Activation Energy For Bulk Diffusion Research Articles

Probing the onset of wurtzite phase formation in (V,Al)N thin films by transmission electron microscopy and atom probe tomography

Single-phase metastable cubic (V,Al)N thin films with columnar microstructure were grown by high power pulsed magnetron sputtering at 440 °C and the thermal decomposition mechanisms were systematically investigated by post-deposition vacuum annealing from 600 to 900 °C. The onset of spinodal decomposition into isostructural V- and Al-rich cubic nitride phases is demonstrated after cyclic vacuum annealing at 700 °C. Moreover, at this temperature, evidence for aluminum diffusion to grain boundaries and triple junctions is provided by correlation of transmission electron microscopy and atom probe tomography data. The formation of Al-rich regions can be understood by the more than 25% lower activation energy for bulk diffusion of aluminum compared to vanadium as obtained from ab initio calculations. It is reasonable to assume that these Al-rich regions are precursors for the formation of wurtzite AlN, which is unambiguously identified after annealing at 800 °C by microscopy and tomography. The significantly larger equilibrium volume of wurtzite AlN compared to the cubic phase explains its initial formation exclusively at triple junctions and grain boundaries. In contrast, twin boundaries are enriched in vanadium. Interestingly, the formation of the wurtzite phase at grain boundaries and triple junctions can be tracked by resistivity measurements, while X-ray diffraction and nanoindentation data do not support an unambiguous wurtzite phase formation claim for annealing temperatures <900 °C. Hence, it is evident that previously reported formation temperatures of wurtzite AlN in transition metal aluminum nitrides, determined by other characterization techniques than chemical and structural characterization at the nanometer scale and/or resistivity measurements, are overestimated.

Effects of Deposition Potential and Temperature on Co(II) Reduction and Electrocrystallization for Preparing Nanocrystalline Co Coatings in Ethylene Glycol Solution

Based on cyclic voltammetry (CV) and chronoamperometry (CA) experiments, the effects of deposition potential and temperature on Co(II) reduction and electrocrystallization in ethylene glycol (EG) are reported. CV observations show that Co(II) reduction reaction is irreversible, and both the equilibrium potential and nucleation overpotential, η nucleation, move to more positive values as the temperature is improved. The diffusion coefficient of Co(II) is 2.24 × 10−6 cm2 s−1 at 323 K and the activation energy for bulk diffusion (E bd) of Co(II) is 22.03 ± 0.8 kJ mol−1. CA observations show that the Co electrocrystallization mechanism is a 3D nucleation and growth of a new phase under diffusion control. Its mechanism is an instantaneous regime at 323 K at various applied potentials, however, the mechanism changes from a progressive nucleation to an instantaneous nucleation at −1.10 V as the temperature increases. The cobalt nuclei are spherical microstructure, and these innumerable spherical crystal particles can align compactly to form many wirelike crystals bundles. XRD observation confirms the characteristic peak of crystalline Co with a preferred orientation direction and the average size of Co grains is 19 nm. It is also found that the Co coating thickness is affect by the potential and temperature.

Characteristics of oxygen diffusion induced by Al and Li interaction in Al-implanted zinc oxide ceramics

We studied on the oxygen diffusion in two kinds of Al implanted ZnO samples (100 keV, 4 × 1014 ions/cm2). Parts of samples were annealed to diffuse simultaneously Al and Li impurity, and then same concentration distribution of Al and Li were observed. Through the oxygen diffusion experiments, the feature of bulk diffusion in the pre-annealed samples was separated at 950°C so that the large activation energy for bulk diffusion was observed below the pre-annealing temperature. Oxygen diffusion feature was explained as result from interaction between Al and Li for the pre-annealing.

