Three Dimensional Electron Microscopy Study On Oxidation Behavior Of ?/?' Structure Single Crystal Superalloys

The engine is the heart of the aircraft,and the superalloy is applied to the turbine blade in the aircraft engine.It directly determines the aircraft's flight distance,flight speed,and service life.Oxidation is one of the main failure modes of superalloys,so studying the oxidation behavior of superalloys has a guiding significance for the development of superalloys.In this thesis,typical first,second and third generation nickel-based single crystal superalloys and new cobalt-based superalloys were selected as the research objects.Nickel-based single crystal superalloys were oxidized at 900^oC,sub-micro needle sample of nickel-based single crystal superalloys were oxidized at 450^oC,and sub-micro needle sample of new cobalt-based superalloy were oxidized at 350^oC.Oxidized samples were characterized by XRD,SEM,FIB,and TEM.The three-dimensional morphology and element distributions of the oxidized samples were obtained by electron tomography.Through the three-dimensional reconstruction model of the oxidized superalloy,differences in oxidation behavior among them were analysized.The results show that the structure of the oxide layer of the first-generation superalloy is different from that of the second-generation superalloy and the third-generation superalloy.The oxide layers of the first generation superalloy from outer to inner are Ti-rich layer,Cr-rich layer and Al-rich layer.Oxide layers of the second generation superalloy and the third generation superalloy from outer to inner were Ni-rich layer,Cr-rich layer and Al-rich layer.In the first generation superalloy,the outermost Ti O₂ oxide layer and the inner oxide layer have poor adhesion and were prone to flaking.The rapid diffusion of Ti during the oxidation process will destroy the Al₂O₃ oxide layer structure,the continuous Al₂O₃ oxide layer turning to discontinuous Al₂O₃,the?'phase with high Ti/Al ratio is more prone to oxidation.In the second generation superalloy,a dense Al₂O₃ and Cr₂O₃ oxide layer was formed during the oxidation process.The oxide layer was well combined with the alloy matrix.Due to the low content of Re in the alloy,the volatile Re₂O₇ generated during the oxidation process couldn't destroy the oxide layer structure.In the third generation superalloy,due to the high Re content,a large amount of volatile oxides were produced to destroy the Al₂O₃ oxide layer,resulting in the reduction of the oxidation resistance of the alloy.And the?phase with a high Re/Al ratio is more prone to be oxidized.The above alloys were prepared into sub-micro-scale needle samples with FIB.After oxidized at 450^oC for 4 h,the three-dimensional morphology and element distribution were reconstructed.After comparative analysis of different algorithms,the Expectation Maximization algorithm and HAADF-EDS Bimodal Technique(HEBT)algorithm was selected for three-dimensional reconstruction of morphology and element distribution.The research results show that the structure of the oxide layer of the needle-shaped sample at the sub-micrometer scale is different from that of the bulk sample.The first-generation superalloy oxide layer has a porous gap between the alloy matrix and the oxide layer.outer oxide layer of the?-phase is Ni-rich oxide,the inner oxide layer of the?phase is a mixed oxide of Cr and Al,the outer oxide layer of?'phase is rich in Ni,and the inner oxide layer is rich in Cr,Al and Ti.It can be seen from the three-dimensional reconstruction model of the oxidized superalloy morphology that the?'phase is more easily to be oxidized during the oxidation process.The outer oxide layer of the second generation superalloy is mainly a mixed oxide of Ni and Co.Cr,Al and Re have segregation at the interface between the outer oxide layer and the alloy matrix,and it is more obvious at?'phase to form a protective oxide film.There is no pore-shaped gap between the oxide layer and the alloy matrix,and there is a bulge-like protrusion structure at the?phase in second generation superalloys.The third-generation superalloy also forms a bulge-like protrusion structure in the?phase,the outer oxide layer is a mixed oxide of Ni and Co,the?'phase has a protective Al₂O₃ layer,and the?phase has no protective oxide layer.Among the three nickel-based single crystal superalloys,the oxidation behavior of the second generation superalloy performed better.The new-type cobalt-based superalloy was prepared as needle-shaped samples and subjected to constant temperature oxidation at 350^oC for 4 h for getting three-dimensional reconstruction of morphology and element distribution.The three-dimensional reconstruction calculation also uses the EM algorithm and HEBT algorithm.The results show that the cobalt-based superalloy also produces a bulge-like protrusion structure at the position of the?phase during the oxidation process.The outer oxide layer at the?phase is spalled due to growth stress and thermal stress,and between the outer oxygen layer and the inner oxide layer at the?'phase has smaller porous structure.The inner Al₂O₃ oxide layer plays a major role in the anti-oxidation performance of the cobalt-based superalloy.During the oxidation process,the?phase is more prone to be oxidized.