Dislocation Diffusion Coefficients Research Articles

Precipitation Behavior of MN(M=V,Nb) Phase in the 7Cr-1.5Mo Ferritic Heat Resistant Steels

The relationship between precipitation behavior of MX(M=V, Nb) nitrides and mechanical property during aging in 7Cr-1.5Mo ferritic heat resistant steels were investigated in this study. Artificial aging was carried out at 700°C for different time from 100 to 10000 hours to simulate the environment of fossil power plant. The Ostwald ripening equation is related to the containing elements can explain the accelerated coarsening of MX in the model alloys. The Ostwald ripening equation was modified by introducing the effective diffusion coefficient as a sum of the lattice diffusion coefficient and dislocation diffusion coefficient, based on an assumption that V, Nb and Ti might diffuse not only by the lattice diffusion mechanism but also by the dislocation diffusion mechanism.

Accelerated Coarsening of MX Carbonitrides in 12%Cr Steels during Creep Deformation

MX carbonitrides (M=Nb, V, Ta/X=C, N), which are finely dispersed in the matrix of 12%Cr ferritic heat resistance steels, increase the creep strength of the steels by the precipitation strengthening mechanism. However, MX particles usually coarsen during high temperature creep deformation, and this results in a decrease of creep strength. In order to clarify the coarsening process of MX particles in 12%Cr steels (10.5Cr-0.1C-(Nb, V, Ta)), TEM samples were prepared from the grip portion (creep stress free zone) and the gauge portion (stress zone) of ruptured creep test pieces. The changes of MX particle radii in the gauge portion were considerably larger than that in the grip portion, indicating that creep stress accelerated Ostwald ripening of the particles in the gauge portion. The Ostwald ripening equation was modified by introducing the effective diffusion coefficient as a sum of the lattice diffusion coefficient and dislocation diffusion coefficient, based on an assumption that Nb and V might diffuse not only by the lattice diffusion mechanism but also by the dislocation diffusion mechanism. The effect of dislocation diffusion in the gauge portion was larger than that in the grip portion because dislocations in the gauge portion were moved by creep stress, and a rate of solute atoms diffusing along the dislocations might be increased. The coarsening process of MX particles in the gauge portion could be simulated successfully by means of a modified ripening equation.

Volume and dislocation diffusion of iron, chromium and cobalt in CVD ß-SiC

Impurity tracer diffusion of 59Fe, 51Cr and 57Co in CVD b-SiC has been studied in the temperature range between 973 and 1873 K. The temperature dependence of the volume diffusion coefficients of iron and chromium can be expressed by linear Arrhenius equations. The preexponential factor and the activation energy are estimated to be 8.7 × 1015 m2 s-1 and 111 kJ mol-1 for iron, respectively, and 9.5 × 1015 m2 s-1 and 81 kJ mol1 for chromium, respectively. The diffusion coefficients of iron and chromium are much higher than those of the self-diffusion in ß-SiC. Furthermore, the activation energies for the diffusion of iron and chromium are about one-tenth of those for carbon and silicon in b-SiC. Therefore, it seems that an interstitial mechanism is predominant for the diffusion of iron and chromium in b-SiC. On the other hand, the diffusion coefficient of cobalt above 1673 K is higher than that of iron, while at lower temperatures it is much lower than that of iron. The difference in the diffusion coefficients at 1173 K is more than three orders of magnitude. Thus, the temperature dependence of the diffusion coefficients of cobalt shows a strongly curved Arrhenius relation. This suggests that cobalt atoms diffuse by an interstitial mechanism at higher temperatures and by a substitutional mechanism at lower temperatures. From the deeper regions of the penetration profiles of iron, chromium and cobalt the dislocation diffusion coefficients of them have been estimated.