A strategy for designing stable nanocrystalline alloys by thermo-kinetic synergy

Aiming to design stable nanocrystalline (NC) materials, so far, it has been proposed to construct nanostructure stability maps in terms of thermodynamic parameters, while kinetic stabilization has seldom been considered, despite the synergy of thermodynamics and kinetics. Consequently, the thermodynamically stabilized NC materials may be easily subjected to grain growth at high temperatures due to the weakly kinetic stabilization. Starting from the thermo-kinetic synergy, a stabilization criterion is proposed as a function of intrinsic solute parameters (e.g. the activation energy for bulk diffusion and the segregation enthalpy), intrinsic solvent parameters (e.g. the intrinsic activation energy for GB migration and the GB energy) and processing parameters (e.g. the grain size, the temperature and the solute concentration). Using first-principles calculations for a series of combinations between fifty-one substitutional alloying atoms as solute atoms and Fe atom as fixed solvent atom, it is shown that the thermal stability neither simply increases with increasing the segregation enthalpy as expected by thermodynamic stabilization, nor monotonically increases with increasing the activation energy for bulk diffusion as described by kinetic stabilization. By combination of thermodynamic and kinetic contributions, the current stabilization criterion evaluates quantitatively the thermal stability, thus permitting convenient comparisons among NC materials involved by various combinations of the solute atoms, the solvent atoms, or the processing conditions. Validity of this thermo-kinetic stabilization criterion has been tested by current experiment results of Fe-Y alloy and previously published data of Fe-Ni, Fe-Cr, Fe-Zr and Fe-Ag alloys, etc., which opens a new window for designing NC materials with sufficiently high thermal stability and sufficiently small grain size.

INFLUENCE OF TEMPERATURE ON THE THERMODYNAMICS AND KINETICS OF COBALT ELECTROCHEMICAL NUCLEATION AND GROWTH

From potentiodynamic and potentiostatic experiments, the cobalt electrochemical nucleation and growth process, onto the glassy carbon electrode surfaces, from an aqueous solution containing 10−2M CoCl2 and 1M NH4Cl (pH=4.66) at different temperatures (15–60°C) is reported. It was found that while the equilibrium potential moves to more negative values as the temperature of the system was increased, the contrary was observed for the nucleation overpotential, ηnucleation, required for the onset of cobalt nucleation onto the surfaces of vitreous carbon. From Tafel plots recorded at the different temperatures considered, both: the transfer coefficient, α, and the exchange current density, j0, associated with the Co(II)/Co(0) system were assessed, and from the Arrhenius plot (ln j0 vs. T−1) the activation energy for Co(II) reduction of (27.9±0.3) kJmol−1 was estimated. From analysis of the potentiostatic current density transients according with the formalism proposed by Palomar-Pardavé et al. (Electrochim. Acta 50 (2005) 4736-4745), the total current density was de-convoluted into individual contributions, due to formation of multiple mass-transfer controlled 3D Co nuclei and that associated to the proton reduction occurring simultaneously on the growing surface of the Co nuclei. The latter contribution was practically null at 10°C, however, it drastically increased as the temperature of the system did, provoking that the cathodic efficiency for Co deposition diminished. Furthermore, it was found that both the number density of active sites, N0, and the nucleation rate, A, for Co nucleation depend exponentially with ηnucleation, regardless of temperature, except at 60°C where N0 diminished linearly with ηnucleation. From variations of the Co(II) ions diffusion coefficient with temperature, a value of (8.5±0.2) kJmol−1 was obtained for the activation energy for bulk diffusion. SEM analysis showed that the cobalt nuclei are bigger as the temperature increases; however, the coverage of the electrode surfaces becomes lower.

Diffusion Kinetics of Indium in TiO2 (Rutile)

This work determines the self‐diffusion coefficients of indium in TiO2 single crystal (rutile). Diffusion concentration profiles were imposed by deposition of a thin surface layer of InCl3 on the TiO2 single crystal and subsequent annealing in the temperature range 1073–1573 K. The diffusion‐induced concentration profiles of indium as a function of depth were determined using secondary ion mass spectrometry (SIMS). These diffusion profiles were used to calculate the self‐diffusion coefficients of indium in the polycrystalline In2TiO5 surface layer and the TiO2 single crystal. The temperature dependence of the respective diffusion coefficients, in the range 1073–1573 K, can be expressed by the following formulas: urn:x-wiley:00027820:media:jace12244:jace12244-math-0001and urn:x-wiley:00027820:media:jace12244:jace12244-math-0002The obtained activation energy for bulk diffusion of indium in rutile (316 kJ/mol) is similar to that of zirconium in rutile (325 kJ/mol). The determined diffusion data can be used in selection of optimal processing conditions for TiO2–In2O3 solid solutions.

