Homologous Temperature Research Articles

Влияние интенсивной пластической деформации на механические свойства чистого цинка

Biodegradable materials, which have the ability to resorb in the body, are new and promising materials for medical implants. Currently, scientists carry out the investigations according to three directions: Mg, Fe, and Zn alloys. Zinc-based alloys and zinc have good solubility in the body, which meets the clinical requirements of implants. However, pure zinc has low mechanical properties, including hardness and tensile strength. Therefore, at present, the world scientific community is seeking ways to improve the properties of pure zinc by alloying. Another known approach is the ultrafine-grained (UFG) structure formation by the severe plastic deformation (SPD) methods, which are based on the large plastic deformations under high pressure and relatively low homologous temperatures. In this work, the authors studied the influence of high pressure torsion of pure zinc with various numbers of revolutions. The paper presents calculations of shear deformation after SPD. The authors investigated the dependence of mechanical properties and microstructure on the deformation degree. Tension tests at room temperature were carried out, and microhardness was measured. The authors studied the structure using scanning electron microscopy and optics. The study identified that the use of high pressure torsion leads to an increase in the tensile strength of pure zinc up to 140 MPa and ductility up to 40 % resulting from dynamic recrystallization.

Cryogenic and Room Temperature ECAP Consolidation of Blended Elemental Powders of Aluminum and Copper

The effect of temperature was investigated on the consolidation of blended elemental powders of aluminum and copper by equal channel angular pressing (ECAP). Aluminum and Copper powders (1:1% vol.) were blended and consolidated in a 90° ECAP die at room (RT) and cryogenic temperatures (CT - ~77 K). ECAP samples were pressed until 4 passes at room temperature in route Bc. As a reference, a sample was obtained by conventional uniaxial pressing. The obtained results indicated a much denser (>99.5%) and harder structure by cryogenic ECAP. The hardness after one pass at CT was comparable with 4 passes at room temperature. Tensile tests performed at CT for materials with similar chemical composition showed a simultaneous increase in strength and ductility at CT, corroborating the results obtained by ECAP. The partial suppression of dynamic recovery and the activation and the transition between deformation mechanisms at CT, as well as stacking fault energies (SFE) of such metals, played an important role in these results. Copper presented a much higher capability of strain hardening than aluminum, due to its lower SFE and much lower homologous temperature. X-ray diffraction indicated a strong correlation between the variation of average microstrain and the variation of hardness on both metals. The results of this study demonstrated the great potential of the application of very low temperatures for the obtaining of deformation metal-metal composites.

Alloying effects in high temperature molten salt corrosion

This study investigates dealloying of Fe-(Cr)-Ni model alloys in molten chloride salts that simulate the heat-transfer circuits of molten salt nuclear reactors. Classical dealloying with porosity formation is observed as the dominant form of corrosion at low homologous temperature, and the usual, fundamental, geometric parting limit for dealloying, 55–60 at% of the reactive element(s) (Fe and Cr), is decreased by several percent. This deviation is due to increased surface diffusivity of the more-noble element, Ni, in molten Cl salts. To better highlight deviations from conventional dealloying theory, results are compared with dealloying of similar Fe-(Cr)-Ni alloys in concentrated caustic solutions.

Crystallographic Preferred Orientation (CPO) Development Governs Strain Weakening in Ice: Insights From High‐Temperature Deformation Experiments

AbstractStrain weakening leads to the formation of high‐strain shear zones and strongly influences terrestrial ice discharge. In glacial flow models, strain weakening is assumed to arise from the alignment of weak basal planes—the development of a crystallographic preferred orientation, CPO—during flow. However, in experiments, ice strain weakening also coincides with grain size reduction, which has been invoked as a weakening mechanism in other minerals. To interrogate the relative contributions of CPO development and grain size reduction toward ice strain weakening, we deformed initially isotropic polycrystalline ice samples to progressively higher strains between −4 and −30°C. Microstructural measurements were subsequently combined with flow laws to separately model the mechanical response expected to arise from CPO development and grain size reduction. Magnitudes of strain weakening predicted by the constitutive flow laws were then compared with the experimental measurements. Flow laws that only consider grain size do not predict weakening with strain despite grain size reduction. In contrast, flow laws solely considering CPO effects can reproduce the measured strain weakening. Thus, it is reasonable to assume that strain weakening in ice is dominated by CPO development, at least under high temperature ( ≥ 0.9) and high stress (>1 MPa), like those in our experiments. We speculate that at high homologous temperatures ( ≥ 0.9), CPO development will also govern the strain weakening behavior of other viscously anisotropic minerals, like olivine and quartz. Overall, we emphasize that geodynamic and glaciological models should incorporate CPOs to account for strain weakening, especially at high homologous temperatures.

