In Situ TEM Investigation Of The Mechanical Properties And Oxidation Behaviors Of High Entropy Alloy Nanoparticles

As a kind of extremely highly concentrated complex solid solution,high entropy alloys(HEAs)always exhibit unique mechanical properties.Conducting atomic-scale researches to establish structure-property relationships can provide theoretical and experimental foundations for alloy design.Based on recent advances in the micro-scale concentration fluctuations of high-entropy alloys,we used the transmission electron microscopy-in-situ nanoindentation system to investigate the mechanical properties of the NiCoFeCuAgPdPtAu high-entropy alloy nanoparticles,and investigated the working scale of the regulation of concentration wave.The size related mechanical performance was studied,and the stacking fault energy and short-range chemical ordering tendency in high-entropy alloy nanoparticles were studied using molecular dynamics(MD)simulations and density functional theory(DFT)calculations.Considering the possible influence of oxidation in the mechanical results,we firstly performed in situ oxidation test using environmental Cs corrected TEM to investigate the oxidation behavior of the NiCoFeCuAgPdPtAu high-entropy alloy nanoparticle before the mechanical test.It is observed that due to the extremely strong surface effect of the nanoparticles,the differences in the oxygen affinity of different elements in the high-entropy alloy nanoparticles are weakened,an amorphous oxide layer with complex constituent was formed as soon as the particle contact oxygen.The amorphous oxide layer undergoes a complex reaction in the subsequent oxidation process and is partially crystallized to form a convoluted amorphous-crystalline structure,which further slows down the diffusion rate of oxygen.This,together with the intrinsic sluggish diffusion effect of high-entropy alloys,greatly improves the oxidation resistance of high-entropy alloys and ensures reliability of the mechanical test data.Mechanical tests on high-entropy alloy nanoparticles show that when the size of the nano-particles is reduced from 220 nm to 50 nm,the high-entropy alloy exhibits abnormal size effects,including changes of deformation behavior from a stable superplastic-like mode to an unstable one with local shearing and dislocation avalanche,an abnormal "smaller and weaker" size effect of the yield strength,and the sharply decreased work hardening ability as the decrease of particle size.Molecular dynamics(MD)simulations show that because of the mixture of multiple elements,the statistic average effect of stacking fault energy(SFE)would break down under certain characteristic size.And due to the randomness of the arrangement of elements,the absolute values of the SFE in each slip plane are different.Therefore,smaller samples have higher possibility containing planes with low lattice friction and make dislocations easier to slide out.At the same time,the enormous differences in element properties in NiCoFeCuAgPdPtAu high-entropy alloy nanoparticles introduce non-intrinsic heterogeneous distribution of stacking fault energy,further increasing the size dependence of stacking fault energy and lead to the breaking down of average statistic rule of the SFEs in a larger scale compared to Cantor alloys.The research in this paper reveals the oxidation behavior of high-entropy alloys and the protective effect of complex oxide layers on the alloys,confirms the regulation of local chemical environment on mechanical properties,and shows the size dependence of this regulation.The researches in this thesis are of great importance in further structure-property research,application and alloy design.