| High-temperature structural materials have excellent thermo-mechanical properties and are widely used in aviation,aerospace,industry,energy,military fields.As the basic unit of high-temperature structural components,the improvement of the high temperature structural materials attracted world-wide interest.The fracture at high temperature and stress is the main form of high temperature structural material failure.Fracture failure usually brings catastrophic consequences or high repair and replacement costs.The atomic mechanisms of high temperature fracture failure is the fundamental and critically important physical issues in the development and application of high temperature structural materials.Revealing the deformation and fracture atomic mechanisms of high-temperature structural materials at high temperature and stress are critical to the development of materials and the advancement of science.Effective experimental methods are very important to reveal the fracture mechanism of high temperature structural materials,but the development of atomic-scale thermos mechanical experimental methods has always been a bottleneck problem for researchers at home and abroad.In this paper,we report the development of a microelectromechanical systems-based thermo-mechanical testing apparatus that enables mechanical testing at temperatures reaching 1556 K inside a transmission electron microscope for in situ investigation with atomic-resolution.This method was used to systematically study the high-temperature fracture atomic mechanism of traditional high-temperature structural materials-tungsten(W)and nickel-based single crystal superalloy.At the same time,this method was used to study the atomic mechanism of the ultra-fast crack self-healing of the emerging high-temperature structural material-high-entropy alloy.1.An in situ atomic-resolution high-temperature mechanical testing system in TEM was developed based on micro piezoelectric ceramic actuator and MEMS miniature heater.The device can systematically study the atomic-scale deformation and fracture mechanisms of the samples in thermo-mechanical testings.The device can heat the sample to a set temperature in the TEM and achieve in-situ deformation such as stretching,compression,bending or indentation.At the same time,it can perform realtime dynamic observation on the evolution of the microstructure of the sample at the atomic scale.The device contains two basic components,one in-situ thermal component based on MEMS micro-heaters,and the other in-situ mechanical component based on micro piezoelectric ceramic actuators.Among them,the in-situ thermal component can load the test sample from room temperature to 1150? to obtain a temperature loading with an accuracy of ± 5?.The in-situ mechanical component can apply a GPa-level stress load to the test sample and provide a 4 ?m range displacement drive with a drive displacement resolution of up to 0.1 nm.These two components were built on a double-tilt sample holder(illustration(e)),also developed in-house.The holder allowed the sample to tilt about the longitudinal and transverse axes for ±15°.This capability is essential for good atomic-scale resolution,which requires precise alignment of the electron beam to a low-index zone axis of the sample during thermomechanical testing.The emergence of this experimental platform filled the technical gap of atomic scale in situ thermo-mechanical research.2.The systematically research of the fracture mechanisms of single crystal W at high temperature was used the self-developed in situ atomic-resolution hightemperature mechanical testing system.The results of the study explain for the first time the atomic mechanism of single crystal W ductile fracture at high temperature.We first uncovered that tungsten fractures at 973 K in a ductile manner via a strain-induced multi-step body-centred cubic(BCC)-to-face-centred cubic(FCC)transformation and dislocation activities within the strain-induced FCC phase.This Breaks the traditional perception that the b=? <111> screw dislocation activities at the crack tip of BCC metal is the main mechanism of the ductile fracture at high temperature.This provides a new understanding of the ductile fracture mechanism of single crystal W at high-temperature.It is reasonable to also expect that this mechanism is applicable for other highbrittleness BCC metals at high-temperatures.3.For the first time,the ultra-fast self-healing behavior in Hf-Nb-Ti-V highentropy alloy was found by the self-developed in situ atomic-resolution hightemperature mechanical testing system.The self-healilng rate at 950? exceeded 100nm/s that is 500 times that of the reported materials at the same temperature.Combined with the focused ion beam technology,it was confirmed that the holes and cracks formed by the irradiation damage can still be healed spontaneously.This means that the Hf-Nb-Ti-V high-entropy alloy has the potential to replace W as the first wall material for nuclear protection.At the same time,using atomic scale high-temperature mechanical in-situ TEM observations combined with first-principles calculations revealed atomic mechanisms of the self-healing anisotropy and the important role of HCP elements for ultra-fast self-repair in the alloy system.4.In-situ thermo-mechanical testing of DD6 nickel-based single crystal superalloy in TEM was implemented through in situ atomic-resolution high-temperature mechanical testing system.For the first time in the process of high-temperature crack propagation,in-situ observation of the crack tip region at atomic-scale was carried out.The experimental results confirmed the theory that the ?/?? interface hindered crack propagation.For the first time it revealed the fracture mechanisms of the nickel-based single crystal superalloy at the atomic scale.The DFZ region at the crack tip no longer existed during fracture at high temperature,which is different from fracture mechanisms at room temperature.The proposed mechanism fills the gap in the atomic mechanism of fracture of superalloys at high temperature. |
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