Phase Formation And Its Revolution Of Al_0.3Cu_0.5CoCrFeNi High-Entropy Alloy

It has been more than 10years since the first time high-entropy alloys?or,alternatively,multi-principal-element alloys?has been proposed as a novel alloy design concept since 2004.In sharp contrast to conventional alloys,which are basically based on one principal element,the high-entropy alloys consist of multiple principal elements.The huge amount of potential alloy systems,special properties and their underlying scientific problems,along with promising application potentials presented by different compositions of this kind of new metallic materials,make the research of high-entropy alloys an increasing hot spot in the research society.In the current thesis,Al_0.3Cu_0.5CoCrFeNi was chosen as the model material.Room temperature microstructures with different extent of deviation from thermodynamic equilibrium were prepared by using different cooling rate.The phase composition,especially the morphologies,distribution,chemical compositions and interface structures were systematically studied via multiple characterization methods that span a wide range of length scale.The lattice distortion effect of high-entropy alloys makes all kinds of lattice defects exist in a matrix that has higher energy level than conventional alloys.Thus,the nucleation,motion and other behaviors of lattice defects are affected by this factor.At the same time,the low stacking fault energy trend of high-entropy alloys would promote the onset of twinning during deformation.The current work made systematic observation and study on the microstructure evolution and interaction between different lattice defects,especially the interaction between growing twins and pre-existing dislocation substructures under severe plastic deformation.Furthermore,the spatial relocation of matter during severe plastic deformation provides an alternative and novel way to achieve compositional homogeneity in materials other than conventional thermal/mechanical process.The compositional homogeneous microstructure could be taken as the important base and start point for further design and modification of microstructure.Previous results showed that,under severe plastic deformation,opposite homogenization behavior presented in different binary alloy systems.For complex alloy systems like high-entropy alloys that simultaneously contain multiple principal elements,the homogenization behavior and underlying controlling factors were studied in this work.The main findings and conclusion of this work are as follows:1.Under both cooling rates,room-temperature microstructures of Al_0.3Cu_0.5CoCrFeNi exhibit inhomogeneity of spatial distribution of principal elements.For slower cooling rates,the matrix contains large amount of ordered precipitates,the interfaces between different phases are coherent.These precipitates significantly increase the strength,along with considerable strain hardening and plasticity.For the faster cooling rate,the dynamical factor limits the formation of precipitates.While the alloy exhibits only one crystal structure,elemental segregation on nanometer scale still occurs.2.For the first time,the penetration of growing deformation twins through low-angle grain boundaries was observed.Under severe plastic deformation,twinning acts as an important deformation mechanism and is ubiquitously observed in deformation microstructure of Al_0.3Cu_0.5CoCrFeNi.Previous research in conventional alloys of severe plastic deformation microstructure focused on the interaction between moving lattice dislocations with pre-existing twin boundaries.However,how a growing twin interacts with pre-existing dislocations and dislocation substructures,and the process for a twin overcoming the obstacles and occurrence of penetration have never been reports.The current work proposed a geometric dislocation model to address the mechanism of this process.3.During severe plastic deformation,the moving dislocations act as the carriers of mass transportation,atomic-scale homogenization was achieved in Al_0.3Cu_0.5CoCrFeNi.All the principal elements have distribution close to random solid solution.Due to the sluggish diffusion of high-entropy alloy,the dynamical factor limits the occurrence of long range diffusion during severe plastic deformation.This is one of the key factors to produce bulk non-equilibrium supersaturated solid solution.Though under conventional analysis methods of atom probe tomography data,all principal elements distribute in space quite close to ideal random solid solution,there're certain spatial correlations between different elements.A new analysis method was proposed for quantitative detection of these correlations.And surprisingly,the observed correlations do not follow the expectation from thermodynamical considerations.