Preparation Of Nano Y-Ti-O Dispersion-strengthened FeCrAl Sheets By EBPVD And Their Microstructures And Properties

Hypersonic aircraft metallic thermal protection system（M-TPS） has a huge impact for overall technical and tactical performance. Stability and anti-erosion ability in a variety of harsh environment of TPS high temperature structure are strongly dependent overall performance of M-TPS skin material. In this paper, based on M-TPS needs of skin the size, the deformation process and service conditions, the process of large-size Y-Ti-O type oxide dispersion strengthened（ODS） alloy sheet fabricated by electron beam physical vapor deposition（EBPVD） were studied. Y-Ti-O type Fe20Cr5.5Al ODS super-alloy sheet, with diameter 1000 mm and thickness 0.15 mm, were prepared successfully. The main contents include: Process Design of ODS superalloy sheet fabricated by EBPVD with Ti; The microstructure of as-deposited alloy sheet and its formation mechanism; Subsequent toughening treatment process and its effects on the mechanical properties of materials and impact mechanism; Affect laws and mechanism of the toughening treatment process on electrochemical corrosion under 3.5% Na Cl aqueous conditions at room temperature. The composition and microstructure are characterized by X-ray fluorescence spectroscopy（XRF）, Scanning electron microscopy（SEM）, X-ray diffraction（XRD）, Transmission electron microscopy（TEM） and High resolution transmission electron microscopy（HRTEM）.Based on the EBPVD technology which can achieve condensate with highly dispersed complex phase, the study of preparation process for high-temperature ODS alloy sheet is carried out. To carry out the research of as-deposited sheet composition, using a fixed substrate deposition technology, the sheets with quasi-continuous composition are obtained based on the difference of incident angle and the target-to-substrate distance. To control the Ti element content exactly, with significant differences in saturated vapor pressure compared with other evaporation elements, a separate Ti resource is used. According to the needs of the uniformity of composition and thickness distribution of a large size alloy sheet, based on the interaction study of these process parameters, such as the intensity of the electron beam gun, the target-to-substrate distance, substrate material, substrate temperature and rotation speed, by microstructure analysis of the as-deposited alloy sheet, the EBPVD process design methods for the deposition of ODS alloy sheet is obtained.The microstructure of Fe20Cr5.5Al base ODS alloy sheet was characterized and analyzed, which revealed that affect law and action mechanism of Ti element and Y₂O₃ on grain size, morphology and micro-voids defects distribution of alloy sheet. XRF, SEM, transmission electron microscopy（TEM） and high resolution transmission electron microscopy（HRTEM） were used to study the effects of Ti on micro-alloyed, the research results showed that the composition and crystal structure had a significant difference when the rate of Ti / Y₂O₃ content was different. Based on the special micro-voids defects of as-deposited sheets prepared by EBPVD, almost through overall thickness of the as-deposited alloy sheet beside the columnar crystals, the formation mechanism was proposed based shadow effect under high speed PVD process.The tensile properties of as-deposited alloy sheet, with different composition, were tested at room temperature, 500℃,700℃ and 900℃. Based on analysis of the brittle characteristics from micro-voids defects, the treatment processes which could eliminate these defects were studied. The most effective way eliminating micro-voids defects, cold rolled / recrystallization annealing for small deformation, was obtained. And the microscopic mechanism eliminating the microstructure defects of as-deposited sheet was proposed based on critical deformation recrystallization.Electrochemical corrosion polarization curves and electrochemical impedance spectroscopy（EIS） of the sheets treated by different methods were analyzed at room temperature and in 3.5% Na Cl solution. Based on the study results of the surface morphology before and after corrosion, the electrochemical corrosion law of the sheets with different microstructure was obtained. For electrochemical corrosion characteristics of the sheets prepared by EBPVD, the microscopic mechanism of columnar grain boundaries micro-voids local electrochemical corrosion behavior is put forward.According to the analysis results of mechanical and electrochemical corrosion properties of the sheets with different microstructure, the most e ffective way was obtained to eliminate the defects of the as-deposited sheet, 6.2% cold rolling/1300℃33h recrystallization annealing processes.