Development And Alpplication Of Intergrated High-temperature Thermo-mechanical Platform For In Situ Tem

The microstructure of materials determines its properties.In recent years,in-situ transmission electron microscopy(TEM)technology provides a powerful weapon for studying the microstructure evolution behaviors and mechanisms of materials under the action of external fields,which has become an indispensable key technology in materials,physics,chemistry,biology.In view of the strategic demand of high temperature structural materials,such as superalloy,by simulating the real service conditions,atomic scale studying of high temperature elastic-plastic behavior by in situ TEM will provide intuitive theoretical and experimental supports for the establishment of the relationship between microstructure and properties of materials,as well as the design and preparation of high-performance materials.In this paper,we develop a set of in situ TEM high temperature mechanical testing platform,with the functions of steady thermo-mechanical loading and real-time atomic scale characterization.Based on micro-electromechanical system(MEMS)technology,the design and manufacture of integrated thermomechanical chip in the platform successfully resolved a serious of key issues subjected to the millimeter sized operating space in TEM,such as low heating temperature,high heating power,mutual influence of high temperature heating and mechanical loading,and incompatible with the double-axis tilting function of TEM sample holder.The platform realized in situ atomic resolution of microstructure evolution of materials by large angle double-tilt functionality under the condition of thermomechanical coupling field.Utlizing this platform,the high temperature deformation behaviors of plycrystalline Pt films was characterized by in situ TEM,and the high temperature oxidation mechanisms of?/?₂dual-phase and?/?twin lamellae in novel lightweigh high temperature structural material-Ti Al alloy was studied combining ETEM.The main contents and results of this paper are as follows:1.Based on the two-step sputtering strategy,the durability temperature of Pt film was increased to over 1000?,and the enhancement mechanism of its high temperature stability was elucidated.The microheater for in situ TEM heating experiments was fabricated based on the Pt film heating resistor.The degradation behavior and mechanism of Pt films above 1000?was studied by in situ(S)TEM.The Pt film degraded mainly through nucleation and growth of voids on the Pt-Si N_x interface.The voids are preferentially formed at the intersections of grain boundaries and triple-junctions with the interface.At temperatures above 1040?,the voids nucleated at both the grain boundaries and inside Pt grains.The stress simulation of the suspended membrane suggests the existence of local tensile stress in the Pt film,which promotes the nucleation of voids at Pt-Si N_x interface.The grain-boundary-dominated mass transportation renders the voids grow preferentially at GBs and triple-junctions in Pt film.Meanwhile,under the influence of current,the voids nucleated inside Pt grains could develop to large size and accelerate the degradation of Pt film.2.Using the innovative suspended membrane heating structures,the design of integrated MEMS chips was optimizated,On the basis of the integration of thermal and mechanical coupling,the heating power is reduced and the TEM imaging resolution is improved.By replacing the traditional Si-based heating structure by Si N_x film,the heating power of the thermomechanical coupling test in TEM was reduced by more than one order of magnitude.According to the research requirements of various materials,an electrothermal actuator with wide driving force and displacement was designed.The design of integrated MEMS chip greatly reduces the interference of high temperature field on mechanical loading,TEM spatial resolution and other functions,and ensures the stable thermal mechanical coupling of micro/nanosize samples in TEM and the realization of high-quality in-situ TEM characterization at the same time.3.The process flow of integrated MEMS chip combined with surface and bulk micromachining process was formulated and optimized,and the integrated MEMS chip was fabricated.The calibration results show that the integrated MEMS chip can realize the high temperature heating above 1115?in TEM.Combined with the integrated V-beam electrothermal actutor,the MEMS chip can provide high precision driving displacement loading above 2?m during the high temperature heating.The in-situ TEM thermomechanical platform based on integrated MEMS chip was developed,with tilting function of?±25°and?±15°and high spatial imaging resolution of 0.13 nm.4.The deformation behavior of polycrystalline Pt films was in-situ characterized at nano and atomic scale using the developed high-temperature mechanical platform.The transformation of deformation behavior of polycrystalline Pt films from intergranular slip at room temperature to grain boundary slip at high temperature was observed,and the atomic scale in-situ observation of grain boundary crack propagation behavior at high temperature was realized for the first time.5.The oxidation behavior and mechanism of nanoscale?₂-Ti₃Al lamellae and?-Ti Al twin lamellae were studied by in-situ TEM combining the ETEM.The early oxidation behavior of the alloy was in-situ studied at atomic sacle,and the enhanced internal diffusion of oxygen at the?/?₂ interface was found.Combined with the element quantitative analysis,the internal oxidation behavior of Al at the?/?₂ phase boundary and the?/?twin boundary was observed.The results provide a new understanding of high temperature oxidation mechanism of Ti Al alloy and reveal the important influence of microscale interface structure on the oxidation behavior of the alloy.