Strain-induced grain growth of cryomilled nanocrystalline Al in trimodal composites during forging

Abstract Grain growth of nanocrystalline aluminum ( nc Al) in trimodal Al metal-matrix-composites (MMCs) during hot forging was investigated. The nc Al phase formed through cryomilling of inert gas-atomized powders in liquid nitrogen has an average grain size down to 21 nm, exhibits excellent thermal stability. However, substantial grain growth of nc Al up to 63 nm was observed when the Al MMCs were thermo-mechanically processed even at relatively low temperatures. Grain growth of the cryomilled nc Al phase in trimodal Al MMCs after hot forging was documented with respect to temperature ranging from 175 °C to 287 °C, true strain ranging from 0.4 to 1.35 and strain rate ranging from 0.1 to 0.5 s −1 . Hollow cone dark field imaging technique was employed to provide statistically confident measurements of nc Al grain size that ranged from 21 to 63 nm. An increase in forging temperature and an increase in true strain were correlated with an increase in grain size of nc Al. Results were correlated to devise a phenomenological grain growth model for forging that takes strain, strain rate and temperature into consideration. Activation energy for the grain growth during thermo-mechanical hot-forging was determined to be 35 kJ/mol, approximately a quarter of activation energy for bulk diffusion of Al and a half of activation energy for static recrystallization.

Hydrogen adsorption, absorption and diffusion on and in transition metal surfaces: A DFT study

Periodic, self-consistent DFT-GGA(PW91) calculations are used to study the interaction of hydrogen with different facets of seventeen transition metals—the (100) and (111) facets of face-centered cubic (fcc) metals, the (0001) facet of hexagonal-close packed (hcp) metals, and the (100) and (110) facets of body-centered cubic (bcc) metals. Calculated geometries and binding energies for surface and subsurface hydrogen are reported and are, in general, in good agreement with both previous modeling studies and experimental data. There are significant differences between the binding on the close-packed and more open (100) facets of the same metal. Geometries of subsurface hydrogen on different facets of the same metal are generally similar; however, binding energies of hydrogen in the subsurface of the different facets studied showed significant variation. Formation of surface hydrogen is exothermic with respect to gas-phase H2 on all metals studied with the exception of Ag and Au. For each metal studied, hydrogen in its preferred subsurface state is always less stable than its preferred surface state. The magnitude of the activation energy for hydrogen diffusion from the surface layer into the first subsurface layer is dominated by the difference in the thermodynamic stability of these two states. Diffusion from the first subsurface layer to one layer further into the bulk does not generally have a large thermodynamic barrier but still has a moderate kinetic barrier. Despite the proximity to the metal surface, the activation energy for hydrogen diffusion from the first to the second subsurface layer is generally similar to experimentally-determined activation energies for bulk diffusion found in the literature. There are also some significant differences in the activation energy for hydrogen diffusion into the bulk through different facets of the same metal.

Evaluation of the Role of Au in Improving Catalytic Activity of Ni Nanoparticles for the Formation of One-Dimensional Carbon Nanostructures

In situ dynamic imaging, using an environmental transmission electron microscope, was employed to evaluate the catalytic activity of Au/SiO(2), Ni/SiO(2), and Au-Ni/SiO(2) nanoparticles for the formation of one-dimensional (1-D) carbon nanostructures such as carbon nanofibers (CNFs) and nanotubes (CNTs). While pure-Au thin-film samples were inactive for carbon deposition at 520 °C in 0.4 Pa of C(2)H(2), multiwalled CNTs formed from Ni thin films samples under these conditions. The number of nanoparticles active for CNF and CNT formation increased for thin films containing 0.1 mol fraction and 0.2 mol fraction of Au but decreased as the overall Au content in thin films was increased above 0.5 mol fraction. Multiwalled CNTs formed with a root growth mechanism for pure Ni samples, while with the addition of 0.1 mol fraction or 0.2 mol fraction of Au, CNFs were formed via a tip growth mechanism at 520 °C. Single-walled CNTs formed at temperatures above 600 °C in samples doped with less than 0.2 mol fraction of Au. Ex situ analysis via high-resolution scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) revealed that catalytically active particles exhibit a heterogeneous distribution of Au and Ni, where only a small fraction of the overall Au content was found in the portion of each particle actively involved in the nucleation of graphitic layers. Instead, the majority of the Au was found to be segregated to an inactive capping structure at one the end of the particles. Using density-functional theory calculations, we show that the activation energy for bulk diffusion of carbon in Ni reduces from ≈1.62 eV for pure Ni to 0.07 eV with the addition of small amounts (≈0.06 mol fraction) of Au. This suggests that the enhancement of C diffusion through the bulk of the particles may be responsible for improving the number of particles active for nucleating the 1-D carbon nanostructures and thereby the yield.

