Multiple Influences of Molybdenum on the Precipitation Process in a Martensitic PH Stainless Steel

Molybdenum has been found to influence the complex precipitation process in a martensitic precipitation hardening stainless steel during aging at 475 degrees C in several different ways. Three steels with different Mo content (0, 1.2 and 2.3 at.%) were investigated. Studies of the microstructure were performed with atom probe tomography and energy filtered transmission electron microscopy. It is shown that, at the initial stage of aging, a faster nucleation of Cu-rich clusters takes place with increasing Mo content. The Cu-clusters act as precipitation sites for other solute elements and promote the nucleation of Ni-rich phases. During further aging, a higher Mo content in the material instead slows down the growth and coarsening of the Ni-rich phases, because Mo segregates to the interface between precipitate and matrix. Additionally, Mo promotes decomposition of the matrix into alpha and alpha\u27 regions. After longer aging times (>40 h) quasicrystalline Mo-rich R\u27 phase forms (to a greater extent in the material having the highest Mo content). The observations serve to understand the hardness evolution during aging

Fracture of Cr2O3 single crystals on the microscale

Studying cleavage properties of protective oxide scales is imperative to understand their fracture behaviour, since transgranular fracture is observed in many cases. The small thickness and polycrystalline structure of such scales makes it difficult to identify active cleavage planes directly from mechanical testing. To resolve this issue for Cr2O3, we present an approach to experimentally identify cleavage planes through micro-cantilever bending. Single crystal wafers are used to prepare micro-cantilevers of pentagonal cross-section in different orientations, targeting possible cleavage planes. Fracture surface imaging showed rhombohedral and pyramidal fracture, though surface energy studies predict rhombohedral as the dominant plane. There does exist a preference for rhombohedral fracture over pyramidal, which is also revealed from the experiments

Cold sprayed Cr-coating on Optimized ZIRLO™ claddings: the Cr/Zr interface and its microstructural and chemical evolution after autoclave corrosion testing

Cr-coated Optimized ZIRLO™ cladding material fabricated with the cold-spray deposition process is studied. Microstructure and chemistry of this material are investigated before and after exposure to autoclave corrosion testing with scanning electron microscopy, energy dispersive spectroscopy analysis, electron backscattered diffraction, transmission electron microscopy and atom probe tomography. The results are used to assess what changes have occurred upon autoclave exposure. The formation of a compact, 80 – 100 nm thick Cr2O3 layer is observed on the surface of the exposed samples. Nucleation of ZrCr2 intermetallic phase is discovered at the Cr/Zr interface. This Laves phase nucleates inside the intermixed bonding layer that can be found in both pristine and exposed samples, and decorates the interface in the form of small particles (less than 50 nm in size). Using transmission electron microscopy and atom probe tomography the growth of a Zr-Cr-Fe phase was detected. This phase is found in the region of the Zr-substrate immediately adjacent to the coating, up to a few hundred nanometres distance from the Cr/Zr interface. A small degree of recrystallization occurs upon autoclave exposure in the 1-2 \ub5m thick nanocrystalline layer produced on the Zr-substrate by the cold spray deposition method utilized for the fabrication of the Cr-coating

Comparing CrN and TiN Coatings for Accident-Tolerant Fuels in PWR and BWR Autoclaves

The development of coatings for accident-tolerant fuels (ATFs) for light water reactor (LWR) applications promises improved corrosion resistance under accident conditions and better performances during operation. CrN and TiN coatings are characterized by high wear resistance coupled with good corrosion resistance properties. They are generally used to protect materials in applications where extreme conditions are involved and represent promising candidates for ATF. Zr cladding tubes coated with 5 \ub5m-thick CrN or TiN, exposed in an autoclave to simulated PWR chemistry and BWR chemistry, were characterized with SEM, EDS, and STEM. The investigation focused on the performance and oxidation mechanisms of the coated claddings under simulated reactor chemistry. Both coatings provided improved oxidation resistance in a simulated PWR environment, where passivating films of Cr2O3\ua0and TiO2, less than 1 \ub5m-thick, formed on the CrN and TiN outer surfaces, respectively. Under the more challenging BWR conditions, any formed Cr2O3\ua0dissolved into the oxidizing water, resulting in the complete dissolution of the CrN coating. For the TiN coating, the formation of a stable TiO2\ua0film was observed under BWR conditions, but the developed oxide film was unable to stop the flux of oxygen to the substrate, causing the oxidation of the substrate

The effect of additive manufacturing on the initial High temperature oxidation properties of RE-containing FeCrAl alloys

