Activation Energy For Self-diffusion Research Articles

The influence of irradiation induced vacancies on the mobility of helium in boron carbide

Boron carbide, used as a neutron absorber, undergoes nuclear reactions producing relevant quantities of He. The understanding of He kinetics at the atomic scale in the material is still in its infancy, in spite of decades of experimental work devoted to the characterization of He containing, irradiated, boron carbide samples. The interplay of He itself with intrinsic defects created by irradiation on kinetics is still almost completely unknown.In this paper we present an exhaustive study of vacancies and substitutional helium impurities in boron carbide using Density Functional Theory. Analyzing the stability and mobility of these defects allows us to consider diffusion mechanisms other than the known interstitial mechanisms.We find that vacancies trap He interstitials, raising the activation energy of 2D diffusion to approximately 2 eV. The trapping mechanism is different according to the charge state of the vacancy: in p-type conditions, when vacancies are neutral or positive, He diffuses via a dissociative mechanism and is trapped in a substitutional position; in n-type conditions, negative vacancies trap He atoms traveling in an adjacent {111} plane by a charge transfer driven distorsion.No favorable vacancy assisted diffusion mechanism was identified for substitutional He atoms, except the dissociative one previously mentioned. Other possible vacancy diffusion mechanisms, which we also analyzed, are hindered by the high activation energy of vacancy self-diffusion.

Un estudio adicional de la cinética de recristalización y crecimiento de grano del acero twip laminado en frío

Muestras de acero TWIP laminadas en caliente en laboratorio de composición, 22% de Mn, 0,6% de C (% en masa) se laminaron en frío a reducciones de 40%, 50%, 60% y 70% para recocerlas isotérmicamente en el intervalo de temperatura 450 ºC ≤ T ≤ 1100 ºC. El objetivo fue estudiar la precipitación y su efecto razonable en la recristalización estática y cinética de crecimiento de grano. Se encontraron dos tipos de precipitados en 600 ºC ≤ T ≤ 700 ºC a tiempos largos de recocido: (Fe, Mn)3C – Cementita y Carbonitruros de Vanadio. El tamaño de grano recristalizado fue muy fino D0 ≤ 2 μm. Además, se encontró que la textura de laminación se hereda en la recristalización muy debilitada. La energía de activación para la recristalización calculada fue Qsoft = 281 ± 70 kJ·mol-1 que se corresponde prácticamente con la energía de activación para la autodifusión en austenita (270 kJ·mol-1) y para la difusión del Mn en austenita (265 kJ·mol-1). Sin embargo, la energía de activación de crecimiento de grano obtenida fue significativamente mayor QGG = 384 ± 60 kJ·mol-1 con un exponente de crecimiento de grano, nGG ~ 4. Consecuentemente, la explicación más razonable es que la cantidad de precipitados es suficiente para tener un efecto de fijación en la migración de las juntas de grano durante el crecimiento debido a que la longitud media entre precipitados, Lprec, es menor que algún valor umbral de tamaño de grano, Lprec < Dumbral, siendo D0

Use of the Wilshire Equations to Correlate and Extrapolate Creep Data of HR6W and Sanicro 25.

Advanced power plant alloys must endure high temperatures and pressures for durations at which creep data are often not available, necessitating the extrapolation of creep life. Many methods have been proposed to extrapolate creep life, and one of recent significance is a set of equations known as the Wilshire equations. With this method, multiple approaches can be used to determine creep activation energy, increase the goodness of fit of available experimental data, and improve the confidence level of calculating long-term creep strength at times well beyond the available experimental data. In this article, the Wilshire equation is used to extrapolate the creep life of HR6W and Sanicro 25, and different methods to determine creep activation energy, region splitting, the use of short-duration test data, and the omission of very-short-term data are investigated to determine their effect on correlation and calculations. It was found that using a known value of the activation energy of lattice self-diffusion, rather than calculating from each data set, is both the simplest and most viable method to determine . Region-splitting improved rupture time calculations for both alloys. Extrapolating creep life from short-term data for these alloys was found to be reasonable.

Developing constitutive equations of flow stress for hot deformation of AZ31 magnesium alloy under compression, torsion, and tension

A comparative study was carried out on AZ31 (Mg-3Al-1Zn) magnesium alloy to understand the effect of deformation mode on the hot flow stress. Accordingly, the peak flow stress resulted from hot compression, hot tension, and hot torsion tests were correlated to the Zener-Hollomon parameter using the Sellars-McTegart approach. Based on the analysis, by applying the lattice self-diffusion activation energy of magnesium (135 kJ/mol) as the deformation activation energy, the hyperbolic sine exponents were determined to be ~ 5. Consequently, by consideration of creep exponent of 5, some reliable semi-empirical constitutive equations were proposed for high-temperature deformation of AZ31 alloy and the effect of deformation mode was quantified.

