Ideal Strength And Stress-induced Martensitic Transformation Of Magnesium-scandium Alloys

Magnesium-based alloy is an ideal light structural material,which is widely used in electronic devices,transportation vehicles and medical apparatus.Magnesium metal has a hexagonal close-packed（hcp）structure,and its mechanical properties need to be adjusted by alloying because of its few slip planes and poor plastic mechanical properties.Metal scandium is stabilizing element for theβphase（bcc phase）of magnesium alloys.β-MgSc alloy has stress-induced martensitic transformation at low temperatures,and has shown superelasticity.However,the Angle range of XRD patterns before and after martensitic transformation given in literature is very narrow,and it is only speculated that the superelastic behavior of MgSc alloy is similar to that ofβ-Ti alloy,so the MT transformation mechanism of MgSc is not clear at present.In this work,the mechanism of stress-induced martensitic transformation of MgSc alloy was explored from the perspective of theoretical calculation,providing useful reference for the development of shape memory functional materials.In this thesis,the ideal tensile strength（ITS）of HCP phase（αphase）and BCC phase（βphase）in MgSc alloys with high symmetry axis was investigated by EMTO-CPA method.At the same time,the equilibrium properties ofβphase and corresponding martensitic phase of single crystal MgSc alloy were systematically analyzed to clarify the mechanism of stress-induced martensitic transformation.The results show that ITS of MgSc alloy increases significantly in the directions ofβphase[001],[110]and[111]andαphase[0001]with the increase of Sc component,indicating that Sc alloying has a significant stabilizing effect on Mg phases.The[001]_βis the weakest stretching resistence direction,indicating that the mechanical properties of BCC phase are unstable at the initial stage of stress loading,which is the source of BCC phase metastability.The ITS of[110]_βis the largest,and the ductility is better than[001]_βand[111]_βdirections.When Sc concenration increases from 15 at.%to 25 at.%,ITS in[001]_βdirection increases by25%,that in[110]_βand[111]_βdirection increases by 10%and 9%,respectively.Inαphase MgSc alloys,ITS increases by 13%when Sc concenration increases from 10 at.%to 25 at.%.In order to understand the mechanical properties of equilibrium structure and ITS point from the nature of atomic bonding,we calculated the total density of states（DOS）and the corresponding partial total density of states（PDOS）of each electron forβandαphase MgSc alloys.The valence electron states at Fermi surface mainly come from the electron states in the p and d orbitals of Mg atom and the electron states in the d orbital of Sc atom,while the contribution of the electrons in the s and p orbitals of Mg atom is very small.When close to the-0.1 Ry,the d-electron of Sc increases obviously,that is,the introduction of d-electron of Sc element leading to a strengthening of atomic bonding of MgSc alloy systems.Based on the experimental findings,in order to clarify the mechanism of stress-induced martensitic transformation,we were systematically analyzed theβphase and corresponding orthorhombic martensitic phase equilibrium properties of single crystal MgSc alloy.The strain critical point of martensitic transformation of MgSc alloy under tensile stress is calculated.It is found that the energy barrier of alloy transformation decreases with the increase of Sc component.When β→α（?）_ABAB and β→α（?）_ABAC phase transition occurs,the energy barrierΔE_α’_→β is only about 0.47-0.53 m Ry and 0.46-0.42 m Ry respectively,approximately corresponding to-190℃.The predicted Burgers path is consistent with the latest experiment,which indicates that the tensile stress-induced martensitic transformation path designed in this study can give a reasonable insight into the phase transition.