Grain Boundaries In Ni3Al Research Articles

Solution of the Atomic Structure of the Σ=5 (310) [001] Grain Boundary in Nial by Hrem and Atomistic Simulations

AbstractThe atomic structure of the Σ = 5 (310) [001] grain boundary in NiAl has been determined by a synergistic approach combining high resolution electron microscopy (HREM) and atomistic structure calculations. A bicrystal of controlled orientation was produced by diffusion bonding and imaged with the electron beam parallel to the [001] tilt axis. The results showed that the material remains chemically ordered up to the boundary plane. Atomistic structure calculations employed N-body empirical potentials developed for the NiAl phase to examine the changes in interfacial energy due to the incorporation of various point defects at the grain boundary. A self-consistent model structure was determined which was of lowest energy and produced calculated images which matched experimental images of the boundary. Monte Carlo simulations confirm the stability of this structure at finite temperatures.

Atomistic Studies of Grain Boundaries in NiAl

AbstractThis paper presents the results of atomistic studies of grain boundaries in NiAl B2 alloy. The interatomic forces are described by Finnis-Sinclair type N-body potentials, and are fitted to properties of NiAl. The results show that the structure, energy and cohesive strength of a grain boundary depend strongly on its chemistry, and a grain boundary possessing more Al is the weakest. Energies of antisite defects at the grain boundary ∑5 {210} are also calculated, and the results suggest that Al has much larger tendency to segregate at a grain boundary than Ni does.

High-resolution electron microscopy of the Σ = 5 [001] (310) grain boundary in NiAl

Abstract The structure of a highly oriented NiAl Σ = 5 [001] (310) grain boundary has been imaged by high-resolution electron microscopy and analysed by multislice image calculations. This grain boundary exhibits an asymmetry produced by a rigid body displacement of approximately 1/2d 1 30 along the grain boundary, but does not show any measurable expansion of the grain boundary. The match between the simulated and experimental images of this grain boundary is improved by changing the stoichiometry of the boundary, which can be accomplished by adding antisite defects and vacancies. The final model structure incorporates 50% constitutional vacancies on the nickel sites adjacent to the boundary plane and a replacement of the aluminium sites adjacent to the boundary plane with 50% nickel antisite defects and 50% vacancies.

Structure of the Σ=5 (310) interface in NiAl

The atomic structure of the Σ=5 [001] (310) grain boundary in NiAl was examined by high resolution electron microscopy and multislice image simulation. As in most other intermetallic compounds, the grain boundaries in NiAl are intrinsically brittle at low temperatures. Although there have been few studies on this alloy, the energies of NiAl grain boundaries have been calculated using embedded atom potentials for both stoichiometric and non-stoichiometric structures. These studies are consistent with the results of Bradley and Taylor, which indicate that nickel-rich compositions result from nickel antisite defects on the aluminum sublattice, while aluminum-rich compositions produce constitutional vacancies on the nickel sublattice, and with recent field ion microscopy results on nickel-rich alloys.The Σ=5 grain boundary was prepared by diffusion bonding at 1000 °C. A JEOL 4000EX was used for HREM imaging and the NUMIS multislice simulation program was used to simulated images. Analysis of these images considered the effects of grain boundary expansion, rigid body displacements along the boundary, grain boundary stoichiometry, and point defects at the boundary.

High resolution transmission electron microscopy and atomistic calculations of grain boundaries in metals and intermetallics

A review of grain boundary studies in metals and alloys using high resolution transmission electron microscopy and atomistic calculations is presented, with an emphasis on the relevance of the results to grain boundary structure and properties. While a number of studies on boundaries in both pure metals and alloys will be discussed, the paper will focus on symmetric tilt boundaries in aluminum and the determination of compositional ordering at grain boundaries in Ni3Al. The conclusions to be drawn from these comparisons will be discussed, and future prospects for joint experimental and theoretical studies will be presented.

