Grain Boundaries In Ni3Al Research Articles

Synergistic effect of Mn and B on the cohesive properties of grain boundaries in Ni3Al: A first-principles study

Ni-based superalloys are known as potential structural materials under extreme conditions due to their good high temperature properties. However, some alloys such as Ni3Al exhibit intrinsic brittleness which severely restricts their practical applications as structural materials. It is well known that the moderate addition of B can significantly improve the ductility of Ni3Al grain boundaries (GB) but restricted to the Ni-rich side. Adding Mn together with B can improve the ductility of Ni3Al in a wider composition range. To provide some fundamental understandings on such a cooperative effect, the segregation of Mn or/and B to the Ni3Al GBs and their effects on the cohesive properties of the GBs are systematically investigated by using a first-principles method. It is shown that B tends to segregate to the Ni-rich holes of the GBs, which means that B prefers to form stronger covalent bonds with the Ni atoms. The calculation of Griffith’s work shows its improved effect on the cohesion of the GBs. Different from site occupation tendency in bulk Ni3Al, Mn tends to segregate to the GBs and substitute the Ni sites at the GBs. It is interesting to notice that B can segregate to the Ni-deficient hole and improve the cohesive properties of the GBs in the presence of Mn substituting Ni atoms at the GBs. Whether at the Ni-rich holes or Ni-deficient holes of B segregation, Mn substituting the Al sites won’t take place at the GBs. The improvement of the ductility of composition-dependent Ni3Al extends from the Ni-rich side to the stoichiometric side with the co-doping of Mn and B. The electronic structure mechanism behind is discussed based on the calculated electronic density of states. It is suggested that the beneficial cooperative effect of Mn and B on the cohesive properties of the GBs is attributable to the increasing B-Al interaction.

Effect of P impurity on NiAlΣ5 grain boundary from first-principles study**Project supported by the National Natural Science Foundation of China (Grant No. 51201181) and the Scientific Research Fund of Civil Aviation University of China (Grant No. 08QD14X).

First-principles calculations based on the density functional theory (DFT) and ultra-soft pseudopotential are employed to study the atomic configuration and charge density of impurity P in NiAl Σ5 grain boundary (GB). The negative segregation energy of a P atom proves that a P atom can easily segregate in the NiAl GB. The atomic configuration and formation energy of the P atom in the NiAl GB demonstrate that the P atom tends to occupy an interstitial site or substitute a Al atom depending on the Ni/Al atoms ratio. The P atom is preferable to staying in the Ni-rich environment in the NiAl GB forming P–Ni bonds. Both of the charge density and the deformation charge imply that a P atom is more likely to bond with Ni atoms rather than with Al atoms. The density of states further exhibits the interactions between P atom and Ni atom, and the orbital electrons of P, Ni and Al atoms all contribute to P–Ni bonds in the NiAl GB. It is worth noting that the P–Ni covalent bonds might embrittle the NiAl GB and weakens the plasticity of the NiAl intermetallics.

First-principles study of the effect of boron on grain boundary in NiAl

The grain boundary (GB) is critical to the mechanical properties of NiAl. The addition of B in NiAl can lead to the increase of yield strength due to its segregation in GB. To elucidate the B-induced enhancement of GB, we investigate the segregation state and its strengthening mechanism of the additive B in the typical ∑5(310)[001] GB of NiAl at atomic scale using the first-principles method. Our calculations shown that the additive B atom prefers to segregate at the interstitial I1 site of GB, in which case it is surrounded by four first-nearest-neighboring (1NN) and two second-nearest-neighboring (2NN) Ni atoms. It is found the segregated B prefers to bond with its neighboring Ni instead of Al atoms. Our calculations further shown not only the intergranular cohesion is improved by the covalent bonding between the segregated B and its four 1NN Ni atoms, but also the intra-planar cohesion of GB is enhanced by the covalent bonding between the segregated B and its two 2NN Ni atoms. The covalent bonding between B and Ni atoms mainly stem from the contribution of B-p and Ni-d states, partly from the weak interactions of B-s and Ni-s states. Our first-principles tensile test shown that the yield stress and ductility of NiAl are indeed improved due to the segregation of B in GB.

