Cryogenic sample preparation: Comparative analysis of Ga⁺ and Xe⁺ FIB milling for TEM and APT examination of zirconium

Specimen preparation is a key step in the characterisation of materials systems. For high-resolution characterisation techniques such as transmission electron microscopy (TEM) and atom probe tomography (APT), it is necessary to have a sample preparation method that creates the nano-scale samples required for analysis but does not significantly modify the initial microstructure. The preparation of hexagonal close-packed materials by focussed ion beam milling (FIB) and electropolishing has previously been shown to be complicated by hydride formation. The formation of hydrides can be reduced by the application of cryogenic temperatures during the final stages of Ga⁺ ion FIB milling, which are often conducted at low accelerating voltages in order to minimise irradiation-induced damage. Xe⁺ ion plasma FIBs are now commonly used in the preparation of samples due to their higher milling rates. However, the severity of the hydride formation in hexagonal close-packed materials during Xe⁺ ion milling is unclear. In this paper, we compare Xe⁺ and Ga⁺ FIB milling to prepare Zr samples at ambient and cryogenic temperatures. By studying TEM and APT samples, we are able to compare the levels of hydride formation after FIB preparation caused by the different preparation techniques. APT is used to estimate the levels of hydrogen in the samples. These results represent an important contribution to researchers who use FIB preparation to create TEM and APT specimens from hexagonal close-packed metals such as zirconium.