Hydrogen pick-up leading to hydride formation is often observed in commercially pure Ti (CP-Ti) and Ti-based alloys prepared for microscopic observation by conventional methods, such as electro-polishing and room temperature focused ion beam (FIB) milling. Here, we demonstrate that cryogenic FIB milling can effectively prevent undesired hydrogen pick-up. Specimens of CP-Ti and a Ti dual-phase alloy (Ti-6Al-2Sn-4Zr-6Mo, Ti6246, in wt.%) were prepared using a xenon-plasma FIB microscope equipped with a cryogenic stage reaching −135 °C. Transmission electron microscopy (TEM), selected area electron diffraction, and scanning TEM indicated no hydride formation in cryo-milled CP-Ti lamellae. Atom probe tomography further demonstrated that cryo-FIB significantly reduces hydrogen levels within the Ti6246 matrix compared with conventional methods. Supported by molecular dynamics simulations, we show that significantly lowering the thermal activation for H diffusion inhibits undesired environmental hydrogen pick-up during preparation and prevents pre-charged hydrogen from diffusing out of the sample, allowing for hydrogen embrittlement mechanisms of Ti-based alloys to be investigated at the nanoscale. Hydrogen contamination in metals during sample preparation for high-resolution microscopy remains a challenge, especially when hydrogen itself is being investigated. Here, the authors show that using cryogenic milling significantly reduces hydrogen pick-up during sample preparation of titanium and titanium alloys.

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Ti and its alloys as examples of cryogenic focused ion beam milling of environmentally-sensitive materials.

Review—Factors Influencing Sulfur Induced Corrosion on the Secondary Side in Pressurized Water Reactors (PWRs)

A solution to FIB induced artefact hydrides in Zr alloys

Could face-centered cubic titanium in cold-rolled commercially-pure titanium only be a Ti-hydride?

Orientation-dependent irradiation hardening in pure Zr studied by nanoindentation, electron microscopies, and crystal plasticity finite element modeling

An atom probe tomography study of Pb-caustic SCC in alloy 800

High resolution characterization of Pb-SCC in Alloy 800 exposed to 330 °C mildly caustic environments

High resolution characterization of sulfur-assisted degradation in alloy 800

The mechanical properties of zirconium alloys are affected by the presence of hydrides. The strain fields around hydrides, which are affected by the size, orientation, and hydride phase, are believed to influence the apparent hysteresis between solubility limits on heating and cooling. TEM characterization of dislocation fields near hydrides in Zircaloy-4 specimens, which were exposed to 300 °C primary-water conditions for 600 h, was performed both before and after a heating and cooling cycle. In addition, EELS characterization is provided before heating. In situ TEM imaging/recording and nano-diffraction allowed monitoring of the morphology of dissolving hydrides throughout the temperature cycling. No dislocations in the matrix surrounding the hydrides were visible prior to heating; however, when the hydrides dissolved, dislocations were visible in the space the hydrides had previously occupied, providing a map of the original hydride distribution. These dislocation ‘nests’ are likely the preferential sites for subsequent hydride precipitation and elucidate the so-called ‘memory effect’. Advancing the understanding of hydride formation kinetics, hydride morphology, and hydrogen solid solubility limits can help to reduce uncertainties and conservatism when addressing the risks of hydrogen embrittlement and hydride cracking in zirconium components.

S.Y. Persaud

Papers

A solution to FIB induced artefact hydrides in Zr alloys

An atom probe tomography study of Pb-caustic SCC in alloy 800

High resolution characterization of Pb-SCC in Alloy 800 exposed to 330 °C mildly caustic environments

High resolution characterization of sulfur-assisted degradation in alloy 800

Advanced Characterization of Hydrides in Zirconium Alloys