A Multi-Scale Modeling Approach to Simulate Intergranular Crack Propagation in Textured Polycrystalline Materials

In this paper, a multiscale modeling approach has been developed to simulate the intergranular crack propagation in textured polycrystalline materials. Embedded Atom Method (EAM) and Molecular Dynamics (MD) simulations were carried out to determine the energy and fracture strength of different types of grain boundaries in Ni3Al. Subsequently, the atomistic model has been integrated with the microstructure based model of crack propagation using the Voronoi-Markov Chain-Monte Carlo approach. The model has been utilized to evaluate the crack length for various scenarios and reasonable results are obtained.

Formation of a coplanar O–Al bonding cluster: the effect of O impurity on aΣ = 5 NiAl grain boundary from first-principles

The site occupancy, structure, and bonding properties of O in an NiAl grain boundary (GB)have been investigated by employing a first-principles total energy method based on densityfunctional theory with the generalized gradient approximation and ultrasoft pseudopotential. TheΣ5(310)/[001] tilt GB of NiAl has been chosen because (i) theΣ = 5 GB has been observed to be a higher fraction in NiAl experimentally, and (ii) theΣ5(310)/[001] is energetically favorable in comparison with theΣ5(210)/[001]. The NiAl GB is shown to favor the O segregation with a segregation energy of−1.75 eV, indicating that most of the O impurity will distribute in the NiAl GBthermodynamically. Moreover, O is shown to prefer occupying the interstitialsites rather than the substitutional sites in the GB according to the calculatedformation energies. The O–Al bond is energetically favorable as compared withthe O–Ni bond due to different electronegativity of Al and Ni in reference toO. Charge distribution and the density of states further indicate the intrinsicbonding properties of O–Al that contain obvious covalent characteristics. It isinteresting to find that O is coplanar with the surrounding Al atoms in bothinterstitial and substitutional cases with lower formation energies, forming strongercoplanar O–Al bonding clusters. Such stronger bonding clusters in the GB canembrittle the NiAl intermetallics and thus are not beneficial to the plasticity ofNiAl. Our results will provide a useful reference for improving the mechanicalproperties and for understanding the oxidation effect of the NiAl intermetallics.

Anharmonic analysis of defective crystals with many-body interactions using symmetry reduction

In this paper we first classify and formulate various types of defects with respect to their symmetries and then show that with this general formulation, one can study the structures and energetics of defects in different crystalline materials effectively. We present all the calculations for the embedded-atom-type potentials but the formulation can, in principle, be applied for any many-body interatomic potential. As examples of defects in crystals with many-body interactions, we study point defects, free surfaces and grain boundaries in fcc Cu and grain boundaries in NiAl. We discuss free surface modelling by relaxing both a semi-infinite lattice and a slab with increasing finite thickness. We demonstrate through several numerical examples that our general framework of anharmonic lattice statics can be used for comparing different interatomic potentials in terms of the structure and energy they predict for a given defect. In the case of fcc Cu, we show how both the structures and energetics of different defects can strongly depend on the choice of potentials.

Low-energy sputtering events at free surfaces near anti-phase and grain boundaries in Ni3Al

