Open Access

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and high-resolution scanning electron microscopy are well-acknowledged tools in materials characterization. The ability to map chemical species on the surface of an investigated sample with often low mass detection limits makes ToF-SIMS an essential tool in fields where many question marks concerning Degradation processes and damage mechanisms exist. The aim of this paper is to describe the power of data fusion of ToF-SIMS and high-resolution scanning electron microscopy results employing computational methods for multivariate data Analysis such as principal component analysis. As a case study the investigation of hydrogen distribution in an artificially charged Duplex stainless steel microstructure is presented aiming on a better understanding of hydrogen embrittlement.

The following presentation provides an overview on some of the challenges with metallic materials used as structural materials in the energy sector for the transport and storage of hydrogen. The presentation contains explanations about the methodologies utilized to characterize the susceptibility of these materials for hydrogen.

The ferroelectric ceramic NBT and its solid solutions with barium titanate (BT) are examples for the most promising lead free materials to substitute the dominant Pb(Zr,Ti)O3 (PZT). Lead based material should generally be disregarded due to environmental and health reasons. However, there is no class of lead-free piezoelectric materials that can replace PZT entirely. Not only the often inferior ferroelectric properties but also the lack of understanding of the defect chemistry of NBT is still a challenge for the replacement of lead containing piezo-ceramics. Just recently it could be shown by Li et al.[1] that NBT obtains extraordinarily high oxygen ionic conductivity when doped with Mg as acceptor. It was actually expected that doping just leads to a hardening of the ferroelectric properties. However, it became clear that the known defect chemical behavior of PZT cannot be extrapolated to NBT materials. Hence, to gain information on general doping effects a better defect chemical investigation is needed. This is particularly important for the applications with high reliability demands to investigate possible degradation and fatigue mechanisms. In the present work Time-of-flight-secondary ion mass spectrometry (ToF-SIMS) was used in order to observe the influence of different acceptor dopants (Fe, Ni, Al) on the oxygen diffusion in NBT. ToF-SIMS holds the ability to gain a full elemental distribution in a sub-micron resolution and was therefore chosen to provide the essential information on the favorable diffusion paths of oxygen in NBT in dependence of the chosen doping element. 18O was used as a tracer for oxygen as its natural abundance is only 0.2%. The doped NBT samples have been annealed for 5 h at 500°C in a 0.2 bar 18O-tracer atmosphere to be able to detect oxygen ion diffusion. ToF-SIMS investigations were conducted on a ToF-SIMS IV (ION TOF GmbH, Münster, Germany) using a Bi+ primary ion beam (25KeV in collimated burst alignment mode with a beam diameter of ~150 nm) and a Cs+ sputter beam (3KeV). The results illustrate the different impact of dopants on the diffusion properties which is evidence for a highly non-linear dependence on dopant type and concentration. 1) Li, M., et al., Nature Materials, 2014. 13(1): p. 31-35.

Among the very few techniques to localize hydrogen (H) at the microscale in steels, Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was proven to be a reliable tool. The necessity to detect hydrogen stems from its deleterious effects in metals, that are often used as structural components and to obtain better understanding of the underlying metallurgical mechanisms of hydrogen embrittlement (HE) which are still unclear. Austenitic stainless steels are nowadays commonly used in a wide variety of application, from hydrogen transport and storage facilities to petrochemical and offshore applications where they are exposed to aggressive environments and therefore prone to HE. One of the greater risks in the austenitic class is the embrittlement of the material due to the instability of the γ austenite and its transformation into a brittle α martensitic phase. This transformation takes place due to the local stresses that are induced by the uptake of hydrogen during service. Nonetheless, it was shown that this transformation can occur as an artefact during SIMS analysis itself where Cs-sputtering is necessary not only to remove surface contaminations but mainly to enhance H/D secondary ion yield. In the following contribution we show the influence of different sputtering conditions on AISI 304L austenitic stainless steel in order to distinguish the artefact from the hydrogen induced transformation. The material was charged electrochemically in a deuterium based electrolyte. Deuterium (D) must be in these experiments as a replacement for hydrogen which cannot be used because adsorbed hydrogen superimposes hydrogen originating from charging the sample in the SIMS images. ToF-SIMS analyses were conducted by ToF SIMS IV (IONTOF GmbH, Münster, Germany). The experiments were carried out on deuterium charged and non-charged samples. The structural characterization was carried out by SEM and EBSD examinations before and after charging, both with a Leo Gemeni 1530VP field-emission scanning electron microscope and a Zeiss Supra 40 instrument (Carl Zeiss Microscopy GmbH, Oberkochen, Germany). The results showed that the use of 1keV Cs+ beam induces stacking faults while higher sputter beam energies results in γ→α transformation.