Summary of Prior Work on Joining of Oxide Dispersion-Strengthened Alloys

There is a range of joining techniques available for use with ODS alloys, but care should be exercised in matching the technique to the final duty requirements of the joint. The goal for joining ODS alloys is a joint with no local disruption of the distribution of the oxide dispersion, and no significant change in the size and orientation of the alloy microstructure. Not surprisingly, the fusion welding processes typically employed with wrought alloys produce the least satisfactory results with ODS alloys, but some versions, such as fusion spot welding, and the laser and electron-beam welding technologies, have demonstrated potential for producing sound joints. Welds made using solid-state spot welding reportedly have exhibited parent metal properties. Thus, it is possible to employ processes that result in significant disruption of the alloy microstructure, as long as the processing parameters are adjustment to minimize the extent of or influence of the changes in the alloy microstructure. Selection among these joining approaches largely depends on the particular application and component configuration, and an understanding of the relationships among processing, alloy microstructure, and final properties is key. Recent developments have resulted in friction welding evolving to be a prime method for joining ODS sheet products, and variants of brazing/diffusion bonding have shown excellent promise for use with tubes and pipes. The techniques that come closest to the goal defined above involve solid-state diffusion bonding and, in particular, it has been found that secondary recrystallization of joints made by pulsed plasma-assisted diffusion can produce the desired, continuous, large alloy grain structure through the joint. Such joints have exhibited creep rupture failure at >82% of the load needed to fail the monolithic parent alloy at 1000 C

Noise reduction in CCD measurements by improving the quality of dark-reference images

This publication is a systematic investigation of the effect the improvement of dark-reference images has on the resulting bright-field images. For this, data were acquired with three different charge-coupled device cameras attached to two different transmission electron microscopes. Multi-frame acquisitions and methods to correct X-ray noise are introduced and quantified as options to improve the dark-reference images. Furthermore, the influence of X-ray noise on transmission electron microscopy measurements is discussed and observations on its composition are shared

High temperature oxidation of ultra-high purity Fe-Cr-Al model alloys

A field emission gun scanning electron microscope (FEGSEM), equipped with an EDX analysis system was used to investigate the oxidation behaviour of Fe-20Cr-5Al based ultra-high purity alloys, which were specifically prepared with controlled levels of Zr, Ti, Hf, La, Si, and Y additions. The oxidation temperature was up to 1200°C for 3000 hours. Numerous studies dedicated to this field have shown that impurities and additions strongly affect the oxidation/spallation behaviour in many different ways. In this pilot study, the effect of carefully controlled combinations of the added elements on α-alumina scale formation, growth and spallation have been examined. The model alloy which contained approx. 300-450 ppm Hf, Ti and Y behaved the best with respect to scale spallation, whereas, the alloy containing only 250 ppm of La spalled the most. Another interesting finding is that in the case of the alloys which contain Hf+Ti, Zr+Ti and Zr+Hf in addition to Y, the scale fractured/spalled in a cohesive manner, whereas in the case of the alloys containing Si +Y and La the scale spalled at the scale/metal interface (adhesive failure). Also a “sunflower” type oxide grain structure was observed in the spalled regions in the case of the commercial alloy YHfAl and the model alloys containing Zr+Hf, and Zr+Ti in addition to Y.In this paper the different oxidation behaviour of the alloys will be discussed, in order to shed light on the influence of the various selected combinations of additional elements

Selective Laser Melting of Oxide Dispersion Strengthened Steels

Oxide Dispersion Strengthened (ODS) alloys are a long established class of materials manufactured using powder metallurgy techniques. These alloys can offer exceptional high temperature strength and resistance to radiation damage, thus are envisioned to be used in a number of future nuclear and fossil energy power applications. However, due to the manufacturing steps involved, the overall cost to build components with these materials can be high. This paper presents work conducted to assess the feasibility of applying Selective Laser Melting (SLM) techniques to either coat or direct build on substrates with Fe-based Oxide Dispersion Strengthened (ODS) alloys. SLM is a rapid prototyping technique which can be used to manufacture near net-shape solid components from layered metallic powder beds. Two different geometries were of interest in this study — a simple button configuration with a nickel-base superalloy (IN939) substrate and a more complex hexagonal shaped wall with a mild steel substrate. Powders of PM2000 (a FeCrAl based ODS alloy) were deposited in both cases. Heat treatments were subsequently conducted on these structures to investigate effects of temperature on the bond characteristics and secondary recrystallisation. Electron microscopy examination revealed significant amounts of diffusion between the nickel and the ODS powders which enhances the bond strength. The studies have revealed the existence of a strong bond between the substrate and the interface even after prolonged exposure at elevated temperatures.</jats:p