The innovation engine for new materials

Maita Roberts

Maita Roberts


Saarland University


Materials Science


Colin Stewart

Faculty Sponsor(s): 

Carlos Levi
Tresa Pollock

Faculty Sponsor's Department: 


Project Title: 


Project Description: 

Novel Co-based γ/γ’ alloys may offer potentially higher operation temperatures for turbine engines than commercial Ni-based superalloys. In order to be deployed in an engine, these alloys need to have some intrinsic oxidation resistance, which is achieved by an external α-Al2O3 layer. During oxidation in dry air at 1100 °C, unoptimized Co-based γ/γ’ alloys may form various non-protective oxides instead. The morphology, chemical composition and order of formation of these oxides must be investigated, to understand the evolution of a protective α-Al2O3 layer. A baseline alloy (Co-Ni-Al-W-Cr-Ta) and a Lanthanum doped alloy were compared. The surfaces and the cross-sections of these samples were investigated by Raman spectroscopy, scanning electron microscopy (SEM) combined with energy-dispersive x-ray spectroscopy (EDX) and transmission electron microscopy (TEM). It has been shown that the adherence of the oxide layers is affected by the surface preparation, with more finely polished samples spalling less than surface ground, whereas the formed oxide phases seem to be unaffected. The morphology is of mixed character with internal and external α-Al2O3, though only the Lanthanum doped samples achieve a significant amount of external α-Al2O3. Recent results suggest that different chromium and aluminum rich spinel and tungsten and tantalum oxides form on top of α-Al2O3, but this must be investigated further by high resolution EDX in a TEM. To determine the order of oxide formation, the samples have been oxidized for one hour and for 10 minutes respectively. All oxide layers appear to have developed after 10 minutes. Shorter oxidation times are carried out to determine when an external α-Al2O3 layer begins to form.