Investigation on the Thermodynamic Stability of Nanocrystalline W-Based Alloys: A Combined Theoretical and Experimental Approach.

The stability of nanostructured metal alloys is currently being extensively investigated, and several mathematical models have been developed to describe the thermodynamics of these systems. However, model capability in terms of thermal stability predictions strongly relies on grain boundary-related parameters that are difficult to measure or estimate accurately. To overcome this limitation, a novel theoretical approach is proposed and adopted in this work to identify W-based nanocrystalline alloys which are potentially able to show thermodynamic stability. A comparison between model outcomes and experimental findings is reported for two selected alloys, namely W-Ag and W-Al. Experimental results clearly highlight that W-Ag mixtures retain a segregated structure on relatively coarse length scales even after prolonged mechanical treatments. Moreover, annealing at moderate temperatures readily induces demixing of the constituent elements. In contrast, homogeneous nanostructured W-Al solid solutions are obtained by ball milling of elemental powders. These alloys show enhanced thermal stability with respect to pure W even at high homologous temperatures. Experimental evidences agree with model predictions for both the investigated systems.

Stress sensitivity of high-temperature microstructures in ice, with potential applications to quartz

Quartz and ice both exhibit distinctive microstructures when deformed at low stress and high homologous temperature, known as grain-boundary migration (GBM) microstructures. These are difficult to reproduce experimentally in silicate minerals, and no correlation has been established between quantifiable aspects of the microstructure and deformational conditions. We carried out direct shear experiments to investigate the effect of stress and temperature (T) on GBM microstructures in ice, which is crystallographically analogous to quartz. Differential stress was 0.7–6.0 MPa, confining pressure 4–9 MPa, and T −3° to −25 °C. There is a clear transition at ∼ −12 °C from granular microstructures produced by rotation recrystallization at high stress and low T, to GBM microstructures at low stress and high T, accompanied by an increase in strength of the crystallographic preferred orientation (CPO). Samples with GBM microstructure show lobate grain-boundaries and “island grains” where several distinct and separate areas have the same crystallographic orientation, representing lobes of a single grain isolated on the cut surfaces. The size of lobes and island grains define an array with respect to stress with a slope of 0.9, similar to but offset from the slope of published stress/grain-size data. This suggests the possibility of determining paleostress from GBM microstructures in both ice and quartz.

The N-body interatomic potential for molecular dynamics simulations of diffusion in tungsten

Tungsten, as the most refractory metal, is applied in fusion reactor in parts subjected to high temperatures and strong neutron irradiation. These factors lead to intense diffusion processes causing degradation of the material. Experimental investigations under such conditions are usually highly complicated and cannot provide a comprehensive understanding of the occurring phenomena. Therefore, their combination with theoretical approaches is required. One of the most robust approaches to simulate diffusion processes is molecular dynamics simulations based on classical interatomic potentials. It allows modeling relatively large samples consisting of several grains, grain boundaries, dislocations, and other types of defects for a reasonable computational time. The reliable simulations of the diffusion process require interatomic potentials satisfying the following criteria: prediction of melting point and thermal expansion as close as possible to the experimental values because the diffusion coefficient strongly depends on the homologous temperature and size factor. In the present paper, we present the new interatomic potential for tungsten, developed within the N-body approach, which reproduces the experimental value of melting temperature (3695 K) and thermal expansion at temperatures up to a melting point. The calculated diffusion coefficient demonstrates adequate agreement with experimental results. The constructed potential is applicable for simulation of processes involving diffusion, one of which is the irradiation damage.

