Ni3Al Interface Research Articles

Theoretical study on hydrogen permeation and fault slip of γ-Ni/γ'-Ni3Al interface with alloying elements (Zr, Nb, Ru, Re, and Hf)

In order to elucidate the role of hydrogen behavior at γ-Ni/γ'-Ni3Al interface with alloying elements, we performed comprehensive research of the permeation and slip properties by using DFT calculations. The result figures out that the alloying elements (Zr, Nb, Ru, Re, and Hf) tends to occupy corner-point sites in γ-Ni phase, while Re has the lowest substitution formation energy. The octahedron interstitial site is the favorable position for hydrogen adsorption due to its lower solution energy compared to the tetrahedron site. The diffusion energy barrier of hydrogen follows the influence order of Zr < Ru< Hf < Re < Nb. Meanwhile, Nb-doped interface has the highest hydrogen diffusion coefficient and permeability compared to other doped interfaces. By analyzing the partial density of states, the hybridization between Re-d orbital and neighbor Ni-d orbital is conducive to affect the permeability behavior of hydrogen. The generalized stacking fault energy reveals that all the alloying elements can improve the creep strength of γ-Ni/γ'-Ni3Al interface. As a barrier, Hf element is more effective to improve the dislocation slip resistance. These results can help people deepen the understanding of hydrogen behavior with alloying elements at the γ-Ni/γ'-Ni3Al interface and provide fundamental theories data for the further design of Ni-based superalloy.

Effects of reactive elements and Pt doping on the adhesive strength and tension property of α-Al2O3/γ'-Ni3Al-S interface

The enhancement of cyclic oxidation performance in ultra-high temperature protective coatings for turbine blades in new generation aero-engines can be achieved through the reactive elements or Pt doped β-NiAl single phase. However, it is important to note that the content of Al in NiAl gradually decreases during high temperature service, leading to an inevitable phase transition from β-NiAl to γ’-Ni3Al. We only studied the reactive element effects on the adhension and stability of the α-Al2O3/β-NiAl interface previously, but the influence mechanism for α-Al2O3/γ'-Ni3Al interface is not clear. Therefore, it is imperative to investigate the adhension and stability of the γ’-Ni3Al/oxide interface. This paper investigates the effect of Pt or reactive elements (Hf, Zr, Dy, Y, La) doping on the α-Al2O3/γ'-Ni3Al interfacial adhesion and tensile properties with or without impurity element S, using the first-principles calculation method. The results indicate that the influence of Pt or REs doping on interface adhension is contingent upon the existence of S, and the effect on static interface bonding or dynamic tensile properties varies as well. It will improve the comprehensive design system of Ni-Al coatings, and provide theoretical guidance for the development of new metallic protective coating materials.

Influence behavior and mechanism of γ′- or γ′′-precipitated phase types on damage resistance of heterogeneous interface in nickel-based superalloys

The microstructure adjustment of Ni-based superalloys focuses heavily on the types of precipitated phases because the failures of Ni-based superalloys often occur at γ-matrix/γ′- or γ′′-precipitated phase heterogeneous interfaces. As different types of precipitate phases result in different interface damage resistances, the influences of the types of precipitated phases on the damage resistances of heterogeneous interfaces were investigated in this work. Based on the results of this work, the interfacial works of adhesion for the three interfaces composed of the γ-matrix and typical precipitated phases (γ′-Ni3Al, γ′-Ni3Ti, and γ′′-Ni3Nb) are 4.81, 3.88, and 4.26 J/m2, respectively, indicating that the γ-Ni/γ′-Ni3Al interface shows the highest cleavage fracture resistance. This can be attributed to the highest interfacial average charge density among all three interfaces. During interface tensile processing, all fractures occur in the precipitated phase, indicating that the ductility of heterogeneous interfaces is determined by that of the precipitated phase. Due to the highest ductility of γ′-Ni3Ti, the γ-Ni/γ′-Ni3Ti interface demonstrates the best plastic fracture resistance with the smallest Rice ratio of 4.89 and the largest maximum strain of 10 %, followed by the γ-Ni/γ-Ni3Al and γ-Ni/γ′′-Ni3Nb interfaces. The γ-Ni/γ′-Ni3Ti interface also exhibits the highest fatigue fracture resistance attributed to its strongest ability in hindering dislocation slipping with the lowest unstable stacking fault energy, followed by the γ-Ni/γ′′-Ni3Nb and γ-Ni/γ′-Ni3Al interfaces. These results conclude that Ni-based superalloys with γ′-Ni3Al as the main precipitated phase demonstrate high strength, while those with γ′-Ni3Ti as the main precipitated phase demonstrate high plastic and fatigue fracture resistances.

