| High-temperature materials represented by Ni-based single crystal superalloy have become irreplaceable key materials in aerospace field due to their excellent high-temperature mechanical performance and oxidation resistance.Studying the microstructural evolution of superalloys under harsh conditions and establishing the cross-scale correlation between microstructure and properties can provide both experimental and theoretical support for the optimization of alloy composition and thermal mechanical treatment.Most of the international experimental studies on the mechanical deformation and oxidation mechanism of superalloys carried out at temperatures above 600? are carried out by ex-situ methods.However,it is still an international methodology problem to apply high-temperature and stress in TEM and observe the microstructural evolution of materials in real time at nano-to-atomic scale,so as to accurately reveal the high-temperature plastic deformation mechanism of materials,and there still lacks of such commercial instrument both at home and abroad.With the support of national major scientific research equipment,our group has developed an in-situ high-temperature mechanical testing system with atomic-scale resolution based on TEM.The system can simulate the working conditions of aeroengine turbine blades at 1150?and 137 MPa,providing a new experimental method for studying the high-temperature plastic deformation mechanism of materials at atomic scale.The system can be used to study the oxidation mechanism at atomic scale when working in conjunction with a spherical-aberration corrected environmental transmission electron microscope(ETEM).This thesis mainly focused on developing the overall mechanical system of the in-situ high-temperature atomic-scale TEM holder and studied the high-temperature oxidation mechanism of superalloy in situ in a TEM by using the abovementioned testing system.The main research contents and conclusions of this paper are as follows:(1)The mechanical outer body of the sample holder with high compatibility with the TEM chamber and ultra-high vacuum achievability is developed.The dimensions of the sample holder are designed as a four-section hollow structure.According to the working conditions of different sections.the front end of the sample holder is fabricated with high-strength titanium alloy,and the other three sections are made of light aluminum alloy.The structure of the four sections is optimized and examined.On the premise of satisfying structural and functional use,the internal space of each section is optimized to expand the space for double-tilt and electrode parts.The mode,material,structure and dimensions of the sealing connection between each section of the sample holder are optimized according to the structure,dimensions and working condition of each section.The seal detection system of the sample holder is independently built to quickly and conveniently detect the location of possible leak on the holder,with a minimum leakage rate of 0.01×10-12 Pa·m3/s,providing an effective pre-detection tool for the self-sealing of the holder and the high-vacuum sealing of the sample and TEM.Test results show that the outer body of the sample holder precisely matches the dimensions of the TEM chamber and allows for the ultimate vacuum demand of the TEM.(2)A double-tilt mechanism compatible with both the front end of the sample holder and the narrow space between the pole pieces of the TEM is develop to support the integrated platform.the tilting table is the driven executive component,the connecting rod is the intermediate transmission component,and the miniature PZT motor connecting the drive shaft with high precision,high vacuum compatibility,no magnetism is used as the driving link.Grating displacement sensors are used to provide high precision displacement feedback.During operation,the forward and backward reciprocating motion of the driving link is transformed into the rotation of the tilting table around the?axis of the rotation center.A software system adapted to the PZT motor is designed to realize the automatic control of tilting.The tilting mechanism can achieve -20°?27°tilting around the?axis within a 5.4 mm TEM pole piece.In combination with the ±20°tilting capability of the goniometer of the TEM,it is easy to observe and analyze the microstructure of materials at atomic scale under high temperature and straining conditions.An external vacuum chamber compatible to scanning electron microscope(SEM) is designed to precisely calibrate the?axis tilting angle in SEM.The relationship between forward and backward displacement and the tilting angle of PZT motor is established and the influence of machining and assembling errors on tilting accuracy is effectively eliminated.The cumulative error of the full cycle(-20°?27°) is reduced to 0.01° and the tilting accuracy is better than 0.01°.(3)An integrated platform compatible with the double-tilt mechanism is designed,which is smaller than 3 mm×9 mm×0.6 mm and consists of a MEMS heating chip,a miniature actuator and electrode wires.Sample preparation method,the gap of the buffer structure and the preloading force during assembly of the platform and the tilting table are determined according to load bearing condition of the platform.By doing so,the fracture of the sample during assembly is effectively prevented and the success of in-situ high-temperature mechanical experiment is ensured.An electrical connection device composed of a printed circuit board(PCB)and a flexible printed circuit(FPC) is designed specially for the small integrated platform.This method solves the technical difficulty of the interference between the tilting mechanism and the wires during the tilting process,ensures the normal operation of the tilting mechanism and the reliability of the wires and realizes the introduction and output of 9-16 multi-signals in a small space.The integrated platform and wire-introduction mechanism are broadly applicable and can support thermal,mechanical or electrical loading as well as the coupling of these stimuli.(4)The assembly of the whole sample holder system is completed combined with the MEMS heating chip and the miniature actuator,and atomic-scale in-situ high-temperature tensile experiment is carried out at 1150?for the first time.The sample holder system has been in operation for 2 years,which verifies the reliability of the mechanical structure and the electrical transmission system designed in this paper.(5)In situ oxidation corrosion experiments of superalloy were carried out in ETEM and the initial oxidation rule of superalloy was revealed.Two oxidation conditions were selected for this experiment:1.The oxygen pressure gradually increased from 10-7 mbar to 0.5 mbar under a constant temperature of 800?,and 2.the temperature rose gradually from room temperature to 900? while maintaining a constant oxygen pressure of 0.5 mbar.The results of experiment are as follows:The?/??interface junctions initiated outwards oxidation at 450°C under an oxygen pressure of 0.5 mbar.The oxide network forms along the?/??interfaces with increasing temperature;Ni,Co and Re are depleted from the interface junction first,then from the?/??interfaces,and finally,they are depleted from the?and??phases.The other alloying elements in the?phase,namely Ni,Ta,and Nb,are also depleted by interface/surface oxidation effects;After severe oxidation at 900°C,Al is left in the??and Al,Cr and W are left in?phase;the stable and protective Al2O3 and Cr2O3 coatings formed by the Al and Cr cannot completely protect other alloying elements from oxidation.The alloying elements can diffuse outwards to the interfaces and surfaces to be oxidized.Due to the migration of these alloying atoms,vacancies and pores can be created inside the?and??phases and create space and channels for oxygen molecules to diffuse inwards into the?and??phases. |
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