Activation Energy For Self-diffusion Research Articles

Atomic-scale interfacial diffusion of diamond into titanium: Phase transition and layer dependence

The diffusion wear of diamond is a commonly recognized wear mode at high temperatures, while the atomic-scale diffusion behavior and the initiation mechanism are scarcely reported. In this work, we conduct a reactive molecular dynamics study to unveil the diffusion mechanism of diamond into titanium from an atomistic perspective. It is found that substantial diffusion of carbon atoms into titanium at a nanoscale temperature occurs at the temperature over 3000 °C, and the diffusion intensifies rapidly with the further increase of temperature. By studying the evolution of the phase transition, we find that the dangling atoms with a coordination less than two play a critical role in initiating the diffusion of diamond, and subsequently more sp-bonded carbon phases are involved in promoting the diffusion process during the non-equilibrium period. Moreover, the diffusivity of carbon atoms in diamond is found to have a strong layer dependence, which can be attributed to the unique phase composition and activation energy for self-diffusion. Eventually, an effective activation energy for the interfacial diffusion from diamond to titanium of 543.9 kJ/mol is obtained. This study provides a new insight into the initiation mechanism of the interfacial diffusion of diamond materials.

Prediction of flow stress characteristics of P92 steel using a simple physically-based constitutive modelling

This study reports the flow stress behavior of three P92 steels with different compositions. Uniaxial compression tests were conducted in the deformation temperature range of 575 °C to 650 °C and strain rate range of 0.001-0.5 s-1 using a Gleeble® 3500 thermo-mechanical simulator. A simple physically-based constitutive model was used to analyse the effects of deformation conditions (temperature and strain rate) on the metal flow stress behavior during the deformation process. The method accounts for the temperature dependence of Young’s modulus and the lattice self-diffusion coefficient of Fe in the ferrite. Constitutive equations describing the flow stress behavior of the three P92 steels were developed. From the results, the stress exponent n of 26.13(steel A), 21.61(steel B) and 27.55 (steel C) were obtained using the self-diffusion activation energy in the physically-based constitutive equation. From the results, the three steels had variation in the stress exponent values, which was attributed to differences in elemental content, such as chromium and tungsten. The developed constitutive equations were verified using statistical parameters: Pearson’s correlation coefficient (R) and average absolute relative errors (AARE). Statistical analysis showed that the three steels had the same R of 0.98, while AARE was: 1.68 (steel A), 1.72 (steel B), and 1.82 (steel C). The constitutive equations developed showed a good correlation between the experimental and predicted flow stress data. Hence, the method is applicable in describing flow stress behavior in the metalworking process in the industry.

Improvement of the Electromigration Resistance of Al Thin-Film Interconnection by Controlling the Crystallinity of Grain Boundaries

The degradation process of the crystallinity of grain boundaries in Al thin-film interconnections was visualized by applying EBSD analysis under high current density. The grain boundary diffusion was accelerated by electromigration only along random grain boundaries with low crystallinity. The crystallinity (IQ value obtained from EBSD analysis) of grain boundaries varied drastically as strong functions of manufacturing process such as deposition method, temperature of thin-film deposition, gas pressure during the deposition, substrate material, and so on. The effective activation energy of self-diffusion of component element along random grain boundaries was found to be dominated by the crystallinity of random grain boundaries. The measured activation energy obtained from the low-crystallinity interconnections was much lower than that obtained from the thermodynamically stable bulk material, and therefore, the diffusion constant due to electromigration was drastically increased, resulting in the drastic decrease of the lifetime of the interconnections. The lifetime of the interconnections was, therefore, improved by increasing the crystallinity of grain boundaries.

