Simulation Of Site Preference And Mechanical Properties Of Some Selective Alloy Phases

As the idea of Materials Genome Innovation(MGI)is recognized extensively,the high throughput computational material science becomes an effective means to acquire rich materials genomes,and even become an edge develop tool for some advanced materials.In this thesis,based on the previous research work in our research group,the analytical method of Sub Lattice Model(SLM)was developed.Based on SLM and First-Principle Calculations(FPC),combined with Quasi-Harmonic Approximation(QHA),the Site Occupying Fractions(SOF)were predicted.Using the Random Distribution on Sublattice(RDS)base on SOF,the mechanical properties of some selective structural intermetallic phases with significant importance in technique were predicted in detail,and the mechanical properties were compared with those from the traditional Special Quasirandom Structures(SQS)method,which consider the alloying atoms distributing randomly on full lattices.The selective intermetallic compounds were taken from the research and develop practice of advanced alloys in our group,where the relationship of their fine microstructure and properties are unclear yet,include the dispersion strengthening phase Zn Zr Al₂ with a L1₂structure in ZA43 alloy,Laves phase and?phase presented in high temperature alloys,as well as the orthorhombic Ti₂Al Nb phase,hexagonal structure Ti₄Nb Al₃(B8₂structure)and Ti(Al_0.75Nb_0.25)(C6 structure)phases which are always involved in ternary Ti-Al-Nb system to develop lightweight and high temperature alloys.Present theoretical predictions are considerably accurate,and rich materials genome data were accumulated,which can be used potentially in material design.The main research contents and results were presented in the following.1.Based on basic crystallography structure information,the calculated site occupying configuration of L1₂ Zn Zr Al₂ is(Zr)_1a(Zn_0.3333Al_0.6667)_3c or(Zn)_1a(Zr_0.3333Al_0.6667)_3cand it is invariant when heat treatment temperature changes.Zn and Zr atoms can exchange the positions each other.Al and Zr atoms,as well as Al and Zn atoms have a strong interaction with a covalent bond in L1₂ Zn Zr Al₂ intermetallics.And a metallic bonding character exists between Zn and Zr atoms.Based on QHA and quasistatic approximation,all the elastic constants(C_ij)of Zn Zr Al₂decrease with the increase of heat treatment temperature,and C₁₁decreases significantly.The thermal expansion coefficient of Zn Zr Al₂is much lower than that of the L1₂ Al₃Sc compound,while the bulk modulus is higher than that of the L1₂ Al₃Sc compound.The difference of the lattice parameters among Zn Zr Al₂,FCC Al and?(Zn Al)phases is considerably small at a wide temperature range.The mismatch of the main crystal plane is considerable small.According to the crystal lattice match theory of heterogeneous nucleation,Zn Zr Al₂ intermetallic is thus an effective heterogeneity nucleating agents for FCC Al and?(Zn Al)phases.2.Based on strict crystallography structure information,the SLM were constructed,and the formation of Gibbs free energy(SLM-G)database at definited temperature,as well as the and formation of enthalpy(SLM-H)database at ground state(0 K)of some end-member compounds for C15 Laves,respectively.The site occupying fractions of alloying elements on sublattices were calculated based on the thermodynamic database of end-member compounds.For Nb Cr₂-based C15 Laves phase alloyed with M(M=W,Mo,Ti,V),the predicted results based on SLM-G agree better to experimental results than those based on SLM-H.The predicted site occupying configuration of ternary C15 Laves phase Hf₂₅V₆₀Nb₁₅ homogenized at 1473K by SLM-G is(Hf_0.75Nb_0.240V_0.010)_8a(Nb_0.105V_0.895)_16d,while the result predicted by SLM-H is(Hf_0.464Nb_0.489V_0.047)_8a(Hf_0.004Nb_0.032V_0.964)_16d,and the former agrees better with experimental results even better than other simulation.In all,the results predicted by SLM-G are more reliable.3.The crystal structure of?phase is rather complicate,which contains five kinds of sublattices,so only SLM-H thermodynamic database at ground state for end-member compounds of?phase were constructed in this thesis.For the?phase with a composition Fe_0.245Cr_0.555Co_0.2,Co atoms mainly occupy 2a and 8i₂ sublattices,Cr atoms mainly occupy 4f,8i₁ and 8f sublattices,Fe atoms mainly occupy 2a,8i₁ and 8i₂ sublattices.The predicted site occupying configuration of?phase Fe_0.245Cr_0.555Co_0.2homogenized at 900K by SLM-H is(Fe0.168Cr0.032Co0.8)2a(Cr)4f(Fe0.169Cr0.592Co0.239)8i1(Fe0.664Cr0.026Co0.310)8i2(Fe0.043Cr0.957)8j.The predicted results in this work agree well with neutron diffraction results in available literatures.The predicted bulk modulus of Fe_0.245Cr_0.555Co_0.2 homogenized at 900K by RDS based on SOF is211.495GPa.4.The elastic module of Ti-25Al-25Nb O phase at high temperature configuration were predicted by RDS,and compared with SQS method.Due to the ordering behaviors of atoms occupying on sublattice were neglected in the SQS,the bulk modulus of Ti-25Al-25Nb orthorhombic phase were underestimated a little bit compared with those by RDS.In Ti₄Nb Al₃alloy,Al atoms mainly occupy 2c sublattice,Nb atoms mainly occupy 2a sublattice,Ti atoms mainly occupy 2a and 2d sublattice,which agree considerably well with the available experimental results.The site occupying configuration of B8₂structure Ti₄Nb Al₃ homogenized at800 K is(Al_0.0006Nb_0.4865Ti_0.5129)_2a(Al₁Nb₀Ti₀)_2c(Al_0.0067Nb_0.0054Ti_0.9879)_2d.The elastic modulus of Ti₄Nb Al₃(B8₂)predicted by RDS is 131.699 GPa.The elastic anisotropy of Ti₄Nb Al₃with B8₂structure is obvious.In C6 structure Ti(Al_0.75Nb_0.25),Ti atoms occupy 1a and 2d₂sublattice,Al atoms occupy 2d₁ and 1b sublattices,and Nb atoms occupy 1b and 1a sublattices.The effect of hydrostatic pressure on properties of the fully ordered Ti₂Al Nb O phase was also studied using FPC based DFT.When the external hydrostatic pressure is loaded up to 63GPa,the difference of the normalized lattice constants(a/a₀,b/b₀,c/c)is still considerably small,which reveals that the elastic anisotropy of the fully ordered Ti₂Al Nb O phase is weak.All the lattice vibrations,phonon frequency,transverse,longitudinal and mean elastic wave velocities(v_t,v_l,v_m),Debye temperature((48)_D)and all C_ij increase with the increase of external hydrostatic pressure.