Effects of Fe, Cr and Mn on inter-diffusion in CoNi-containing multiple principal alloys

Inter-diffusion in eight alloys from the binary CoNi alloy to the quinary FeCrMnCoNi high entropy alloy (HEA) was studied by the diffusion couple experiments. It was found that the FeCrMnCoNi HEA does not exhibit sluggish diffusion in both the absolute temperature scale and the homologous temperature scale. Effects of specific elements were analyzed: Mn increases inter-diffusion in the absolute temperature scale but decreases it in the homologous temperature scale; Cr decreases inter-diffusion in both temperature scales; Fe decrease inter-diffusion in the absolute temperature scale but in the homologous temperature scale, the interaction between Fe and other elements could become important. The relationship between the pre-exponential factor and homologous activation energy indicates that the atomic diffusion process may not change qualitatively from binary to multiple principal alloys.

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.

Deformation-induced grain boundary segregation mediated high-strain rate superplasticity in medium entropy alloy

Superplasticity refers to extremely high plastic deformation of crystalline materials at homologous temperature. In particular, high-strain rate superplasticity (HSRS) at a strain rate equal to or higher than 10−2 s−1 draws great technological interest in the shape-forming of engineering materials. Besides primary mechanism as grain boundary sliding, the formation of liquid phase at grain boundaries/interfaces can assist HSRS. With the advent of multi-principle element alloys (MPEAs), designing grain boundary segregation of a low melting temperature element with the appropriate choice of composition can bring the possibility to produce the HSRS accommodated by the partial melting in MPEAs. Here, we show the trace of the liquid phase in Al0.3CoCrNi MPEA at the homologous temperature of 0.65. Careful microstructural examination reveals for the first time that deformation-induced Al-segregation occurs at grain boundaries/interfaces. The present work expedites a new path to tailoring microstructure for promoting the HSRS in high-strength materials by grain boundary segregation engineering.

Investigation of the \u03b3\u2032 Precipitates Dissolution in a Ni-Based Superalloy During Stress-Free Short-Term Annealing at High Homologous Temperatures

The equiaxed Ni-based superalloy René 108 was subjected to short-term annealing at five temperatures between 900 °C and 1100 °C. The phase composition, phase lattice parameters, microstructure, stereological parameters, and chemical composition of γ′ precipitates were investigated by thermodynamic simulations, X-ray diffraction, scanning and transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Analysis of the γ and γ′ lattice parameters using the Nelson-Riley extrapolation function showed that the misfit parameter for temperatures 900 °C to 1050 °C is positive (decreasing from 0.32 to 0.11 pct). At 1100 °C, the parameter becomes negative, δ = − 0.18 pct. During the short-term annealing, γ′ precipitates dissolution occurred progressing more rapidly with increasing temperatures. The surface fraction of γ′ precipitates decreased with increasing temperature from 0.52 to 0.34. The dissolution of γ′ precipitates did not only proceed through uninterrupted thinning of each individual precipitate, but also included more complex mechanisms, including splitting. Based on transmission electron microscopy, it was shown that after γ′ precipitates dissolution, the matrix close to the γ/γ′ interface is strongly enriched in Co and Cr and depleted in Al.

Mechanical behavior at elevated temperatures of an Al–Mn–Mg–Sc–Zr alloy manufactured by selective laser melting

A high temperature compression and compression creep study was undertaken performed on Al–Mn–Mg–Sc–Zr alloy produced by selective laser melting (SLM). High temperature compression tests in the temperature range from 150 to 300 °C were conducted for the as-fabricated (AF) and aged (HT) SLM specimens. The 0.2% proof stress (σ0.2) showed comparable values at different loading directions, which reveal an isotropic mechanical behavior. σ0.2 of the HT samples exceeded that of the AF at temperatures up to 200 °C. At 250 °C, σ0.2 of AF surpasses that of HT, due to the interaction of solid solution strengthening above 0.5 of the homologous temperature and the formation of new precipitates in this temperature range. For AF and HT σ0.2 becomes similar again at 300 °C. Creep tests were conducted at 250 °C in a stress range of 100–150 MPa. The Norton stress exponent was determined to (5.1 ± 0.2) indicating that creep is dislocation climb controlled. An activation energy for creep of (198 ± 11) kJ/mol was found. It is suggested that solid solution strengthening in conjunction with a dispersed L12 Al3Sc precipitates improved the creep resistance compared to conventionally cast-manufactured Al–Sc–Zr alloys.

