Abstract
Hydrogen is the lightest and most abundant element in the universe. It has an ambiguous relationship with metals. In titanium alloys, hydrogen is a β stabilizing element and widely used as a temporary alloyinglement to modify microstructure, improve mechanical properties and enhance the processability of titanium products. In the meantime, it has been extensively reported that uncontrolled hydrogen introduction during manufacturing and service degrades fracture toughness, dwell fatigue resistance, etc. in titanium alloys. However hydrogen behaviors in materials remain elusive because of the experimental challenges to directly observe them, especially at near-atomic scale. Here, by using atom probe tomography (APT) combined with transmission electron microscopy (TEM) and atom probe crystallography, we imaged hydrogen as solute atoms in bcc β phase, hydrides and segregation to interfaces in a set of titanium alloys: commercially-pure Ti, Ti-2at.% Fe and Ti-2at.% Mo binary model alloys, Ti-6Al-2Sn-4Zr-6Mo alloy and additive manufactured Ti-6Al-4V alloy (see Fig. 1). It is possible that the H is partly introduced during specimen preparation by focused-ion beam. We show that hydrogen concentration could locally reach up to ~50 at.%, forming titanium hydride in CP Ti and binary alloys, even though the bulk average H level was less than 0.1 at.%. The hydride phase was confirmed by both atom probe crystallography and correlation with TEM. They were mainly observed to have grown at α grain boundaries and α/β interfaces. In Ti64 and Ti6246 specimens, no hydride was observed, but most hydrogen was enriched inside β phase and/or along interfaces/boundaries. The higher H solubility in bcc β phase than in α phase was clearly revealed. Residual H level in APT analysis chamber was evaluated by detected H content in pure α-Ti phase which has negligible H solubility. The quantification of H by APT will be discussed with respect to the electric field at the surface of specimen. Direct observation of H opens an access for more closely understanding the H behaviors and furthermore hydrogen embrittlement mechanism within materials.