Interfacial behaviour of the high-entropy alloy CuCoCrFeNi and TC4 joint obtained by vacuum diffusion welding

In this work, good bonding between the high-entropy alloy (HEA) CuCoCrFeNi and TC4 titanium alloy was obtained through vacuum diffusion welding at a joining temperature of 1000 °C for 60 min under a pressure of 5 MPa. The results showed that the typical interfacial microstructure of the CuCoCrFeNi/TC4 joint was TC4/diffusion layer/island structure/dendritic structure/diffusion layer/HEA. Compared with Ti atoms, atoms such as Cr and Co from the CuCoCrFeNi substrate were prone to diffuse into the other material. Intermetallic compounds Cu0.5Fe0.5Ti and Co0.15Ni0.85Ti, solid solutions Ti(Fe, Cr)ss and amorphous materials were produced in the joint. The self-diffusion activation energy formula [see formula in PDF] can be used to approximate the order of diffusion capacity of elements, which follows in Cr >Fe> Co > Ni.

Активационные характеристики вязкоупругих свойств жаропрочного сплава на основе системы Ni-Co-Cr, упрочненного объемным азотированием

The results of measurements of the high-temperature internal friction background of the heat-resistant alloy VZh171 in the initial state and after volume nitriding presented. The obtained data used to study the regularities of the transition from the elastic to the viscoelastic state of the material at elevated temperatures. Nitriding the alloy leads to an increase in the transition temperature to the viscoelastic state from 571 K (initial state) to 763 K (after nitriding). The values of activation energy of the high-temperature internal friction background obtained: U f = 0.63 ± 0.01 eV for the alloy VZh171 in the initial state and U f = 0.59 ± 0.01 eV for the alloy after volume nitriding. The activation energies U 0 of the processes responsible for the formation of the viscoelastic properties of the substance are estimated: U 0 ≈ 2.52 eV for the alloy in the initial state, U 0 ≈ 2.36 eV for the nitrided alloy. The obtained values U 0 correspond to the self-diffusion activation energy in nickel, the main component of the alloy matrix. The appearance of dispersed nitride phases in the matrix leads to a decrease in the activation energy of viscoelasticity processes by about 6%. Possible reasons for the change in the transition temperature and the activation energy of the viscoelastic state at moderately high temperatures as a result of nitriding discussed.

Amorphization, mechano-crystallization, and crystallization kinetics of mechanically alloyed AlFeCuZnTi high-entropy alloys

Equiatomic and non-equiatomic AlFeCuZnTi high-entropy alloys (HEAs) were prepared by high energy ball milling of elemental powders. For the equiatomic HEA with chemical composition at the boundary of solid solution and amorphous phase regions, an amorphous phase was obtained at moderate milling times, which was mechano-crystallized into a BCC solid solution by continued milling. For the non-equiatomic HEA with chemical composition well within the solid solution region, the same BCC solid solution was obtained during mechanical alloying. The crystallization kinetics of the amorphous HEA was studied by differential scanning calorimetry (DSC) thermal analysis. The crystallization activation energy (calculated based on the Kissinger equation) was correlated to the mean self-diffusion activation energy of the constituting elements, which was related to the multi-component nature of HEAs.

Evolution of microstructure during tensile creep deformation of nickel-based disk superalloy

The aim of the present investigation is to study the tensile creep behavior of nickel-based disk superalloy, which is widely used in the manufacturing of turbine disks for the aero engines. Tensile creep tests were performed over a wide range of stresses (100–900 MPa) and temperatures (625–850 °C) and rupture time varies from 93.1 to 9473 h. Furthermore, stress rupture strength was predicted using the time-temperature parameter at different temperatures for longer durations. The stress dependence of minimum creep rate for the alloy is found to follow the power law. The stress exponent (n) decreased from 31.2 to 7.8 with increase in the test temperature from 625 °C to 700 °C. Subsequently, the rate-controlling mechanism of creep is identified as dislocation climb by adopting the threshold stress analysis in the temperature range of 625–700 °C. At higher temperature, the n values are drastically reduced to 2.9, 2.7 and 2.2 at 750, 800 and 850 °C, respectively, due to dislocation annihilation and dissolution of precipitates. This indicates that the rate-controlling mechanism of creep changes from viscous glide to grain boundary sliding (n = 2.2). The activation energy for creep (QC) has been determined by using the modified power law in the temperature range of 625–700 °C and is found to be 598 kJ/mol, whereas the obtained QC is also decreased significantly to 435 kJ/mol at higher temperature range (750–850 °C). The calculated QC values are found to be 52.5% and 34.7% higher than the activation energy for lattice self-diffusion of nickel (284 kJ/mol). Post creep microstructural examination using transmission electron microscopy (TEM) revealed extensive deformation in the microstructure is accommodated through the γ′ precipitates, formation of stacking faults and deformation twins within the larger γ′ precipitates. The major findings are well in strong agreement with the hardness characterization, where prominent increase in hardness within localized deformation is mainly due to extensive dislocation-interactions with the γ′ precipitates.