On the Self - and Impurity Diffusion in High Entropy Alloys

General trends in self- and impurity diffusion data are analyzed in high entropy alloys. Our analysis is based on the similarity of inter-atomic potentials in metals, which is in fact equivalent to a three-parameter description of the system (the mass, m, the lattice spacing, a, and the melting point, Tm, are only used). This leads to the so-called law of corresponding states in metals, manifested in many empirical rules (e.g. compensation laws or the proportionality between the self-diffusion activation energy and the melting point) if one uses dimensionless/reduced variables (like the homologous temperature: T*=T/Tm). It was shown in our previous papers, using the concept of a hypothetical crystal composed of simple atomic species whose properties are an average of the components, that the tracer diffusion of any species (let it be either one of the constituent atoms or a foreign atom) can be considered as impurity diffusion in the pure many-component matrix. Using this concept, we illustrate that the diffusion coefficients, Di, follow the same rule which obtained for impurity diffusion in pure metals: lnDi=A(T*)(Tmi/Tm-1)+r, with the same fitting parameters A(T*) and r. According to this, the diffusion of the constituent elements in high entropy alloys indeed shows some sluggish character, which can be attributed to a more or less temperature independent factor.

Low-Temperature Anelastic Property of Nanocrystalline Ag Fabricated by Gas Deposition Method

It was reported that the internal friction (Q-1) of nanocrystalline (n-) Au and n-Cu showed a rapid increase linearly with temperature above ~200 K. Since the rapid increase in Q-1 decreased with the progress in the grain growth, it was suggested that the anelasticity of grain boundaries in n-Au and n-Cu was thermally activated above ~200 K. In order to pursue the intrinsic behavior of the grain boundaries in n-metals, the internal friction of n-Ag was measured and the result was compared with those of n-Au and n-Cu. Similar to n-Au and n-Cu, Q-1 of n-Ag was also showed the increase linearly with temperature above ~200 K. However, the onset temperature for the linear increase in Q-1 of n-Ag was slight lower than those of n-Au and n-Cu and it can be attributed to the lower activation energy of the grain boundary self-diffusion in Ag.

Calculations of activation energy of diffusion and self-diffusion

The methodology is given for the evaluation of activation energy of diffusion and self-diffusion based on the application of spatial-energy parameter (Р-parameter). The corresponding calculations are made for 57 structures. The calculation results are in accordance with the experimental data.

Temperature-induced structural evolution in liquid Sn85Zn15 eutectic alloy

Temperature-dependent atomistic structural evolution in liquid Sn85Zn15 eutectic alloy has been investigated by using in situ high energy X-ray diffraction and ab initio molecular dynamics simulations. We reveal a structural crossover at around 900 K in liquid Sn85Zn15 eutectic alloy, below and above which the liquids exhibit different changes in first peak position shifts on S(q) and G(r) curves, the nearest coordination number and activation energies for self-diffusion. This structural crossover is likely ascribed to the spatial connectivity variation of local clusters. Our finding will trigger more studies on the liquid-to-liquid structural transition in metallic liquids.

A theoretical model of grain boundary self-diffusion in metals with phase transitions (case study into titanium and zirconium)

The paper offers a model describing the process of grain boundary self-diffusion in metals with phase transitions in the solid state. The model is based on ideas and approaches found in the theory of non-equilibrium grain boundaries. The range of application of basic relations contained in this theory is shown to expand, as they can be used to calculate the parameters of grain boundary self-diffusion in high-temperature and low-temperature phases of metals with a phase transition. The model constructed is used to calculate grain boundary self-diffusion activation energy in titanium and zirconium and an explanation is provided as to their abnormally low values in the low-temperature phase. The values of grain boundary self-diffusion activation energy are in good agreement with the experiment.

Thermodynamic activation energy for self-diffusion and order-order relaxation in intermetallic compounds

Thermodynamic activation energy for self-diffusion and order-order relaxation in intermetallic compounds

Coarsening Kinetics of Pb Phase in a Nanocomposite Alloy Produced by Mechanical Alloying in Immiscible Al-Pb System and the Influence of Cu Addition on It

Coarsening kinetics of Pb phase in a nanocomposite alloy produced by mechanical alloying in immiscible Al-Pb system and the influence of Cu addition on it were studied by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Results show that when annealing at 573, 623, 673 and 723 K for different time, the relation of average particle size of Pb phase in Al-Pb nanocomposite alloys to time obeys the cube law even though the size of the constituent phase is in the nanometer range. The coarsening activation energy of Pb phase is 84.8 kJ/mol, close to the grain boundary self-diffusion activation energy of Al matrix. This indicates that the coarsening of Pb phase is controlled by grain boundary diffusion. Adding Cu decreases the coarsening rate of Pb phase, primarily by reducing the interfacial energy through Cu segregation at the Al/Pb interface. The coarsening activation energy of Pb phase is increased by the addition of Cu.