Interactions between lattice dislocations and grain boundaries in L1 2 ordered compounds investigated by in situ transmission electron microscopy and computer modelling experiments

In this paper the interaction lattice dislocations and grain boundaries in Ni 3Al is reported as investigated by means of situ transmission electron microscopy deformation experiments and modelling experiments. In particular, the interaction between screw dislocations and a coherent twin boundary was analysed in detail. The results of the situ straining indicate that <110> screw dislocations impinging on a Σ = 3 coherent twin boundary, which have a Burgers vector parallel to the grain boundary plane, can be transmitted to the symmetric slip plane in the other grain under the influence of an applied stress. A one-to-one comparison with the results of a computer modelling study of exactly the same system in Ni 3Al can be made and the experiment agrees with the simulations.

The Effects of Stoichiometry on the Mechanical Behavior of Grain Boundaries in NiAl and FeAl

The Effects of Stoichiometry on the Mechanical Behavior of Grain Boundaries in NiAl and FeAl

Interaction of dislocations with grain boundaries in Ni3Al

The interaction between slip dislocations and grain boundaries in hypo-stoichiometric Ni3Al, with and without boron, has been investigated by using the in situ TEM deformation technique. In both alloys, the slip dislocations were incorporated into the grain boundaries and remained at the point of entry. The difference between the alloys was in the dominant response mode of the grain boundary to the stress concentration associated with a dislocation pileup. In the boron-free material, the stress was relieved primarily by the nucleation and propagation of a crack along the grain boundary. In contrast, in the boron-doped material, relief occurred by the emission of dislocations from the grain boundary. These results are consistent with boron increasing the cohesive energy of the grain boundary. The slip system activated at grain boundaries in the ductile ordered alloy was shown to satisfy the same slip transfer criteria that operate in f.c.c. disordered alloys.

Interactions between lattice dislocations and grain boundaries in Ni3Al investigated by means of in situ TEM and computer modelling experiments

The interaction between lattice dislocations and grain boundaries in Ni3Al has been investigated by means of in situ TEM deformation experiments. The interaction between screw dislocations and a coherent twin boundary could be analyzed in detail. The interaction mechanism found experimentally was compared to the results of a computer modelling study. In the computer modelling study, many-body potentials representing Ni3Al were used. The results of the in situ straining indicate that 〈110〉 screw dislocations impinging on a Σ = 3 coherent twin boundary that have a Burgers vector that is parallel to the grain boundary plane can be transmitted to the symmetric slip plane in the other grain under influence of an applied stress. A one-to-one comparison with the results of a computer modelling study of exactly the same system in Ni3Al can be made and the experiment agrees with the simulations. Also, observations were made of superlattice intrinsic stacking faults (SISF) that were formed as a result of the interaction between gliding dislocations and the dislocations of a low angle grain boundary (cell wall). The creation of jogs in the line of the gliding dislocation may be the cause of the SISF formation.

The role of microscopy in the alloy design of Ni- and Fe-based aluminides

During the last 10 years, there has been a resurgence of interest in ordered alloys for structural uses due to the discovery of the dramatic ductilizing effect of boron on grain boundaries in Ni3Al. With this discovery, it was hoped that the property of an increase in strength as the temperature is increased could be utilized as well as the excellent oxidation resistance. Now, alloys based on Ni3Al are in use in specialized industrial applications, such as high temperature forging dies and being tested for use as turbocharger rotors. Due to the successful control of the grain boundary strength in Ni3Al, other systems were reexamined. For example, Fe3Al was also thought to have inherently brittle grain boundaries, however it was found that with purer alloys the material failed by cleavage. Subsequently, development of practical, inexpensive, oxidation resistant alloys has proceeded. Fe3Al alloys are currently being tested for automobile exhaust applications.