Deformation behavior during hot processing of the alloy of the Al-Mg system economically doped with scandium

Hot deformation behavior of the new alloy 1580 sparingly doped with scandium (Al-5Mg-0.6Mn-0.10Sc-0.11Zr) was studied under the deformation temperature ranging from 350 to 450 °C and strain rate ranging from 0.01 to 10 s−1. Hot deformation by uniaxial compression and torsion was carried out on a Gleeble 3800 thermomechanical simulator. Hot deformation of the alloy 1580 is accompanied by dynamic softening with the establishment of a balance between the rates of nucleation and annihilation of dislocations. Alloy flow stresses increase with increasing strain rate and decreasing temperature. The experimental results for the new alloy are in full agreement with the known concepts that the combined effect of temperature and strain rate on the deformation behavior of metallic materials is described by the Zener-Hollomon parameter. The deformation activation energy of the alloy obtained by the regression analysis of steady-state flow stresses is 175.7 kJ/mol. When using the steady-state flow stresses normalized to the shear modulus, the deformation activation energy is 148.8 kJ/mol and is close to the activation energy of self-diffusion of aluminum. Analytical expressions are obtained for steady-state flow stresses as a function of the Zener-Hollomon parameter, which make it possible to predict the deformation resistance of the alloy 1580 in a wide range of temperatures and strain rates. This result is important for FEM modeling and for the choice of temperature and stain rate of hot deformation of the alloy under study. Comparison with the literature data showed that in the region of high temperatures and low strain rates, the flow stresses are weakly dependent on the scandium content in the alloy. However, with a decrease in the temperature and an increase in the strain rate, an increase in the scandium content in the alloys leads to a noticeable increase in the flow stresses. In torsion tests, a sharp increase in the ductility of the alloy is observed at a temperature of about 400 °C. One of its probable causes may be dissolution of particles Al3(Sc,Zr) during hot deformation in this high-temperature region.

The relationship between self-diffusion activation energy and Soret coefficient in binary liquid mixtures

The Soret effect is an observed phenomenon in which an applied temperature gradient will induce a concentration gradient in multicomponent mixtures. The Soret coefficient is the ratio of thermally driven diffusion to mutual diffusion driven by concentration gradients. It is measured at steady state when the two driving forces are balanced, i.e. the net flux is zero. This study is designed to explore the potential relationship between the activation energy of self-diffusion of the individual components of a binary mixture and the mixture’s resulting Soret coefficient. This study found a trend of increasingly negative Soret coefficients as the difference in the mole-fraction-weighted self-diffusion activation energies of the components increased.

Hydration, self-diffusion and ionic conductivity of Li+, Na+ and Cs+ cations in Nafion membrane studied by NMR

Hydration of Nafion 117 perfluorinated sulfonic cation-exchange membrane in alkaline ion forms was investigated by high resolution 1H NMR. Hydration numbers of Li+, Na+ and Cs+ cations were 5 ± 1, 6 ± 1 and 1 ± 0.2, correspondingly for membrane equilibrated with water vapor at 98% RH. As opposed to Li+ and Na+, which form separate ion pair, Cs+ cation directly contacts with membrane sulfonate group. Cation self-diffusion coefficients were measured by pulsed field gradient NMR technique on 7Li, 23Na and 133Cs nuclei for the first time. Self-diffusion coefficients are changed in the next rows Li+ ≤ Na+ > Cs+. Self-diffusion activation energies of Li+ and Na+ cations are about 20 kJ/mol which is close to water self-diffusion activation energy in these membranes, but Cs+ self-diffusion activation energy is distinctly more (25 kJ/mol). Ionic conductivities calculated on the basis of Nernst–Einstein equation from cation self-diffusion coefficients 1.6∙10−2, 2∙10−2, 6∙10−3 S/cm for Li+, Na+, Cs+ cations, correspondingly, are closely approximating to conductivities measured by impedance spectroscopy: 1.3∙10−2, 1.1∙10−2, 2.3∙10−3 S/cm for Li+, Na+, Cs+ cations, correspondingly, but calculated values are appreciably more compared with experimental meanings.