Effects of Zr-Re/W co-segregation behavior on the thermodynamic stability and fracture strength of γ-Ni/γ'-Ni3Al interface

First-principles calculations based on density functional theory were employed to investigate the Zr-Re/W co-segregations at the γ-Ni/γ'-Ni3Al interface and their effects on the stability and binding strength. It is found W/Re can segregate at the γ-L1-3.52-cp site of the γ-Ni (001) layer in the Zr-segregated interface which is the most stable. The stability of Zr-W/Re co-segregated interface is improved and becomes more stable than clean and Zr-segregated interfaces, due to the existence of pseudogaps at the Fermi level. The co-segregations of Zr-W/Re atoms do not change the final fracture place of γ-Ni/γ'-Ni3Al interface as compared with the Zr-segregated and clean interfaces. The co-segregations slightly enhance the fracture strength of Zr-segregated interface when interfacial fracture occurs in region-1, while is improved and even stronger than the clean interface as the fracture in region-2. Also, the binding energies of Zr-W/Re were calculated to elucidate the interatomic interactions, which contribute to interfacial strength.

Unraveling the formation mechanism of hydrogenated vacancy at γ-Ni/γ'-Ni3Al phase interface and its roles in interfacial stability and strength

γ/γ' phase interfaces are hidden troubles that threaten the service safety of Ni-based single crystal superalloys due to their thermodynamic instabilities and lower mechanical strengths. To gain insights into the role of hydrogenated vacancy in the stability and cohesion of γ/γ' interfaces, the formation mechanism of hydrogenated vacancy at (001) γ-Ni/γ'-Ni3Al interface and its influences on interfacial stability and fracture strength were systematically studied by using first-principles calculations. Our results show that the short-range interplay between monovacancy and single hydrogen atom at (001) γ-Ni/γ'-Ni3Al interface causes the formation of Vac-H complex structure (i.e., hydrogenated vacancy) as separated by a certain distance rather than a direct recombination. The hydrogen atom in the first nearest-neighbor interstices of Ni vacancy (site 6) deviates from the interstitial centers to the vacancy, owing to the interattraction between monovacancy and hydrogen atom. The formation of hydrogenated vacancy at (001) γ-Ni/γ'-Ni3Al interface can promote the interfacial thermodynamic stability as compared with the vacancy-containing γ/γ' interface, but weaken the interfacial fracture strength and change the preferable fracture place of γ/γ' interface. The underlying mechanisms of stabilizing and weakening effects on the γ/γ' interface were revealed by using spin-polarized density of states and differential charge densities.

Impact of crystallography at Ni/NiAl interfaces on the nucleation of Ni3Al

Early stages of phase formation at (Ni)/NiAl interfaces with different crystallographic configurations were studied. As opposed to earlier work, macroscopic diffusion couples and a low annealing temperature of 650°C were applied. The morphology of the newly forming Ni3Al phase, its crystallographic relationships to the different (Ni)/NiAl interfaces and the growth kinetics of Ni3Al were evaluated with appropriate statistics combining EBSD with STEM EDX. The Ni3Al phase forms in two layers and mainly grows into the NiAl phase, establishing a specific crystallographic relationship with respect to the adjoining phases. The majority of Ni3Al grains growing into the NiAl phase maintain Kurdjumov-Sachs orientation relationships. The grains in the second Ni3Al layer mostly grow in a near cube-on-cube orientation relationship (misorientation < 15°) towards the (Ni) phase. The formation of large Al enriched (Ni) grains at the (Ni)/Ni3Al interface is observed and interpreted as diffusion induced recrystallization.

Segregation of alloying elements and their effects on the thermodynamic stability and fracture strength of γ-Ni/γ′-Ni3Al interface

