Ti has a high affinity for hydrogen and are typical hydride formers. Ti-hydride are brittle phases which probably cause premature failure of Ti-alloys. Here, we used atom probe tomography and electron microscopy to investigate the hydrogen distribution in a set of specimens of commercially pure Ti, model and commercial Ti-alloys. Although likely partly introduced during specimen preparation with the focused-ion beam, we show formation of Ti-hydrides along α grain boundaries and α/β phase boundaries in commercial pure Ti and α+β binary model alloys. No hydrides are observed in the α phase in alloys with Al addition or quenched-in Mo supersaturation.

No abstract

Titanium alloys are widely used in the aerospace industry, yet oxygen ingress can severely degrade the mechanical properties of titanium alloy components. Atom probe tomography (APT), electron probe microanalysis (EPMA) and nanoindentation were used to characterise the oxygen-rich layer on an in-service jet engine compressor disc, manufactured from the titanium alloy TIMETAL 834. Oxygen ingress was quantified and related to changes in mechanical properties through nanoindentation studies. The relationship between oxygen concentration, microstructure, crystal orientation and hardness has been explored through correlative hardness mapping, EPMA and electron backscatter diffraction (EBSD). It has been found that the hardening effects of microstructure and crystallography are only significant at very low-oxygen concentrations, whereas interstitial solid solution hardening dominates by order of magnitude for higher oxygen concentrations. The role of microstructure on oxygen ingress has been studied and oxygen ingress along a potential α/β interface was directly observed on the nanoscale using APT.

A method is presented for the registration and correlation of property maps of materials, including data from nanoindentation hardness, Electron Back-Scattered Diffraction (EBSD), and Electron Micro-Probe Analysis (EPMA). This highly spatially resolved method allows for the study of micron-scale microstructural features, and has the capability to rapidly extract correlations between multiple features of interest from datasets containing thousands of data points. Two case studies are presented in commercially pure (CP) titanium: in the first instance, the effect of crystal anisotropy on measured hardness and, in the second instance, the effect of an oxygen diffusion layer on hardness. The independently collected property maps are registered using affine geometric transformations and are interpolated to allow for direct correlation. The results show strong agreement with trends observed in the literature, as well as providing a large dataset to facilitate future statistical analysis of microstructure-dependent mechanisms. Graphical abstract

Atom Probe Tomography is used to show that the α 2 phase (Ti 3 Al) forms upon ageing Ti-7Al at 550 o C with as little as 500 wppm oxygen. Notably, it is found that oxygen consistently partitions away from the α 2 precipitates to the α matrix. The role of aluminium concentration and oxygen solubility on oxygen partitioning preference is also investigated. Based on our observations, we propose a mechanism by which oxygen encourages α 2 formation despite partitioning away from it. Furthermore, nanohardness systematically measured with respect to ageing time and oxygen concentration suggests that α 2 precipitation is not the dominant hardening mechanism during aging.