Viscous flow and self-diffusion in densely and loosely packed metallic melts

The Ni self-diffusion, density, and viscosity of the densely packed liquid ${\mathrm{Ni}}_{66.7}{\mathrm{B}}_{33.3}$ and the more loosely packed liquid ${\mathrm{Ge}}_{66.7}{\mathrm{Ni}}_{33.3}$ were measured with high accuracy using different experimental methods. The viscosity $\ensuremath{\eta}$ was obtained using the oscillating-drop method combined with electrostatic levitation and the oscillating-cup technique in a rheometer. The Ni self-diffusion coefficient ${D}_{Ni}$ was obtained from quasielastic neutron scattering experiments. For ${\mathrm{Ni}}_{66.7}{\mathrm{B}}_{33.3}$, it was found that the activation energies for self-diffusion and viscous flow, ${E}_{D}$ and ${E}_{\ensuremath{\eta}}$, respectively, are almost equal. Hence, the product of the viscosity $\ensuremath{\eta}$ and self-diffusion coefficient $D$ is fairly constant over the entire temperature range measured, spanning almost 800 K. In contrast, for ${\mathrm{Ge}}_{66.7}{\mathrm{Ni}}_{33.3}, D\ensuremath{\eta}$ increases with increasing temperature, indicating that the dynamics in liquid ${\mathrm{Ge}}_{66.7}{\mathrm{Ni}}_{33.3}$ shows better agreement with an activated process. This temperature dependence is in line with the Stokes-Einstein relation.

Stability to oxidation resistance of heat-resistant nickel alloy with γ′-phase structure

Heightened interest to multicomponent alloying of nickel is connected with the search of new compositions of oxidation- and heat-resistant alloys on the basis of nickel solid solution or its intermetallics. In the present work, the author has investigated the resistance to high-temperature oxidation of an alloy of Ni – Al – Mo – W – Nb system which can be used as a basis for creation of dispersion-strengthened inert particles of carbides and nitrides of two-phase thermally stable superalloys with a γ′-phase matrix. Samples of the alloy were subjected to oxidation on air at 900 – 1300 °C during 1 – 125 hours. Weight reduction ( ΔМ , gr) was measured which after that was recalculated into indicators of change of samples weight for a time unit, rationing for the area of initial samples surface ( Δm , gr/(m 2 ·hour)) and “burn” rate of surface layer (scaling loss h, micron/hour). It is shown that at oxidation of Ni – Al – Mo – W – Nb alloy at all temperatures there is a reduction of samples weight because of formation of fragile and friable superficial scale. Dependences of this indicator on oxidation time are close to the linear. With growth of temperature, processes of weight reduction are intensified. It is offered to raise oxidation resistance of Ni – Al – Mo – W – Nb alloy by short-term preliminary oxidation at temperature of 1300 °С on air. The observable effect of increase of oxidation resistance is caused by formation in scale of NiAl 2 O 4 layer, more effectively protecting an alloy from interaction with oxygen. Experiences on oxidation with the use of inert platinum marks have shown that it is necessary to consider oxygen diffusion through oxide film into metal as a mechanism, supervising oxidation of Ni – Al – Mo – W – Nb alloy at high temperatures in case of presence of NiAl 2 O 4 on the surface layer. Activation energy of oxidation of Ni – Al – Mo – W – Nb alloy was calculated at 900 – 1300 °С and without preliminary oxidation. This value is equal to 220,000 J/mol that is characteristic for activation energy of nickel self-diffusion.

