First Principle Calculation On Critical Properties Of Several Transition Metal Alloys

Abstract:During the past decades, transition metal alloys have been widely used as key materials in new recycle energy materials, high-temperature structural materials, novel electronic materials and high-performance magnetic materials, etc. Many properties of transition metal alloys, however, are difficult or expensive to be obtained through experimental tools. The Materials Innovation Infrastructure of the Materials Genome Initiative (MGI) provides a novel concept of using simulation software to design and study advanced materials, especially transition metal alloys, including many noble metals. Inspired by MGI, this thesis aims to obtain and investigate critical properties of transition metal alloys by means of first principle calculation. Three transition metal systems, titanium(Ti)-hydrogen(H) system, tungsten(W)-copper(Cu) alloys, and iridium(Ir) based superalloys are therefore thoroughly studied and the main results are summarized as follows:(1) Ti-H system:Titanium hydrides TiHx (x=l,1.25,1.5,1.75, and2) with the cubic fluorite-type (FCC,8phase) and face-centered-tetragonal (FCT: ε phase, cla<1;γ phase, c/a>1) structures were systematically investigated through first-principles calculation. The calculated results were widely compared with available experimental results in the literature, and could clarify the three controversies regarding atomic configuration, stability, and hydrogen embrittlement of TiHx phases in the literature. Moreover, the mechanical properties of ε and y phases were calculated for the first time and found to play an important role in the brittle/ductile behavior of TiHx phases. The calculated electronic structure suggests that a bonding transition from mainly covalent to mainly metallic as a function of H concentration is the main reason for brittle/ductile behavior of TiHx phases.First principles calculation also reveals that the tetragonal transitions of TiHx (1≤x≤2) could be divided into two types in terms of energy pathway. Calculation indicates that the fundamental reasons for the δ→ε and δ→γ transitions are quite different from each other, i.e., the mechanical instability causes the δ→ε transition and internal symmetry breaking induces the δ→γ transition. In addition, the Poisson ratio of δ phases between x and z axes is proposed for the first time to provide a deeper understanding of intrinsic natures of tetragonal transitions.(2) W-Cu alloys:Various properties of W-Cu are systematically investigated through a combined use of first-principles calculation, cluster expansion, special quasirandom structures (SQS), and lattice dynamics. It is shown that SQS are effective to reflect the intrinsic nature of solid solution, and that the BCC and FCC W100-xCux solid solutions are energetically more stable structures.when0≤x≤70and70≤x≤100, respectively. The thermodynamic and mechanical properties of W-Cu solid solutions are obtained by means of Debye model, and calculated results show that W-Cu solid solutions have outstanding thermodynamic and mechanical properties. For instance, the coefficients of thermal expansion of W100-xCux solid solutions are much lower than those of corresponding mechanical mixtures and the G/B values of W100-xCux solid solutions reach a minimum at x=50, which is fundamentally due to the softening of phonons and a strong chemical bonding between W and Cu with a mainly metallic feature.Moreover, first principles calculation reveals that W-Cu interfaces have high interface strength when the number of overlayers is less than2, and that (111)Cu/(110)W and (110)Cu/(110)W interfaces with one overlayer are both energetically favorable with big negative interface energies. Calculation indicateds that (110)Cu/(110)W could be possibly formed within3Cu overlayers, while (111)Cu/(110)W is energetically more favorable when Cu overlayers are bigger than3.In addition, present first principles calculation reveals that W-rich graded interfaces possess higher interface strength and lower interface energy than Cu-rich counterparts. Calculation also shows that differences of thermal expansion and differences of Young’s modulus between overlayer and substrate of each interface should be decisive factors in the design of Cu-rich and W-rich graded interfaces, respectively. It follows that the optimum graded interface structure would be:W8Cu1/W7Cu2/W6Cu3/Cu6W3/Cu8W1.(3)Ir-based superalloys:The brittleness of Ir has become a challenging and puzzling problem for decades and its fundamental mechanisms are controversial with each other in the literature. It is found that Ir has a normal pressure-dependent mechanical behavior, while the temperature-dependent behavior of Ir is unusual and contrary to that of other FCC metals, and that pressure decreases the brittleness of Ir, whereas temperature increases its brittleness a little bit. Moreover, electronic structure and crystal field theory reveal that Ir has a mainly octahedral bonding, which would be transformed to a mainly cubic bonding under high pressure, while become more octahedral and directional at high temperature. In addition, the implication and importance of the similarities between Ir and semiconductors are also discussed. These common features of deformation between Ir and Ⅳ-semiconductors imply that Ir may be probably cleaved like Ⅳ-semiconductors through bonding breaking, and that further study should be needed to reveal the brittleness of Ir by means of the well-known behaviors of semiconductors.On the other hand, the structural, phonon, electronic, thermodynamic and mechanical properties of Ir and Ir3X(X=Ti, Zr, Hf, and Nb) have been systematically investigated by the first-principles calculation. The calculated phonon and electronic structures suggests the stronger bonding is formed between Ir and Nd than other Ir3X(X=Ti, Zr, and Hf) phases. Present work agrees well with the available experimental data, and provides necessary thermodynamic data for investigating and designing new Ir-based superalloys, and also can be used as a database for Ir-based materials.In addition, it is found that for Ir/Ir3X superalloys, composition change should have much more contribution to temperature-dependent lattice misfit than thermal expansion, and that the coherence as well as lattice misfit should be divided into two groups, i.e., the coherent Ir/Ir3Ti and Ir/Ir3Nb interfaces with constrained lattice misfit, and the semi-coherent Ir/Ir3Zr and Ir/Ir3Hf interfaces with unconstrained lattice misfit. The calculated results are compared with available experimental results in the literature and the agreements between them are fairly good.