Phase Stability, Stacking Fault Energy And Mechanical Properties Of Intermetallics From First-principles Calculations

With the rapid development of modern industrial production, high performance materials becoming one of the most hot spots concerned about. The intermetallic compounds with long range ordered super lattice to other metals or alloys and present unique metallic-covalent double bonding characteristics. In this case, the intermetallics who exhibit low density, high hardness and melting point, good corrosion resistance, excellent creep and oxidation resistance and excellent high temperature service ability are widely used in the fields of aerospace, automotive and marine industry and so on. And, it is expected to be one of the most prosperous high performance high temperature structure materials. However, the inherent brittleness severely limits their use in such applications. From the lattice structure, the inherent brittleness is attributed to their low structural symmetry under equilibrium lattice and cannot provide a sufficient number of equivalent slip systems. Experiments show that the stabilization of high symmetry metastable phase can provide more activated slip systems to satisfy von Mises criterion to improve their plastic. However, these mechanisms are not very clear; especially the researches based on the micro electronic mechanism are very scarce. Therefore, it makes sense to investigate the influence of material properties for alloying elements to several kinds of representative intermetallic compounds.In this paper, the phase structural stability, mechanical properties and stacking fault energy of intermetallics MoSi2, TiAl, TiAl3 and Al3 Zr alloyed with ternary elements under various doping concentrations have been calculated using first-principles calculations. With the combination of bonding characteristics, the brittle/ductile mechanisms as well as hardness, melting point and other properties were obtained. The calculated results will provide a theoretical reference and guidance for improve the low temperature ductility and the design of high performance high temperature structure material. The main research contents are presented as follows:(1) Intermetallic compound MoSi2 is consisted by Mo and Si atoms. Due to its specific dual characteristics of ceramic and metal, and exhibits high melting point, moderate density and excellent high temperature stability. But the inherent brittleness limits its technological assignment. The phase structural stability has been calculated with ternary alloying elements Al, Mg and Ge contains. It is found that Al and Mg can effectively stabilize the C40 metastable phase. And there is a phase transition from C11 b to C40 phase when the concentrations of Al and Mg reach 7 at.% and 6 at.%, respectively. Furthermore, the ductility has a good improvement, which is good agreement with Dasgupta's experimental observation. In addition, it has a slightly decrease of the hardness and melting point with Al and Mg contains. But it is found that it has little influence for Ge contains without phase change. The effects of Nb and Nb-M(M=Al, Mg, Ge) codoping are also discussed. The results show the mechanical properties improved only with Nb-Mg codoping. The results indicate that the phase transformation is the key factor to improve the mechanical properties of MoSi2. Finally, the density of states is used to analysis the effects of alloying elements on the mechanical properties. The bonding characteristics show that the improvement of ductility is due to the wreaked Mo-Si covalent interactions.(2) As one of the important candidates of excellent lightweight high temperature structural materials, Ti Al intermetallic compound exhibits its excellent and unique properties to other traditional metals and alloys. In this paper, the phase structural stability and mechanical properties have been calculated with transition metal elements W and Mo as well as rare earth elements Sc and Yb alloying using first-principles calculations. The site preference performances show rare earth elements Sc and Yb are always preferentially occupy the Ti sites both for L10 and B2 phase, and transition metals W and Mo have a weak site preference for the Ti site in L10 phase, while they have a strong site preference for the Al site in B2 phase. The calculated formation enthalpies show that the B2 phase is stabilized with W and Mo contains and there will be a structure phase transition from L10 type to a B2 type with increasing doping concentration when W ups to 10.50 at.% and Mo ups to 11.50 at.