PNNL

Abstract

Analyzing hydrogen distribution in nuclear structural materials at sub-nanometer scale spatial resolution has been challenging by electron microscopy. Methods such as thermal desorption spectroscopy only provides a bulk estimate of hydrogen concentration with no nanoscale spatial resolution. In the last decade, cryogenic transfer atom probe tomography (APT) has been emerging as an invaluable characterization technique for analyzing the nanoscale distribution of hydrogen isotopes in materials [1, 2]. At Pacific Northwest National Laboratory, a complete suite of vacuum shuttle, environmental transfer hub (ETH), with cryogenically cooled sample transfer carousal was developed and connected to the buffer chamber of a CAMECA LEAP 4000 XHR [3]. We have now also established methods to electrochemically charge deuterium into electropolished steel needle samples and immediately plunge freeze the charged samples in liquid nitrogen (Fig. 1 (a-d)). The plunge frozen samples are then transferred to a cryogenic dual focused ion beam scanning electron microscope (FIB/SEM) using a Quorum Technologies vacuum shuttle for removing the frost and electrolyte residue by cryogenic FIB milling (Fig. 1 (e-f)). The sharpened needle sample is then transferred into the ETH attached to the APT buffer chamber (Fig. 1(g)) and into APT analysis chamber and then analyzed (Fig. 1(h)). The photograph of the Thermo Fisher Scientific Quanta FIB/SEM with cryogenic preparation stage and cryogenic workstation are given in Fig. 2(a) along with the photograph of the ETH in Fig. 2(b). The time it takes between the stopping of electrochemical charging and plunge freezing of sample was maintained to be within 180 seconds. Through automation, we are attempting to reduce this time to under 10 seconds to reduce any loss of hydrogen from the stopping of charging. Occasional issues arose during transfer from cryogenic FIB to ETH where vacuum leak occurred in which case the APT result will only show signatures from the frost. By optimizing this transfer procedure, we could avoid frost formation and get successful APT analysis of both ferritic and austenitic steels using cryogenic transfer APT. An example mass-to-charge state ratio spectra of a pure Fe sample before charging (Fig. 2(c)) and after 2H charging (Fig. 2(d)) clearly showed enhanced peak at 2 Da coming the 2H charging. The APT reconstruction showing the distribution of 1H+ and 2H+ are given in Fig. 2 (e) and (f). A capability to integrate a Ferrovac ultrahigh vacuum cryogenic transfer module with a cryogenic glove box, cryogenic plasma focused ion beam, and a CAMECA LEAP 6000 XR is currently under construction also in PNNL. With these capabilities, we now have the unique ability to analyze hydrogen segregation to grain boundaries and deformation induced defects in bulk samples of steels providing valuable insights to understand material degradation mechanisms of nuclear materials as well as steels used for hydrogen pipelines and hydrogen storage tanks [4].