The Research On Microstructural Damage Evolution Mechanism At Crack Tip And Fracture Resistance Of High Entropy Alloy

Crack failure is widely existed in engineering practice,and it is one of the main factors for structural function degradation and safety accidents,which brings great hidden dangers to people's property and personal safety.To improve the corresponding damage tolerance performance,on the one hand,the traditional structural design should be optimized continuously;on the other hand,it is necessary to recognize the origin of microscopic damage mechanism of materials,especially to understand mechanism of interactions between crack tip and microstructure in detail on different length scales.This requires objective measurement and evaluation of the cracking resistance of materials,especially the emerging engineering materials,which can provide a reference for their potential structure application.Then the damage evolution law of microstructures at crack tip should also be explored and understood on a microscopic scale for revealing the inherent damage mechanism which possesses mechanical and material meanings.However,to try and realize this more intrinsic,cross-over and multi-scale study,there are still huge challenges in terms of traditional research ideas and methods.This paper takes one of the emerging engineering alloys,high entropy alloy?HEA?,as the research object and carry out a series of scientific research aiming at exploring the crack failure mechanism of the ductile alloy materials,and revealing the crack growth resistance mechanism.Through a variety of in-situ and multi-scale experimental characterization methods,a large number of interaction forms of crack tip and microstructure are obtained corresponding to different load conditions and service environment.And micro deformation mechanisms were further explored from the point of view of mechanics and materials.Furthermore,the microscopic mechanism of crack resistance of these alloys is revealed,and some scientific views for promoting the understanding of crack propagation behavior and damage deformation mechanism are proposed innovatively.In addition,some suggestions with practical engineering significance for improving the crack resistance of the alloys are provided.The main work of this thesis is as follows:?1?The crack growth behaviors and fracture mechanism of a CoCrFeNiMo_0.2 five-element HEA under static load were studied.The crack tip opening angle?CTOA?of the alloy was measured by in-situ scanning electron microscope?SEM for short?mechanics experimental system.The measured critical CTOA value of the alloy is 18°,indicating that the alloy has elevated fracture resistance.Meanwhile,the ductile fracture mechanism of void coalescence of this alloy was revealed in real-time.It is found that the intermetallic compound particles randomly distributed in matrix are the main locations of void nucleation and influence crack propagation path locally.High-energy ductile fracture phenomena such as crack tip blunting,crack branching and deflection are frequently observed,which are consistent with the elevated fracture resistance of the alloy.Furthermore,microscopic deformation morphology near the crack tip indicates that there are multiple deformation mechanisms,involving slip and twinning,which are associated with the good plastic deformation ability of the matrix.?2?The small fatigue crack growth behaviors and crack retardation mechanisms of a CoCrFeNiMn five-element HEA were investigated,and influence mechanism between crack propagation driving force and crack resistance was clarified.A crystallographic model is used to quantitatively characterize the crack retardation effects at GBs,revealing that the retardation effect is significantly enhanced when the micro-texture difference between neighbouring grains is large?twist angle is greater than 50°?.Due to the significant micro-roughness feature on the fracture surface,the crack closure effect is found to be motivted to resist the crack growth.The origin of the micro-roughness feature is further explored.It is believed that the dislocation plane slip mechanism of the alloy is the intrinsic reason for the formation of such fracture microstructure.?3?Fatigue crack growth performances of minor Mo alloyed CoCrFeNiMo_0.2 HEA and CoCrFeNi HEA were compared and the micro-mechanism of minor-element alloying to improve the fatigue crack growth resistance of HEAs was studied.The FCGRs in Paris region of the two alloys were measured by in-situ SEM fatigue experimental system,it is found that the CoCrFeNiMo_0.2 HEA exhibits a lower FCGR than CoCrFeNi HEA.The measured cyclic cumulative irreversible strain at crack tip are compared under the same effective driving force,and the results indicate that the CoCrFeNiMo_0.2 HEA has a better capability of deformation reversibility.Based on dislocation cyclic motion theories,it is considered that this better capability is attributed to the addition of Mo element which leads to a combination of decreasing the stacking fault energy and increasing the lattice friction stress and shear modulus.The synergy of these changes leads to an enhanced slip planarity in CoCrFeNiMo_0.2 HEA which contributes to an improved cyclic reversibility.In addition,the stacking fault mediated deformation mechanism found in CoCrFeNiMo_0.2 HEA is considered beneficial in resisting cyclic plastic damage at crack tip.?4?The effects of high temperature on the fatigue crack growth behavior and crack growth retardation mechanism of CoCrFeNiMn HEA were investigated.Compared with the experimental results at room temperature?293K?,it is found that the crack growth resistance of the alloy does not degrade seriously with the decrease of strength.This is closely related to the change of crack retardation mechanism caused by high temperature.Firstly,distinguishing microscopic mechanisms at high temperature,the micro-crack toughening mechanism?473K?and the crack tip periodic blunting mechanism?673K?,play an important role in inhibiting crack growth.Secondly,the phenomenon of crack deflection and blunting are more commonly found at high temperature,which result in reducing the effective driving force at crack tip frequently.Thirdly,the plastic zone size increases with temperature as the temperature increases,contributing to enhanced level of crack closure.The synergistic changes of these mechanisms benefit to offset the degradation in crack growth resistance of the alloy at high temperatures caused by decrease in strength.