%. The brittle/ductile behaviors are predicted based on Pugh ratio. It is shown that production of B2 phase can transform its intrinsic brittleness into ductility. Furthermore, it is shown the low concentration of rare earth elements can also improve the ductility but the effect is very limited, which indicates that the enhancement of ductility is due to the phase transition from the less slip systems L10 to a more slip systems B2 phase of TiAl based alloys, which is good agreement with previous observations. The hardness and elastic anisotropy are also calculated. The electronic structure show that the brittleness is due to the strong covalent bonding between Ti and Al atoms and the improved ductility is originated from the weakened Ti and Al covalent interactions and the improved metallic interactions with the addition of ternary elements.(3) As a kind of TiAl based intermetallics, TiAl3 has a long range ordered D022 structure and it has been expected to be the candidate for lightweight high temperature structural materials due to its very low density, excellent mechanical properties and high temperature stability. However, the lack of any significant plastic deformation and the intrinsic brittleness at room temperature is one of the main obstacles for its application in practice. In this work, our aim is to elucidate the phase structural stability and mechanical behaviors of TiAl3 with ternary ds-area transition elements Cu, Zn and Ag contains using first-principles calculations. The formation enthalpy indicates that the cubic L12 phase with adequate activated slip systems can be effectively stabilized with these elements substitute Al site. The calculated Pugh ratio, Cauchy pressure and Poisson's ratio show that the stabilized L12 phase can improve the ductility. The reduced unstable stacking fault energy and antiphase boundary energy of <110>{001} slip systems in D022 phase should be used to give a reasonable explanation of the phase transition from D022 phase to L12 phase. Furthermore, the improved ductility may be largely ascribed to the activated <110>{111} slip system in D022 phase. Rice and ZCT criteria are also introduced to understand the brittle/ductile behaviors based on Griffith's fracture theory with combination of calculated stacking fault energy and cleavage energy. The results show that the cause of brittle of D022 is due to the microcrack initiation in D022 phase. Finally, the DOS structures indicate that the improved ductility is mainly due to the weakened 3p-3d interactions between Al and Ti atoms but enhanced d-d interactions between Ti and alloying atoms, and the bonding forces tend to a more even distribution in TiAl3 with alloying atoms.(4) Long range ordered structure intermetallic of Al3 Zr has been also investigated in this paper. The alloy of Al-Cu-Zr in this work was prepared using Al-35 wt.%Cu, Al-4wt.%Zr and high-purity Al(99.99%). After melting, the alloy was cast into a direct chill method to cylindrical bars with a diameter of 12 mm and a length of 110 mm. The microstructure and composition of the cast alloy was examined by transmission electron microscopy after heat treated and subsequently water-quenched. The results show that it has Al3 Zr dispersed precipitate in the matrix. Connection the analysis of diffraction pattern with EDX results, these Al3 Zr particles is identified as Al2.5Cu0.5Zr. It indicates that the substitution of Al by Cu increases the stability of L12 phase Al3 Zr. The calculated formation enthalpy shows that substitution of Al by Cu in Al3 Zr precipitates can effectively stabilize the L12 phase relative to D023 phase from first-principles calculations. Furthermore, it is shown L12-Al3 Zr along Al<001> zone axis with planar faults parallel to the {001} planes of L12 phase. The calculations show the <110>(001) faults energy of D023 phase reduced obviously; On the contrary, increased tremendously in L12 phase with the addition of Cu atom. It is suggested that Cu can effectively stabilize the L12 phase with the combination of experiment and theory calculations. The Pugh and Poisson's ratios show the ductility is improved due to the stabilization of L12 phase. It has the same effects for Zn and Ag contains. The fracture toughness IcK as well as Rice and ZCT criterions is used to give a prediction of brittle/ductile behaviours based on Griffith's brittle fracture theory. However, it has some inconsistencies between brittle fracture theory and elastic theory. It may be due to the rough theory for Griffith's view based on the sheer brittle fracture; while there must be other more complex dislocation in real materials and the simple sheer brittle fracture or unstable fault energy is too rough for real materials. So, it is necessary from the electronic structure to further analyze. Finally, the DOS structures show that the brittleness is mainly due to the strong directional covalent bonding between Zr and Al atoms, and the improved ductility is due to the weakened Zr-4d and Al-3p interactions with the addition of ternary transition metal elements Cu, Zn and Ag, and the bonding force tends to a more even distribution in Al3 Zr intermetallic compound.