Microstructure And Mechanical Properties Of Y₂O₃ Strengthened Ni Based Supper Alloy Foil Prepared By EB-PVD

Metallic thermal protection system (TPS) is one of the key technologies for developing reusable launch vehicle (RLV). Compared with the traditional ceramic tiles, the metallic TPS has many merits, such as lighter wight, excellent thermal shock resitance, more simple structure, easier installation and removal, high reliability. Moreover, the most key part in the MTPS is surface material of resistance to elevated temperatures and oxidation. In this paper, large-scale ODS Ni-based alloy thin foils with high strength are prepared by electron beam physical vapor deposition (EB-PVD) technology. Surface mechanical attrition treatment (SMAT) and heat treatment were used to elevate the mechanical properties of the alloy thin sheet. The influences of the EB-PVD technology on the microstructure were studied systematically. The microstructure, mechanical properties and oxidation resistance are investigated by means of modern forecasting and analysis methods. The investigation contents include preparation technique of superalloy thin sheets, the research of SMAT and heat treatment on the alloy sheet, the analysis of structures, mechanical properties and the investigation of oxidation resistance in high temperature.The results indicated that fine equiaxed crystals are formed on the surface near to the substrate; moreover, columnar crystals are formed on the surface apart from the substrate. The width of the columnar crystals is about 3-5μm. The XRD results show that the phase composition of the as-deposited sheets isγ-Ni. TEM results reveal that the columnar crystals are composed of twin crystal. The pole figures indicate that the columnar crystals have a certain crystallographic orientation. The result suggests that a predominantly <111> fiber texture is formed. The tilt angle of the columnar crystals changes gradually according to the radius of the substrate. The vaulue of the angle can be estimated by the theory model established by through the relationship between the thicknesss and radius. The mechanical properties of the alloy sheets can be influenced by the tilt angle of the columnar crystals. The larger tilt angle is, the higher ultimate tensile strength is. However, for the samples at the edge of the sheet, since the variational range of VIA during the deposition is very large, the deposition procedure is complicated. Therefore, the rule is invalid here. The substrate side and deposition side exhibit different mechanical properties since the microstructure of these two sides are totally different. The fracture mode of the as-deposited sheets is a typical brittle inter-granular fracture mode. The cracks are much easier propagating along the columnar grain boundaries because of the loosely-bonding between columnar grains. The ultimate tensile strength of as-deposited sheets is 1080 MPa, while the elongation is almost 0. The tensile strength of the as-deposited sheets at 980℃is only 54 MPa, a little higher than the alloy without oxides.SMAT was used to treat the as-deposited sheet. The results indicate that the grain size of the treated surface decreases significantly, and hardness and yield strength increases greatly after SMAT. However, the tensile properties was not changed. By means of DSC and hardness test, 800℃and 1100℃was selected as the temperature of heat treatment. The strength and ductility can be improved effectively by means of heat treatment. After having been annealed at 800℃for 3 h, the ultimate tensile strength and elongation percentage of the sheet increased to 1220 MPa and 0.94. While, for samples treated at 1100℃, the strength decrease to 930 MPa and the ductility was enhanced to 2.42% due to the precipitates of YAG and the increase of grain size.The oxidation kinetics of as-deposited and heat treated alloy at 1000℃follows a parabolic power law. The compact oxide layer is mainly composed of Cr₂O₃ and NiCr2O4. Internal oxidation was observed in the as-deposited alloy since the interface of the columnar crystals can provide transport channel of oxygen. It is shown that heat-treated alloy have better oxidation resistant performance according to structure and constituent of oxide scale. After exposure at 1000℃for 100 h, the mass gain of the as-deposited alloy is 1.53 mg/cm², while the value is only 0.65 mg/cm² for the heat treated alloy.