Diffusion-controlled true grain-boundary sliding in nanostructured metals and alloys

Based on the study of grain-boundary internal friction, it has been demonstrated that the temperatures of onset and intensive development of true grain-boundary sliding (grain-boundary microsliding) as well as the activation energy of this process in submicrocrystalline and nanocrystalline metals and alloys with imperfect grain boundaries relative to coarse-grained ones with perfect grain boundaries decrease. For all grain structures studied the activation energy of grain-boundary internal friction is found to exceed the respective one for grain-boundary diffusion but to be lower than the activation energy of bulk diffusion. The diffusion-controlled mechanism of true grain-boundary sliding is validated.

Diffusion of nickel in single- and polycrystalline Cr2O3

Chromia protective layers are formed on many industrial alloys to prevent corrosion by oxidation. Their role is to limit the inward diffusion of oxygen and the outward diffusion of cations. A number of chromia-forming alloys contain nickel as a component, such as steels, FeNiCr and NiCr alloys. To ascertain if chromia is a barrier to outward diffusion, nickel diffusion in chromia was studied in both single crystals and polycrystals in the temperature range 900–1100°C at an oxygen pressure of 10−4 atm (argon + 100 ppm O2). A nickel film of ∼35 nm thick was deposited on the chromia surface and, after diffusing treatment, nickel penetration profiles were established by secondary ion mass spectrometry (SIMS). Two diffusion domains appear in polycrystals, the first domain is assigned to bulk diffusion and the second is due to diffusion along grain boundaries. For the bulk diffusion domain and diffusion in single crystals, using a solution of Fick's second law for diffusion from a thick film, bulk diffusion coefficients were determined at 900 and 1000°C. At the higher temperature, a solution of Fick's second law for diffusion from a thin film could be used. For the second domain in polycrystals, Le Claire's model allowed the grain boundary diffusion parameter (αD gb δ) to be established. Nickel bulk diffusion does not vary significantly according to the microstructure of chromia. The activation energy of grain boundary diffusion is slightly greater than the activation energy of bulk diffusion, probably on account of segregation phenomena. Nickel diffusion was compared with cationic self-diffusion and with literature data on Fe and Mn heterodiffusion in the bulk and along grain boundaries. All results were analyzed in relation to the oxidation process of stainless steel.

Impurity diffusion of 141Pr in LaMnO3, LaCoO3 and LaFeO3 materials

The impurity diffusion of Pr(3+) in dense polycrystalline LaMnO(3), LaCoO(3) and LaFeO(3) was studied at 1373-1673 K in air in order to investigate cation diffusion in these materials. Cation distribution profiles were measured by secondary-ion mass spectrometry and it was found that penetration profiles of Pr(3+) had two distinct regions with different slopes. The first, shallow region was used to evaluate the bulk diffusion coefficients. The activation energies for bulk diffusion of Pr(3+) in LaMnO(3), LaCoO(3) and LaFeO(3) were 126 +/- 6, 334 +/- 68 and 258 +/- 75 kJ mol(-1), respectively, which are significantly lower than previously predicted by atomistic simulations. The bulk diffusion of Pr(3+) in LaMnO(3) was enhanced compared to LaCoO(3) and LaFeO(3) due to higher concentrations of intrinsic point defects in LaMnO(3), especially La site vacancies. Grain-boundary diffusion coefficients of Pr(3+) in LaCoO(3) and LaFeO(3) materials were evaluated according to the Whipple-Le Claire equation. Activation energies for grain-boundary diffusion of Pr(3+) in LaCoO(3) and LaFeO(3) materials were 264 +/- 41 kJ mol(-1) and 290 +/- 36 kJ mol(-1) respectively. Finally, a correlation between activation energies for cation diffusion in bulk and along grain boundaries in pure and substituted LaBO(3) materials (B = Cr, Fe, Co) is discussed.

Diffusion Mechanisms in Nanocrystalline and Nanolaminated Au-Cu

Thermal anneal treatments are used to identify the temperature range of the two dominant diffusion mechanisms – bulk and grain boundary. To assess the transition between mechanisms, the low temperature range for bulk diffusion is established utilizing the decay of static concentration waves in composition-modulated nanolaminates. These multilayered structures are synthesized using vapor deposition methods as thermal evaporation and magnetron sputtering. However, at low temperature the kinetics of grain-boundary diffusion are much faster than bulk diffusion. The synthesis of Au-Cu alloys (0-20 wt.% Cu) with grain sizes as small as 5 nm is accomplished using pulsed electro-deposition. Since the nanocrystalline grain structure is thermally unstable, these structures are ideal for measuring the kinetics of grain boundary diffusion as measured by coarsening of grain size with low temperature anneal treatments. A transition in the dominant mechanism for grain growth from grain boundary to bulk diffusion is found with an increase in temperature. The activation energy for bulk diffusion is found to be 1.8 eV·atom-1 whereas that for grain growth at low temperatures is only 0.2 eV·atom-1. The temperature for transitioning from the dominant mechanism of grain boundary to bulk diffusion is found to be 57% of the alloy melt temperature and is dependent on composition.

