Heating of TEM specimens during ion milling

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

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.

Electron backscatter diffraction applied to lithium sheets prepared by broad ion beam milling

Due to its very low hardness and atomic number, pure lithium cannot be prepared by conventional methods prior to scanning electron microscopy analysis. Here, we report on the characterization of pure lithium metallic sheets used as base electrodes in the lithium-ion battery technology using electron backscatter diffraction (EBSD) and X-ray microanalysis using energy dispersive spectroscopy (EDS) after the sheet surface was polished by broad argon ion milling (IM). No grinding and polishing were necessary to achieve the sufficiently damage free necessary for surface analysis. Based on EDS results the impurities could be characterized and EBSD revealed the microsctructure and microtexture of this material with accuracy. The beam damage and oxidation/hydration resulting from the intensive use of IM and the transfer of the sample into the microscope chamber was estimated to be <50 nm. Despite the fact that the IM process generates an increase of temperature at the specimen surface, it was assumed that the milling parameters were sufficient to minimize the heating effect on the surface temperature. However, a cryo-stage should be used if available during milling to guaranty a heating artefact free surface after the milling process.

Measurement and estimation of temperature rise in TEM sample during ion milling

The actual temperature rise was measured during ion-milling process used in the transmission electron microscopy (TEM) sample preparation. Special probes were fabricated for the measurements, one with shielded, floating thermocouple mounted onto a 3mm grid to compute the thermal load at the sample, and the other, a bare probe with a polymer coating to measure the maximum temperature attained. The temperature measured in the most commonly used ion-milling system reached up to 295 degrees C when the typical milling conditions, 6keV ion-energy and an incident angle of 80 degrees, were used. Based on the temperature profiles that were obtained by the shielded probe, two unknown parameters, the amount of heat deposited by the energetic ions/neutrals to the sample and the thermal conductivities between the materials, were estimated and used to compute the temperature rise in commonly adopted materials. The calculation showed that the temperature of the glass sample reached more than 300 degrees C under typical ion-milling conditions. The calculated value was confirmed with the experimental result of the crystallization of an amorphous Si on the glass under the typical ion-milling condition, which gave the same extent as annealing at 350 degrees C.