Mechanical Behavior And Microstructure Of Y₂O₃ Dispersion Strengthened FeCrAl Foil Fabricated By EBPVD

Oxide Dispersion Strengthened (ODS) alloy is potential for the application in reusable launch vehicle metallic thermal protection systems as panel material for their excellent high temperature properties. However, current ODS fabrication methods are confronted with difficulties in the aspects of deformation processing and controlling the oxide particle size when producing large size foils. Electron Beam Physical Vapor Deposition (EBPVD), as a method for coating fabrication, has unique advantages for the fabrication of ODS alloy foils, considering its simple procedure, rapid deposition speed, convenient control of oxide content, size, and distribution, as well as its near net shape characteristic. However, it is urgent and significant to investigate the keys of controlling the microstructure and improving the mechanical properties.This paper investigated yttria dispersion strengthened FeCrAl high-temperature alloy foils fabricated by EBPVD technique, covering four aspects: the microstructure and growth mechanisms of as-deposited material; the characterization and stability investigation of yttria dispersoids, and the strengthening mechanisms of the material; room temperature and high termperature tensile behavior, and the controlling deforming mechanisms; and creep behavior and creep equation of the material. The investigation methods and main conclusions are summarized in the following part.The surface and section morphology of foils deposited with various substrate rotation speeds and vapor incidence modes was examined by means of scanning electron microscopy (SEM). The textures at different depths of foils deposited under various vapor incidence modes were characterized via X-ray pole figure method. It was founded that the microstructures of ODS alloy foils fabricated by EBPVD counld be arranged by controlling the technical parameters. Microstructures composed of helical or columnar grains could be chosen. Using the geometrical model proposed in this paper, which was based on the supposition that the direction of grain growth follows the incident vapor flux, the column inclination angle of foils deposited under unsymmetrically variant vapor incidence angle could be calculated semi-quantatively. The influence of vapor incidence mode on the microstructural morphology and orientation was described and interpreted. The mechanisms controlling the microstructure and texture evolution was clarified. The substructrure of the material, the structure, chemical state, size, and distribution of the yttria particles, as well as the size and distribution of dispersoids after high-temperature and long-time treatment were characterized by means of X-ray diffraction (XRD), high resolution/transmission electron microscopy (HR/TEM), X-ray photoelectron spectroscopy (XPS) and electron backscatter diffraction techniques (EBSD). It was founded that the yttria particles in spherical or ellipsoidal shapes were distributed in the matrix grains homogeneously and dispersively. The sizes of the yttria particles were in the range of several nanometers to several tens of nanometers. Both of the matrix grain size and the dispersoid particle size could be controlled by adjusting the technical parameters. The addition of yttria particles significantly improved the room temperature strength of the foils via Hall-Petch mechanism and Orowan mechanism. Different from ODS alloys fabricated by mechanical alloying, in which Y and O were combined with other metal element and formed complex oxides, in ODS iron-based alloys fabricated by EBPVD, Y and O exsisted in the form ofα-Y2O3 with stable body centered cubic structure. Both of the matrix grains and the yttria particles are with good stability during 800℃long-time preservation.Room temperature and high temperature tensile properties were examined in terms of foils with 0, 0.8wt.%, and 1.2wt.% yttria. Microstructures of deformed specimens after tensile were characterized by TEM. The strengthening mechanisms were discussed. It was founded that the addition of yttria dramatically improved the high-temperature strength of the foil. The strength of foil in 800℃was improved by 51.2 percent via adding 0.8wt.% yttria, and the strength in 1000℃was improved by 64.0 percent via adding 1.2wt.% yttria. It is better to determine the yttria content according to the application temperature. High yttria content will lead to high fragility.Creep properties of foils without yttria and with 1.2wt.% yttria were investigated. Microstructures of deformed specimens after creep tests were characterized by TEM and EBSD. Corresponding creep equations were constructed. It was founded that the stable state creep rate was decresead by 95.55 percent, and the creep activation energy was increased from similar with the self diffusion acitivation energy to 2.45 times of self diffusion acitivation energy via adding 1.2wt.% yttria under 900℃and 20MPa.