Radiation enhanced diffusion of Ti in Al2O3

Titanium bulk and dislocation diffusion coefficients at 1000 °C have been determined for Ti in single crystal α-Al2O3(0001) for three types of samples: (i) Ti evaporated onto Al2O3, (ii) Ti+48at 100 eV deposited onto Al2O3, and (iii) Ti+48 at 100 eV deposited onto radiation damaged Al2O3 (damage inflected by implantation of Ti+46 at 7 keV). Diffusion penetration profiles were obtained by using secondary ion mass spectrometry depth profiling techniques. For the deposited and implanted Ti+ samples, the bulk diffusion coefficients (D) are 5.4×10−21 and 5.6×10−17 m2 s−1, respectively, and dislocation diffusion coefficients are 3.6×10−17 and 1.4×10−12 m2 s−1, respectively. Comparing the D's for Ti in the undamaged and damaged Al2O3, the D's for the latter samples are higher by a factor of 104–105, reflecting the radiation enhanced diffusion due to the defect structure inflected by the implanted 7 keV Ti+46. Comparing the D's for Ti deposited onto Al2O3 (both the evaporated and 100 eV Ti+48 samples) with those for Cr deposited onto Al2O3, the Ti D's are larger by a factor of 10, reflecting the influence of the valence state of the cation. These results show that cationic diffusion coefficients in Al2O3 can be controlled by varying the level of defects in the crystal.

ＭｇＯ中の陽イオン拡散のシステマティックス

Diffusion of cations (self-, impurities) in single-crystal MgO have been studied. The diffusion penetration profiles were obtained with the aid of a secondary ion mass spectrometry (SIMs) . The diffusion profiles contained lattice and dislocation diffusion contributions. The temperature dependence of lattice and dislocation diffusion for 16 species has been determined. The dislocation diffusion coefficients were 4 to 6 orders of magnitude larger than those of lattice diffusion, although the activation energies for lattice and dislocation diffusion on the same element were not so different. A linear correlation between pre-exponential factor and activation energy for lattice and dislocation diffusion has been confirmed. The elements were separated into two trends of lines by the geometrical condition. The slope of these lines on lattice and dislocation diffusion corresponds with that expected from Zener's elastic theory.

Formation of cylindrical n/p junction diodes by arsenic enhanced diffusion along interfacial misfit dislocations in p-type epitaxial Si/Si(Ge)

Arsenic enhanced diffusion along individual misfit dislocations in Si/Si(Ge) heterostructures has been detected and imaged using scanning electron microscopy (SEM) and in the electron beam induced current (EBIC) mode. The formation of buried cylindrical, or conical, diodes surrounding misfit dislocations has been observed. The diffusion enhancement is not uniform for each dislocation. EBIC/SEM micrographs reveal a dark recombination contrast in the vicinity of the dislocation core and a white generation signal within the space-charge region of the surrounding n/p diode. Based on an experimental isoconcentration etching profile and a simple model for enhanced diffusion, the dislocation diffusion coefficient for arsenic is estimated to be up to six orders of magnitude higher than that in the host crystal.

Self-diffusion along dislocations in single-crystals MgO

Abstract Self-diffusion of 26 Mg in single crystal MgO has been investigated at low temperature side (1573-973 K). The lattice and dislocation diffusion coefficients were determined from diffusion penetration which was measured by SIMS. The dislocation pipe diffusion was 4–5 orders faster than the lattice diffusion at the same temperature.

Direct determination of grain-boundary and dislocation self-diffusion coefficients in silver from experiments in type-C kinetics

The use of an implanted, mass-separated carrier-free 105Ag radiotracer improves the sensitivity for measuring grain-boundary (GB) penetration profiles. In this way the GB self-diffusion coefficient DGB in silver was directly determined from experiments in type-C kinetics conditions. When multiplying the DGB values with a GB width δ=0.5 nm the results are in agreement with GB diffusivities P=δDGB measured in the same Ag material in the usual type-B kinetics experiments which were performed between 911 and 540 K and extrapolated to the low-temperature regime of type-C kinetics experiments (455–353 K). From the near-surface part of the type-C diffusion profiles the dislocation diffusion coefficient Dd was directly determined. These Dd values are compared with experiments of Turnbull and Hoffman [Acta Metall. 2, 419 (1954)] who applied the dislocation pipe model for a low-angle-tilt GB in silver. The good agreement confirms a dislocation core diameter of about 0.5 nm.

Impurities dislocation diffusion in single-crystal MgO

Tracer diffusion of Li +, Na +, Si 4+ and P 5+ in single-crystal MgO has been performed. The lattice and dislocation diffusion coefficients were determined from diffusion penetration measured by SIMS. Temperature dependence of the lattice and dislocation diffusion among these ions was argued to connect with the property of ions.