Grain boundaries in Ni 3Al—I. The local compositional order

The local compositional order in the vicinity of grain boundaries in Ni 3Al, with and without boron, was examined using lattice imaging techniques. It was shown that small angle twist, tilt and mixed boundaries and large angle (near Σ = 5) twist boundaries were ordered up to very close to the interface plane. A compositionally disordered region ∼ 1.5 nm thick is present in the vicinity of a large angle general boundary in boron-doped Ni 3Al. Two large angle general boundaries are ordered up to very close to the interface in boron-free Ni 3Al. Image simulations were performed to identify possible imaging artifacts due to inclination of the boundary plane away from being edge-on. It was shown that the experimental observations of a locally disordered region cannot be explained as being an imaging artifact.

The Effect of Temperature on the Structure of Grain Boundaries in Ni3Al with and Without Boron

The Effect of Temperature on the Structure of Grain Boundaries in Ni3Al with and Without Boron

Boron and carbon in nickel, iron, and Ni3Al

Small amounts of boron and carbon in nickel, iron, and Ni3Al behave similarly except that carbon, when segregated into grain boundaries in Ni3Al and other nickel rich alloys, promotes rather than suppresses intergranular embrittlement. It is suggested that this stems from the absence of a stable carbide in the nickel system, as contrasted with the readiness to form carbides and borides in the Fe–C, Ni–B, and Fe–B systems. Excess carbon in nickel rich grain boundaries is thus expected instead to form graphite-like structures, so weakening the boundaries. This behaviour is attributed to a large electron concentration, leading to some occupation of antibonding spd hybrid states and a positive heat of solution.MST/1449

The local compositional order at grain boundaries in Ni 3Al

The local compositional order at grain boundaries in Ni 3Al

Mechanism of boron-induced strengthening of grain boundaries in Ni3Al

Abstract Poor cohesive strength of grain boundaries in strongly ordered Ni3Al is due to porous structure of grain boundaries with crack-like microcavities which open up under small tensile stresses. Interstitial B atoms in Ni-rich Ni3Al reduce the strength of directional bonding between Ni and Al atoms in the interior of the grains. When two such grains with reduced Ni—Al bonding meet each other, the atoms near the grain boundary can relax more easily and the size of the microcavities is reduced. Furthermore, interstitial B atoms increase ductility in the interior of grains so that dislocations can be easily generated to shield the tips of cracks initiated from the cavities, providing increased resistance to intergranular fracture. Evidence for the above mechanism of B-induced strengthening of grain boundaries in Ni3Al is presented.

Structure of Grain Boundaries in L12 Alloys at Finite Temperatures: Effects of Deviations from Stoichiometry

ABSTRACTAtomic structures of grain boundaries in Ni3Al and Cu3Au has been studied using a Monte Carlo relaxation technique and many body central force potentials to describe interatomic forces. The purpose of this work is to investigate the temperature dependence of the compositional order in the boundary region for both stoichiometric and non-stoichiometric alloys. In the former case the boundaries in Ni3Al remain ordered up to very high temperatures while boundaries in Ni3Al disorder at relatively low temperatures. In the non-stoichiometric case, the surplus of nickel in the bulk leads to segregation to grain boundaries and to a significant chemical disordering in the boundary region. The surplus of aluminum also leads to segregation but without disordering in the boundary region. Using these results we discuss the intrinsic brittleness of grain boundaries in Ni3Al as well as the effect of stoichiometry on the intergranular brittleness.

Fracture Strengths of Individual Grain Boundaries in Ni3Ai Using a Miniaturized Disk Bend Test

