Phase Stability And Elastic Properties Of Al And Mg And Ti Based Alloys Studied By First Principles

With the development of computing science and especially the outcome of highperformence computer, computation materials science has become one of the mostactive areas in materials science. First-principles method, which based on the quantumtheory, does not rely on any empirical parameters to predict the physical and chemicalproperties of materials and has been widely used in designing new materials, owing tothe low cost and high efficiency.Magnesium, aluminum, titanium, and their alloys are widely using in automobile,aerospace, medicine care, and so on, due to their light weight and special physical andchemical properties. This thesis uses first-principles method to study the mechanismsof alloying element and impurity influence on the phase stability and elastic propertiesof such kind alloys. Results presented here provide us a deep understanding on themicromechnisms of light metal alloys and a guidline for to the design of newmaterials.For the pure metals, relationships between the electronic structure and the essentialproperties, i.e., the cohesive energy, the structural parameters, and elastic moduli havebeen established. All the calculated values are consistent with experimental results. Aparabolic relationship between an electronic structural parameter, the charge density atthe bond critical point and bulk modulus of metals is fitted. It is found that the chargeat bond critical point could act as an medium to make the connection between theelectronic structure and physical properties, such as lattice volume, cohesive energy,and elastic modulus, of metals.Three special metals, Mg, Al, and Ti are selected to study in detail in this thesis. ForMg and Al metals, mechanisms of the alloying elements X (X=Al, Mg, Ti, V, Cr, Fe,Ni, Cu, Zr, Nb, Mo, Sn, Ta, and W) and the impurities X (X=H, C, N, and O) influenceon the phase stability and elastic properties were studied. Results show that Sn is aspecial element, it increases the stability in Mg but shows an opposite effect in Al. InMg, impurities H and O prefer to occupy the tetrahedral site, while C and N prefer tooccupy the octahedral site. The occupation behavior of C in Al is same with that in Mg.It prefers the octahedral site, while other three impurities prefer the tetrahedral site. Interm of the mechanical stability, all Mg-X and Al-X alloys satisfy the mechanicallystable criteria. The value of (C11-C12) of Mg-X is very sensitive to the alloyingelements, and a very small C44was obtained in the Al-Zr alloy implying that theobstacle for shear deformation is weak in this alloy. The analysis of electronicstructures shows that the bonding interaction beween alloying elements and theirneighboring atoms of host is the key factor that governs above behaviours. Further studies for the binary Mg17Al12alloy were also performed. It shows that alloyingelements Ca, Zr and La strongly affect the phase stability of Mg17Al12, but Ni, Cu andZn show weak influence on the phase stability. O prefers to occupy the tetrahedral sitesurrounded by four Mg atoms.For Ti, mechanisms of alloying elements (Al, Cr, Cu, Fe, Mo, Nb, Ni, Sn, Ta, V, W,and Zr) and impurities (H, C, N, and O) influence on the phase stability and elasticproperties and on the martensite transformation of Ti were investigated. Resultsillustrate that all alloying elements stabilize the β phase and show similar influence onthe stabilization of α and α phases, but only Al and Sn increase the stability of ωphase, and the effect of alloying elements on the phase stability is relativelyindependent on the concentrations of alloying element in Ti. It is also shown that theNb significantly enhances the stability of β phase, and the effect is enhanced if otheralloying elements associate with Nb. Results also show that all impurities gainnegative occupation energy in Ti regardless the phases, and their occupation behaviorsdo not depend on the concentrations of impurity except the case that6.25at.%O in theα phase. The impurity changes the distribution of Ti d orbitals at the Fermi energylevel stabilizing the stability of the α and α phases over the β phase. The influence ofalloying elements on the martensite transformation of Ti is originated from affecting ofalloying elements on the mechanical stability. It is worth to note that the mechanicallystable criteria are satisfied for the α phase with impurity but not for the α phase withimpurity. Ti2448(Ti-24Nb-4Zr-7.9Sn) alloy shows good stability and low Young’smodulus, which are consistent with experimental results.Fially, the adsorption of O atom on γ-TiAl surface and of H in Mg/TiAl sanwdishedsystem is studied. Results show that there is a linear relationship between theadsorption energy of O atom on γ-TiAl surface and integrals of overlaps between thePDOSs of O, Ti and Al. The O-Al and O-Ti bonding interactions compete each otherwhen oxygen is adsorbed. In general, the O-Ti bonding is stronger than the O-Albonding. This means oxygen is usually found in a Ti-rich environment where it islikely to generate TiO2. For the Mg/TiAl sanwdished system, the unsaturated bonds ofMg, Ti or Al atoms in the interface zones offer capturing centers to bond H atom tostabilize the system. Therefore the Mg/TiAl sanwdished system can act as hydrogenstorage media, which is consistent with experimental result.