Nanomolding of Gold and Gold-Silicon Heterostructures at Room Temperature.

Nanofabrication techniques are limited by at least one of the required characteristics such as choice of material, control over geometry, fabrication requirements, yield, cost, and scalability. Our previously developed method of thermomechanical nanomolding fulfills these requirements, although it requires high processing temperatures. Here, we demonstrate low-temperature molding where we utilize the enhanced diffusivity on "eutectic interfaces". Gold nanorods are molded at room temperature using Au-Si alloy as feedstock. Instead of using alloy feedstock, these "eutectic interfaces" can also be established through a feedstock-mold combination. We demonstrate this by using pure Au as feedstock, which is molded into Si molds at room temperature, and also the reverse, Si feedstock is molded into Au molds forming high aspect ratio Au-Si core-shell nanorods. We discuss the mechanism of this low-temperature nanomolding in terms of lower homologous temperature at the eutectic interface. This technique, based on enhanced eutectic interface diffusion, provides a practical nanofabrication method that eliminates the previous high-temperature requirements, thereby expanding the range of the materials that can be practically nanofabricated.

An Experimental Investigation of the Effect of Grain Size on “Dislocation Creep” of Ice

AbstractThe creep behavior of ice is believed to be dominated by dislocation creep at high stresses (1 MPa) and high homologous temperatures (0.9). Dislocation creep of ice is often described by the Glen law, , with a canonical value of 3, and is independent of grain size. Laboratory studies of ice deformation at elevated pressures, however, suggest that dislocation creep of ice is characterized by a value of 4. Here, we deformed ice samples with initial grain sizes of 0.23 and 0.63 mm, at 263 K and confining pressures of 10 or 20 MPa. The experiments yield a value of 3.6 using the peak stresses obtained at a strain 0.02, and reveal a marked dependence of the stress on initial grain size. The nominally constant flow stresses at larger strains, up to 0.2, yield a value of 3.9, and no influence of the initial grain size on the stress is observed. The lower value of at peak stresses and the decrease in peak stress with decreasing initial grain size suggest a contribution to the strain rate from a grain‐size‐sensitive process, such as grain boundary sliding. Our data yield two flow laws, one for isotropic ice and one for highly anisotropic ice deformed to large strains, the latter of which may be used to model flow in high‐strain natural environments.

Enhanced Phase Stability and Carrier Suppression of Ternary Cation-Based Amorphous Oxide Semiconductor Thin Film Transistors

