In order to improve the efficiency of thermodynamic processes, higher operation temperatures are required, which, however, are mainly limited by the maximum application temperature of the nowadays-used materials. The most effective strengthening mechanism for metal alloys at elevated temperatures represents the incorporation of nanometer-sized oxides, also known as oxide-dispersion-strengthening. Nevertheless, the production of this alloy class via mechanical alloying is still time-consuming and therefore, expensive, limiting the economic feasibility. In this context, this thesis investigates the effect of cryogenic mechanical alloying as a possible means to improve the efficiency of mechanical alloying and gives insights into the mechanism of mechanical alloying of an fcc FeCrMnNiCo oxide-dispersion strengthened alloy. Pre-alloyed gas-atomized FeCrMnNiCo powders were milled with 1 m.% Y2O3 using a novel cryogenic milling setup and the powders were analyzed using state-of-the-art methods including high-resolution scanning electron microscopy, high-energy X-ray diffraction, transmission electron microscopy, atom probe tomography and positron annihilation spectroscopy. The powders were consolidated via spark plasma sintering and inductive heating within the synchrotron beamline in order to analyze the oxide-precipitation and defect evolution during consolidation ex-situ and in-situ. Cryogenic mechanical alloying was found to increase defect densities of the fcc FeCrMnNiCo matrix and is suggested to improve the refinement of yttria during mechanical alloying and thus increases the milling efficiency towards shorter milling times. Furthermore, the refinement of yttria is assumed to occur in two different size ranges of yttria, with 10 nm as the transition, while in both ranges, cryomilling reduces the size of yttria. Investigation regarding crystal structure identified yttria larger 10 nm as crushed remnants of the initial yttria, whereas independent of the milling temperature, approximately two-thirds of the added yttria is smaller than 10 nm and postulated to be dissolved into the matrix forming nanoclusters. The formation and stability of these nanoclusters were further suggested to be strongly related to vacancies, and thus, increased vacancy densities within the cryomilled sample suggest smaller and more stable nanoclusters after milling at cryogenic temperatures. These more stable nanoclusters are proposed to cause a later precipitation of Y and O during heating which further results in a higher fraction of YCrO3 within the cryomilled sample, whereas after room temperature milling, mostly Y2O3 formed. It is further suggested that the proposed increased cluster stability retains the, in defects stored, internal energy towards a higher temperature during heating promoting abnormal grain growth in the cryomilled specimen, whereas this effect was not observed in the room temperature milled specimen.
|Translated title of the contribution||Temperatureinfluss während des mechanischen Legierens auf die Veränderung von Yttriumoxid bei der Verarbeitung einer ODS FeCrMnNiCo-Legierung|
|Publication status||Published - 2023|
Bibliographical noteno embargo
- high-entropy alloys