Volume 55, Issue 6

The mechanical properties and microstructures of alloys play an important role in the different steps of the coin manufacturing process. Only few data are available for the precious metals gold and silver as well as for their respective alloys. Several different circular coin blanks were metallographically examined in order to provide a better estimation of the impact of the production steps on the material. An electrochemical polishing method was developed aiming at obtaining contrast-rich and artifact-free micrographs. The microhardness was systematically measured on the whole surface of polished cross sections in order to generate distribution mappings. Object of the examination was fine gold (Au 999.9) and ducat gold (Au 986) which are primarily used to manufacture bullion and collector coins.

Friction-stir processing (FSP) imposed a remarkable grain refining effect on Al-5Mg-0.2Sc alloy, changing typical dendritic microstructure into fine and equiaxed recrystallized grains. Such effect was found to be related to intense plastic deformation and high-temperature exposure, resulting in occurrence of dynamic recrystallization. During this process, Al 3 Sc dispersoids due to the addition of 0.2 % Sc in a high density were precipitated and effectively hindered the migration of grain boundaries and subsequent grain growth, giving rise to a significant grain-refining strengthening effect. On the other hand, these Al 3 Sc precipitates caused an additional substructure strengthening and precipitation strengthening effect. An attractive comprehensive property of Al-5Mg-0.2Sc alloy subjected to FSP was obtained.

The scope of this research project covers the examination of the microstructure of different austenitic steels with regard to H embrittlement. It is known from the literature that the alloy X2CrNi18-9 is susceptible to H embrittlement, whereas steel X2CrNiMo18-14-3 is highly resistant to the embrittlement process. Austenitic steels with a low γ stability are susceptible to form α‘ martensite during plastic deformation. The γ stability can be estimated using empirical formulas, while one drawback of this approach is that only a limited amount of alloying elements is considered by those equations. A thermodynamic approach has proven effective for the estimation of the γ stability of multicomponent alloys. Hence, results from the microsegregation analysis for alloy X2CrNi18–9 using energy dispersive X-ray spectroscopy are presented and their influence on the local γ stability is discussed based on property distribution images.

A spectacular damage occurred at the end of 2005 in Münsterland when a large number of power towers in 110 kV transmission lines collapsed due to increased loads of ice and snow, which raised awareness amongst experts for the evaluation of a continuous safe operation of historical towers. At the time, it was found that embrittlement by strain ageing was a contributing factor. The in-house client ordered the condition evaluation of a power tower located on his premises. The evaluation was performed by testing four so-called structural members from this tower. Mechanical-technological material tests were performed exclusively on samples from the flat, mostly not cold-formed areas of the structural members' angle irons. The results of all these tests were inconspicuous, including the notch impact tests at −20 °C. Therefore, it is safe to say that the material at hand is generally not in an embrittled condition. In this context it is also interesting that the nitrogen content of the batch the angle irons of the examined structural members were made from, did not exceed the embrittlement sensitivity threshold of 0.01 percent by weight set by the Federal Institute for Materials Research. From a material-technological perspective, there is no objection to a continued safe operation of the power tower on the client's premises. Further tests and repairs were not considered necessary.