Maria A Auger

The impact of plasma nitriding on the microstructure and the hardness of a recently developed 4 wt.-% medium manganese steel are presented. In contrast to standard quench and tempering steels, the investigated material achieves its martensitic microstructure by air-cooling from the forging heat, which enables the reduction of the carbon footprint of the forged components. The influence of nitriding on this grade of steel has not been investigated so far, but fundamental differences in comparison to standard nitriding steels are expected due to the increased manganese concentration. To address this issue, nitriding treatments with different temperatures (350 °C, 580 °C and 650 °C) have been performed, followed by examinations of the microstructure, the phase composition, the obtained hardness profiles and the tensile properties of the bulk material after nitriding, accompanied by thermodynamic equilibrium calculations. It is demonstrated that after nitriding at 580 °C similar hardness profiles like standard nitriding steels are achieved, with a shorter process as austenitization and hardening were omitted, reaching a hardness of approximately 950 HV0.1. Furthermore, it was demonstrated that austenite can be stabilized by manganese and nitrogen partitioning to room temperature during nitriding in the intercritical phase region.

Reduced activation ferritic (RAF) steels are expected to be widely used in challenging nuclear industrial applications under severe thermo-mechanical regimes and intense neutron loads. Therefore, actual research panorama is facing the strengthening strategies necessary to maximize both performance and endurance under these conditions. Oxide dispersion strengthened (ODS) RAF steels are leader candidates as structural materials in fusion energy reactors thanks to the reinforcement obtained with a fine dispersion of nanosized oxides in their matrix. In this study, the influence of the initial powder particle size and the selected processing route on the final material has been investigated. Two RAF ODS steels coming from atomized pre-alloyed powders with nominal particle powder sizes of 70 and 30 µm and composition Fe-14Cr-2W-0.4Ti-0.3Y2O3 (wt. %) were manufactured by mechanical alloying. Alloyed powders were compacted by hot isostatic pressing, hot crossed rolled, and annealed at 1273 K. Initial powder particle size differences minimize after milling. Both steels present an almost completely recrystallized material and similar grain sizes. The same type and distributions of secondary phases, Cr-W-rich, Ti-rich, and Y-Ti oxide nanoparticles, have been also characterized by transmission electron microscopy (TEM) in both alloy samples. The strengthening effect has been confirmed by tensile and Charpy impact tests. The two alloys present similar strength values with slightly better ductile brittle transition temperature (DBTT) and ductility for the steel produced with the smaller powder size.

Carlin-type gold deposits are one of the most important gold mineralization styles in the world. Despite their economic importance and the large volume of work that has been published, there remain crucial questions regarding their metallogenesis. Much of this uncertainty is due to the cryptic nature of the gold occurrence, with gold occurring as dispersed nanoscale inclusions within host pyrite rims that formed on earlier formed barren pyrite cores. The small size of the gold inclusions has made determining their nature within the host sulfides and the mechanisms by which they precipitated from the ore fluids particularly problematic.

This study combines high-resolution electron probe microanalysis (EPMA) with atom probe tomography (APT) to constrain whether the gold occurs as nanospheres or is dispersed within the Carlin pyrites. APT offers the unique capability of obtaining major, minor, trace, and isotopic chemical information at near-atomic spatial resolution. We use this capability to investigate the atomic-scale distribution of trace elements within Carlin-type pyrite rims, as well as the relative differences of sulfur isotopes within the rim and core of gold-hosting pyrite.

We show that gold within a sample from the Turquoise Ridge deposit (Nevada) occurs within arsenian pyrite overgrowth (rims) that formed on a pyrite core. Furthermore, this As-rich rim does not contain nanonuggets of gold and instead contains dispersed lattice-bound Au within the pyrite crystal structure. The spatial correlation of gold and arsenic within our samples is consistent with increased local arsenic concentrations that enhanced the ability of arsenian pyrite to host dispersed gold (Kusebauch et al., 2019). We hypothesize that point defects in the lattice induced by the addition of arsenic to the pyrite structure facilitate the dissemination of gold. The lack of gold nanospheres in our study is consistent with previous work showing that dispersed gold in arsenian pyrite can occur in concentrations up to ~1:200 (gold/arsenic). We also report a method for determining the sulfur isotope ratios from atom probe data sets of pyrite (±As) that illustrates a relative change between the pyrite core and its Au and arsenian pyrite rim. This spatial variation confirms that the observed pyrite core-rim structure is due to two-stage growth involving a sedimentary or magmatic-hydrothermal core and hydrothermal rim, as opposed to precipitation from an evolving hydrothermal fluid.

A nanograin sized model oxide dispersion strengthened (ODS) ferritic steel with nominal composition Fe–14Cr–0·3Y₂O₃ (wt-) was produced by mechanical alloying and consolidated by hot isostatic pressing. The alloy was submitted to severe plastic deformation by equal channel angular pressing (ECAP). Microstructural and mechanical characterisation was performed before and after ECAP. It was found that ECAP decreases and homogenises grain size without altering the nanoparticle dispersion, in addition to enhancing ductility and shifting the strength drop at high temperatures.

We have studied the surface kinetic roughening of AlN films grown on Si(100) substrates by dc reactive sputtering within the framework of the dynamic scaling theory. Films deposited under the same experimental conditions for different growth times were analyzed by atomic force microscopy and x-ray diffraction. The AlN films display a (002) preferred orientation. We have found two growth regimes with a crossover time of 36min. In the first regime, the growth dynamics is unstable and the films present two types of textured domains, well textured and randomly oriented, respectively. In contrast, in the second regime the films are homogeneous and well textured, leading to a relative stabilization of the surface roughness characterized by a growth exponent β=0.37±0.03. In this regime a superrough scaling behavior is found with the following exponents: (i) Global exponents: roughness exponent α=1.2±0.2 and β=0.37±0.03 and coarsening exponent 1∕z=0.32±0.05; (ii) local exponents: αloc=1, βloc=0.32±0.01. The differences between the growth modes are found to be related to the different main growth mechanisms dominating their growth dynamics: sticking anisotropy and shadowing, respectively.