A predictive model of impurity diffusion coefficients in face-centered-cubic metallic systems based on machine-learning

This paper develops models for diffusion coefficient prediction to provide parameters for atomic mobility databases and to assist material design in a multi-scale simulation framework for face-centered-cubic (fcc) alloys. Models of impurity-diffusion activation energy (QI) and self-diffusion activation energy (Qs) are trained using machine-learning with experimental diffusion data and basic physical properties. The values of Qs in body-centered cubic (bcc), fcc and hexagonal close-packed (hcp) can be well-predicted using melting temperature, electronic configuration, atomic properties and elasticity parameters. Estimates of QI in fcc metallic systems calculated using a model with six features agreed well with experimental data. Compared with previous models of Qs and QI, the newly developed models exhibit higher coefficients of determination (R2) and significantly lower mean absolute errors. The self- and impurity-diffusion coefficients in fcc metallic systems can be simulated by these models. The models are also successfully applied during the assessment process of the Ni–Ti binary atomic mobility database. Thus, the developed models provide an easy and reliable method for estimating the self- or impurity-diffusion coefficients of fcc alloys when they are unavailable.

Novel WC-reinforced iron-based composites with excellent mechanical properties synthesized by laser additive manufacturing: Underlying role of reinforcement weight fraction

Abstract Laser powder bed fusion (LPBF) process was utilized to prepare high-performance steel matrix composites (SMCs) consisting of a tool steel (1.2767 L) matrix with various contents of submicron-sized WC reinforcing particles. The influence of the content of WC reinforcing particles on constitutional phase, microstructural evolution, densification rate and mechanical properties of SMCs was investigated. It shows that the microstructures and mechanical properties of SMC are highly sensitive to the WC-content. A higher weight fraction of WC leads to a reduced martensite start temperature (Ms) since more W and C atoms are dissolved within the iron matrix during the LPBF-process. The microstructure consists of more retained austenite using a higher WC-content. The addition of 2 wt% WC particles enables a significant grain refinement of the iron-based matrix, due to the formation of a (Fe,W)6C carbidic network that restricts the growth of the sub-grain boundaries of the parent austenite. This refinement-effect is less pronounced at higher WC content due to the reduced self-diffusion activation energy of the Fe atoms in the parent austenite. Increasing WC-content also increases the thickness of the carbidic network, leading to a reduced directionality in the texture of the composite-microstructure. Compared with the unreinforced steel parts, the composites reinforced with 2 wt% WC show a synergetic reinforcing effect in compressive strength of ∼3210 MPa and fracture strain up to ∼30.2 % and ultimate tensile strength of ∼1677 MPa and elongation of ∼8.5 %. The improved mechanical properties result from the combined effect of transformation-induced plasticity (TRIP) effect, grain refinement, non-equilibrium grain-boundary strengthening, and nano-scaled precipitation.

EBSD analysis of hot deformation behavior of Cu-Ni-Co-Si-Cr alloy

Cu-Ni-Si alloy is widely used in electronics industry due to its high electrical conductivity and high strength. In this paper, the Cu-Ni-Co-Si-Cr alloy is prepared by ZG-0.01 vacuum frequency induction melting furnace. The hot compression tests of the Cu-1.5Ni-1.1Co-0.65Si-0.2Cr alloy were carried out using the Gleeble-1500 simulator at 0.001–10 s−1 strain rates and 500–900 °C deformation temperatures. The microstructure changes of the alloy at 500–900 °C were studied. The changes in the hot deformation process for the Cu-1.5Ni-1.1Co-0.65Si-0.2Cr alloy were analyzed by EBSD. The dislocation density was sacrificed to promote recrystallization. The DRX grains texture of the alloy at 700 °C is {001} <100> cubic texture, which is replaced by the {011} <100> Goss texture at 900 °C. And the higher deformation temperature makes the texture more random by lowering the texture intensity. The precipitates of the Cu-1.5Ni-1.1Co-0.65Si-0.2Cr alloy are mainly disk-shaped and rod-shaped (Ni, Co)2Si. The constitutive equation of the Cu-1.5Ni-1.1Co-0.65Si-0.2Cr alloy was established to predict the behavior of the flow stress, and the activation energy is 634.45KJ/mol. Due to precipitation-hardened caused by (Ni, Co)2Si, the activation energy of the Cu-1.5Ni-1.1Co-0.65Si-0.2Cr alloy was much higher than the self-diffusion activation energy of pure copper. And the CDRX and DDRX mechanisms are two dominant DRX mechanisms for the Cu-1.5Ni-1.1Co-0.65Si-0.2Cr alloy.

