protonation degree of amine oxide molecules, favoring hydrogen bonding, a significant improvement in foam stability was only observed when using the long chain surfactant (C14DAO). This behavior was attributed to the synergy between increased attractive hydrophobic interactions of alkyl chains and reduced repulsion among ionic surfactant heads, which led to formation of large surfactant aggregates in bulk, as demonstrated by the significant increase in viscosity of the aqueous phase. There was also a decrease in foam drainage and coarsening rates for the foams using C14DAO in brine, compared to results in deionized water, suggesting that formation of hydrogen bonds is not enough to arrest the destabilization in CO2-foams. Moreover, the linear rate of bubble growth obtained for these foams (∝t) was higher than that reported for N2– or air-foams (∝t1/2), likely due to the high solubility of CO2 in the aqueous phase, which accelerated the coarsening phenomena. The results of this work offer insight on the future design of foaming formulations for stabilizing CO2-foams in applications that require the use of high salinity brines, such as carbon sequestration and oil recovery.

Role of Ti in the microstructure evolutions and mechanical properties of cast Al-2.5Li-1.5Cu-1Zn-0.5Mg(−0.2Zr) alloys

Transition elements are important alloying elements in aluminum alloys, whilst there was a blank of the role of Ti in cast Al-Li-Cu alloys. In this study, the impact of 0.1 and 0.2 wt% Ti additions on Al-2.5Li-1.5Cu-1Zn-0.5Mg(−0.2Zr) alloys has been investigated by microstructure characterizations and tensile tests. The optical microscopy results showed that the grains were significantly refined as the transition elements presented. There was little poisoning phenomenon in the alloys containing both Zr and Ti for the limited holding time during the smelting process. The age-hardening and mechanical properties were prominently enhanced with the addition of Ti and Zr after ageing at 150 ºC. The results of transmission electron microscopy showed that dispersed and dense θ՛ (Al2Cu) precipitates distributed in the alloy containing 0.2 wt% Ti, while T1 (Al2CuLi) phases dominated in the alloy containing 0.2 wt% Ti and 0.2 wt% Zr, accompanied by a moderate number of S՛ (Al2CuMg). The coarsening rates of δ՛(Al3Li) and δ՛-precipitation free zones (PFZs) decreased gradually with the content of transition elements increasing, but the case in the 0Zr2Ti alloy was abnormal for the interaction between θ՛ and δ՛. The best comprehensive mechanical properties of YS = 374 MPa, UTS= 527 MPa and EL = 7.1% could be obtained by 2Zr2Ti alloy after ageing at 150 °C for 64 h.

Effects of alloying additions on the microstructure, lattice misfit, and solvus temperature of a novel Co–Ni-based superalloy

In this study, the effects of Ti, Nb, Mo and W addition on the microstructure, lattice misfit, and solvus temperature of a novel Co–Ni–Al–V–Ta-based superalloy are presented. Only a γ/γ′ two-phase microstructure was detected in the studied alloys after equilibrating heat treatment. Ti, Nb, Mo and W are confirmed as γ′-phase-forming elements, the addition of which increases the solvus temperature. In addition, the addition of strong γ′ formers like Ti and Nb, which have high partition coefficients, significantly increases lattice misfit, whereas the lattice misfit in Mo- and W-containing alloys is at the same level as that of the base alloy. The γ′ phase coarsening behaviour of the studied alloys follows LSW theory, and all alloying elements increase the coarsening rate. The activation energy of the 2Ti alloy is 355.9 ± 12.5 kJmol-1 at 900–1000 °C, which is higher than the activation energy of most solutes in the γ-Co matrix. In particular, the 2Mo and 2W alloys have lower densities than the Co–Ni–Al–Mo-based and Co–Ni–Al–W-based alloys, respectively, and the γ′ phase solvus temperature of the 2Mo alloy is higher than that of the Co–Ni–Al–Mo-based alloys. Moreover, the specific yield strengths of 2Ti and 2Nb alloys at 700 °C are higher than those of Co–Al–W-based and traditional Co-based (Mar-M-302) superalloys.

