Volume 57, Issue 8

At moderate and high temperature applications, nickel-based superalloys are widely preferred as suitable candidate materials due to their high mechanical properties and corrosion/oxidation resistance, however, difficulties in mass production of these alloys and their cost cause the use of these alloys as a part of dissimilar weld joints with any structural material. In this study, a dissimilar weld joint between a nickel-based superalloy (UNS 10276) and unalloyed steel (SA 516 GR 70) was successfully performed by gas tungsten arc welding method by using ErNiCrMo-3 filler metal. Initially, the microstructural features of base materials, heat affected zone and weld metal were characterized and cellular, coarser cellular and epitaxial growth dendrites as matrix phases and several Mo-rich and Nb-rich precipitates embedded in the solidified structure were observed in weld metal. In heat affected zones and fusion lines of the dissimilar joints, some metallurgical phenomena like partially melted zone, solidification at grain boundary, migrated grain boundary and decarburization were also examined. All these microstructural features affected the mechanical properties.

Boundary-layer visualization in Functionally Graded Metal Matrix Composites (FGMMC) is imperative for a fundamental evaluation of the shape, size, gradient and orientation of the reinforcement phase. In this context, FGMMCs are particularly challenging as they require high-quality mechanical preparation. The reason for this is the central importance of the boundary layers where the local reinforcement phase is located and which require high edge retention to be achieved during the preparation process. On the other hand, a potential debonding of the reinforcing particles from the matrix may complicate the preparation process. The aim of this research was the development of a suitable preparation routine in order to evaluate the boundary layer of FGMMCs produced by means of composite peening. A suitable preparation method could be found for the aluminium-alloy matrix material EN AW1050 reinforced with alumina particles.

Intergranular corrosion refers to a selective corrosion attack at the grain boundaries of polycrystalline materials. In engineering Al-Mg alloys of the AA 5xxx series it is caused by the formation of chemicallyless noble β-Al 8 Mg 5 phases along the grain boundaries. The sensitivity of a material to intergranular corrosion can be assessed based on the mass losses in the so-called nitric acid mass loss test (NAMLT) according to ASTM G67. However, a detailed investigation of the underlying corrosion mechanisms requires that the β phases are made directly visible in the microstructure by metallographic methods. In the present work, the NAMLT mass losses of three AA 5xxx alloys with different Mg contents are compared with the results from two different etching methods. On the one hand, the alloys are etched in diluted phosphoric acid, a substance routinely used to examine the grain boundary occupancy in AA 5xxx materials. On the other hand, a newer etching method using a dilute ammonium persulfate solution is tested which etches the β-Al 8 Mg 5 phases in the microstructure in a way that they can be examined at higher magnifications; even examinations in the scanning electron microscope are possible.

Even very simple cases of component failure can represent an enrichment for this section Failure Analysis . Besides the instructive value of an easily understandable content, they allow those failure analysts becoming acquainted with the field to gradually get to know the most important material-related mechanisms governing component failure cases. A look back by the authors on the section Failure Analysis of Practical Metallography cases revealed only one single erosion failure case in the past 10 years. It won't therefore do any harm to address this interesting failure mechanism once again. The erosion failure of a fan impeller presented in this article was very easily solved However, it was rather unexpected. Who would have considered it possible that an air extraction system in a factory workshop is subject to such high erosive distress? Hence, the main message of this short article is that the experts tasked with investigating the component failures should be very thorough and propose all sorts of damage hypotheses, sometimes even those which, at first glance, appear unlikely, and may only reject them when the examination results indicate the contrary.