The Investigation On Fracture Mechanism And Fracture Resistance Of The FeCrNi Medium Entropy Alloy

Medium entropy alloy is a new alloy with multiple principal elements,considered to be promising for future application in complex engineering due to its excellent strength and toughness.Its ultra-high fracture toughness has also been reported in famous journals such as Science and Nature.However,most of the research on medium entropy alloy focuses o n its static strength properties and deformation mechanism,while the research on fracture properties and f racture mechanism is relatively insufficient,which greatly reduces the space for its application in key engineering structures,and restricts the further exploration of medium entropy alloy.FeCrNi medium entropy alloy is a new alloy in the Fe-Cr-Ni series.Compared with other alloys,the FeCrNi medium entropy has better combination of strength and ductility,but its fracture toughness and fracture mechanism need further study and analysis.Therefore,in this paper,a medium entropy alloy containing three elements of Fe,Cr,Ni,which is taken as the research object,and the fracture nature of the FeCrNi medium entropy alloy is systematically studied from three aspects.Firstly,the fracture toughness of the alloy was evaluated from the macroscopic aspect.Secondly,the fracture mechanism is revealed by in-situ experimental characterization technology at the micro aspect.Finally,four micromechanical models were established from the mechanical aspect based on the typical experimental phenomena to further reveal the fracture nature of the FeCrNi medium entropy alloy.The main works and innovations of this thesis are as follow:(1)Based on the combination of the mechanical experiment system and scanning electron microscope,the fracture toughness of FeCrNi medium entropy alloy was tested.The crack tip opening displacement,crack tip opening angle and J-R curve were obtained.From the macroscopic point of view,it is revealed that the FeCrNi medium entropy alloy has a relatively poor ability to resist crack propagation.(2)The fracture mechanism of entropy alloy in FeCrNi was studied based on in-situ SEM-EBSD mechanical experiment system.The results show that the formation of micro-crack induced by local deformation and its aggregation with the main crack are the dominant fracture modes in FeCrNi medium entropy alloy.This is because it has relatively low intrinsic lattice strength,high obstacle strength for grain boundary and strong resistance of dislocation movement between grains.During the deformation process,the dislocation movement within the grain is obviously blocked and accumulated,and the compatibility of deformation in grains is usually contributed by the local dislocation slip.Therefore,this will lead to the formation of local deformation bands and the accumulation of high-density dislocation.When the stress generated by dislocation accumulation is higher than the strength of the matrix in the grain,it will lead to the cracking of the local deformation bands,and then the microcracks rapidly form and aggregate with the main cracks,reducing the toughness of the material.(3)Based on the typical experimental results,four micromechanical models were established using distributed dislocation technique to further analyze the influence of dislocation behavior and microcrack formation on the main crack propagation from the mechanical perspective.For these models,the effects of grain boundary to hinder dislocation slip,dislocation emission angle,dislocation activation state and the formed microcrack on main crack propagation were considered respectively.The results further revealed the fracture mechanism of the FeCrNi medium entropy alloy from the mechanical perspective.