Cation diffusion in La 1− xSr xFeO 3− δ, x = 0 and 0.1 measured by SIMS

Cation tracer diffusion coefficients in La 1− x Sr x FeO 3− δ , ( x = 0 and 0.1) at temperatures ranging from 1173 to 1373 K have been investigated by secondary ion mass spectrometry (SIMS) for both A-site and B-site diffusion applying Y 3+and Cr 3+as tracer ions. B-site impurity (represented by Cr) diffusion was found to be faster than A-site impurity (represented by Y) diffusion for LaFeO 3 in the whole temperature interval investigated and for La 0.9Sr 0.1FeO 3 at temperatures less than 1273 K. The obtained activation energies for bulk diffusion in La 0.9Sr 0.1FeO 3− δ were 168 ± 24 kJ/mol for the Y tracer and 150 ± 30 kJ/mol for the Cr tracer, which are significantly lower than activation energies reported from diffusion couple experiments. The large variation in reported activation energies can be explained in terms of the difference in cation vacancy populations between the two experimental techniques. The diffusion coefficients were also clearly affected by a first order phase transition at approx. 1263 K in pure LaFeO 3. The obtained grain boundary diffusion coefficients were about five order of magnitude higher than the bulk diffusion coefficients, with an activation energy of 128 ± 28 kJ/mol.

First-principles study of carbon diffusion in bulk nickel during the growth of fishbone-type carbon nanofibers

Ab initio plane wave density functional theory calculations are performed to investigate the carbon diffusion in bulk nickel during the growth of fishbone-type carbon nanofibers (CNFs). Results indicate that the octahedral interstitial sites are preferred for C dissolution relative to the tetrahedral sites. And the heat of solution of C in paramagnetic (PM) Ni is larger than that in ferromagnetic (FM) Ni because the induced C atom quenches the magnetic moments of neighboring Ni atoms. The bulk diffusion has been successfully described under two different C concentrations. At the initial CNF growth stage, the C concentration in bulk Ni is low and the calculated energy barriers for the diffusion of an isolated C atom are 1.641 eV and 1.678 eV in the Ni FM and PM state, respectively. When the C content is increased to 20 at.%, two models are established. In one case, it is assumed that all C atoms hop in the same direction at the same time, and the calculated activation energies are 1.137 eV and 1.126 eV. In the other case, only one C atom is permitted to move with the neighboring C atoms fixed at the octahedral sites and the corresponding barriers are decreased to 0.972 eV in the Ni FM state. Through these calculations, it is concluded that the magnetic state has a minor effect on the diffusion energy barrier which can be substantially lowered by the increase of C concentration in bulk Ni. Comparing the activation energy for bulk diffusion with the surface diffusion results, the reason for the formation of different CNF morphologies has been revealed.

Study of Fe Diffusion in Cr&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; by Secondary Ion Mass Spectrometry

Chromia protective layers are used to prevent corrosion by oxidation of many alloys, such as the stainless steels, for instance. To check if chromia is a barrier to the outward diffusion of iron in these alloys, iron diffusion in chromia was studied in both polycrystals and oxide films formed by oxidation of Ni-30Cr alloy in the temperature range 700-1100°C at an oxygen pressure equal to 10-4 atm. An iron film of about 70 nm thick was deposited on the chromia surface, and after the diffusing treatment, the iron depth profiles were established by secondary ion mass spectrometry (SIMS). Using a solution of the Fick’s second law for diffusion from a thick film, effective or bulk diffusion coefficients were determined in a first penetration domain. Then, Le Claire’s and Hart’s models allowed both the bulk diffusion coefficient and the grain boundary diffusion parameter (aDgbd) to be obtained in a second penetration domain. Iron bulk and grain boundary diffusion does not vary significantly according to the nature-microstructure of chromia. The activation energy of grain boundary diffusion is at least equal or even greater than the activation energy of bulk diffusion, probably on account of segregation phenomena. Iron diffusion was compared to cationic self-diffusion and related to the protective character of chromia.

