Plastic Behaviors Of Multi-principal Element High Entropy Alloys

High entropy alloys（HEAs）are new type of metallic materials,which are different from traditional metallic materials with the single principal element design.High entropy alloys are usually composed of five or more elements in equal atomic ratio or approximately equal atomic ratio.High entropy alloys usually include the chemical disordered high entropy crystal alloys and high entropy metallic glasses（HE-MGs）with both long-range chemical disorder and topological disorder.Thanks to the multi-principal elements,the high entropy crystal alloys exhibit the high mixing entropy,severe lattice distortion,sluggish diffusion and cocktail effect,and alloys have excellent mechanical properties such as high strength,high hardness,and high temperature softening resistance;the high entropy metallic glasses have the dense atomic structure and good glass-forming ability.In this work,Ti Zr Hf Cu Ni Be high entropy metallic glass and Al_0.3Co Cr Fe Ni high entropy crystal alloy were selected as the carriers to investigate the effect of the multi-principal elements on the plastic deformation of the alloys.At the same time,based on the shear transformation zone and stick-slip models,the effects of rate and aspect ratio on the inhomogeneous plastic deformation of HE-MGs were analyzed.The high-temperature flow behavior and dynamic strengthening mechanism of chemical disordered Al_0.3Co Cr Fe Ni high entropy crystal alloy were investigated.The main research conclusions are as follows:（1）Under the framework of the flow unit model and free volume model,the influence of high entropy alloy characteristics on the high-temperature deformation mechanism and glass-forming ability of Ti Zr Hf Cu Ni Be HE-MG were studied.Under the glass transition temperature,the activation volume of Ti Zr Hf Cu Ni Be HE-MG is smaller than that of conventional MGs,and the non-exponential parameterβ_kww is larger than that of conventional metallic glass,which indicates that the system of HE-MG is more uniform and stable than conventional MGs.In the supercooled liquid region,the high mixing entropy caused by multiple principal elements makes the viscosity of Ti Zr Hf Cu Ni Be HE-MG smaller than that of conventional MGs.The viscosity changes more slowly with temperature than traditional alloys near the glass transition temperature,and the corresponding fragility index（m=33）is less than that of most conventional amorphous alloys.It indicates that Ti Zr Hf Cu Ni Be HE-MG is the strong glass,which echoes its excellent glass-forming ability.Based on the free volume model,the defect concentration of Ti Zr Hf Cu Ni Be HE-MG during high-temperature flow is much smaller than that of conventional MGs,implying that the system of HE-MG is more uniform.The lower defect concentration is not conducive to atomic migration and rearrangement to nucleation,and it facilitates the formation of amorphous.（2）Based on the shear transition zone and stick-slip model,the effect of rate on the inhomogeneous plastic deformation of HE-MG was studied.Nanoindentation,indentation creep and room temperature quasi-static compression were performed on Ti Zr Hf Cu Ni Be HE-MG.The results show that the plastic deformation of HE-MG under different loading methods is rate-dependent.The smaller strain\loading rate makes the deformation more inhomogeneously（larger serration and smaller strain rate sensitivity）.Due to HEA characteristics（sluggish diffusion）,the average serration sizes of Ti Zr Hf Cu Ni Be HE-MG are smaller than the traditional MGs.The inhomogeneous plastic deformation of Ti Zr Hf Cu Ni Be HE-MG can be considered as the self-organized critical（SOC）behavior,indicating that the shear band interaction affects the inhomogeneously plastic deformation of HE-MG.In addition,the standard deviation of serration size is relatively large when the strain rate is lower,implying that the serration size is more abundant and the shear band interaction is more complicated as the lower strain rate.Based on the cooperative shear model,it is found that the STZ size of HE-MG during plastic deformation is negatively correlated with the loading rate.It implies that as the lower rate,the larger STZ will promote the activation and propagation of shear bands,which will lead to the rate dependence of serration and strain rate sensitivity.On the other hand,the topographies of the compressed samples show that density distribution and shear-bands interaction are more complex for the lower rate.（3）The effect of the aspect ratio on the inhomogeneous plastic deformation of HE-MG at room temperature is studied by analyzing the size-dependent serrations and shear bands.The results show that the average time for local stress drop is shorter for larger samples,which implies that the propagation of shear bands is more hindered in smaller samples.Based on the stick-slip model,the shear band velocities are positively related to the aspect ratio,which means the shear band spreads faster in a larger sample until it breaks.Lower aspect ratio makes the larger shear-band stability index and the more stable motion of shear bands.The plasticity and serration size are not monotonically correlated because the serration is determined by the competition between motion and shear-bands interaction.As the aspect ratio increases,there exists the transition from ductility to brittleness and the corresponding existence of the self-organized critical changes to a chaotic mode state.（4）The high-temperature flow behavior of chemical disordered Al_0.3Co Cr Fe Ni high entropy crystal alloy was studied.The high-temperatures compression tests with various strain rates were performed.Combining with the theory,experimental data,and high-resolution transmission electron microscopy images,the flow stress,the stress exponent n,and the threshold stress were analyzed to know the strengthening behavior and deformation mechanism of the high entropy crystal alloy.The results show that the temperature and strain rate significantly influence the steady-state flow stress,and the corresponding constitutive model is given.The calculated stress exponent n is greater than five in general at different temperatures due to the precipitation strengthening formed by the dynamic precipitation phase.The strengthening mechanism is the dislocation line interaction with the precipitates phase.Simultaneously,the stress exponent n decreases rapidly with the raised temperature at the initial stage and changes slower,because the atoms move faster,and the threshold stress reduces with the temperature increasing.Therefore,the high-temperature deformation mechanism of Al_0.3Co Cr Fe Ni alloy is dislocation climbing,accompanied by precipitation strengthening.