Alessandro Mottura

A new TriBeam system for three-dimensional multimodal materials analysis

The unique capabilities of ultrashort pulse femtosecond lasers have been integrated with a focused ion beam (FIB) platform to create a new system for rapid 3D materials analysis. The femtosecond laser allows for in situ layer-by-layer material ablation with high material removal rates. The high pulse frequency (1 kHz) of ultrashort (150 fs) laser pulses can induce material ablation with virtually no thermal damage to the surrounding area, permitting high resolution imaging, as well as crystallographic and elemental analysis, without intermediate surface preparation or removal of the sample from the chamber. The TriBeam system combines the high resolution and broad detector capabilities of the DualBeam(TM) microscope with the high material removal rates of the femtosecond laser, allowing 3D datasets to be acquired at rates 4-6 orders of magnitude faster than 3D FIB datasets. Design features that permit coupling of laser and electron optics systems and positioning of a stage in the multiple analysis positions are discussed. Initial in situ multilayer data are presented.

Diffusion anisotropy of poor metal solute atoms in hcp-Ti

Atom migration mechanisms influence a wide range of phenomena: solidification kinetics, phase equilibria, oxidation kinetics, precipitation of phases, and high-temperature deformation. In particular, solute diffusion mechanisms in α-Ti alloys can help explain their excellent high-temperature behaviour. The purpose of this work is to study self- and solute diffusion in hexagonal close-packed (hcp)-Ti, and its anisotropy, from first-principles using the 8-frequency model. The calculated diffusion coefficients show that diffusion energy barriers depend more on bonding characteristics of the solute rather than the size misfit with the host, while the extreme diffusion anisotropy of some solute elements in hcp-Ti is a result of the bond angle distortion.

Interstitial diffusion of O, N, and C in α-Ti from first-principles: Analytical model and kinetic Monte Carlo simulations

The high affinity of O, N, and C with α-Ti has a serious detrimental influence on the high-temperature properties of these alloys, promoting the formation of α-case. These elements dissolve in interstitial sites and diffuse very fast in α-Ti (10(3)-10(8) times higher than the self-diffusivity of Ti) at high temperature accelerating the growth of α phase surface layer. Understanding the diffusion mechanisms of these elements is crucial to the design of high-temperature Ti alloys. This work aims to determine the stable interstitial sites and migration paths of O, N, and C in α-Ti. Diffusion coefficients were evaluated applying an analytical model, the multi-state diffusion method, and kinetic Monte Carlo simulations informed by first-principles calculations. The results show the reliability of these two methods with respect to the experimental data. In addition to octahedral sites, less traditional interstitial sites are shown to be stable configurations for these elements instead of tetrahedral sites. This requires to update the transition pathway networks through which these elements have been thought to migrate in α-Ti.