Atomic recoil events on free surfaces orthogonal to two different anti-phase boundaries (APBs) and two grain boundaries (GBs) in Ni3Al are simulated using molecular dynamics methods. The threshold energy for sputtering, E sp, and adatom creation, E ad, are determined as a function of recoil direction. The study is relevant to FEG STEM (a scanning transmission electron microscope fitted with a field emission gun) experiments on preferential Al sputtering and/or enhancement of the Ni–Al ratio near boundaries. Surfaces intersected by {110} and {111} APBs have minimum E sp of 6.5 eV for an Al atom on the Ni–Al mixed (M) surface, which is close to the value of 6.0 eV for a perfect M surface. High values of E sp of an Al atom generally occur at a large angle to the surface normal and depend strongly on the detailed atomic configuration of the surface. The mean E sp, averaged over all recoil directions, reveals that APBs have a small effect on the threshold sputtering. However, the results for E ad imply that an electron beam could create more Al adatoms on surfaces intersected by APBs than on those without. The equilibrium, minimum energy structures for a (001) surface intersected by either Σ5[001](210) or Σ25[001](340) symmetric tilt grain boundaries are computed. E sp for surface Al atoms near these GBs increases monotonically with increasing recoil angle to the surface normal, with a minimum value, which is only about 1 eV different from that obtained for a perfect surface. Temperature up to 300 K has no effect on this result. It is concluded that the experimental observations of preferential sputtering are due to effects beyond those for E sp studied here. Possible reasons for this are discussed.

Segregation and structural transformations at Σ=3 grain boundaries in NiAl: A Monte-Carlo study

Grain boundary structure and grain boundary segregation in stoichiometric and Ni-rich compositions of NiAl are studied by molecular statics and grand-canonical Monte-Carlo simulations in conjunction with an embedded-atom potential. Significant Ni segregation is found at the Σ = 3(1 1 1) and Σ = 3(2 1 1) grain boundaries, with segregation isotherms showing a saturation effect. The amount of Ni segregation increases and the excess free energy of the grain boundaries decreases with increased Ni bulk concentration. In strongly Ni-rich bulk compositions, both boundaries undergo transformations to new structures capable of accommodating more excess Ni atoms than the initial structures. The structural transformation is clearly manifested by a non-monotonic behavior of the segregation isotherm. In the Σ = 3(2 1 1) grain boundary, the transformation is accompanied by a relative tangential translation of the grains.

Interphase boundary fracture and grain boundary precipitation of Ni 3Al(γ′) phase in β-NiAl bicrystals

The effect of the crystallography of film-like Ni 3Al(γ′) precipitates along grain boundaries of NiAl(β) on the fracture stress at room temperature was examined using β bicrystals with controlled orientations. The selected variant of γ′-film satisfied the Kurdjumov–Sachs (K–S) relation with a neighbouring β crystal, and deviated from the relation with an adjacent β crystal. In the course of tensile deformation at ambient temperature, fracture occurred preferentially at the (β/γ′) interphase boundary deviating from the K–S relation, which showed no plastic flow, and the fracture stress decreased with increasing deviation angle. In contrast, slip transfer from γ′-film to β crystal across coherent (β/γ′) interface was observed, when the variant of γ′-film satisfied the K–S relation with both neighbouring β crystals. To clarify the relation between the interphase boundary fracture and the deviation angle from the K–S relation, the boundary structure of incoherent (β/γ′) interfaces was discussed using O-lattice theory and transmission electron microscopic observations.

Effect of coherency on interphase boundary sliding in NiAl(β) bicrystals with film-like Ni 3Al(γ ′) precipitate along boundary

Effect of the crystallography of Ni 3Al(γ ′) precipitates along grain boundaries of NiAl(β) on the interphase boundary sliding of (β/γ ′) interface was examined using β bicrystals. Interphase boundary sliding occurred preferentially at incoherent (β/γ ′) interface and the sliding displacement increased with increasing deviation angle from the Kurdjumov–Sachs orientation relationship.

Vacancies in selected special tilt grain boundaries in Ni 3Al

The structure of selected special ( Σ=5) symmetrical tilt grain boundaries in Ni 3Al with tilt axis [1 0 0] was simulated by the Monte Carlo method. Vacancies were generated both in Al and Ni sites in their relaxed structure and the vacancy formation and migration energy was estimated.

