First Principles Study On Phonon And Thermodynamic Properties Of MgSc Alloys

Shape memory alloy is a class of functional materials exhibiting superelasticity,pseudo-plasticity and shape memory effect,the underlying physics are the reversible phase transformation from a high-temperature austenitic phase to a low-temperature martensitic phase.These unique properties make them having wide range of application prospects in the fields of medicine,architecture and aerospace engineering.The recent research results of Ogawa et al.bring the possibility for light Mg-based shape memory alloys.MgSc alloys with Sc content of about 20 has body centered cubic(?-BCC)structure,which exhibits high plastic isotropy.Therefore,the?phase MgSc alloys are also expected to be a good ductile matertials with shape memory effect,while the research on its phase stability and phonon as well as thermodynamic properties are still lacking.This thesis studied the phonon spectra,dynamical phase stability and thermodynamic properties of MgSc alloys with different composition and phase structure.A special emphasis is put on the effect of long-range order on elastic constants and lattice vibration of bcc phase MgSc alloys.For hexagonal and orthorhombic phase,the effect of different atomic stacking is also addressed.In terms of mechanical properties,the long-range ordering of Sc atoms in?-MgSc alloys has a significant effect on the elastic coefficient.The disordered phases are mechanically stable in 15-25 at.%Sc alloys,but show poor ductility.Long range Sc ordering causes the tetragonal shear modulus of the alloys to be very small(about 4GPa),so Sc ordering would lead to a shear instability in?-MgSc alloys.For dynamical stability,the imaginary mode of ideal disordered Mg_81.25Sc_18.75and Mg₅₀Sc₅₀alloys are removed by long range Sc ordering.The lattice instability of the ordered BCC phase Mg_87.5Sc_12.5alloys is due to the solid solution state,which is more likely to from a hexagonal phase structure.For cubic phase at 15-25 at.%in Sc component,the lattice is dynamically stable,and when Sc concentration is 50 at.%,the alloy only shows the dynamic stability in ordered phase(B₂).When the Sc concentration is 20.83 at.%,the orthorhombic phase with Cmcm space group and hexagonal phase with P6₃/mmc space group show dynamical stability.As a model system of martensitic transformation,a strong anharmonic effect is present in high temperature?phase of MgSc.Therefore,the dynamically unstable ordered BCC phase Mg_87.5Sc_12.5alloys is studied by the temperature dependent effective potential method,and the the phonon spectra at elevated temperatures are obtained.When the temperature rises to 50K,the imaginary frequency is eliminated,thus the phonon-phonon scattering due to temperatures increase would hardening or softening of some phonon modes.From the perspective of electronic structure,The Fermi surfaces of ordered and the disordered phase are obviously different,the disordered Fermi surface has a clear translation vector,while the reconstruction of Fermi surface makes the lattice distorted.Moreover,the partial electron density of states of Mg atom and Sc atom between ordered and disordered are also significantly different.The alloying of Sc makes pure Mg introduce strong p-d hybridization.There are some SQS structures have the same number of atoms for different phase.Electronic structure of Mg-12.5at.%Sc has the biggest difference,while the cubic phase has an obvious peak near the Fermi surface,which is the reason for the dynamic instability of this structure.The Cmcm of orthogonal phase and P6₃/mmc of hexagonal phase is very close,and the number of electronic states near the Fermi surface does not have strong changes,which is also consistent with the phonon dispersion relationship.From the perspective of thermodynamics,the instability of cubic phase under different components is verified by calculating the difference between Gibbs free energy and entropy value.It is proved again that BCC phase,as the parent phase of MgSc alloys,is possible to appear the martensitic phase transform to the Cmcm of orthorhombic phase and P6₃/mmc of hexagonal phase in 12.5-20.83 at.%Sc.In addition,we propose that in the stable Cmcm of orthorhombic phase and P6₃/mmc of hexagonal phase,the atomic stacking in 4H will be slightly more stable than the ideal HCP(2H).