The γ/γ’ phase interface, being the main plane defect in nickel-based single-crystal superalloys, can have a significant effect on the microstructural stability and mechanical properties of superalloys. To improve the thermodynamic stability and fracture strength of the γ-Ni/γ’-Ni3Al interface and gain insight into the underlying micro-mechanisms, the segregation tendency of 12 alloying elements X at the γ-Ni/γ’-Ni3Al interface and their effects on the interfacial formation and Griffith fracture works were investigated by using first-principles calculations. It is found that all alloying elements X can segregate to the corner-point site of the γ-Ni (001) layer except the element Y. The Re-, Ti-, Mo-, W-, Cr-, Nb-, Ta-, Hf- and Zr-segregated interfaces are more stable than the unalloyed interface due to the presence of pseudogap and lower values of densities of states at the Fermi level. The segregation of Ru, Co, Re, Mo, W, Cr, Nb and Ta strengthens the interfacial fracture strength, which can be mainly attributed to the enhanced bonding strengths of X–Ni bonds formed in these segregated interfaces. The interfacial segregation of Cr, Re, Mo, W, Nb and Ta can not only improve the thermodynamic stability but also enhance the fracture strength of the phase interface. The segregation of Ta and Re is able to improve the thermodynamic stability and fracture strength of the γ-Ni/γ’-Ni3Al interface to the maximum extent, respectively. The effect of interfacial segregation of alloying elements on the fracture strength and thermodynamic stability of the γ-Ni/γ’-Ni3Al phase interface and the underlying mechanisms are studied.

Insight into γ-Ni/γ′-Ni3Al interfacial energy affected by alloying elements

Interfacial energy (σγ/γ′) of the γ-Ni/γ′-Ni3Al interface is critical to understand and design Ni-based superalloys. In the present work, a first-principles methodology with constrained relaxations has been used to study the effect of dilute alloying element (X) on σγ/γ′ of the (100) coherent interface, where the sixteen X's include Al, Co, Cr, Fe, Hf, Mo, Nb, Pd, Pt, Re, Ru, Ta, Ti, W, Y, and Zr. σγ/γ′=19mJ/m2 has been predicted for the unalloyed γ/γ′ interface, agreeing well with the previous estimations. It is found that the σγ/γ′ value is the lowest when alloying element is in γ-Ni, while the addition to γ′-Ni3Al increases σγ/γ′ due mainly to the in-plane lattice expansion. Specifically, our calculations show that alloying elements Mo, W, and Re have the largest effect on the σγ/γ′ value by decreasing it to 4–5mJ/m2 when partitioned to γ-Ni; Ru and Pt are shown to increase greatly the σγ/γ′ value up to 25–28mJ/m2 when partitioned to γ-Ni; all ternary additions substituting for Ni in γ′-Ni3Al increase σγ/γ′ except for Pt; and for Ti and Ta, which segregate strongly to γ′-Ni3Al and substitute for Al, the σγ/γ′ values increase to 32 and 35mJ/m2, respectively.

First principles study of the diffusional phenomena across the clean and Re-doped γ-Ni/γ’-Ni3Al interface of Ni-based single crystal superalloy**Project supported by National Basic Research Program of China (Grant No. 2011CB606402) and the National Natural Science Foundation of China (Grant No. 51071091).

Density functional theory calculations in conjunction with the climbing images nudged elastic band method are conducted to study the diffusion phenomena of the Ni-based single crystal superalloys. We focus our attention on the diffusion processes of the Ni and Al atoms in the γ and γ’ phases along the direction perpendicular to the interface. The diffusion mechanisms and the expressions of the diffusion coefficients are presented. The vacancy formation energies, the migration energies, and the activation energies for the diffusing Ni and Al atoms are estimated, and these quantities display the expected and clear transition zones in the vicinity of the interface of about 3–7 (002) layers. The local density-of-states profiles of atoms in each (002) layer in the γ and γ’ phases and the partial density-of-states curves of Re and some of its nearest-neighbor atoms are also presented to explore the electronic effect of the diffusion behavior.

Influence of alloying elements upon the theoretical tensile strength of Ni-based model superalloy: γ-Ni/γ′-Ni3Al multilayer

The theoretical tensile strength of Ni-based model superalloys was investigated by density functional theory calculations. The superalloy was modeled as a γ-Ni/γ′-Ni3Al multilayer, and the strengthening effect induced by alloying elements X (X=Re, Ru, Cr, Mo, W, or Ta) close to the γ/γ′ interface was investigated. Theoretical calculations show that the tensile strength of the model superalloy is ∼1GPa higher than that of the γ′-Ni3Al phase, although the critical strain is ∼38% lower than that of the pure γ′-Ni3Al phase. Except for Ta, doping at the Al site of the γ-Ni/γ′-Ni3Al interface significantly increases the theoretical tensile strength (>7%) or critical strain (>10%) of the γ-Ni/γ′-Ni3Al multilayer. Local atomic disorder is observed in the Ni–Al sublayer of the γ′-Ni3Al block of the superalloy, wherein the Ni–Al atomic layer decomposes into an Al layer and a Ni layer when the strain exceeds a critical value. The charge density difference and partial density of states provide further insight into the changes of chemical bonds and charge redistribution during the tensile process. Charge transfer from Al to Ni in the γ′-Ni3Al precipitate of the superalloy could account for its higher tensile strength than the single γ′ phase. Moreover, covalent-like Ni–X bonds inhibit deformation of the system, resulting in tensile strengthening of doped γ-Ni/γ′-Ni3Al multilayers.