Transparent optoelectronic materials have gained significant attention for application in high performance devices such as next generation displays1-2, photovoltaics3, electrochromic devices4, and sensors5-6. Metal oxide-based semiconductors, specifically, are a promising group of transparent optoelectronic materials, now considered to be the vital building blocks for future device applications due to the unique combined properties of excellent optical transparency (from the visible to near-infrared regime), and high electrical conductivities7-8. Metal oxides have already seen wide implementation in various optoelectronic devices, depending on their electrical conductivity, as transparent electrodes by degenerately-doped oxides such as indium tin oxides (ITO) and doped zinc oxides (ZnO), as semiconducting active layers based on In2O3, ZnO or SnO2, and as insulators (e.g., SiO2, Al2O3, HfO2) for dielectrics or encapsulations.The key pixel driving switches in display technologies, thin film transistors (TFTs), require a vast array of demanding material properties such as high carrier mobilities (usually defined as the “TFT field effect mobility”), low thermal budgets during processing, resilient phase stability, and reliable device performance under high thermal and bias stress conditions. Amorphous oxide semiconductors (AOSs), specifically those based on indium oxides, are receiving unique attention for TFT implementation due to their promisingly high carrier mobilities (> 5-20 cm2/Vs) compared to conventional amorphous Si (<~1 cm2/Vs), low processing temperature requirements (ambient to 200 °C), superior mechanical flexibility to their crystalline counterparts, and large area process-ability. Indium oxide-based binary and ternary cation material systems show tremendous promise for use in next generation displays as these materials exceed the aforementioned material property and fabrication requirements. Unfortunately, undoped In2O3 experiences a rapid onset of microstructural crystallization at very low homologous temperatures (T/Tm<0.19) at 150 °C and struggles to maintain its amorphous phase structure. The inclusion of Zn in In2O3 has revealed a viable and promising binary cation material which specifically addresses the structural instability of undoped In2O3, Indium Zinc Oxide (IZO), as the addition of Zn into In2O3 proves to stabilize the temperature-sensitive amorphous phase of indium oxide. Furthermore, the reported carrier mobility of IZO has shown to be as high as 20-40 cm2/Vs for both Hall and 15-30 cm2/Vs for TFT field effect mobilities. Studies have dug deeper to further unveil the effects of doping of In2O3-basd materials, and the ternary cation system of indium gallium zinc oxides (IGZO) has proven to be even more popular than binary systems as the addition of Ga in IGZO allows for the controllable suppression of channel carrier density during TFT applications (preferred for TFT devices where a low device off-state current is desired). In addition to Ga, more third cation species have been investigated as suitable material candidates such as Hf, Si, and Zr, but the carrier mobilities (~3-10 cm2/Vs) are around 3-10 times lower than that of In2O3 or IZO.Therefore, securing strategies to develop a material system which maintains both high carrier mobility (e.g., >~20 cm2/Vs) and suppresses carrier generation for TFT channel application is of significant importance, and is necessary to expedite the realization of next-generation transparent displays which possess reliable performance, fast switching speed, and, consequently, ultra-high definition resolution. In this study, the ternary cation oxide system of indium aluminum zinc oxide (IAZO) is investigated with varying Al concentration in IAZO thin films. The IAZO thin films were deposited using magnetron co-sputtering at room temperature. The structural, optical, and electrical properties were systematically characterized as a function of Al concentration and compared to baseline IZO samples. The carrier transport characteristics, as well as the dominant mechanisms for carrier density and resistivity and their relation to Al concentration, are discussed. Furthermore, amorphous IAZO-based TFTs were developed to objectively compare and validate device performance and parameters against IZO-based TFTs.The authors gratefully acknowledge the financial supports of the U.S. NSF Award No. ECCS-1931088; the Purdue Research Foundation (Grant No. 60000029); and the Improvement of Measurement Standards and Technology for Mechanical Metrology (Grant No. 20011028) by KRISS.References Nomura et al. Nature 2004, 432 (7016), 488-492.Lee et al. Applied Physics Letters 2014, 104 (25), 252103.Fortunato et al. MRS Bull. 2007, 32 (3), 242-247.Maho et al. Journal of The Electrochemical Society 2017, 164, H25-H31.Venkatanarayanan et al. Elsevier: Oxford, 2014; pp 47-101.Zhao et al. The Journal of Physical Chemistry C 2015, 119 (26), 14483-14489.Lewis et al. MRS Bull. 2000, 25 (8), 22-27.Coutts et al. MRS Bull. 2000, 25 (8), 58-65. Figure 1

Electrochemistry of High Temperature Corrosion of Alloys in Molten Salts Relevant to Future Nuclear Reactors