ABSTRACTThe results of our initial efforts at measuring the fracture strengths of grain boundaries In Ni3Al using a miniaturized disk-bend test are presented. The samples tested were 3 mm in diameter and between 150 and 300 μm thick. An Ingot of directlonally-solidlfled, boron-free Ni3Al containing 24% Al was annealed between 1300 and 1350 °C to induce grain growth, producing many grain boundaries In excess of 1.5 mm in length. Specimens were cut from these In such a way that one long grain boundary was located near a diameter of the specimen. The relative orientations of the grains on either side of the boundary were determined from electron channeling patterns. Low-angle boundaries are so strong they do not fracture; Instead the samples deform In a completely ductile manner. High-angle boundaries always fracture, but only after considerable plastic deformation of the two grains flanking them. Fracture is Indicated by a load drop in the load vs. displacement curves. A method involving extrapolation of the elastic portion of these curves to the displacement at fracture is used to estimate the fracture stresses. This procedure yields consistent values of the fracture strengths of high-angle boundaries. The measured stresses are large (∼2 to 3 GPa), but considerably smaller than those required for the fracture of special boundaries, as predicted by computer simulations. No correlation was found between the fracture stresses or loads and the geometry of the high-angle boundaries, many of which are close to, but deviate from, coincident site lattice orientations.

Effect of cerium on grain boundaries in Ni3Al

Effect of cerium on grain boundaries in Ni3Al

Brittle fracture and grain boundary chemistry of microalloyed NiAl

The room-temperature tensile properties, fracture mode, and grain boundary chemistry of undoped stoichiometric NiAl, as well as NiAl doped with boron, carbon, and beryllium, have been investigated, Pure, stoichiometric NiAl fractures with limited tensile ductility in a predominantly intergranular manner. Auger analyses revealed that the grain boundaries in NiAl are extremely clean and free of any segregated impurities, indicating that they are intrinsically brittle. Boron, when added to stoichiometric NiAl at a bulk level of 300 wt. ppm, segregates to the grain boundaries and suppresses intergranular fracture. However, there is no attendant improvement in tensile ductility because boron is an extremely potent solid solution strengthener in NiAl, more than doubling its yield strength. As a result, any potential benefit of improving grain boundary strength is more than offset by the increase in yield strength. Unlike boron, both carbon (300 ppm) and beryllium (500 ppm) are ineffective in suppressing intergranular fracture in NiAl, and Auger analyses of the C-doped alloy revealed that carbon did not affect the fracture mode because it did not segregate to the grain boundaries. Although neither beryllium nor carbon suppressed grain boundary fracture, their effects on the tensile ductility of NiAl were quite different: the ductility of the Be-doped alloy was higher than that of the B-doped alloy because beryllium, unlike boron, has a rather modest strengthening effect in NiAl, whereas the C-doped alloy was brittle like the B-doped alloy, because carbon is a potent solid solution strengthener, just like boron. These observations were rationalized by considering a hard-sphere model for interstitial and substitutional sites in NiAl. It was concluded that boron and carbon occupy interstitial sites, whereas beryllium dissolves substitutionally. In all the alloys that were investigated, the Ni and Al contents of the grain boundaries were not significantly different from the bulk levels, and no evidence was found for B–Ni cosegregation.

Theoretical studies of grain boundaries in Ni 3Al with boron or sulfur

It is well known that grain boundaries (GB) can have pronounced effects on the physical properties of materials (mechanical properties, corrosion resistance, fracture path, resistivity, etc.). Accordingly, a great deal of effort has been devoted to trying to understand the structure, energetics, and properties of grain boundaries. Significant experimental and theoretical progress has been made in understanding grain boundaries in pure systems, while the understanding of grain boundaries in alloy systems is much less developed. Also the mechanical properties of the grain boundaries are not well understood. In the present report, the authors summarize recent results on atomistic simulations of grain boundaries in the Ll/sub 2/ ordered alloy Ni/sub 3/Al. Understanding grain boundaries in this material is of particular importance since intergranular fracture limits the applicability of this otherwise promising material. To put these results into perspective, additional simulations were performed on grain boundaries in pure Ni and Al. Many features of grain boundaries in the ordered alloy may be understood in terms of the results on pure Ni and Al grain boundaries. The authors also consider the effect of boron, sulfur, and nickel segregation on the strength of grain boundaries in Ni and Ni/sub 3/Al.