Effects of Lanthanium on corrosion of NiAl in molten carbonate

Lanthanium is added to NiAl which is a promising coating material for the wet-seal area of the separator in molten carbonate fuel cells. The effect of lathanium on the corrosion of NiAl in molten carbonate is investigated. Lathanium is precipitated at the grain boundaries of NiAl and reduces the grain size by one-fifth to one-eighth when added above 1.0 at.% by arc-melting. Though precipitated lathanium shows reactive effects and slightly increases the adherence of the oxide layer in cyclic oxidation, the oxide layer is disrupted at the grain boundaries when corroded in molten carbonate salt for more than 100 h. This behaviour is due to the internal stress which results from the oxidation.

Effect of Interphase Boundary Character on Mechanical Properties in NiAl(β) Bicrystals with Ni 3Al(γ') Precipitates along Grain Boundary

ABSTRACTRole of the crystallography of Ni 3Al(γ') precipitates along grain boundaries of NiAl(β) in the mechanical properties was systematically investigated using β bicrystals with controlled orientations. γ' phase preferentially precipitated along βgrain boundaries showing a film-like shape. The selected variant of γ'-film satisfied the Kurdjumov-Sachs (K-S) relation with a neighboring βcrystal but it deviated from the relation with another adjacent βcrystal. In the course of tensile deformation at room temperature, fracture occurred preferentially at incoherent (β/γ') interphase boundary deviating from the K-S relation and the fracture stress decreased with increasing deviation angle. In contrast, the interphase boundary sliding occurred preferentially at irrational (β/γ') interface at 1073K. The sliding displacement increased with increasing deviation angle. Thus, the mechanical properties of β bicrystals with γ'-film were found to depend strongly on the interphase boundary character.

Concurrent design for NiAl-based (β/γ′) two-phase alloys by controlling microstructure and texture

Control of the crystallography of Ni3Al (γ′) precipitates along grain boundaries of NiAl(β) was systematically studied using β bicrystals with controlled orientations. γ′ phase preferentially precipitated along β grain boundaries showing a film-like shape. The variants of γ′ precipitates were uniquely selected, which satisfies the Kurdjumov–Sachs K–S) relation with a neighboring β grain and deviates from the relation with another adjacent β grain. In the course of tensile deformation, fracture occurred preferentially at the (β/γ′-film) interface deviating from the K–S relation and the fracture stress decreased with increasing deviation angle from the K–S relation. For improvement of the coherency at the irrational (β/γ′-film) boundaries, the control of microstructure and crystal orientation distribution in (β/γ′) two-phase polycrystals was next attempted by thermomechanical processing. After hot-compression in β phase region and subsequently annealing in (β/γ′) two-phase region, γ′ phase transformed from β phase with 〈111〉β fiber texture satisfying the K–S relation,resulting in the formation of 〈110〉γ′ fiber texture. In particular, a large number of ′/γ′-film) boundaries became partially coherent. This thermomechanical processing was effective in controlling the crystallography of γ′-film along β grain boundaries and leads to the harmonic design of strength and ductility for (β/γ′) two-phase alloys.

EELS as a Probe of Local Electronic Structure and Cohesion

There is an intimate connection between the electronic structure of a material and its physical properties. to change one, is to change the other. Some of the most striking illustrations of this relationship can be found at grain boundaries in metals and their alloys. Here, the most important changes in cohesion can be described by changes in the local density of states (LDOS), which in turn can be measured using EELS [1]. The first demonstration that EELS could be used to connect the electronic and mechanical properties of a material was in revealing the role that boron has in restoring a bulk-like bonding to grain boundaries in Ni3Al [2,3]. Boron was known to change the fracture mode in Ni3Al from intergranular to transgranular, possibly by enhancing grain boundary cohesion.What interested me in this project, when John Silcox first suggested it as a thesis topic, was the potential of using the EELS fine structure to measure materials properties directly.

