The first principle method based on density functional theory and generalized gradient approximation is used to investigate the interaction of Ta and Re elements in Ni/Ni<sub>3</sub>Al interface and their influence on the interface strength. According to the calculation of the dissolution energy of these two alloying elements at 7 different positions, it can be concluded that in most of the stoichiometric ranges, Ta atoms preferentially occupy Ni site in the γ phase, while Re atoms occupy preferentially Al site in γ' phase. The doping positions do not change when these two atoms are co alloyed. The calculation of Griffith fracture work of Ni/Ni<sub>3</sub>Al interface system shows that the doping of Ta atoms can improve the interface fracture strength of the phase boundary region between the γ/γ' coherent atomic layer and γ atomic layer. The interface is easier to fracture at the phase boundary area between γ/γ' coherent atomic layer and γ' atomic layer after Ta atoms doped. Doping of Re atom can improve the interface fracture strength of the phase boundary region between γ/γ' coherent atomic layer and γ' atomic layer. The interface is easier to break at the phase boundary area between γ/γ' coherent atomic layer and γ atomic layer. The calculation results of the unstable stacking fault energy under the interface slip system before and after Ta and Re alloying show that the doping of these two types of atoms increases the value of the unstable stacking fault energy of the interface, and the slip system becomes difficult to start, which enhances the ability of the interface to block the movement of dislocations, thus enhances the creep strength of the nickel base superalloy. When doping Re atoms, the effect is greater, and the unstable stacking fault energy of the interface increases by 11.1%, which is better for improving the creep strength of the system. By studying the influence of alloying atoms on the path of vacancy migration and the energy barrier, it is concluded that the doping of Ta and Re atoms can increase the vacancy formation energy and the potential barrier of vacancy migration at the interface. The doping of Re atoms increases the migration energy barriers on both sides of the interface, and the doping of Ta atoms increases the migration energy barriers of γ phase. The increase of the migration barrier hinders the emission and absorption of vacancies, thereby improving the creep ability of the alloy.
This investigation aims at the Zr-doping in -TiAl alloy systems in which Ti (or Al) atoms are partly replaced and the impurity concentrations are 1/54, 1/36, 1/24 and 1/16 (molar ratio), respectively. The structural, energy, plastic and electronic properties of the alloys are calculated and studied by using the first-principles method based on the density functional theory and other physical theory. From geometry optimization results it is shown that doping with Zr can change the structural symmetry of the -TiAl systems. These results also suggest that the cubic degree of Zr-doped -TiAl alloys can be increased due to the Zr-substitution. For instance, the cubic degrees of Ti12Al11Zr and Ti18Al17Zr systems are enhanced distinctly, which are positive for improving the mechanical properties of the alloys. The average formation energies obtained indicate that the Ti atom replaced by Zr can slightly decrease the formation energy of the system (0.003 eV/atom); while Zr substituting the Al atom can increase the formation energies of the systems (0.07 eV/atom). Accordingly, when Zr atoms are introduced in the -TiAl system, they tend to substitute Ti atoms, and can also substitute Al atoms with a certain possibility. Thus, various Zr-doped -TiAl regions can be produced in the system. The integral effects are of significance for improving the performance of the -TiAl based alloys by means of Zr-doping method. Comparing the axial ratios of Zr-doped -TiAl systems with that of pure -TiAl system, we find that Zr substituting Al atom can reduce the axial ratio of the Zr-doped alloys, which is responsible for the ductility of the materials. It should be mentioned that when the impurity concentration is in the range of 1.85 at%-6.25 at%, the doping effect will be most distinct and the axial ratio of the alloys is close to unity. It is expected that the Ti12Al11Zr system has a good ductility for its axial ratio equals to 1.007. The band structures of Zr-doped -TiAl systems show that they all have metallic conductivities. After Zr atom substitutes the Al atom in the -TiAl system, the intensity of covalent bond between Zr atom and its nearest neighbour Ti atoms in Ti12Al11Zr system reduces evidently and the bond length increases (0.032 ), which is indicated by the obtained overlap population (decrease by 0.21) and the densities of states in the Zr-doped and pure -TiAl systems. These results in the decrease of average intensity of Ti-Al(Zr) bonds and the increase of metallic bonds in Ti12Al11Zr system, which is an important factor for improving the ductility of -TiAl based alloys.
By using a first-principles pseudopotential method based on the density functional theory and Vienna ab initio Simulation Package (VASP), we investigate the multiple trapping of C by Ni vacancy (VNi) and its temperature effects in NiAl intermetallics. A single C atom is energetically and favorably situated at the Ni-rich octahedron interstitial site that surrounds Ni vacancy, which is shown via calculating the formation energy of C atom in NiAl with Ni vacancy system. Single C atom prefers to interact with neighboring Ni atom and Al atom to form a covalent bond. In NiAl intermetallics, C atoms prefer to be trapped in the Ni vacancy in the sequential way, thus easily forming the CnVNi (n=1, 2, 3, 4) clusters, in which the C4VNi clusters are most energetically favorable. It is interesting to find that when C atoms are trapped by Ni vacancy, all the C atoms themselves prefer to be combined with each other to form a bond, surrounding Ni vacancy. With the C atoms further added, both the charge density and the deformation charge prefer to bind with each other despite the Ni or Al environment and the intrinsic bonding properties of CC bond contain obvious covalent characteristics. Furthermore, using first-principles calculations combined with statistical model, we quantitatively predict point defect concentration as a function of temperature in NiAl intermetallics. It is concluded that the concentration of intrinsic Ni vacancies (VNi) will obviously increase as temperature increases. With the increase of temperature, the concentration of C atoms in the CnVNi cluster is higher than that at the intrinsic position. Besides, it indicates that most of C atoms in NiAl intermetallics are trapped by Ni vacancy, which is due to the larger binding energy of the CnVNi clusters and most of the C atoms are trapped directly by vacancies at room temperature or high temperature to form CnVNi clusters. Since the formation of CnVNi clusters is a process of heat releasing which will further increase the temperature of the NiAl system and produce more and more Ni vacancies, we can conclude that much more vacancies are created as a result of the presence of C impurity in NiAl intermetallics. However, the Ni vacancies exist in the form of CnVNi clusters from our calculation in a certain temperature range (less than 700 K). The existence of this kind of CnVNi cluster can effectively restrain the generations of cracks in the vacancies, which will produce some influences on the mechanical properties of NiAl intermetallic compound. Consequently, our results will provide a valuable reference for understanding the effects of C and vacancy on the mechanical properties of the NiAl intermetallics.
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