Calcium Diffusion along High‐Diffusivity Paths in Single‐Crystal MgO

The lattice and dislocation diffusion coefficients for Ca2+ in single‐crystal MgO have been determined in the temperature range 1743 to 1293 K from the diffusion penetration determined using secondary ion mass spectrometry. The dislocation diffusion coefficients were 4 to 5 orders of magnitude larger than the lattice diffusion coefficients in the same temperature range. The activation energy of Ca2+ lattice diffusion was comparable to that of ionic conductivity. The diffusion mechanism of Ca2+ is related to Mg vacancies.

Oxygen self-diffusion along high diffusivity paths in forsterite.

Diffusion profiles of 18O tracer in single crystals of forsterite following anneals at 1100° and 1200°C have been determined by a depth profiling technique using secondary ion mass spectrometry. The diffusion penetration profiles showed error function forms with developed tails. The profiles were analyzed in terms of diffusion in the forsterite lattice together with a contribution from dislocation. The present results of lattice diffusion lie in the range of the extrapolation of lines on Arrhenius plots reported previously for intracrystalline diffusion in forsterite. The dislocation diffusion coefficients are about 104 times faster than the lattice diffusion at the same temperature. Diffusion along the dislocations takes place within a pipe with radius of about 0.1 nm surrounding the dislocation line. Based on the above results, O diffusion along high diffusivity paths, such as dislocations and grain boundaries, play an important role on the diffusional creep in olivine under laboratory condition.

Impurity diffusion in NiO grain boundaries

The diffusion of 139Ce and 51Cr in polycrystalline NiO has been studied in the temperature range 600–1100°C in oxygen at a pressure of 1 atm. These impurities were chosen because of their different effective charges and segregation behaviour and because of their relevance to the oxidation of metals at elevated temperature. The solubility of Ce in the NiO lattice is negligible, but Ce is soluble at NiO grain boundaries and dislocations. Consequently Ce tracer only diffused along these pathways and the resulting profiles were analysed straightforwardly to give grain boundary and dislocation diffusion coefficients. The lattice solubility of Cr, on the other hand, is not negligible and Cr also segregates strongly to NiO grain boundaries. A procedure for analysing penetration profiles of such an impurity has been developed in which parameters describing segregation and grain boundary diffusion are deduced self-consistently. When combined with previous measurements of Ni and Co diffusion it is found that grain boundary coefficients decrease in the order Co, Ni, Cr, Ce as is also found for lattice diffusion (except for Ce, which has negligible lattice solubility). The implication of the results for the distribution of Cr and Ce in NiO films formed by metal oxidation is also discussed.

Mechanical behavior of polycrystalline tungsten at elevated temperature

The influence of stress, temperature, grain size and subgrain size on the steady state creep rate of polycrystalline unalloyed tungsten has been examined based on the extensive published data available. It was shown that the steady state creep rate of polycrystalline tungsten, and other metals, is given by ϵ ̇ = Sλ 2 Deff( σ E ) 7 where λ = subgrain size, σ = creep stress, E = average unrelaxed elastic modulus, and S is a universal creep constant equal to about 3 × 10 40 cm −4. The effective diffusion coefficient, Deff, equals ( D Lƒ L + D Dƒ D ) where D L is the lattice diffusion coefficient, D D is the dislocation diffusion coefficient, and ƒ L and D are the fraction of atoms participating in lattice and dislocation diffusion respectively. When subgrains form during creep, as in normal behaving materials, λ = kσ −1 ( k is a material constant) and the above equation yields the generally observed five power stress law for creep. When subgrains do not form during creep (as in many refractory metals produced by powder metallurgy techniques), λ in the above equation is replaced by the grain size. In this case ϵ ̇ αL 7 and ϵ ̇ ;αL 2 , predictions in agreement with experimental evidence. The power law breakdown for creep at high stresses can be explained by one of two possibilities 1. (i) the creation of excess vacancies by plastic deformation which increases D L in the Deff term 2. (ii) the increase in dislocation density with increasing stress, which influences the dislocation diffusion term, D Dƒ D , in Deff.