On the physical basis of a Larson-Miller constant of 20

Larson-Miller (LM) and Orr-Sherby-Dorn (OSD) parameters are widely used when formulating and predicting creep rupture life tr. The LM constant C and the activation energy Q in the OSD parameter characterize temperature dependence of tr. Q = QLSD (QLSD: activation energy for lattice self-diffusion) and C = 20 are often assumed in the formulation. Creep rupture datasets of 11 heats of nickel alloy A617, 304H and 304J1 stainless steels, 6 heats of A3004N aluminum alloy and four Mg-Al alloys are formulated with the parameters for determining C and Q of each dataset. Based on the correlation between C and Q among similar materials, it is discussed what is the C value equivalent to Q = QLSD. It is also examined how the C value changes with melting temperature of the material. The value of C equivalent to Q = QLSD varies from 14 to 9.5 depending on average values of log(rupture life) and of reciprocal temperature of the dataset. These values of C point out that C = 20 is not equivalent to Q = QLSD. The equivalent C values are similar in all the materials independently of their melting temperatures.

Study on hot deformation behavior and workability of squeeze-cast 20 vol%SiCw/6061Al composites using processing map

The hot deformation behavior and workability of squeeze-cast 20vol%SiCw/6061Al composites were investigated by isothermal compression test and development of processing map, which was verified by uniaxial upsetting and two-directional forging of big cylindrical billets. Due to dispersed strengthening caused by SiC whiskers, the activation energy of 20vol%SiCw/6061Al composites was much higher than the self-diffusion activation energy of pure Al. Both the DMM and MDMM were reliable in predicting the safe deformation regions. The instability regions predicted by the Prasad's and Murty's instability criterion were smaller than the actual ones, while the Gegel's instability criterion was not appropriate in predicting the instability regions of 20vol%SiCw/6061Al composites. The optimum deformation region was at higher temperature of 450–500°C and higher strain rate of 0.1–1s−1, in which the compressed samples exhibited perfect surface without cracks and microstructure with few of cavities due to the occurrence of partial melting and dynamic recrystallization. With the increase of deformation temperature and strain rate, the deformation mechanism changed from dynamic recovery to dynamic recrystallization. Squeezing casting technique with high filtration pressure may also contribute to the increase of strain rate of DRX, which was helpful for hot forging of Al matrix composites with high productivity.

The superplasticity and microstructure evolution of coarse-grained Ti40 alloy

ABSTRACTThe superplastic deformation characteristics of coarse-grained Ti40 alloy have been studied in the temperature and strain rate range of 760–880°C and 5 × 10−4 to 1 × 10−2 s−1, respectively. The alloy exhibited good superplasticity in all test conditions except at 760°C and strain rate higher than 5 × 10−3 s−1, with the maximum elongation of 436% at 840°C, 1 × 10−3 s−1. The activation energy value was found to be close to the self-diffusion activation energy of Ti40 alloy, suggesting that the rate controlling mechanism was lattice diffusion. The coarse grain was elongated and refined which can be attributed to the occurrence of dynamic recovery and continuous dynamic recrystallisation. These processes were promoted by the subgrain formation and evolution, resulting in the good superplasticity of Ti40 alloy with coarse grains.

A Study at the Workability of Ultra-High Strength Steel Sheet by Processing Maps on the Basis of DMM

AbstractThe hot workability of the ultra-high strength steel BR1500HS has been investigated by processing maps. A series of hot deformation tensile tests were carried out on a Gleeble-3500 thermal simulator in the deformation temperature range of 773–1,223 K and strain rate range of 0.01–10 s–1. The obtained flow stress curves reveal that the peak stress increases with the rising of strain rate and decreases with the rising of temperature. Based on dynamic materials model (DMM), the processing maps at the strains of 0.05, 0.10 and 0.15 were developed, and the optimum hot working conditions were recommended as the temperature range of 1,200–1,223 K and the strain rate range of 0.01–0.1 s–1, where the peak power dissipation efficiency is about 37 % revealing the occurrence of typical dynamic recrystallization (DRX). The main instability defects are deformation twinning and micro-crack occurring mainly at the temperature range of 773–873 K with the strain rate higher than 1 s–1. In order to deeply understand the microstructure mechanisms, the Zener–Hollomon parameter is solved, and then the self-diffusion activation energy is compared with the apparent activation energy Q at different deformation temperatures and strain rates.

