Investigations Of The Phase Structure And Elastic Mechanical Properties Of The Multi-principal Elements High-entropy Alloys

The high entropy alloys(HEAs)is a kind of innovative alloy design conception in the past twenty years.However,the alloy theory and alloy design method are still immature,especially the relationship between the fine structure of site occupying behavior and mechanical properties are unclear.Systematic and intensive researches are required to explore the rich family of innovative alloys.In this thesis,a thermodynamic database includes HEAs FCC and BCC phase was established by combining first-prinicples calculations based on density functional theory(DFT)with computational thermodynamics based on quasiharmonic approximation(QHA).The phase structure,thermodynamic function,the temperature-depedence of site fraction of alloying elements,and the effect of site occupying ordering behavior on the mechanical properties of the selective HEAs CoCrFeNiM(M=Cu,Mn)and NbTaVWM(M=Mo,Ti)were studied systematically by combining the first-principles calculations with calculation of phase diagrams(CALPHAD)approach.The main results were presented in the following.The end-member thermodynamic database constructed by using the Gibbs free energy of formation can precisely predict the phase structure,thermodynamic function,and site occupying behavior of element in HEAs.The results shows that in the temperature range of 298K to 1498K,CoCrFeNiM(M=Cu,Mn)are stable as a single FCC phase structure,and in the temperature range of 298K to 2698K,NbTaVWM(M=Mo,Ti)are stable BCC single-phase structure,and no spinodal decomposition present in both cases.With the temperature rises up,the mixing enthalpy and mixing entropy increase,the Gibbs free energy of the HEAs decrease,and the entropy of mixing gradually reaches the theoretical value of the Boltzmann hypothesis.The site preference of the elements in HEAs also affected significantly by the equilibrium heat treatment temperature.Taking CoCrFeNi alloy as an example,the occupancy configuration is(Co0.519Cr0.240Fe0.014Ni0.227)1a(Co0.160Cr0.253Fe0.329Ni0.258)3c at 298K,where Co atom tends to 1a sublattice,Fe atom tends to 3c sublattice,while Cr and Ni atoms have no obvious site preference.While the occupancy configuration is(Co0.433Cr0.112Fe0.193Ni0.262)1a(Co0.189Cr0.296Fe0.269Ni0.246)3c at 1498K,except Cr atom slightly biases to 3c sublattice,others elements tend to occupy the sublattice disorderly with nearly equal site fractions.So we conclude that the site preference of alloying elements exhibit a significant temperature-dependence,and most of the elements shifted from an orderly tendency to a disordered tendency with the increase of temperature.We quantitatively reported the site fraction for the first time to identify the fine microstructure.The atomic arrange array models of the alloy phase were further established,and the elastic mechanical properties of the alloy phase were calculated based on the site occupancy fraction(SOF)model,as well as the traditional special quasi-random structure(SQS)model.The former considers the actual situation,where the alloy atoms have a site prefence to occupy each sublattice,while the latter uses the traditional SQS model,where suppose the atoms ocuppy all kinds of sublattices randomly only.The calculated elastic mechanical properties based on SOF and SQS have little difference and reflect the intrinsic plasticity of the selective high-entropy alloys involved in this study.However,based on the occupancy modeling,the modeling is truly reflect the fine microstructure and the research content is more profound.For CoCrFeNiM HEAs,the elastic properties of the three materials rank as:CoCrFeMnNi>CoCrFeNi>CoCrCuFeNi,meanwhile,for NbTaVWM HEAs,the elastic properties of the three materials rank as:MoNbTaVW>NbTaVW>NbTaTi VW.The site occupying behavior of the element influences the mechanical properties of HEAs considerably.With the increase of equilibrium heat treatment temperature,the order degree decreases,the distortion of the crystal lattice increase,and the volume increase besides the thermal expansion,thus the bond strength decrease,and the elastic properties descend.