The Thermal Shock Resistance Behavior Of In-Situ TiC_x-Ni₃?Al,Ti?/Ni Functionally Graded Composites

Up to date,nickel-based functionally gradient composites have become the most widely used alloy materials in the field of aerospace due to its remarkable chemical stability,oxidation and corrosion resistance and excellent high-temperature mechanical properties.However,because of the harsh environment of the application,nickel-based gradient materials are exposed to huge temperature difference,high stress and high thermal shocks.So it puts forward higher requirements for thermal properties and thermal shock resistance of materials.In this paper,a novel TiC_x-Ni₃?Al,Ti?/Ni functional gradient composites prepared by in-situ reaction were used as experimental materials to study the thermal shock resistance at different temperatures and cycle times.Phase analysis was characterized by X-ray diffractometer?XRD?.Microstructural analysis and surface morphology were observed by using Scanning Electron Microscope?SEM?.Elemental analysis was performed on an Energy Spectrometer?EDS?.The changes of phase and microstructure of materials before and after thermal shock were studied and the reasons for the changes of properties were explained.Moreover,The scale and distribution of thermal stress were simulated by ANSYS finite element software.These results are expected to provide valuable information for the further design and the high-temperature application of this novel FGM.The results show were showed as following:?1?The surface of TiC_x-Ni₃?Al,Ti?/Ni FGM was oxidized to different extent after10 thermal shocks cycles at different temperatures,It's mainly the oxidation of TiC and Ni₃?Al,Ti?,The phases of each layer almost did not change after thermal shocks at the desired temperature,compared with that of the FGM before the thermal shock.It shows the thermal stability of FGM.?2?The microstructure of FGM did not change before and after the thermal shock.However,with the increase of thermal shock temperature and number of thermal shock,the number and size of holes and microcracks on FGM surface increased.In the process of microcrack propagation,obvious migration and branching occurred,which reduced the probability of interlayer crack formation.FGM maintains a good overall structure and has no interlaminar cracks with unstable propagation.It is the special gradient structure that reduces the difference of physical parameters,eases the thermal stress and reduces the damage.?3?Compared with the strength of 1329MPa without thermal shock,when the temperature of the thermal shock reached 800?and 1000?respectively,with 10repeated heat-water quench cycles,the residual flexural strength of FGM decreased slightly,but it could still reach 1100MPa.Notably,the strength can still maintain at 843MPa after 10 cycles of thermal shock at 1200?.?4?The excellent thermal shock resistance of FGM is mainly attributed to three aspects:first,the good interface between the phases and the gradient layers of the composite material maintains the overall structure of FGM,and the transition of the gradient layers is uniform Second,the stress field formed by the oxide layer reduces the power of crack propagation,hinders the internal oxidation of FGM,and improves the thermal shock resistance of FGM.Thirdly,the deflection and bifurcation of microcracks caused by thermal shock dissipate the strain energy of interlayer crack propagation,thereby improving the thermal shock resistance.?5?By optimizing the thickness of each layer of FGM composite material,the thermal stress of FGM decreases significantly.The maximum thermal stress of FGM optimized by the Screening method decreases from 443MPa to 315MPa.The thermal stress difference between the transition layer and the composite material of each layer is smaller and evenly distributed,which can improve the stability of the material.The optimum thickness range of each layer composite material is determined by three optimization methods.?6?The thermal stress of TiC_x-Ni₃?Al,Ti?/Ni FGM can be effectively reduced by introducing a gradient layer with less component difference.With the introduction of35Ti₃AlC₂/Ni and 45Ti₃AlC₂/Ni composite layers,the maximum thermal stress of FGM decreased by 50MPa and 45MPa respectively.In contrast,after the introduction of both35Ti₃AlC₂/Ni and 45Ti₃AlC₂/Ni composite layers,the maximum thermal stress of FGM plummeted to 252MPa and the decline amplitude reached 191MPa,and the thermal stress distribution between the layers was more uniform,showing a better thermal stress relief effect.