Experimental investigation of the drainage characteristic and stability mechanism of gel-stabilized foam used to extinguish coal fire

For further clarifying the stability characteristic of gel-stabilized foam (GSF), the foam half-life, drainage process, microcosmic liquid film structure, and stability mechanism were investigated in this paper. In addition, the thermostability and fire-resistance characteristics of GSF were also tested by exposing foam to high-temperature coal fire. The results indicated that the GSF exhibited the special three-stage drainage process of GSF, different from the two-stage theory of traditional two-phase foam (TTF). The enhanced foam stability was mainly ascribed to the formation of gel structure in GSF system by chemical crosslinking reaction, which provided larger resistance to the liquid drainage in bubble film dominated by gravity, finally resulting in zero drainage before 60 h. Moreover, SEM-EDS analysis revealed that the gelified film of gel-stabilized foam contained O, C, and Na elements (more than 95 wt%), mainly deriving from polymer and surfactant molecules in the raw materials of foam. Microstructure and theoretical analysis revealed that gelified film in GSF could delay the coarsening rate of bubbles, slow down its drainage rate, as well as reduce the coalescence and rupture rate of foam. Due to the enhanced foam stability after forming gel in liquid film, the GSF showed the superior fire-resistance characteristic and coverage effect on the heat source, and the foam could remain stable within 30 min even heating by coal-burning furnace. The gel-stabilized foam demonstrated the good application prospect for covering the burning coal to isolate oxygen and reduce temperature to extinguish coal fire.

Scaling law and universal drop size distribution of coarsening in conversion-limited phase separation

Phase separation is not only ubiquitous in diverse physical systems, but also plays an important organizational role inside biological cells. However, experimental studies of intracellular condensates (drops with condensed concentrations of specific collections of proteins and nucleic acids) have challenged the standard coarsening theories of phase separation. Specifically, the coarsening rates observed are unexpectedly slow for many intracellular condensates. Recently, Folkmann, et al. [Science {\bf 373}, 1218 (2021)] argued that the slow coarsening rate can be caused by the slow conversion of a condensate constituent between the state in the dilute phase and the condensate state. A consequence of this conversion-limited picture is that standard theories of coarsening in phase separation (Lifshitz-Slyozov--Wagner Ostwald ripening and drop coalescence schemes) no longer apply. Surprisingly, I show here that the model equations of conversion-limited phase separation can instead be mapped onto a grain growth model in a single-phase material in three dimensions. I further elucidate the universal coarsening behavior in the late stage using analytical and numerical methods.

Phase-field simulation of diffusion-controlled coarsening kinetics of γ’ phase in Ni–Al alloy

Abstract Morphology evolution and coarsening kinetics of γ’ (Ni3Al) precipitates in Ni–Al alloy were investigated quantitatively using the interface diffusion-controlled, the bulk diffusion-controlled, and the interface diffusion-controlled elastic strain models. By calculating the γ’ phase number density, volume fraction, and particle radius, the effects of diffusion and elastic strain on the γ’ phase growth and coarsening were clarified. The coarsening rate constants are smaller in the bulk diffusion-controlled and the interface diffusion-controlled elastic strain models than those of the interface diffusion-controlled model, which is due to the long-range diffusion of Al atoms and the retardant effect of coherent strain between the γ’ and γ phases, respectively. The linear fitting on the cubic exponent shows the comparable associate coefficient that gives no superiority to the different models in Ni–Al alloy.