Eutectic molten salts are coolant candidates for molten salt-cooled nuclear reactors; however, alloy corrosion is the key materials-compatibility issue [1]. Corrosion in molten salts may involve thermodynamic considerations, thermal gradient-driven corrosion, dissimilar material corrosion, dealloying, and impurity-driven corrosion [2, 3]. New alloys must be developed, and their corrosion behaviour merits special attention at a fundamental level. We aim to understand industrial alloy behaviour through study of model alloys, leading to insights relevant to materials performance in Molten Salt Reactors.In the field of molten salt corrosion, dissolution of alloying elements is mostly discussed in terms of one-dimensional diffusion of a more easily dissolved element from the bulk to the surface, which necessarily involves lattice diffusion of metals. We report on electrochemical study of corrosion mechanisms in Fe-(Cr)-Ni model and industrial alloys, and report on a study of critical alloy compositions and porosity formation upon dealloying of one or more electrochemically reactive components in molten salts.Dealloying is selective electrolytic dissolution of one or more active elements from a metallic solid solution or intermetallic compound [4, 5]. The operative mass transport process in aqueous dealloying is diffusion of the more-noble component at the solid-electrolyte interface – enhanced by poorly-understood electrolyte effects on the diffusivity. The parting limit in dealloying is the minimum content of less-noble element(s) for dealloying, below which the dealloying is prevented by a passive layer of more-noble elements on surface formed at initial stage of corrosion. The more usual case (i.e. AgAu, CuAu, etc.) is a threshold of ca. 55-60 at. % less-noble element (Ag and Cu, respectively) [6]. According to Artymowicz et al. [7] the underlying parting limit is very close to 60 at. %, but increasing kinetics of surface diffusion could drop the parting limit to ca. 55 at. % in systems studied to that date.Fe-(Cr)-Ni model alloys were prepared using a Cold Crucible Induction Levitation Melter. Electrochemical studies were done using a well-controlled electrochemical cell for corrosion study in molten chloride salts. We have developed Mg|Mg2+ reference electrode (RE) for our eutectic chloride salts and it is found to be a reliable RE, as far as we can see from our electrochemical measurements (open circuit potential, cyclic polarization, and polarization resistance). Impurity contents were monitored electrochemically, and final water removal was carried out using Mg. Characterization is being carried out by analytical electron microscopy, X-ray diffraction and secondary ion mass spectrometry.We found that up to a certain temperature, there is dealloying of the type observed in aqueous solutions, with porosity formation and parting limits. In brief, porosity and parting limits were observed in molten salts, exactly as predicted, except that the de-alloying threshold for electrolytic dissolution of the less-noble element(s) was dropped by several percent, compared with aqueous solutions. This is due to the very fast surface diffusion of more-noble metal in the molten chloride salt. Dealloyed layers were nearly pure nickel, but with residual Fe and/or Cr at the ligament cores in the porous structure. Correspondingly, the porosity is very coarse, and shows new features such as secondary corrosion through the ligament cores, as shown in Fig. 1.More Ni suppresses dealloying in both Fe-Ni and Fe-Cr-Ni model alloys, but more Fe and Cr promote oxide formation in binary and ternary alloys where dealloying propagates below an oxide layer of Fe and Cr in binary and ternary alloys. So, to some extent there is a balance, masking the underlying dependency of dealloying on Ni content.This type of dealloying is mediated by surface diffusion. At higher temperatures, there is a shift to different mechanisms involving lattice diffusion in the metal and porosity changes its appearance. Even then, there is not necessarily a planar dealloying front – more of a “negative dendrite” type of interface would appear [8]. At very high homologous temperature the dealloying feature will revert to that mostly described in the molten salt literature, with planar interfaces.

The microstructure of the rapidly solidified foil of the hypoeutectic alloy Sn – 4.4 wt. % Zn

