TY - THES

T1 - The effect of cryogenic conditions during mechanical alloying on an oxide dispersion strengthened steel

AU - Kusztrits, Florian

N1 - embargoed until 19-11-2023

PY - 2018

Y1 - 2018

N2 - The aim of this work was to determine the influence of cryogenic conditions during mechanical alloying on the temper resistance and the microstructure of an oxide dispersion strengthened steel. The cryogenic conditions were applied by indirect cooling of the milling container with liquid nitrogen. Cryogenic milling temperatures are assumed to enhance the presence of crystal defects and therefore the dissolution of thermodynamically not soluble elements for the generation of supersaturated powder materials. Consequently, a higher density of oxides and hence, improved mechanical properties at elevated temperatures might be generated in the subsequent consolidated steel. In order to elucidate the effects of the so-called cryomilling process four different powder materials were produced and investigated. As benchmark materials high alloyed steels in the unmilled and room temperature milled but unreinforced state were used. Furthermore, two additional powders were reinforced with 0.5 m.% Y2O3 and milled at room temperature and at cryogenic temperatures. To analyse the temper resistance of all four produced materials hot isostatic pressing and tempering was performed. Hardness testing with varying temperatures shows a general increase in hardness through milling and a shift of the hardness decrease to higher temperatures for the reinforced materials. The cryomilled material showed an elevated temper resistance compared to room temperature milled materials. The properties of all powder states were investigated by means of chemical composition, scanning electron microscopy and X-ray diffraction to get information about incorporation of foreign elements, morphology and microstructure. Additionally, to investigate the microstructure of mechanical alloyed oxide dispersion strengthened steel after consolidation optical light microscopy, scanning electron microscopy and electron back scatter diffraction investigations were performed. It can be concluded that a higher amount of dissolved interstitial elements and higher density of carbo-nitrides as well as finely distributed reinforced yttrium oxides are the reason for the improvement of hardness and temper resistance of the cryomilled specimen.

AB - The aim of this work was to determine the influence of cryogenic conditions during mechanical alloying on the temper resistance and the microstructure of an oxide dispersion strengthened steel. The cryogenic conditions were applied by indirect cooling of the milling container with liquid nitrogen. Cryogenic milling temperatures are assumed to enhance the presence of crystal defects and therefore the dissolution of thermodynamically not soluble elements for the generation of supersaturated powder materials. Consequently, a higher density of oxides and hence, improved mechanical properties at elevated temperatures might be generated in the subsequent consolidated steel. In order to elucidate the effects of the so-called cryomilling process four different powder materials were produced and investigated. As benchmark materials high alloyed steels in the unmilled and room temperature milled but unreinforced state were used. Furthermore, two additional powders were reinforced with 0.5 m.% Y2O3 and milled at room temperature and at cryogenic temperatures. To analyse the temper resistance of all four produced materials hot isostatic pressing and tempering was performed. Hardness testing with varying temperatures shows a general increase in hardness through milling and a shift of the hardness decrease to higher temperatures for the reinforced materials. The cryomilled material showed an elevated temper resistance compared to room temperature milled materials. The properties of all powder states were investigated by means of chemical composition, scanning electron microscopy and X-ray diffraction to get information about incorporation of foreign elements, morphology and microstructure. Additionally, to investigate the microstructure of mechanical alloyed oxide dispersion strengthened steel after consolidation optical light microscopy, scanning electron microscopy and electron back scatter diffraction investigations were performed. It can be concluded that a higher amount of dissolved interstitial elements and higher density of carbo-nitrides as well as finely distributed reinforced yttrium oxides are the reason for the improvement of hardness and temper resistance of the cryomilled specimen.

KW - mechanical alloying

KW - cryogenic

KW - cryomilling

KW - powder metallurgy

KW - steel

KW - mechanisches Legieren

KW - kryogen

KW - cryomilling

KW - Pulvermetallurgie

KW - Stahl

KW - ODS

M3 - Master's Thesis

ER -