Self-diffusion coefficients of hexamethylphosphoric triamide and ethylene glycol molecules in ethylene glycol solutions

The self-diffusion coefficients of the molecules of hexamethylphosphoric triamide (HMPT) and ethylene glycol (EG) in ethylene glycol solutions in the concentration range 0-16 mol % HMPT and molecules of pure HMPT in the temperature range 30–60°С are measured by the spin-echo method on protons. Activation energies for the corresponding processes of self-diffusion were calculated. The obtained data are discussed in terms of solvophobic effects in the EG–HMPT system. The self-diffusion coefficient of pure HMPT was 0.344 × 10−5 cm2/s at 33.2°C, and the self-diffusion activation energy was 3.86 kcal/mol.

Microstructure and hot deformation behavior of A356/Al2O3 composite fabricated by infiltration method

The hot deformation behavior of an A356/Al2O3 composite fabricated by the infiltration method was characterized in the temperature range of 300-500 °C and strain rate range of 0.001-1/s using compressive tests. The composite consists of an Al-Si based matrix and nano-sized Al2O3 particulates. A constitutive model was established based on the hyperbolic sine Arrhenius type equation and its hot workability was evaluated by means of processing maps based on Dynamic Material Modeling. The activation energy for hot deformation was calculated to be 223 kJ/mol, which is higher than the activation energy for self-diffusion of pure aluminum (142 kJ/mol). The optimum processing condition for the hot working of the composite was found to exist at 500 °C with a strain rate of 1/s, where a dynamic recrystallized microstructure was observed and the maximum efficiency was exhibited in the processing map. Voids were frequently detected at 500 °C with lower strain rates, deteriorating the workability of the composite.

Microstructure Stability of a Fine-Grained AZ31 Magnesium Alloy Processed by Constrained Groove Pressing During Isothermal Annealing

In this study, an AZ31 magnesium alloy plate was processed by constrained groove pressing (CGP) under three deformation cycles at temperatures from 503 to 448 K. The process resulted in a homogeneous fine grain microstructure with an average grain size of 1.8 μm. The as-processed microstructure contained a high fraction of low-angle grain boundaries (LAGB) of subgrains and dislocation boundaries that remained in the structure due to incomplete dynamic recovery and recrystallization. The material's yield strength was found to have increased from 175 to 242 MPa and with a significant weakening of its initial basal texture. The microstructure stability of the CGP-processed material was further investigated by isothermal annealing at temperature from 473 to 623 K and for different time. Abnormal grain growth was observed at 623 K, and this was associated with an increased in nonbasal grains at the expense of basal grains. The effect of annealing temperature and time on the grain growth kinetics was interpreted by using the grain growth equation, Dn+D0n=kt, and Arrhenius equation, k=k0 exp (−(Q/RT)). The activation energy (Q) was estimated to be 27.8 kJ/mol which was significantly lower than the activation energy for lattice self-diffusion (QL = 135 kJ/mol) and grain boundary diffusion (Qgb = 92 kJ/mol) in pure magnesium. The result shows that grain growth is rapid but average grain size still remained smaller than the as-received material, especially at the shorter annealing time.

Comparative study of severe plastic deformation at elevated temperatures of two aluminium alloys during friction surfacing

Aluminium alloys 5083 and 6082 were deposited by Friction Surfacing (FS) under the same process conditions. Process characteristics including torque and forces, temperatures and the deposit microstructure were compared. The observed differences are discussed with regard to material strength, thermal softening rate and recrystallization mechanisms.AA 6082 plasticises faster, reaching ≈30K higher temperatures, thicker and wider coatings and a higher material efficiency. The specific energy required for plastification is in the same order of magnitude as the activation energy for self-diffusion, emphasising the influence of dynamic recrystallization (DRX) mechanisms. A tendency for lower grain size and larger variations in grain boundary misorientation observed for AA 5083 points towards a shift in the steady-state DRX balance towards dislocation generation, due to the higher Mg content of this alloy. This corresponds to the lower process speeds required for AA 5083. AA 6082 may undergo more localized shear because of its high thermal softening rate and additional loss of strength through dissolution of Mg2Si with increasing temperature. This may contribute to a higher energy and material efficiency for plastification and deposition of AA 6082 by FS.

Atomistic origin of the thermodynamic activation energy for self-diffusion and order-order relaxation in intermetallic compounds I: analytical approach

The general state of knowledge of diffusion and order-order relaxation in a long-range ordered system is summarised. A consistent atomistic description of self-diffusion and the order-order relaxation process is discussed in terms of effective atomic jump frequencies and the current degree of chemical long-range order. It is demonstrated that the thermodynamic activation energies of self-diffusion and the order-order relaxation can be expressed in terms of the activation energies of more elementary processes. The derived expressions differ from each other indicating that although both processes are controlled by the same vacancy-mediated elementary atomic jumps, the values of their thermodynamic activation energies can be different.