Study On The Thermal Stability Of Self-Organization Structure In Metallic Materials Induced By Service Behaviours

Self-organizing structure refers to a kind of ordered structure formed spontaneously after the materials are coordinated with each other according to certain rules of their own composition and organizational structure under certain external conditions and without the help of external environmental instructions.In this study,the dislocation wall structure in the copper single crystal after low cycle fatigue and the raft structure in the nickel-based single crystal superalloy after creep were taken as the research objects to study the thermal stability of these two typical self-organized structures.At present,it is generally believed that during the annealing process,the twins in the fatigued copper single crystal grow with the recrystallization grain boundary migration.In this study,the scanning electron microscope electron channel contrast technique（SEM-ECC）,focused ion beam（FIB）,and environmental transmission electron microscope（ETEM）were used to anneal the[1^<sub>39]oriented fatigue Cu single crystal.It provides a useful supplement to the growth theory of annealing twins.On the other hand,nickel-based single crystal superalloys are used in important mechanical devices such as aircraft engines and turbine blades of internal combustion engines due to their excellent high temperature mechanical properties.During service,oxidation failure has always been the main factor affecting its service life.In this study,through macro and micro oxidation experiments,SEM,FIB,energy spectrum analysis（EDS）,ETEM,and X-ray diffraction（XRD）were used to study the structural stability of the rafted CMSX-4 superalloy during oxidation and the migration direction of each element.The research results are as follows:At a constant plastic strain amplitude of 1×10^-3,the[1^<sub>39]oriented Cu single crystal was fatigue-cycled to saturation,and an annealing study was carried out on the fatigue sample.Experiments show that when the cumulative plastic strain is not high.After annealing at 300℃,the dislocation structure has no obvious recovery,and it is still the PSB ladder wall structure and vein structure;After annealing at 500℃,the dislocation structure recovered obviously,from the PSB ladder wall structure to a loose cell structure;After annealing at 800℃,the dislocation structure disappeared and a small amount of fine annealed twins appeared,the process of formation and growth of the twins has nothing to do with the growth of recrystallized grains and the migration of grain boundaries.It is a structure with lower energy derived from the dislocation configuration evolution.Thermal stability results for CMSX-4 superalloy raft structure is as follows:The migration direction of each element between the two phases in the in-situ oxidation experiment is:Elements Co,Cr,Ta,Ti,Re,Hf,and W migrate fromγphase toγ′phase,and Ni elements migrate fromγ′phase toγphase,Mo and Al is no significant migration,the O element is concentrated in theγphase.Macro-oxidation experiments show that two oxidized regions（dendrite stem and interdendrite）are formed in the superalloy after oxidation.The main components of the outer oxide layer on the dendrite stem are NiO and Co₂O₃,Re accumulates at the interface between the layer and the unoxidized area of the substrate and is not oxidized.There are a large number of voids in the oxide layer,and stress concentration is easily generated in the voids,causing failure of the superalloy.The oxide layer between the dendrites is very thin,its main components are Al₂O₃,Cr-O and TiO,the oxide layer is very dense and can protect the substrate from further oxidation.