Effects of aluminum additions to gas atomized reaction synthesis produced oxide dispersion strengthened alloys

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Spicher, Alexander
Iowa State University
Advanced Ultra-Supercitical; Coal Fired Power Plant; Fossil Energy; Gas Atomization; Oxide Dispersion Strenghtened
thesis / dissertation description
The production of an aluminum containing ferritic oxide dispersion strengthened (ODS) alloy was investigated. The production method used in this study was gas atomization reaction synthesis (GARS). GARS was chosen over the previously commercial method of mechanical alloying (MA) process due to complications from this process. The alloy compositions was determined from three main components; corrosion resistance, dispersoid formation, and additional elements. A combination of Cr and Al were necessary in order to create a protective oxide in the steam atmosphere that the boiler tubing in the next generation of coal-fired power plants would be exposed to. Hf and Y were chosen as dispersoid forming elements due to their increased thermal stability and potential to avoid decreased strength caused by additions of Al to traditional ODS materials. W was used as an additive due to benefits as a strengthener as well as its benefits for creep rupture time. The final composition chosen for the alloy was Fe-16Cr-12Al-0.9W-0.25Hf-0.2Y at%. The aforementioned alloy, GA-1-198, was created through gas atomization with atomization gas of Ar-300ppm O2. The actual composition created was found to be Fe-15Cr-12.3Al-0.9W-0.24Hf-0.19Y at%. An additional alloy that was nominally the same without the inclusion of aluminum was created as a comparison for the effects on mechanical and corrosion properties. The actual composition of the comparison alloy, GA-1-204, was Fe-16Cr-0Al-0.9W-0.25Hf-0.24Y at%. An investigation on the processing parameters for these alloys was conducted on the GA-1-198 alloy. In order to predict the necessary amount of time for heat treatment, a diffusion study was used to find the diffusion rate of oxygen in cast alloys with similar composition. The diffusion rate was found to be similar to that of other GARS compositions that have been created without the inclusion of aluminum. The effect of heat treatment time was investigated with temperatures of 950°C, 1000°C, 1100°C, and 1200°C. A large precipitate phase, FeHf2 ht, was found in the 950°C and 1000°C samples through SEM. This was confirmed through XRD analysis where it was found that the 1100°C sample may have had clusters. These clusters could act as a location for the origination of cracks during future rolling operations. For this reason, an attempt to look at the hold time and ramp rates on the formation this phase. It was found that a 1200°C hold for 5 hours was able to homogenize the sample to prevent precipitation of the FeHf2 ht phase during a subsequent hold at 1000°C, the rolling temperature used in this study. For this reason a heat treatment at 1200°C for 5 hours was used in both alloys. Both alloys were rolled to 70% reduction in thickness and evaluated through microhardness, tensile testing, and corrosion testing. Microhardness showed high strength for the aluminum containing GA-1-198 and significantly more isotropic properties than mechanically alloyed ODS materials. Tensile testing showed GA-1-198 strength between MA956 and PM2000 for temperatures below 600°C and slightly lower strengths than MA856 at 800°C. GA-1-204 was not protective in either atmosphere; air at 1200°C and air with 10 vol% H2O at 1100°C. GA-1-198 showed increased mass gains due to sub-optimal oxygen content in the alloy. GA-1-198 had spallation in the air at 1200°C atmosphere, but remained protective up to 1000 hr in the water containing atmosphere.