The effect of additive manufacturing on the high temperature oxidation properties of FeCrAl materials was investigated. For this purpose, additively manufactured Kanthal AM100 cut parallel and perpendicular to the building direction and hot-rolled Kanthal AF as a reference were exposed to air at 900 and 1100 \ub0C for 168 h. AM100 performed slightly better than AF in terms of mass gain. Nevertheless, an oxide scale with local differences in thickness formed on AM100 due to the bimodal grain structure of the underlying metal, which was composed of coarse-grained cuboidal repeating units (100 μm wide), separated by fine-grained rims

High-temperature oxidation behavior of additively manufactured IN625: Effect of microstructure and grain size

High-temperature oxidation of additively manufactured (AM) Ni-base alloy IN625 has been studied in air and Ar-5%H2-3%H2O at 900–1000 \ub0C. AM material is found to oxidize faster than the conventionally manufactured (CM) IN625 due to severe intergranular oxidation observed in the former. The AM IN625 was heat treated at 1100–1250 \ub0C and hot rolled at 980 \ub0C in order to modify the AM microstructure, primarily grain size, and analyze its role in alloy oxidation behavior. Grain size is shown to affect overall oxidation kinetics but not the intergranular oxidation morphology

CrN–NbN nanolayered coatings for enhanced accident tolerant fuels in BWR

The accident tolerant fuel (ATF) concept has emerged in the years after the 2011 Fukushima accident as part of a renewed effort in research for light water reactors. The primary focus is to further improve safety measures under and beyond design basis accident conditions, and to improve fuel cladding performance in normal operation. The application of a coating on zirconium claddings can achieve both these aims without extensive changes to the reactor design. Metallic chromium coatings have been profusely studied as solution for pressurized water reactors, but the search for an effective ATF coating able to withstand the environment inside boiling water reactors (BWRs) is still ongoing. In this work, two different versions of a novel nitride coating composition were studied. Zirconium claddings coated with 8 \ub5m thick layers of superlattice CrN–NbN and a nanolayered CrN–NbN were tested in autoclave under BWR operating conditions for 60 days. Scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy, electron back-scattered diffraction, x-ray diffraction, and atom probe tomography were employed to characterize as-deposited and autoclaved samples of these two materials. During exposure, both coating versions formed a stable, dense and passivating oxide scale (200–300 nm thick) on the surface, demonstrating improved oxidation protection under operating conditions. Some differences in the oxide growth mechanism were observed between the superlattice and the nanolayered CrN–NbN coatings, which allowed to glimpse at the effect of the layer thickness on the oxidation protection provided by these coatings. The nano-structured morphology of both coatings remained unaffected by the autoclave test, but a 35 nm thick Zr-Cr-N phase was found at the coating-substrate interface of the superlattice CrN–NbN coated cladding

Characterization of as-deposited cold sprayed Cr-coating on Optimized ZIRLO™ claddings

As-produced Cr-coated Optimized ZIRLO™\ua0cladding material\ua0fabricated with the cold-spray (CS)\ua0deposition process\ua0is studied. Cross-sectional\ua0electron microscopy, nano-hardness profiling,\ua0transmission electron microscopy, transmission Kikuchi diffraction, and\ua0atom probe tomography\ua0(APT) were performed to investigate the nature of the CS Cr-coating/Optimized ZIRLO™ interface, the microstructure of the coating, and the effects of the deposition on the Zr-substrate microstructure. The former surface of the Zr-substrate was found to have a highly deformed nano-crystalline microstructure, the formation of which was attributed to dynamic recrystallization occurring during\ua0coating deposition. This microstructural change, evaluated with\ua0electron backscattered\ua0diffraction and nano-hardness profiling, appeared to be confined to a depth of a few microns. Through APT analysis, a 10–20\ua0nm thick intermixed bonding region was observed at the interface between coating and substrate. The chemical composition of this region suggests that this layer originated from a highly localized shearing and heating of a thin volume of the outermost former surface of the substrate. The study of the intermixed bonding region\u27s crystalline structure was performed with\ua0high resolution transmission electron microscopy\ua0and revealed a distorted hexagonal close-packed structure

Oxidation of an inward grown aluminide diffusion coating studied using SEM and TEM

Oxidation of an inward grown aluminide diffusion coating on Ren\ue9 N4 after isothermally oxidation in air at 1050\ub0C, between 0.5 and 2000 h was studied using SEM and TEM. For preparation of TEM foils an improved method using focused ion beam (FIB) milling, combined with the lift-out technique, was applied. Studies of the microstructure and chemistry of the oxide scale revealed existence of three different types of voids; (a) interfacial voids, (b) a row of voids dividing the oxide scale in two parts with different morphology and structure, and (c) internal voids within the α-Al2O3 grains. It is believed that the different types of voids are the result of different mechanisms. TEM/EDS analyses show that the outermost part of the scale is enriched in Cr and has different morphology than the inner part. This is partly explained by the change in oxygen partial pressure (pO2) at the metal/oxide interface during the oxide growth