Kinetics of spontaneous phase transitions from wüstite to magnetite in superparamagnetic core-shell nanocubes of iron oxides.

Iron oxide nanoparticles with a wüstite structure have been prepared by thermal decomposition. In air, they undergo a spontaneous transition into a thermodynamically more stable magnetite structure that grows from the surface. The thickness of this magnetite shell increases with time, thereby producing a series of core-shell nanoparticles. We investigated the kinetics of this phase transition in 23 nm nanocubes using time-resolved XRD, from which the fractions of individual phases were determined by the Rietveld refinement. This kinetics is described theoretically using three coupled reaction-diffusion master equations for the concentrations of oxygen, wüstite, and magnetite, in which both the diffusion of oxygen and its reaction with wüstite are thermally activated. The coefficients of these terms were adjusted so that the predictions of the model reproduce the XRD data at 298 K and 353 K, whereas the predictive capability of the model was assessed by comparing its predictions with measurements at 403 K.

Surface and Subsurface Analysis of Stainless Steel and Titanium Alloys Exposed to Ultrasonic Pulsating Water Jet.

This article deals with the effect of periodically acting liquid droplets on the polished surfaces of AISI 316L stainless steel and Ti6Al4V titanium alloy. These materials were exposed to a pulsating water jet produced using an ultrasonic sonotrode with an oscillation frequency of 21 kHz placed in a pressure chamber. The only variable in the experiments was the time for which the materials were exposed to water droplets, i.e., the number of impingements; the other parameters were kept constant. We chose a low number of impingements to study the incubation stages of the deformation caused by the pulsating water jet. The surfaces of the specimens were studied using (1) confocal microscopy for characterizing the surface profile induced by the water jet, (2) scanning electron microscopy for detailed surface observation, and (3) transmission electron microscopy for detecting the changes in the near-surface microstructure. The surface described by the height of the primary profile of the surface increased with the number of impingements, and was substantially more intense in the austenitic steel than in the Ti alloy. Irregular surface depressions, slip lines, and short cracks were observed in the Ti alloy, whereas pronounced straight slip bands formed in the austenitic steel. The dislocation density near the surface was measured quantitatively, reaching high values of the order of 1014 m-2 in the austenitic steel and even higher values (up to 3 × 1015 m-2) in the Ti alloy. The origins of the mentioned surface features differed in the two materials: an intense dislocation slip on parallel slip planes for the Ti alloy and mechanical twinning combined with dislocation slip for the austenitic steel.