Enhanced thermal-stability of AlCuMg alloy by multiple microalloying segregation of Mg/Si/Sc solute

The multiple microalloying influences of Si & Sc on the aging behavior and thermal stability of Al-4Cu-0.5Mg alloy were systematically studied. The result revealed that precipitation evolution of Al-4.0Cu-0.5Mg alloy was closely relevant to the content of Si. Si enhanced formation of the Q′ phases serving as nucleation sites for the θ′-Al2Cu phases, resulting in a refined distribution of θ′ phases. Al-4.0Cu-0.5Mg-0.40Si-0.1Sc alloy is a combination of high strength and outstanding heat resistance because of the finer and denser Q' & θ′ as well as the segregational structure of Mg, Si and Sc. Especially, the Sc-segregation at the interfaces of θ'/Al is essential to stabilize the microstructure of θ′ precipitates at high temperatures. The solute segregation of Sc atoms in θ′ phases inhibits their coarsening rate. The high-temperature strength of Al-Cu-Mg-Si-Sc alloy are increased dramatically compared with that of typical heat resistant 2618 aluminum alloy.

A comparison of theory and simulation with microgravity experiments on phase coarsening

A quantitative understanding of microstructure evolution during phase coarsening is crucial to the optimization of processing, final structure, and properties of materials at high homologous temperatures. For example, microgravity experiments on phase coarsening in Pb-Sn solid-liquid mixtures allowed the kinetics of phase coarsening to be followed more closely and more accurately by eliminating sedimentation and convective melt flow. In this study we developed phase-field simulations to study microstructure evolution during solid-liquid phase coarsening. Physical properties for Pb-Sn solid-liquid mixtures were calculated using CALPHAD. Simulated microstructure features during phase coarsening are compared quantitatively with the microstructure evolution and kinetics extracted and archived from microgravity experiments. In particular, experimental particle size distribution and coarsening rate constant are predicted from theory, and deduced from microgravity experiments, then calculated from phase-field simulations. The new results from phase field simulations agree with the results from archived microgravity experiments for a lower volume fraction alloy, and still acceptably for Pb-Sn alloys with higher volume fraction.

Effect of Ru addition on γ/γ′ microstructural stability in a low-density CoNi based superalloy

The present work explores the effect of ruthenium (Ru) addition to the γ/γ′ strengthened Co-30Ni-10Al-5Mo-2Nb superalloy. A 2 at.% Ru increases the γ/γ′ microstructural stability at 900°C. XRD and APT revealed a reduction of γ/γ′ lattice misfit by ∼ 26% and segregation of Ru at γ/γ′ interface, respectively that contribute to the decrease of γ/γ′ interfacial energy. The temporal evolution of γ′ precipitates at 900°C shows the classical matrix diffusion controlled LSW mechanism with a coarsening rate constant (Kr) ∼ 6.03 nm3/s similar to the values for Co-Al-W based superalloys. Furthermore, Ru addition increases the degree of anomalous strengthening with a peak 0.2% proof stress (PS) value of ∼ 630 MPa at 770°C. The results show Ru as a promising alloying addition to increase the γ/γ′ microstructural stability at high temperatures for the low-density class of CoNi-based superalloys.

Co-strengthening of dislocations and precipitates in alumina-forming austenitic steel with cold rolling followed by aging

The microstructural evolutions of the alumina-forming austenitic (AFA) steel subjected to different cold-rolled reductions and then aging at 700 °C were investigated by multiple characterization methods, and the mechanical properties were also examined. The results show that the increased dislocations (especially, edge dislocations) after cold rolling can effectively enhance the volume fraction and refine the particle size of precipitates during initial aging and subsequently accelerate the coarsening rate of precipitates in the AFA steel, resulting in a high coarsening exponent. The coarsening rate of B2–NiAl phase is higher than that of Laves phase. Excellent mechanical properties with a combination of yield strength (1152 ± 32 MPa) and elongation (44.8 ± 4.1%) of cold rolled AFA steel after aging for 2 h can be obtained due to co-strengthening contribution derived from dislocations and precipitates. With the duration of aging, the dislocation strengthening constantly decreases, the effect of precipitation strengthening is contrarily larger than that of dislocations.