Activation Enthalpy Of Self-diffusion Research Articles

Grain boundary self-diffusion in polycrystalline nickel of different purity levels

Grain boundary self-diffusion in Ni materials of two different purity levels (99.6 and 99.999 wt.%) was measured over wide temperature intervals using the radiotracer technique and applying the 63Ni radioisotope. The diffusion experiments were performed in both Harrison’s type B and type C kinetic regimes. The diffusional grain boundary width, δ, was found to be equal to 0.54 ± 0.1 nm, that is reasonably close to the accepted value of δ = 0.5 nm in face-centred cubic metals. The purer the material, the higher the grain boundary diffusivity and the lower the corresponding activation enthalpy of self-diffusion along general high-angle grain boundaries. Using the semi-empirical approach of Borisov et al., the average energy of high-angle grain boundaries was estimated to be about 0.79 and 0.98 J m −2 in the low- and high-purity Ni materials at room temperature, respectively.

Activation Enthalpy of Self-Diffusion in Pure Metals Interpreted as a Measure of their Interatomic Force Constant

The present study of self-diffusion data for pure solid elements has revealed that the diffusivities of body centered cubic (b.c.c.), face centered cubic (f.c.c.) and hexagonal close packed (h.c.p.) metals at their melting point temperatures (Tm) do relate in a systematic manner to the atomic number of the elements and their Period numbers. Also, the activation enthalpy of self-diffusion varies in a regular manner with the atomic number and Period number of the elements. It is surmised from these regularities that the activation enthalpy (Q) of self-diffusion may be considered as a direct measure of the interatomic force constant ε/k = (&phis;/R)Tm where &phis; = Q/Tm is the enthalpy coefficient. For solid metals ε/k = (15 to 19) Tm, K; for non-metals the Tm coefficient is (20 to 35), for rare-earth metals (10 to 13) and for liquid metals (3.32±0.24). A comparison is made of these ε/k values with those derived from the standard enthalpies of dissociation of inorganic compounds to the constituent elements. A critical assessment is made of the experimental self-diffusion data for pure elements which conform to the observed regularities in the self-diffusion parameters in relation to the atomic number and Period number of the elements. Also see Addendum.

Measurement of the Zener Relaxation in High-Purity FeCr Single Crystals

In binary alloys the measurement of the so-called Zener relaxation may be used as a non-destructive method to determine the activation enthalpy of self diffusion. High-purity FeCr single crystals with a Cr content of 53 at% were prepared by containerless inductive zone melting under Ar atmosphere. The internal friction was measured between temperatures of 300 to 1200 K using an inverted torsion pendulum in the frequency range between 2 and 80 Hz. A strong damping is observed below the Curie temperature of about 600 K due to domain wall motion. Above the ferromagnetic transition temperature a relaxation maximum occurs at about 920 K. The relaxation strength and the activation parameters were determined for single crystals with (100), (110) and (111) orientation. The relaxation can be assigned to the stress-induced reorientation of short-range ordered Fe and Cr atoms, respectively. An analysis yielded an effective activation enthalpy of (2.95 ± 0.3) eV showing no significant dependence on the crystal orientation. The orientation dependence of the relaxation strength and shear modulus indicates that jumps between nearest and next-nearest neighbor sites are involved. These results are compared with results of self-diffusion measurements.

Comment on “creep behavior of discontinuous SiCAl composites”

Recently, Park, Lavernia and Mohamed investigated creep in a 30 vol.%SiCp - reinforced aluminium alloy (SiCp-Al alloy 6061) at temperatures from 618 K to 678 K and applied shear stresses ranging from 4.3 [times] 10[sup [minus]4] G to 3.0 [times] 10[sup [minus]4] G (G is the shear modulus). The measured minimum creep shear strain rates covered seven orders of magnitude (3 [times] 10[sup [minus]9]s[sup [minus]1] to 9 [times] 10[sup [minus]2]s[sup [minus]1]). The authors show that at any given temperature, the stress sensitivity parameter of the minimum creep strain rate m[prime] = ([partial derivative]lnjm/[partial derivative]ln[tau])T increases with decreasing stress, reaching a value close to 25 at a creep shear strain rate jm of about 10[sup [minus]8]s[sup [minus]1]. Later, Mohamed, Park and Lavernia showed that the true stress exponent of the minimum creep strain rate is approximately equal to 5, and reported the threshold stress decreases with increasing temperature. Further, the authors estimated the true activation energy of creep, Qc, to 225 kJmol[sup [minus]1], with the activation enthalpy of self-diffusion in the matrix lattice being [Delta]HL = 142 kJmol[sup [minus]1]. In the present paper, the data of the authors are reanalyzed to show that the creep in the composite under consideration ismore » most probably matrix lattice diffusion controlled.« less

Point Defects in Materials Part I: Behavior and Characteristics in Different Material Classes