The article presents the results of a study of the microstructure of the rapidly solidified foil of the hypoeutectic alloy Sn – 4.4 wt. % Zn. It was found that the investigated alloy has a two-phase structure, which consists of solid solutions of tin and zinc. Doping of tin with zinc leads to a decrease in the unit cell parameter. The difference between the unit cell parameters of a rapidly solidified alloy in comparison with an alloy of pure tin tends to decrease during holding, which is due to strong supercooling of the melt during its production at ultrahigh speeds, and the formation of a supersaturated solid solution of zinc in tin, which, due to high homological temperatures, as a consequence, active diffusion processes, decomposes at room temperature. It has been established that a microcrystalline structure is formed in the foil of the alloy under study, in the cross section of which there are uniformly distributed equiaxed dispersed dark zinc precipitates against the background of a light tin matrix; the absence of zinc plates in the foil reduces the ability to brittle fracture. The unequal distribution of the average chord of random secants on the grains in the surface layers A is caused by the release of heat, which leads to a decrease in the supercooling of the subsequent layers of the melt, and an increase in the grain size as the crystallisation front moves. It was found that in the (301) plane along the [103] direction, tin twinning is observed, which occurs under the action of quenching stresses at high crystallisation rates. The alloy under study has a (100) tin texture, the formation of which is associated with the fact that the (100) plane is the most densely packed, which promotes the growth of grains with this orientation at the highest rate.

Noble gas bubbles in bcc metals: Ab initio-based theory and kinetic Monte Carlo modeling

Understanding the interactions of noble gases with metals is of fundamental importance for the design of radiation-resistant structural materials for fission and fusion nuclear reactors. Here we present a unified theory for describing the energetics of He, Ne, Ar, and Kr bubbles in bcc metals in group 5B (V, Nb, Ta), 6B (Cr, Mo, W) and 8B (Fe). Our predictive analytical model is based on the effective-medium and isotropic elasticity theories, and is parameterized using density functional theory (DFT) calculations of small gas-vacancy clusters. By performing kinetic Monte Carlo (KMC) simulations driven by our analytical model, we have predicted the lifetimes of noble gas bubbles and their coarsening by Ostwald ripening. Our most notable finding is the exceptionally higher thermal stability of Ne, Ar and Kr bubbles than He bubbles in bcc metals, conferring them outstanding resistance to Ostwald ripening. The physical origin of the unexpected stability of bubbles formed by large noble gas atoms has been further elucidated. Our theoretical finding is consistent with the experimental observation of He gas bubble superlattice (GBS) coarsening under thermal annealing, and provides new insights on the exceptional stability of fission GBS in bcc U-Mo up to a high homologous temperature of 0.78. The present calculated results also compare favorably with the existing thermal helium desorption spectrometry experiments in the literature.

Disordered interfaces enable high temperature thermal stability and strength in a nanocrystalline aluminum alloy

Lightweighting of structural materials has proven indispensable in the energy economy, predicated on alloy design with high strength-to-weight ratios. Modern aluminum alloys have made great strides in ambient temperature performance and are amenable to advanced manufacturing routes such as additive manufacturing, but lack elevated temperature robustness where gains in efficiency can be obtained. Here, we demonstrate the intentional design of disorder at interfaces, a notion generally associated with thermal runaway in traditional materials, in a segregation-engineered ternary nanocrystalline Al–Ni–Ce alloy that exhibits exceptional thermal stability and elevated temperature strength. In-situ transmission electron microscopy in concert with ultrafast calorimetry and X-ray total scattering point to synergistic co-segregation of Ce and Ni driving the evolution of amorphous intergranular films separating sub-10 nm Al-rich grains, which gives rise to emergent thermal stability. We ascribe this intriguing behavior to near-equilibrium interface conditions followed by kinetically sluggish intermetallic precipitation in the confined disordered region. The resulting outstanding mechanical performance at high homologous temperatures lends credence to the efficacy of promoting disorder in alloy design and discovery.

Fast mobility induced self-lubrication at metallic glass surface

The enhanced surface mobility in metallic glasses (MGs) has been a constant source of fascination due to its unique mechanical properties. We show experimentally that the mobile surface layer of MGs functions as a lubricating layer in friction experiments, which is evidenced by a reduction of a friction coefficient of 50% or less and suppression of dissipative stick-slip behavior with decreasing scratch depth down to nanoscale in the various MGs. The lubrication mechanism could be attributed to easier shearing of the mobile surface layer induced by homogeneous plastic flow. Importantly, the thickness of the self-lubricating layer is inversely proportional to glass transition temperature with a higher homologous temperature yielding a larger thickness. These results extend the fundamental understanding of the ubiquitous MG surface and present a path for the rational design of self-lubricating materials.