| Due to its prominent position in nuclear power plants,zirconium alloy materials have been extensively studied worldwide.However,in order to develop the next generation of zirconium alloys with even better performance,it is also necessary to conduct extensive and in-depth research on the mechanical and thermodynamic properties of monocrystal zirconium in zirconium alloy base materials.There are mainly two reasons:on the one hand,zirconium,as the main component material of zirconium alloys,accounts for more than 95%of the mass,which clearly indicates that the physical,chemical,and mechanical properties of zirconium alloys are highly dependent on the structure and properties of their base material,monocrystal zirconium;on the other hand,so far,there are still few theoretical studies on the structure and properties of monocrystal zirconium.In this paper,a more extensive and in-depth study of the mechanical and thermodynamic properties of various phases of monocrystal zirconium in zirconium alloy base materials will be conducted using first-principles.The results are as follows:(1)The mechanical properties of monocry stal zirconium in zirconium alloy base materials for each existing phase:the bulk modulus,shear modulus,and Young’s modulus,as well as the ratio of the bulk modulus to the shear modulus,are all greater than 85 GPa,20 GPa,60 GPa,and 2.5,respectively.These results indicate that monocrystal zirconium have good hardness and plasticity.Electronic structure calculations also show that it has metallic properties and is a good conductor of heat.(2)Free energy calculations show that the order of stability of various phases at zero pressure is HCP phase>ε phase>δ phase>ω phase>BCC phase>FCC phase.However,the δ and ε phases of monocrystal zirconium exhibit a large number of imaginary frequencies in their phonon spectra at zero temperature and pressure,indicating that they are dynamically unstable phases.Within the considered temperature and pressure range(T<1500K,P<230GPa),the FCC phase always has higher free energy than the other phases,making it structurally unstable as well(and its kinetics are also unstable above 6 GPa).Debye temperature calculations show that due to the sharp decrease of Debye temperature with increasing temperature,the BCC and ω phases of zirconium are unstable at zero pressure and low temperatures(T<150K),and only the HCP phase is stable under these conditions.(3)By calculating phonon spectra and free energy,it is found that at low temperatures,when the pressure increases to 6 GPa,the HCP(including FCC phase)phase becomes unstable and transforms into the ω phase.When the pressure continues to increase to 23 GPa,the ω phase becomes unstable and transforms into the BCC phase.Afterward,when the pressure continues to increase to 175 GPa,the BCC phase remains stable,then both the BCC phases become dynamically unstable,and monocrystal zirconium may transform into a structurally disordered phase.Finally,a broader P-T phase diagram for monocrystal zirconium is calculated and presented. |
