Date of Award

1-1-2012

Document Type

Campus Access Thesis

Department

Mechanical Engineering

First Advisor

Djamel Kaoumi

Second Advisor

Anthony P Reynolds

Abstract

INCOLOY® MA956 is a ferritic ODS alloy. It has very good oxidation resistance by virtue of its large chromium and aluminum concentrations and high mechanical strength and creep resistance at elevated temperatures thanks to oxide dispersion strengthening.

The conventional processing route utilized to obtain this alloy involves two main multistep stages. The first (or front end) stage of the process consists of a dry, high-energy milling process which mixes very fine Y2O3 particles with elemental alloy powders by Mechanical Alloying (MA) in an attritor. The second (or back end) stage of the process consists of consolidating the mechanically alloyed powder by hot extrusion in vacuum-sealed cans at about 1000°C, or by degassing followed by hot isostatic pressing (HIP).

The precipitation of a fine dispersion of yttrium-aluminum-rich oxides (Y-Al-O) during the consolidation is at the origin of the high temperature mechanical strength of this alloy. Three different thermodynamically stable oxides are known to exist for the binary Y2O3:Al2O3 system: Y4Al2O9, YAlO3 and Y3Al5O12. All three of them have been observed in this type of alloys when processed by the route described above. Their size ranges from just a few up to hundreds of nm.

In this work, the applicability of Friction Consolidation to this ODS alloy was investigated in order to tackle the downsides of the conventional processing route (multisteps and extremely high raw material final cost).

For this study, mechanically alloyed INCOLOY® MA956 powder was consolidated through Friction Consolidation under three different sets of processing conditions. As a result, three small compacts of low porosity have been achieved with a refined equiaxed ferritic grain structure smaller than 10 microns and the desired oxide dispersion. Two types of mixed Y-Al oxides were observed by different complementary techniques, Scanning Electron Microscopy (SEM), Electron Dispersive Spectroscopy (EDS) and X-ray diffraction (XRD), YAlO3 and Y3Al5O12. Their size distribution was characterized using electron microscopy at different locations (for precipitates above 50 nm) and showed a larger average precipitate size for larger grain size. The total energy input during processing was correlated with the relative amount of each of the oxides in the disks (observed from XRD experiments): the higher the total processing energy input, the higher the relative proportion of Y3Al5O12 precipitates. The elemental composition of these precipitates was also probed individually by EDS showing an aluminum enrichment trend as precipitates grow in size. Overall, the Friction Consolidated material showed microstructural characteristics comparable to the ones observed in conventionally processed material, which makes it a very promising processing alternative.

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