Abstract: X-ray diffraction (XRD) and electron microprobe microanalysis (EPMA) were used for characterizing the structure and composition of surface layers formed on austenitic Fe-Mn-Si shape memory alloys under vacuum. The XRD results demonstrated that during annealing, face centered cubic austenite is transformed to hexagonal close packed martensite on the alloy surface. The EMPA results revealed that manganese in the surface layer was depleted during annealing. Further, this analysis determined that the thickness of the surface layer of the alloy annealed at 1173 K for 1 h was approximately 20 μm and that value is consistent with the depth detected by XRD. The compositional changes of the surface layers such as manganese depletion by annealing were discussed based on the ternary Fe-Mn-Si phase diagram. Although the formation of body centered cubic ferrite is detrimental to shape memory alloys, the amount of manganese was also observed to change during processing and strongly influence the stability of the shape memory alloys.
Abstract: On Ni and Ni-16wt%.Cr model-alloys compressed at 30 % and 60 % deformation, point-defects and dislocations concentrations are respectively characterized by positron annihilation spectroscopy and x-ray diffraction analysis. The positron results show that only mono-vacancies are formed during compressive test The X-ray results allows us to quantify the dislocation concentration in the systems. Saturation of defect densities is observed in measurements for these high deformation rates. In support to the experimental work, an homogeneous kinetic model is used to characterize point-defect – dislocation interactions to estimate the kinetics of vacancy restoration to equilibrium concentration.
Abstract: Oxidation of a dilute Si(Ge) alloy is modeled using an original protocol based on molecular dynamicssimulation and rules for the oxygen insertions. These rules, deduced from ab-initio calculations,favor the formation of SiO2 against GeO2 oxide which leads to segregation of Ge atoms into the alloyduring the oxidation front advance. Ge condensation is then observed close to the SiO2/Ge interfacedue to the strain induced by oxidation in this region. From the analysis of the simulations process, wepropose a one-dimensional description of Ge condensation which reproduces the evolution of the Geconcentration during oxidation of the SiGe alloy.
Abstract: Self-diffusion in thin nanocrystalline Pt films was investigated using secondary ion mass spectrometry. Our experiments are motivated by recent investigations on stress relaxation where self-diffusion of Pt is supposed to play an important role, especially at temperatures below 250 °C and annealing times of a few hours. For the diffusion experiments, double layers of natPt/194Pt were deposited on oxidized silicon wafers using ion beam sputtering. At 180 °C no significant diffusion induced broadening of the profiles could be observed even after an annealing time of 64 h. However, the concentration of 195Pt in the top layer decreases slightly after an annealing time of 16 h and remains constant for higher annealing times. At 600 °C a broadening of the profiles was observed after an annealing time of 5 minutes. From our results we conclude that at 180 °C only atoms in the grain boundaries are mobile. After about 16 h the isotopes in the grain boundaries are completely interdiffused. From the change of the 195Pt concentration in the top layer we estimate the amount of grain boundary phase in the Pt films to be about 5 %. The broadening of the profile after annealing at 600 °C is attributed to bulk diffusion.
Abstract: We present experiments based on neutron reflectometry in combination with 29Si/natSi isotope multilayers in order to investigate the self-diffusion in amorphous silicon. Such experiments allow the detection of diffusion processes in the amorphous state on length scales below 10 nm. First results at 650 °C show a continuous decrease of the artificial Bragg peak produced by the multilayer, corresponding to a diffusivity of (1.1 ± 0.4) x 10-20 m2/s on a length scale of 2 - 7 nm. The diffusivity is not time-dependent for annealing times between 3 min and 1 h. Compared to recent measurements in silicon single crystals by the same method, the diffusivity is higher by a factor of about 105.