The Effect of Local Symmetry on Atomic Resolution EELS Near-Edge Structures: Predictions for Grain Boundaries In NiAl

Abstract It has been demonstrated that electron energy loss spectrometry (EELS) can be used to probe the electronic structure of materials on the near-atomic scale. The electron energy loss near edge structure (ELNES) observed after the onset of a core edge reflects a weighted local density of final states to which core electrons are excited by fast incident electrons. Lately ‘atomic resolution EELS’ and ‘column-by-column spectroscopy’ have become familiar themes amongst the EELS community. The next generation of STEMs, equipped with spherical aberration (Cs) correctors and electron beam monochromators, will have sufficient spatial and energy resolution, along with the superior signal to noise required, to detect small changes in the ELNES from atomic column to atomic column. Core loss ELNES provides information about unoccupied states, but the structure observed in spectra is sensitive to changes in the underlying occupied states, and thus to the bonding in the material.

Fracture Strength of Grain Boundaries in Ni3Al

ABSTRACTThe fracture strength of grain boundaries in Ni3Al was quantitatively measured by performing tensile tests on miniature bicrystal specimens with various grain boundary types. A fairly good relationship between fracture mode, fracture strength and Σ value was established. The Σ1, Σ3 and Σ9 boundaries were found to be strongly crack-resistant but the adjacent bulk fractured. In contrast, Σ5, Σ7, Σ11, Σ13 and random boundaries were found to be less crack-resistant and fractured at lower stresses than the fracture strength of the bulk. In the latter case, the fracture strength of the Σ11 and Σ13 boundaries was higher than that of the Σ5 and Σ7 boundaries. Most random boundaries were as weak as the Σ5 and Σ7 boundaries, but some exhibited high fracture strength comparable to that of the Σ11 and Σ13 boundaries.

Jerky motion of grain boundaries in NiAl: an atomic force microscopy study

It is known that moving grain boundaries (GBs) interact with the thermal grooves developing along the line where the GB intersects the surface. The questions of the GB migration character (continuous or jerky) is of principal importance for the theory of GB migration and grain growth. Recent computer simulation studies of GB migration in pure materials have shown that the calculated GB mobilities are many orders of magnitude higher than those usually observed in the experiment even in very pure materials. If the jerky GB motion is a common phenomenon then the GB mobility measured in the macroscopic experiments can differ considerably from the true microscopic mobility, since in the former case the time which the GB spent in the locked position is taken into account. In this paper, the authors will demonstrate the potential of atomic force microscopy (AFM) in deducing the real dynamic behavior of the GBs at high temperature from the post-mortem studies of the surface topography in the region of ghost lines.

Computer modeling of grain boundaries in Ni 3Al

Computer simulation of symmetric tilt grain boundaries (GB) Σ=5 [1 0 0] (0 1 2) and Σ=5 [1 0 0] (0 1 3) in alloy Ni 3Al was carried out. The energy of GB was calculated out by a method of construction of γ-surface with using Morse's empirical central-force potentials. Investigations shown that GB have several steady states: one is stable and other – metastable. These states differ by energy and atomic structure GB. It is shown, that GB in model CSL are unstable, the stabilization is achieved by additional displacement on some vector along plane of defect. Directions and value of potential barriers of GB slippage are calculated.

Studies of local bonding and chemistry at internal interfaces using electron energy loss spectroscopy†

Changes in the structure, chemistry and bonding at internal interfaces often have a great influence on the properties of materials. High resolution imaging and Energy Dispersive X‐ray Spectroscopy in the Transmission Electron Microscope have been used extensively to study the structure and composition at interfaces. Spatially resolved Electron Energy Loss Spectroscopy (EELS) in the Scanning Transmission Electron Microscope (STEM) offers a new probe to explore the effect of structure and chemistry changes on the local bonding at interfaces. This paper reviews the relationship between EELS and electronic structure and illustrates the insights obtained from this technique with the help of examples which include: a) The effect of B segregation on grain boundaries in Ni3Al and Ni3Si. b) The effect of Bi segregation on grain boundaries in Cu. c) Bonding changes at two metal‐ceramic interfaces :Nb‐Al2O3 and Cu‐ A12O3.