NiAl Single Crystals Research Articles

Texture Memory Effect in Heavily Cold-Rolled Ni<sub>3</sub>Al Single Crystals

The paper presented the texture evolution during primary recrystallization and following grain growth in the heavily cold-rolled Ni3Al single crystals. It turned out that the texture evolution occurred in the two stages. First, primary recrystallization caused the drastic change of the as-rolled texture. Then, as grain growth proceeded, the texture returned to the same one as the as-rolled textures. This texture return can be designated as Texture memory effect. The mechanism of the texture memory effect was discussed based on the analysis of the orientation relationship between the as-rolled and the primary recrystallization textures.

Texture memory effect in heavily cold-rolled Ni 3Al single crystals

In Ni 3Al the cold-rolled Goss texture changed to a complicated one after primary recrystallization and returned to the original Goss during the subsequent grain growth, which can be referred to as the texture memory effect. In this study, we examined the evolution of grain orientations during the grain growth using the electron backscatter diffraction (EBSD) method. It was found that just after the primary recrystallization most of the grains had a 40°〈1 1 1〉 rotation relationship to the Goss texture, the remaining grains being Goss and other textures. The formation of the 40°〈1 1 1〉 rotated grains can be explained by a multiple twinning mechanism. In the grain growth, the Goss grains, which were surrounded by the 40°〈1 1 1〉 rotated grains, grew preferentially due to the high mobility of the 40°〈1 1 1〉 grain boundaries, leading to the texture memory effect.

Acoustical properties of Ni3Al single crystals alloyed with cobalt and niobium

High-temperature nickel-base alloys (superalloys) are widely adopted in industry as materials for structural elements of devices operating upon high temperatures (aerospace turbine blades, etc.). Successful application of these alloys is based on their high mechanical properties. Today, the range of service conditions expands toward progressively higher temperatures. Thereupon, the investigation of mechanical and thermal properties of nickel high-temperature alloys appears to be a topical question.

The effect of Pt on Ni 3Al surface oxidation at low-pressures

The fully-oxidized surface that forms on (1 1 1) oriented Ni 3Al single crystals, with and without Pt addition, at 300–900 K under oxygen pressures of ca. 10 −7 Torr was studied using XPS, AES, and LEIS. Two main types of surfaces form, depending upon oxidation temperature. At low-temperature, the predominant oxide is NiO, capped by a thin layer of aluminum oxide, which we refer to generically as Al x O y . At high-temperature (i.e., 700–800 K), NiO is replaced by a thick layer of Al x O y . By comparing samples that contain 0, 10 and 20 at.% Pt in the bulk, we find that the effect of Pt is to: (1) reduce the maximum amount of both NiO and Al x O y ; and (2) shift the establishment of the thick Al x O y layer to lower temperatures. Platinum also decreases the adsorption probability of oxygen on the clean surface.

Cross-sectional TEM studies of plastic wave attenuation in shock loaded NiAl

The role of plasticity and overall shear strength of metals subjected to weak and moderate shock waves is not well understood. The plastic response of shocked metals is known to vary with applied pressure although these variations are different for each material. Weakly shocked single crystals will tend to display plastic anisotropy and anisotropic Hugoniots at least up to the overdrive pressure. In the current study, cross-section transmission electron microscopy (XTEM) has been used to elucidate these effects in single crystals of nickel aluminide (NiAl). Single crystals of NiAl (5 mm diameter) were cut and polished down to 100–300 μm along three loading directions 〈0 0 1〉, 〈1 1 0〉, 〈1 1 1〉. Each of the three samples was subjected to laser induced shocks of around 15 GPa. Cross-sectional transmission electron microscopy (XTEM) samples were extracted from shocked samples and were used to observe dislocation structures and lattice rotations with selected area diffraction patterns. Regions within 2 μm of the drive surface exhibited lattice rotation gradients up to about 1400°/mm in 〈1 1 0〉 samples. Following the first few microns, the lattice rotations were found to decay rapidly over the next 5 μm. These trends were also seen in orientation imaging microscopy (OIM) data. The lattice rotation gradients correlated well with pressure decay from 1D hydrodynamic calculations.

The high-temperature damping background in intermetallic alloys

The high-temperature damping background (HTDB) in various intermetallic compounds, such as γ-TiAl based alloys, NiAl single crystals and quasicrystalline d-Ni–Al–Co exhibits viscoelastic behavior which can be observed as damping, increasing exponentially with temperature. The activation enthalpy H was determined from measurements of the HTDB at various frequencies. In γ-TiAl based alloys with various chemical composition the activation enthalpies are in the range of H = 4.2–4.3 eV for Ti–46 at.%Al–9 at.%Nb, and H = 3.8–3.9 eV for Ti–46.5 at.%Al–4 at.% (Cr, Nb, Ta, B). For a Ni 49.5Al 50.5 single crystal an activation enthalpy of H = 3.3 eV was determined. The H-values in TiAl and NiAl agree well with activation enthalpies from creep and self-diffusion experiments. Therefore, the HTDB in TiAl and NiAl is assigned to diffusion-assisted climb of dislocations. Decagonal Al–Ni–Co quasicrystals show different behavior. The HTDB is only observed in polycrystalline material, but not in a single crystal. The activation enthalpies, H = 2.4–3 eV, agree with values obtained by tracer diffusion experiments. This indicates that the HTDB in decagonal Al–Ni–Co quasicrystals is due to an intragrain diffusion process which is equivalent to Nabarro–Herring creep.

