O adsorption and incipient oxidation of the Mg(0001) surface

First principles density functional calculations are used to study the early oxidation stages of the Mg(0001) surface for oxygen coverages 1/16 <= Theta <= 3 monolayers. It is found that at very low coverages O is incorporated below the topmost Mg layer in tetrahedral sites. At higher oxygen-load the binding in on-surface sites is increased but at one monolayer coverage the on-surface binding is still about 60 meV weaker than for subsurface sites. The subsurface octahedral sites are found to be unfavorable compared to subsurface tetrahedral sites and to on-surface sites. At higher coverages oxygen adsorbs both under the surface and up. Our calculations predict island formation and clustering of incorporated and adsorbed oxygen in agreement with previous calculations. The calculated configurations are compared with the angle-scanned x-ray photoelectron diffraction experiment to determine the geometrical structure of the oxidized Mg(0001) surface.Comment: 10 pages, 5 figure

Hot-Wire Measurements of the Influence of Surface Steps on Transition in Favorable Pressure Gradient Boundary Layers

An examination of the effects of surface step excrescences on boundary layer transition was performed, using a unique experimental facility. The objective of the work was to characterize the variation of transition Reynolds numbers with measurable step size and boundary layer parameters, with the specific goal of specifying new tolerance criteria for laminar flow airfoils, alongside a fundamental investigation of boundary layer transition mechanisms. This paper focuses on interpretation of hot-wire measurements, including supporting stability calculations, undertaken as part of the study. The results for both forward and aft-facing steps indicated a substantial stabilizing effect of favorable pressure gradient on excrescence-induced boundary layer transition. These findings suggest that manufacturing tolerances for laminar flow aircraft could be loosened in areas where even mild favorable pressure gradients exist

In-Situ Nuclear Magnetic Resonance Investigation of Strain, Temperature, and Strain-Rate Variations of Deformation-Induced Vacancy Concentration in Aluminum

Critical strain to serrated flow in solid solution alloys exhibiting dynamic strain aging (DSA) or Portevin–LeChatelier effect is due to the strain-induced vacancy production. Nuclear magnetic resonance (NMR) techniques can be used to monitor in situ the dynamical behavior of point and line defects in materials during deformation, and these techniques are nondestructive and noninvasive. The new CUT-sequence pulse method allowed an accurate evaluation of the strain-enhanced vacancy diffusion and, thus, the excess vacancy concentration during deformation as a function of strain, strain rate, and temperature. Due to skin effect problems in metals at high frequencies, thin foils of Al were used and experimental results correlated with models based on vacancy production through mechanical work (vs thermal jogs), while in situ annealing of excess vacancies is noted at high temperatures. These correlations made it feasible to obtain explicit dependencies of the strain-induced vacancy concentration on test variables such as the strain, strain rate, and temperature. These studies clearly reveal the power and utility of these NMR techniques in the determination of deformation-induced vacancies in situ in a noninvasive fashion.

Temperature dependence of the single-crystal elastic constants of co-rich co-fe alloys

The single-crystal elastic moduli, C11, C12, and C44 of three fcc cobalt-iron alloys (Co-6 at. % Fe, Co-8 at.% Fe, Co-10 at.% Fe) were measured in the range 0-315 °c. In addition C11 for the Co-6 at. % Fe alloy, and C' = (1/2)( C11 + C12 + 2 C44) for the three alloys are measured over the temperature range 0-1250 °c. Plots of the elastic moduli vs temperature exhibit a change in slope and deviation from linearity in the neighborhood of the Curie temperature. The temperature variation of the shear anisotropy in the fcc phase A fcc (=2 C44/ C11- C12) differs among the three alloys. A fcc exhibits a highly positive temperature dependence in the Co-10 at. % Fe alloy and a slight negative dependence in the Co-6 at. % Fe and Co-8 at. % Fe alloys. Previous statements in the literature that the hcp → ← fcc transformation in cobalt is preceded by a highly negative temperature dependence of the shear anisotropy ratio A (= C 44/ C 66) in the hcp phase between 523 °K and the transition at about 743 °K is not borne out by the present results. Rather it appears that the hcp → ← fcc transformation involves a change from A >1 in the hcp phase to A < 1 in the fcc phase

Temperature dependence of the single-crystal elastic constants of co-rich co-fe alloys

The single-crystal elastic moduli, C11, C12, and C44 of three fcc cobalt-iron alloys (Co-6 at. % Fe, Co-8 at.% Fe, Co-10 at.% Fe) were measured in the range 0-315 °c. In addition C11 for the Co-6 at. % Fe alloy, and C' = (1/2)( C11 + C12 + 2 C44) for the three alloys are measured over the temperature range 0-1250 °c. Plots of the elastic moduli vs temperature exhibit a change in slope and deviation from linearity in the neighborhood of the Curie temperature. The temperature variation of the shear anisotropy in the fcc phase A fcc (=2 C44/ C11- C12) differs among the three alloys. A fcc exhibits a highly positive temperature dependence in the Co-10 at. % Fe alloy and a slight negative dependence in the Co-6 at. % Fe and Co-8 at. % Fe alloys. Previous statements in the literature that the hcp → ← fcc transformation in cobalt is preceded by a highly negative temperature dependence of the shear anisotropy ratio A (= C 44/ C 66) in the hcp phase between 523 °K and the transition at about 743 °K is not borne out by the present results. Rather it appears that the hcp → ← fcc transformation involves a change from A >1 in the hcp phase to A < 1 in the fcc phase