Seeing solute hydrogen and hydrides in Ti Alloys by atom probe tomography

Seeing solute hydrogen and hydrides in Ti Alloys by atom probe tomography

In this project, we directly image and characterize solute hydrogen and hydride by use of atom probe tomography combined with electron microscopy, with the aim to investigate H interaction with different phases and lattice defects (such as grain boundaries, dislocation, etc.) in a set of specimens of commercially pure Ti, model and commercial Ti-alloys.

Hydrogen embrittlement is a severe problem for many engineering alloys. Several hydrogen embrittlelment mechanisms have been proposed, like hydrogen-induced localized plasticity (HELP) mechanism, hydrogen-enhanced decohesion (HEDE), adsorption-induced dislocation emission (AIDE), etc. Specifically, for hydride forming materials like Ti, Zr alloys, one more mechanism, hydride-induced cracking, was also proposed since the hydrides are brittle and would probably cause premature failure of these materials.

A precise knowledge of the distribution of H among different bulk phases and even on the lattice defects such as grain boundaries, dislocations, etc., is crucial to distinguish these mechanisms. However, observing hydrogen (H) in matter is a formidable challenge. Atom Probe Tomography (APT) is a promising technique owing to its unique capability of direct imaging hydrogen distribution at nano-scale, which is hardly achieved by all other microscopic techniques.

In this project, we directly image and characterize solute hydrogen and hydride by use of atom probe tomography combined with electron microscopy, with the aim to investigate H interaction with different phases and lattice defects (such as grain boundaries, dislocation, etc.) in a set of specimens of commercially pure Ti, model and commercial Ti-alloys. Our results show the formation of Ti-hydrides along α grain boundaries and α/β phase boundaries in commercial pure Ti and two-phase binary model alloys (Fig. 1a). No hydrides are observed in the α phase in alloys with Al addition and higher H solubility in β phase than in α phase was clearly revealed (Fig. 1b).

Fig. 1. a) A hydride formed along α grain boundaries; b) hydrogen content in some Ti systems measured by APT in HV-mode with LEAP 3000HR at MPIE.

We also use cryo-FIB (Focused Ion Beam) to prepare specimen from H/D charged samples in order to 1) prohibit H uptake and hydride formation during specimen preparation by standard FIB (Fig. 2); and 2) prevent H/D out-diffusion from charged samples. We try to image the H possibly trapped at lattice defects, such as precipitates, GBs and dislocations, with aim to gain a fundamental knowledge of the H behaviour in such materials and further reveal the hydrogen embrittlement mechanism.

Fig. 2. Experimental evidences of FIB-induced hydride in uncharged Commercial Pure Ti: a) 3D mapping and 1D composition profile from APT results; b) bright-field micrograph and c) dark-field image from TEM observation.

 

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