Papers by Author: Edeltraud Materna-Morris

Abstract: Refractory materials, in particular tungsten base materials are considered as primary candidates for structural high heat load applications in future nuclear fusion power plants. Promising helium-cooled divertor design outlines make use of their high heat conductivity and strength. The upper operating temperature limit is mainly defined by the onset of recrystallization but also by loss of creep strength. The lower operating temperature range is restricted by the use of steel parts for the in- and outlets as well as for the back-bone. Therefore, the most critical issue of tungsten materials in connection with structural divertor applications is the ductile-to-brittle transition. Another problem consists in the fact that especially refractory alloys show a strong correlation between microstructure and their manufacturing history. Since physical and mechanical properties are influenced by the underlying microstructure, refractory alloys can behave quite different, even if their chemical composition is the same. Therefore, creep and thermal conductivity have been investigated using typical commercial tungsten materials. Moreover, the fracture behavior of different tungsten based semi-finished products was characterized by standard Charpy tests which have been performed up to 1100 °C in vacuum. Due to their fabrication history (powder mixing, pressing, sintering, rolling, forging, or swaging) these materials have specific microstructures which lead different fracture modes. The influence of the microstructure characteristics like grain size, anisotropy, texture, or chemical composition has been studied.

Abstract: 9 – 12%Cr-MoVNb steels are successful materials for the use at higher temperatures. The universal application in turbines and power plants promises an expedient material for fission and future fusion reactors. Different developmental alloys were fabricated, irradiated and mechanically tested to optimize the material properties. Already, after the first irradiation tests could be shown that this type of steel has a reduced swelling and much faster decay of radioactivity than e.g. austenitic Cr-Ni-steels. This inherent property of reduced activation includes a high recycling potential. Further variations in the chemical compositions were carried out to increase the decay of radioactivity and to reduce the hardening and strengthening by neutron irradiation. The state of development today is a 9%Cr-WVTa-alloy, which is called EUROFER. The post-irradiation behavior and the development of material matrix are shown in comparison with further test alloys. The emphasis is to characterize the hardening mechanisms by irradiation at lower temperatures, which appear in helium bubbles, interstitial loops, and α`-precipitates.

Abstract: The intermetallic phase precipitation, mainly sigma () and chi () phases, was studied in a comparative manner in two stainless duplex steels: a duplex type UNS S31803 and a superduplex type UNS S32520. The -phase precipitated at ferrite/ferrite grain boundaries prior to the -phase precipitation, which occurred preferentially at ferrite/austenite interfaces and at ferrite/ferrite grain boundaries. The -phase precipitation is a eutectoid type reaction of ferrite, leading to -phase and austenite (γ). The -phase precipitated at lower temperatures and in smaller amounts than sigma. The -phase is metastable in the studied steels and was consumed during the -phase precipitation.