Elien Wallaert

Thermal desorption spectroscopy study of the interaction of hydrogen with TiC precipitates

Thermal desorption spectroscopy (TDS) was used to study hydrogen-trap interactions for an experimental steel (0.025 wt%C-0.09%Ti). After lab processing, the microstructure consisted of small (∼20 μm) ferrite grains containing nanometer TiC precipitates. After hot and cold rolling, the material contained some hydrogen (originated from the hot rolling) in irreversible traps, the TiC precipitates. After annealing in hydrogen, the TDS spectra consisted of a high temperature peak, attributed to irreversible trapping by TiC precipitates. Annealing slightly increased the TiC precipitate size. Both the peak temperature and peak area increased with increasing annealing temperature. The increase in peak area occurred together with the increase in TiC precipitate size. The TDS spectra for samples annealed at 800 °C, and electrochemically charged, contained (i) a low temperature peak which decreased in height with increasing desorption time, and (ii) a high temperature peak that did not change significantly with desorption time, and was similar to those after gaseous charging. The low temperature peak was attributed to reversible traps such as grain boundaries, whereas the high temperature peak was attributed to irreversible trapping by TiC precipitates. The high temperature TDS peak was composed of constituent peaks with essentially the same activation energy of 145 kJ/mol.

Use of filtration techniques to study environmental fate of engineered metallic nanoparticles: Factors affecting filter performance

We examined the filtration of aqueous suspensions of negatively charged (citrate-stabilized) Ag (14.5±1.1nm) and positively charged CeO2 (7.3±1.4nm) engineered nanoparticles (ENPs) via different filtration techniques such as paper filtration, micro- and ultrafiltration, and evaluated the impact of initial concentration, matrix composition, and filter type and (pre-)treatment, on nanoparticle retention. Solutions of Ag+ and Ce3+ ions were tested in the same way. Significant retention of nanoparticles was observed even for filters having considerably larger pore sizes than the ENPs size. Retention also seemingly increased with decreasing initial concentration, but generally decreased upon preconditioning of the paper or membrane filters with diluted nitric acid or 0.1M Cu(NO3)2, respectively. In ultrapure water, retention appeared to depend more on particle characteristics than on a membrane type. However, in 2mM KNO3, NaCl, or CaCl2, more significant differences in recovery were observed between different membrane materials. Additionally, background electrolytes might reduce nanoparticle or ionic retention, but could also affect their (colloidal) stability, e.g., resulting in enhanced retention of Ag ENPs and Ag+ ions in chloride-containing matrices. Results from centrifugal ultrafiltration recommend using 10kDa filters for nanoparticle removal from the solution, and suggest these filters might potentially be suitable to differentiate between (nano)particulate and dissolved species.

Evaluation of the Effect of TiC Precipitates on the Hydrogen Trapping Capacity of Fe-C-Ti Alloys

The present work evaluates the hydrogen trapping behavior of different laboratory cast generic Fe-C-Ti martensitic alloys. Titanium carbides were precipitated in the materials by well-designed heat treatments. A quenched and tempered martensitic matrix with final strength above 1000 MPa was aimed for and verified by means of hardness measurements. Tempering allowed generating precipitates with different characteristics in terms of coherency, size and distribution due to the secondary hardening effect, as was evaluated by transmission electron microscopy. The hydrogen trapping capacity of the TiC precipitates was investigated by thermal desorption spectroscopy, while melt extraction was performed to determine the amount of hydrogen present after cathodic hydrogen charging. Generally, it could be concluded that the incoherent particles in the quenched material were not able to trap hydrogen, whereas the quenched and tempered material trapped hydrogen at the interface of small probably coherent TiC.

Understanding the Interaction between a Steel Microstructure and Hydrogen

The present work provides an overview of the work on the interaction between hydrogen (H) and the steel’s microstructure. Different techniques are used to evaluate the H-induced damage phenomena. The impact of H charging on multiphase high-strength steels, i.e., high-strength low-alloy (HSLA), transformation-induced plasticity (TRIP) and dual phase (DP) is first studied. The highest hydrogen embrittlement resistance is obtained for HSLA steel due to the presence of Ti- and Nb-based precipitates. Generic Fe-C lab-cast alloys consisting of a single phase, i.e., ferrite, bainite, pearlite or martensite, and with carbon contents of approximately 0, 0.2 and 0.4 wt %, are further considered to simplify the microstructure. Finally, the addition of carbides is investigated in lab-cast Fe-C-X alloys by adding a ternary carbide forming element to the Fe-C alloys. To understand the H/material interaction, a comparison of the available H trapping sites, the H pick-up level and the H diffusivity with the H-induced mechanical degradation or H-induced cracking is correlated with a thorough microstructural analysis.