Diffusion of iron in Cr2O3: polycrystals and thin films

Chromia protective layers are formed on many industrial alloys to prevent corrosion by oxidation. The role of such layers is to limit the inward diffusion of oxygen and the outward diffusion of cations. A number of chromia-forming alloys contain iron as a major component, such as the stainless steels. To check if chromia is a barrier to the outward diffusion of iron in these alloys, iron diffusion in chromia was studied in both polycrystals and oxide films formed by oxidation of Ni–30Cr alloy in the temperature range 700–1100 ◦ C at an oxygen pressure equal to 10 −4 atm. An iron film of about 80 nm thick was deposited on the chromia surface, and after the diffusing treatment, iron depth profiles were established by secondary ion mass spectrometry (SIMS). Two diffusion domains appear whatever the nature of the chromia material, polycrystals or films. In the first domain, using a solution of the Fick’s second law for diffusion from a thick film, effective or bulk diffusion coefficients were determined. With the second domain, Le Claire’s and Hart’s models allowed both the bulk diffusion coefficient and the grain-boundary diffusion parameter (αDgbδ) to be obtained. Iron bulk and grain-boundary diffusion does not vary significantly according to the microstructure of chromia. The activation energy of grain-boundary diffusion is at least equal or even greater than the activation energy of bulk diffusion, probably on account of segregation phenomena. Iron diffusion was compared to cationic self-diffusion in the bulk and along grain boundaries and related to the protective character of chromia. © 2004 Elsevier B.V. All rights reserved.

Mass transport in La 1− xSr xFeO 3( x=0 and 0.1) measured by SIMS

Tracer diffusion coefficients in La 1− x Sr x FeO 3, x=0 and 0.1 from 900 to 1100 °C for both A- and B-site diffusions were examined with secondary ion mass spectrometry (SIMS) applying Y 3+ and Cr 3+ as tracers. In both compositions, the B-site diffusion, represented by Cr, was found to be faster than the A-site diffusion, represented by Y. The obtained activation energies for bulk diffusion in La 0.9Sr 0.1FeO 3 were 168±24 kJ/mol for the Y tracer and 150±30 kJ/mol for the Cr tracer. The 18O-tracer diffusion coefficients were found to be significantly higher in La 0.9Sr 0.1FeO 3 compared with LaFeO 3.

Coarsening kinetics of Cu particles in an Fe-1.5 % Cu alloy

The Ostwald ripening of spherical and rod-shaped Cu precipitate particles in an Fe-1.5 mass% Cu alloy aged between 873 and 1023 K has been examined by measuring both the particle size by electron microscopy and the Cu concentration in the α-Fe matrix by electrical resistivity. The average radius of spherical and rod-shaped particles increases with ageing time t as t 1 / 3 , in agreement with the growth by diffusion-controlled coarsening. The kinetics of the depletion of supersaturation with t obey the predicted t - 1 / 3 time law. The Cu/a-Fe interfacial energy and the diffusivity of Cu in α-Fe have been independently derived from the data on coarsening. The interfacial energies are measured as 0.57 and 1.1 Jm - 2 for the spherical and rod-shaped particles respectively. The pre-exponential factor and the activation energy for bulk diffusion are found to be 4.54 × 10 - 5 m 2 s - 1 and 241 kJmol - 1 .

Coarsening kinetics of Cu particles in an Fe-1.5% Cu alloy

Abstract The Ostwald ripening of spherical and rod-shaped Cu precipitate particles in an Fe-1.5 mass% Cu alloy aged between 873 and 1023 K has been examined by measuring both the particle size by electron microscopy and the Cu concentration in the α-Fe matrix by electrical resistivity. The average radius of spherical and rod-shaped particles increases with ageing time t as t 1/3, in agreement with the growth by diffusion-controlled coarsening. The kinetics of the depletion of supersaturation with t obey the predicted t – 1/3 time law. The Cu/α-Fe interfacial energy and the diffusivity of Cu in α-Fe have been independently derived from the data on coarsening. The interfacial energies are measured as 0.57 and 1.1 Jm–2 for the spherical and rod-shaped particles respectively. The pre-exponential factor and the activation energy for bulk diffusion are found to be 4.54α10 –5 m2s– 1 and 241 kJmol – 1.