308 Daniel Haley et al.
subsequent absorption by other materials (Konda & Chen, 2015). Second, pure vanadium was examined where, at the pressures used in this study, both H and D are in solid solution. Vanadium is unusual in that the phase diagram for V-H is markedly different to V-D (Manchester, 2000). Thus, care must be taken in interpretation of the D interac- tion withinVand the extrapolation of these results to predict the behavior of H. For the purposes of this work, these materials are used to demonstrate the capability of APT to characterize hydrides. The Pd-H system, for which extensive data are available
and for which the Pd-D systemexhibits similar behavior, has a unique two-phase region. At room temperature the two-phase α−β region occurs between 2 and 40 at% hydro- gen, between which the pressure-composition isotherm is relatively flat (Manchester, 2000). The Pd–Rh-H system behaves similarly, and Rh is considered to not alter D solubility considerably within the pressure range in this study (Thiebaut et al., 1995). This two-phase region is slightly wider at the cryogenic temperatures used by APT analysis, but otherwise should not differ for the purposes of this work. Solubility isotherm data indicates that at equilibrium
with 100 kPa (1 bar) hydrogen, the equilibrium solubility should exceed 0.75H atoms per metal atom for both Pd and Pd–Rh. The bulk diffusion rate for Pd-D at room tempera- ture is ~5.5×10− 11m2/s (Flanagan & Oates, 1991), which yields a 95% diffusion time of 5×10−5 s for a 100nm linear diffusion path, indicating rapid equilibriation within the bulk. However, adsorption/desorption kinetics of H to/from the surface of the material are likely to dominate the time to equilibriation (Schwarz et al., 2005). Thus these timescales are likely to be widely underestimated in describing actual diffusion behavior. PureV(Goodfellow, 99.8%; Huntingdon, Cambridgeshire,
UK) and an alloy of Pd–6.25%Rh (Johnson Matthey, Royston, Hertfordshire, UK) both in the form of drawn wires were used in this study. The needle-shaped specimens required for APT were generated using standard electropolishing techniques (Miller et al., 1996). For V a sulfuric-ethanol solution was used,
whereas a two-stage 25% perchloric-75% acetic, then 10% perchloric-acetic mixture was used for Pd–Rh. Prior to deuterium charging, samples were initially
reflectron-equipped LEAP 3000HR system in voltage-pulsing mode at 50K with a 15% pulse fraction. Pd data was obtained at a pulse frequency of 100 kHz to prevent Pd mass “wrap-around” (i.e., ions that are incorrectly assigned a sub- sequent pulse, and thus incorrect time-of-flight due to their long flight time) at low voltages, and V data were obtained at 200 kHz. A voltage reconstruction algorithm (Cameca IVAS 3.6.6, Madison, Wisconsin, USA) was used to generate three-dimensional (3D) data from the APT experiment. A schematic and photograph of the constructed deu-
subject to APT analysis using the laser-pulsing mode to remove surface oxides using an incident laser energy of 0.2 nJ, as laser mode has improved yield. APT analysis was then switched to voltage-pulsing mode to obtain a baseline signal in the undeuterated specimens, incorporating ~5× 105 detected ions. Samples were then removed from the vacuum, charged with deuterium and re-introduced into the atom probe vacuumsystem, with air-transfer times on the order of minutes. Postcharging, sample re-introduction into the cryogenically cooled atom probe analysis chamber was achieved within 45 min from deuterium charge completion. All APT characterizations were conducted using a
terium gas charging system are shown in Figure 1. The apparatus is a pneumatic system primarily constructed of 316 stainless steel, which allows for N2 purging and then subsequent deuterium charging of APT samples. The maxi- mum pressure of the system is limited to 500 kPa (5 bar). APT samples were loaded into the deuterium charging
rig while still within their APT specimen holders for straightforward transfer to/from the LEAP 3000HR. Atmospheric oxygen was purged by at least five cycles of purging gas, N2, at 500 kPa (5 bar) then the pressure reduced to 100 kPa (1 bar) (abs.).Next,D2 was pressurized at 500 kPa (5 bar), then vented to 120 kPa (1.2 bar). This was repeated three times to purify the D2 content in the chamber. Subsequently gaseous charging with D2 gas was conducted.
Figure 1. Instrumentation diagram and photograph of the charging rig developed for this study. The charging chamber in the image is at the bottom right. Atom probe tomography samples in holder are inserted and the chamber is pressurized. Deuterium cylinder is shown in the left side of the photograph.
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