Compressive creep behavior of selective laser melted CoCrFeMnNi high-entropy alloy strengthened by in-situ formation of nano-oxides

Abstract The present study has investigated the high temperature creep behavior of equiatomic CoCrFeMnNi high-entropy alloy (HEA) manufactured via selective laser melting (SLM). Additionally, the effect of in situ oxide, formed during SLM, on the creep property was examined. The SLM-built HEA exhibited a heterogeneous microstructure as well as cellular and columnar structures decorated by dislocations. Nanosized oxides were newly formed at the substructure boundaries due to high oxygen content in the pre-alloyed powders. SLM-built HEA with in-situ formed oxide revealed exceptional creep resistance at 600 °C, as compared to other HEAs, and stress exponents of n = 3.45 and n = 6.45 for low stress region (LSR) and high stress region (HSR), respectively. This indicated that the creep mechanisms of SLM-built HEA were viscous glide at the LSR and dislocation climb at the HSR. The activation energy for creep was determined to be 335 kJ/mol, which was similar to the activation energy of self-diffusion of the constituent elements. The prediction of the Cottrell-Jaswon dragging model on the steady-state creep rate for the SLM-built HEA was that the most sluggish species of the Ni element encouraged the solute drag phenomenon in the LSR. After the creep tests, the SLM-built HEA exhibited an accumulation of geometrically necessary dislocations at the substructure boundaries, unlike in other metallic materials, where the dislocations are concentrated at the grain boundaries. The nanosized oxides that formed during SLM suppressed the dislocation movement and acted as a site of dislocation accumulation. These results could suggest a way for improving the high-temperature creep resistance of HEAs using unique microstructures (i.e., sub-structures decorated by dislocations and in-situ-formed oxide via high oxygen content in pre-alloy powders) developed by additive manufacturing.

Tensile creep and rupture behavior along with evolution of microstructure in a Zr-2.5Nb alloy

Tensile creep behavior of the Zr-2.5Nb alloy has been studied through tests under constant load (stress range ~ 137–371 MPa) between 275 and 375 °C. The minimum creep rate of the alloy is found to vary with applied stress and temperature following a power-law relation. The values of stress exponent (n) obtained in the range of 5.3–6.8 in the temperature interval of 300–375 °C; whereas it is calculated as 1.2 and 7.0 under low and high stresses, respectively at 275 °C. The apparent activation energy of creep (QC) and stress exponent have been determined by analyzing the experimental data. The obtained QC (~198.5 ± 10.7 kJ/mol) is found to be higher than the lattice self-diffusion activation energy of pure zirconium (113 kJ/mol). Using this value, stress exponent ~5.6 ± 0.23 is obtained by the temperature-compensated power law. Microstructural characterization by transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS) analysis has confirmed coarsening of β-(Nb, Zr) precipitates with compositional changes during creep. The dislocation-precipitate interaction as evidenced by TEM observations is considered to be the origin of threshold stress, which decreases with increasing temperature. Considering the threshold stress, true activation energy of creep (Qt) and true stress exponent (nt) are found as ~160.4 ± 6.9 kJ/mol and ~4.8 ± 0.22, respectively. Analysis of creep data has confirmed the role of dislocation climb as the rate-controlling mechanism, along with validity of Monkman-Grant and modified Monkman-Grant relations. Scanning electron microscopy (SEM) of creep fracture surfaces has revealed evidence for prominent ductile fracture. Further, the obtained creep damage tolerance factor value of 1.9, indicating the predominance for cavitation during creep.

