Is it meaningful to quantify vacancy concentrations of nanolamellar (Ti,Al)N thin films based on laser-assisted atom probe data?

Recently, it was proposed to quantify vacancy concentrations based on laser-assisted atom probe tomography (APT) data for a (Ti,Al)N film with a nanolamellar architecture, grown by chemical vapor deposition. To determine, whether vacancy concentrations can be reliably estimated based on APT data, we systematically compare measurements with an ultraviolet (UV) as well as a green laser to ion beam analysis data. First, monolithic TiN and AlN films, which are stoichiometric within the measurement uncertainties of ion beam analysis, are investigated. In case of TiN an electric field strength of 39.4 V nm î\u88\u93 1 and a nitrogen content of 49.0 at.%, consistent with ion beam analysis data, is obtained with both lasers, when using pulse energies of 5 pJ (UV laser) and 0.1 nJ (green laser). However, significant differences can be observed for monolithic AlN as nitrogen contents from 41.9 to 35.8 at.% and 48.4 to 41.4 at.% are measured, depending on variations of the pulse en-ergies of the UV and green laser, respectively. In order to mimic a nanolamellar architecture for correlative compositional analysis by ion beam analysis and laser-assisted APT, a TiN/AlN/TiN trilayer film is synthesized and significant deviations with respect to the nitrogen content are evident for the AlN layer. While the average nitrogen concentration from ion beam analysis is 51 at.%, nitrogen contents from laser-assisted APT data are only 38 at.% (UV laser) or 41 at.% (green laser). Hence, the nitrogen content of the AlN layer is underestimated from laser-assisted APT data by at least 10 and up to 13 at.% and this significant discrepancy cannot be explained by the measurement uncertainties. Thus, the here presented data emphasize that it is not meaningful to quantify vacancy concentrations of nanolamellar (Ti,Al)N thin films solely based on atom probe data, since the measurement accuracy depends on the field evaporation conditions

Mitigating the detrimental effects of galvanic corrosion by nanoscale composite architecture design

Widespread application of magnesium (Mg) has been prevented by its low strength and poor corrosion resistance. Core of this limitation is Mg's low electrochemical potential and low solubility for most elements, favoring secondary phase precipitation acting as effective micro-galvanic elements. Mg-based metal-metal composites, while benefiting strength, are similarly active galvanic couples. We show that related detrimental corrosion susceptibility is overcome by nanoscale composite architecture design. Nanoscale phase spacings enable high-strength Mg-Fe composites with degradation rates as low as ultra-high purity Mg. Our concept thus fundamentally changes today's understanding of Mg's corrosion and significantly widens the property space of Mg-based materials.ISSN:2397-210

Stainless steel reveals an anomaly in thermal expansion behavior of severely deformed materials

Thermal expansion of materials is of fundamental practical relevance and arises from an interplay of several material properties. For nanocrystalline materials, accurate measurements of thermal expansion based on high-precision reference dilatometry allow inferring phenomena taking place at internal interfaces such as vacancy annihilation at grain boundaries. Here we report on measurements obtained for a severely deformed 316L austenitic steel, showing an anomaly in difference dilatometry curves which we attribute to the exceptionally high density of stacking faults. On the basis of ab intio simulations we report evidence that the peculiar magnetic state of the 316L austenitic steel causes stacking faults to expand more than the matrix. So far, the effect has only been observed for this particular austenitic steel but we expect that other magnetic materials could exhibit an even more pronounced anomaly