Internal Oxidation and Equal Channel Angular Pressing of Fe-Y and Fe-Zr-Y Alloys

Abstract

Metal/oxide composites with ultrafine-grained metallic microstructures stabilized by a dispersion of oxide particles exhibit a number of desirable characteristics including high strength and creep resistance at elevated temperatures. These materials are traditionally processed by powder metallurgy, with typical production steps including mechanical alloying of powders, followed by thermomechanical processing to create a fully dense final alloy. Because powder metallurgy is expensive and requires a series of high-energy and high-temperature techniques, alternative fabrication methods are being explored. This work investigates the processing of metal/oxide composites by internal oxidation of a bulk alloy, followed by severeplastic deformation for grain size refinement and oxide dispersion. Internal oxidation rates have been shown to be exceptionally high in alloys exhibiting in situ oxidation. Referred to as a diffusionless internal oxidation, this accelerated oxidation process occurs in materials where the reactive solute phase has negligible solubility anddiffusivity in the matrix. The Fe-Y binary system was initially chosen for this study for its in situ internal oxidation characteristics and similarity in composition to metal/oxidecomposites that are traditionally processed with Fe-based alloys and Y-rich oxides. As a complement to the Fe-Y system, Fe-Zr-Y alloys were also investigated as a candidate for internally oxidized metal/oxide composites. While the oxidation behavior of Fe-Zr-Y alloys has not yet been reported in the literature, ternary Ni-Zr-Y alloys have been known to undergo rapid in situ internal oxidation.The first stage of this study was the characterization of previously made Fe-Y alloys containing 1.5 wt.% yttrium, internally oxidized at 800?C. The oxidized alloys had beendeformed via equal channel angular pressing (ECAP) using route Bc. The post-ECAP microstructure was examined, and an automated technique was developed to analyze particledispersion in these deformed alloys. Based on the characterization of these previously made alloys, a new experimental procedure was designed for the internal oxidation and ECAP deformation of Fe-Y and Fe-Zr-Y alloys to form metal/oxide composites. A new set of alloys was created using a laboratory scale arc-melting furnace, with Fe-Y alloys containing 3 wt.% Y (1.9 at.% Y) and Fe-Zr-Y alloys with (Zr + Y) content equal to 1.9 at.%. The as-cast Fe-Y microstructure consisted of a pure ?-Fe matrix with Fe17Y2 intermetallic structure, while the Fe-Zr-Y alloys were found to have an Fe matrix with a multiphase secondary structure. Alloys were oxidized in air using a thermogravimetric analysis (TGA) furnace at 800?C, 1000?C and 1200 ?C for 1.5 to 12 hours. The new set ofFe-Y alloys experienced internal oxidation up to twice as deep as the previously studied Fe- 1.5 wt.% Y alloys, forming bundles of rod-like oxides at the oxidation front which coarsened into bands of discrete globular particles over time. The oxide structure of the Fe-Zr-Y wasvery different from the Fe-Y alloys, forming an interconnected oxide network. The Fe-Zr- Y alloys oxidized significantly deeper than the Fe-Y, exhibiting through-thickness internal oxidation at 1000C and 1200C.To better understand the microstructure and oxidation behavior of Fe-Zr-Y materials, a new set of ternary alloys was produced with varied Zr/Y ratios. The as-cast alloyswere found to have a multiphase secondary structure featuring a Zr-rich phase presumed to be Fe2Zr, and a Y-rich phase presumed to contain Fe17Y2, the ratios of which were dependent on alloy composition. These alloys were oxidized at 800?C, 1000C and 1200C for three hours under N2 - 5%H2 gas with a controlled dew point of 0C, exhibiting rapid internal oxidation. The internally oxidized microstructures also exhibited both Zr-rich and Y-rich phases in similar distributions to the as-cast alloys, indicating that internal oxidation occurred by an in situ process. The trend in Fe-Zr-Y oxidation rate with composition correlated with that of oxygen diffusivity in yttria-stabilized zirconia, suggesting a rate controlling mechanism of oxygen diffusion through the oxide. All Fe-Zr-Y alloys exhibited internal oxidation more than twice as deep as in binary Fe-Zr and Fe-Y alloys. A new ECAP procedure was developed for the oxidized Fe-Y and Fe-Zr-Y alloys. To mitigate surface cracking during deformation, multiple ECAP passes were performed in succession to induce a back pressure. Internally oxidized Fe-Y alloys were encased in pure Febillets and deformed via ECAP Route A for four passes. The post-ECAP microstructure revealed that there was no redistribution of oxide particles into the matrix. Rather, oxides remained clustered together in bands, and the oxide bands underwent stretching and reshaping within the deformed alloy microstructure, indicating that the effect of ECAP on particle dispersion is highly dependent on original microstructure length scale.