An interplay of sulfur and phosphorus at the γ-Ni/γ′-Ni3Al interface

A first-principles investigation of the interplay of S and P and their role on the cleavage rupture of γ-Ni/γ′-Ni3Al interface is conducted. Trace elements S and P can coexist in the γ-Ni/γ′-Ni3Al interfacial region and their site preferences are scarcely changed by another heterogeneous element. S and P in duplex doping systems tend to simultaneously replace for a Ni and occupy an octahedral interstice adjacent to substituted sub-lattice site to form a doping atom pair, but the interaction between S-doping and P-doping varies with their distributions along or across the interfacial layer. The cleavage fracture of duplex doping interfaces generally occurs in the region between S-doped and P-doped atomic layers, and an extra heterogeneous dopant will further aggravate S- or P-induced embrittlement. It is revealed a strong repulsive interaction between P-doping and S-doping is responsible for the descent of cleavage fracture strengths caused by extra heterogeneous dopants.

The correlation between Re and P and their synergetic effect on the rupture strength of the γ-Ni/γ′-Ni3Al interface

A first-principles investigation on the synergetic effect of Re and P on Griffith rupture works of the γ-Ni/γ′-Ni3Al interface is performed. The calculated results indicate site preference of P at the γ/γ′ interface is not changed by Re-addition but P-induced embitterment can be inhibited even reversed by the addition of Re at the adjacent atomic layer of P. The correlative effect between P and Re at the γ/γ′ interface is also evaluated by a correlative energy function. It is found a strong repulsive interaction between P-doping and Re-addition is limited within a unit cell of γ or γ′ phase and is not profitable for the strengthening of the γ/γ′ interface. The electronic and geometric structures reveal a large interfacial separation and the depletion of electron densities in the interfacial region should be responsible for the potential inter-phase cleavage fracture site, and the deleterious effect of P can be attributed to a mutual influence of local elastic stain energy and atomic bonding energy in the doped system.

Microstructure and abrasive wear performance of chromium carbide reinforced Ni 3Al matrix composite coating

A Ni–Al–Cr 3C 2 welding wire was produced by metal–powder–core technique. When the welding wires were welded on the surface of carbon steel, under the effect of the physical heat of arc, Ni reacted with Al to form Ni 3Al and carbide particles reinforced Ni 3Al matrix composite was formed. Cr 3C 2 was decomposed during welding and dispersed Cr 7C 3 with stripe shape formed, which strengthened the matrix significantly. The Cr 7C 3–Ni 3Al interface has broadened into a zone of interdiffusion and a new phase M 23C 6, which indicates that a good bond has been formed. The pin-abrasion wear test showed that the abrasion resistance of Cr 7C 3/Ni 3Al composite is six times higher than that of Stellite12 alloy at room temperature. The good wear resistance of Cr 7C 3/Ni 3Al composite coating can be attributed to large volume fraction of carbides, high hardness, and good phases interface bond.

Development of New Wear-Resistant Surface Coating at Elevated Temperature

Because of good oxidation resistance at high temperature and excellent mechanical properties of Ni3 Al and high hot hardncss, and good oxidation resistance of chromium carbide, chromium carbide particle reinforced Ni3 Al matrix composite would possess excellent wear resistance at elevated temperature. Cr7 C3 -NiAl-Ni welding wire was produced by pressureless sintering process in vacuum. When the welding wire was welded on the surface of carbon steel, under the action of the physical heat of arc, NiAl reacted with nickel to form Ni3 Al and carbide particle reinforced Ni3 Al matrix composite was formed on the welding layers. Cr3 C2 was dissolved during welding and dispersed Cr7 C3 was formed, which strengthened the Ni3 Al matrix significantly. The Cr7 C3 -Ni3 Al interface was broadened, and a zone of interdiffusion and a new phase M23C6 were formed, indicating that a good bond has been formed. The hardness of Cr7 C3/Ni3 Al composite at room and elevated temperatures is much higher than that of stellite alloys. In addition, Cr7 C3 /Ni3 Al composite possesses better high temperature oxidation resistance than stellite 12 alloy. So Cr7 C3 / Ni3 Al composite can become an attractive potential candidate for elevated temperature wear-resistant surface material.