In this issue of the MRS Bulletin we present five papers that involve point defect phenomena in a wide variety of materials—metals, conducting ceramic oxides, semiconductors, amorphous alloys, and high Tc superconducting oxides. The unifying theme of this issue is point defects—zero-dimensional defects. Even for the high Tc oxides, where planar defects are discussed, it is the ordering of oxygen/vacancy chains that ultimately gives rise to twins in the famous YBa2Cu3O7.δ (1:2:3) oxide superconductor.Hillard Huntington, professor emeritus of physics at Rensselaer Polytechnic Institute, is an early and important pioneer in the study of point defects in metals. A theorist, he has also performed many experiments over the years; for example, he performed key early experiments on electromigration effects. Huntington's article presents a historical review of the research on vacancies and self-interstitial atoms in metals during the period that stretches from the mid-1930s to the mid-1960s. He played a crucial role in this field as a result of his seminal theoretical calculations, with Fred Seitz in 1942, on the enthalpies of formation and migration of vacancies or self-interstitial atoms in pure copper. Huntington's and Seitz's calculations indicated that diffusion occurs predominantly by a monovacancy mechanism since the enthalpy of formation of a self-interstitial atom, the [100] split form, also called the dumbbell form, is too large to be compatible with the activation enthalpy for self-diffusion in copper. It is now well established that the latter is given by the sum of the enthalpy of formation and the enthalpy of migration of a monovacancy in many face-centered-cubic (fcc), body-centered-cubic (bec), and hexagonal-close-packed (hcp) metals.

Cation self-diffusion and impurity diffusion in Fe 2O 3+

Cation self-diffusion D ∗ Fe, parallel to the c axis has been measured as a function of temperature (1100–1300°C) and oxygen partial pressure p o 2 (2 × 10 −3-1 atm) in the same single crystals of Fe 2O 3 as those used by Chang and Wagner. Whereas the p o 2 dependence of D∗ Fe, observed by Chang and Wagner has been confirmed, the absolute value of D ∗ Fe and the activation enthalpy for self-diffusion are much higher than those reported by them. The various diffusion studies indicate that cation self-diffusion occurs by an interstitial-type mechanism. However, the sample-to-sample variations in D ∗ Fe, suggest that all diffusion measurements may have been performed on samples where the defect concentrations are impurity controlled. Impurity diffusion of 60Co, 51Cr, and 88Y has also been measured as a function of p o 2 at 1200°C. The results indicate that these impurities diffuse by an interstitialcy mechanism in Fe 2O 3.

A radio-tracer study of self-diffusion in the smectic phasesiso-butyl 4-(4′-phenylbenzylideneamino) cinnamate [IBPBAC

Average translational self-diffusion coefficients, , have been determined in the smectic E, B and A phases of unaligned samples of iso-butyl 4-(4′-phenylbenzylideneamino) cinnamate. A radiotracer technique was employed which was based on the serial sectioning method used in the study of diffusion in solids. In the smectic E phase is about 1 × 10-14 m2 s-1. The self-diffusion in the smectic B phase fits reasonably well to an Arrhenius law with an activation enthalpy of 82 kJ mol-1. The transition from the smectic B to A phase results in a dramatic rise in . In the smectic A phase the results fit an activation enthalpy for self-diffusion of about 168 kJ mol-1. Similarities between the diffusion in the smectic E and B phases and in plastic solids are observed.

Measurement of the monovacancy formation enthalpy in copper

The monovacancy formation enthalpy in copper, H1vF=(1.30+or-0.05) eV, has been measured using precision electrical resistometry on samples quenched from temperatures in the range 525-650 degrees C. Comparison of this value for H1vF with the activation enthalpy for self-diffusion via a monovacancy mechanism in copper, previously measured over the same temperature range, yields H1vM=0.77 eV for the enthalpy for monovacancy migration. This value compares well with the activation enthalpy for post-quench vacancy annealing, (0.76+or-0.04) eV, measured in the present work. The results are compared with those of previous investigations.

High temperature creep in a Cu-11 at. % Al solid solution

Abstract The creep in a Cu-11 at. % solid solution was investigated by the isothermal tests technique. In the region of lower temperatures and lower creep rates, respectively, the apparent activation energy of creep is stress independent and its value is close to that expected for the activation enthalpy of self-diffusion of aluminium in the solid solution. The stress exponent n = η In δέ/δ In a increases with stress from 4 to 5·5. Viscous motion of dislocations connected with the interaction of aluminium atoms with dislocations was suggested to be the rate-controlling mechanism. In the region of higher temperatures and higher creep rates, respectively, two mechanisms seem to operate. The apparent activation energies are strongly stress dependent and are considerably higher than the activation enthalpy of self-diffusion of copper in the solid solution. The stress exponents n generally depend on stress and temperature. The results for the higher stress interval 6·0–11·0 Kg mm−2 can be correlated with the m...