Ionic liquid dynamics in nanoporous carbon: A pore-size- and temperature-dependent neutron spectroscopy study on supercapacitor materials

The influence of spatial confinement on the thermally excited stochastic cation dynamics of the room-temperature ionic liquid 1-N-butylpyridinium bis-((trifluoromethyl)sulfonyl)imide ([BuPy][Tf_2N]) inside porous carbide-derived carbons with various pore sizes in the sub- to a few nanometer range are investigated by quasi-elastic neutron spectroscopy. Using the potential of fixed window scans, i.e. scanning a sample parameter, while observing solely one specific energy transfer value, an overview of the dynamic landscape within a wide temperature range is obtained. It is shown that already these data provide a quite comprehensive understanding of the confinement-induced alteration of the molecular mobility in comparison to the bulk. A complementary, more detailed analysis of full energy transfer spectra at selected temperatures reveals two translational diffusive processes on different time scales. Both are considerably slower than in the bulk liquid and show a decrease of the respective self-diffusion coefficients with decreasing nanopore size. Different thermal activation energies for molecular self-diffusion in nanoporous carbons with similar pore size indicate the importance of pore morphology on the molecular mobility, beyond the pure degree of confinement. In spite of the dynamic slowing down we can show that the temperature range of the liquid state upon nanoconfinement is remarkably extended to much lower temperatures, which is beneficial for potential technical applications of such systems.

Tailoring grain growth and solid solution strengthening of single-phase CrCoNi medium-entropy alloys by solute selection

In the present study, we selected solutes to be added to the CrCoNi medium-entropy alloy (MEA) based on the mismatch of self-diffusion activation energy (SDQ) between the alloying elements and constituent elements of the matrix, and then investigated their grain growth behavior and mechanical properties. Mo and Al were selected as the solutes for investigation primarily because they have higher and lower SDQ, respectively, than those of the matrix elements; a secondary factor was their higher and lower shear modulus. Their concentrations were fixed at 3 at.% each because previous work had shown these compositions to be single-phase solid solutions with the face-centered cubic structure. Three alloys were produced by arc melting, casting, homogenizing, cold rolling and annealing at various temperatures and times to produce samples with different grain sizes. They were (a) the base alloy CrCoNi, (b) the base alloy plus 3 at.% Mo, and (c) the base alloy plus 3 at.% Al. The activation energies for grain growth of the CrCoNi, CrCoNi-3Mo and CrCoNi-3Al MEAs were found to be ∼251, ∼368 and ∼219 kJ/mol, respectively, consistent with the notion that elements with higher SDQ (in this study Mo) retard grain growth (likely by a solute-drag effect), whereas those with lower values (Al) accelerate grain growth. The room-temperature tensile properties show that Mo increases the yield strength by ∼40 % but Al addition has a smaller strengthening effect consistent with their relative shear moduli. The yield strength as a function of grain size for the three single-phase MEAs follows the classical Hall-Petch relationship with much higher slopes (＞600 MPa μm−0.5) than traditional solid solutions. This work shows that the grain growth kinetics and solid solution strengthening of the CrCoNi MEA can be tuned by selecting solute elements that have appropriate diffusion and physical properties.

Application of SPS in the fabrication of UN and (U,Th)N pellets from microspheres

In this study, the process involved in the fabrication of a potential accident tolerant fuel is described. Homogeneous uranium nitride microspheres doped with different thorium content were successfully manufactured using an internal gelation process followed by carbothermic reduction, and nitridation. Elemental analysis of the materials showed low carbon and oxygen content, the two major impurities found in the products of carbothermic reduction. Uranium nitride microspheres were pressed and sintered using spark plasma sintering (SPS) to produce pellets with variable density. Final density can be tailored by choosing the sintering temperature, pressure and time. Density values of 77–98% of theoretical density (%TD) were found. As expected, higher temperatures and pressures resulted in a denser material. Furthermore, a direct correlation between the onset sintering temperature and thorium content in the materials was observed. The change of onset temperature has been related to an increment in the activation energy for self-diffusion due to the substitution of uranium atoms by thorium in the crystal structure.