Study of anomalous hardening in NiAL single crystals at intermediate temperatures by FEA

The hardening model based on the dislocation mechanics is employed to study the experimentally observed high tensile elongations of NiAl along the [1 1 0] orientation at intermediate temperatures. In the hardening model proposed, a mobility of dislocation is assumed to be restricted to glide through the slip plane by forest dislocation and thermally activated cross-slip event. Overall deformation behavior of NiAl was greatly influenced by temperature-dependent dislocation mobility that both experimental and simulated yield stresses decreased as temperature increased. The results of simulation showed anomalous hardening behaviors analogous to those of experiment at certain circumstances. This behavior occurred due to the hardening contributions generated by cross-slip events that disable the dislocation motion in the primary slip systems. By comparing simulation results with experiments, it is confirmed that the proposed hardening model can represent anomalous tensile elongations due to the hardening by forest dislocations and cross-slip events.

Sputtering of clusters from nickel–aluminium

A NiAl(1 1 1) single crystal was bombarded with 15 keV Ar +, and the resulting secondary neutrals were analysed by laser postionisation secondary neutral mass spectrometry. By measuring the individual cluster photoion intensity as a function of laser power, the sputter yields of 33 individual clusters were determined. The yield of Al n clusters sputtered from NiAl falls with increasing cluster nuclearity as n −8.7 while Ni n and Al m− n Ni n yields are proportional to n −5.9 and n −5.2, respectively. The distribution of thee yields of mixed Al m− n Ni n clusters with n and m is found to diverge significantly from the expected distribution based on a random combinatorial approach, indicating that the energetics due to the chemical bonding in the clusters plays a significant role during cluster formation in the sputtering process.

The effect of surface orientation on oxidation-induced growth strains in single crystal NiAl: An in situ synchrotron study

Abstract The evolution of oxidation-induced growth stresses in single crystal stoichiometric NiAl has been studied; it is specimen dependent and also depends on crystallographic orientation. Epitaxial issues at the NiAl/Al2O3 interface appear to govern initial formation of γ- or θ-Al2O3 and its subsequent transformation to the stable and denser α-Al2O3 polymorph. This transformation generates tensile stresses in α-Al2O3, which relax with further oxidation. Room temperature residual stresses are large, ∼3.3 GPa, and compressive. Confocal photo-stimulated microspectroscopy studies of these residual compressive stresses have revealed large spatial variabilities, which are also attributed to epitaxial issues at the NiAl/Al2O3 interfaces.

Cyclic plasticity of soft oriented NiAl single crystals at 600 K

Cylindrical specimens of stoichiometric NiAl single crystals with axial orientations near [ 2 ¯ 2 3 ] in the stereographic unit triangle were cyclically loaded at 600 K, imposing low plastic strain amplitudes. A main aim was to study the slip phenomena, involving type and persistence of strain localisation, as well as the dislocation patterns occurring after cycling at ɛ pa = 2.5 × 10 −4. Single slip is dominant in the [0 0 1]( 1 ¯ 1 0 ) system on a macroscopic level. Broad cumulative slip bands appear after a sufficiently large number of cycles. Within these bands the strain is considerably concentrated in individual zones of localised slip, although the average strain carried by all of these slip zones is rather small. During a quarter cycle, the slip is highly localised in a small number of thin slip bands. Atomic force microscope (AFM) measurements show that there exists a broad spectrum of local strain amplitudes (LSAs) reaching up to 100%. The mean strain of the LSAs is more than a factor of 150 larger than the average strain of the cumulative zones of localised slip. The cumulative slip bands are not cyclically persistent. The dislocation patterns in a zone of localised slip (ZLS) characteristically differ from those formed in the surrounding matrix. Multiple slip has to be taken into account on a micrometer-scale in order to understand the ZLS dislocation substructure.

Ab-Initio Lattice Instability Analysis on Ni and Ni3Al Single Crystals

The purpose of the present study is to elucidate the “ideal strength” of the Ni and Ni3Al single crystals, the main compositions of Ni-based superalloy, from the viewpoint of the lattice stability. The unit lattices of Ni and Ni3Al, fcc and L12 ordered alloy, are subjected to the [001] uniaxial tension/compression and hydrostatic tension/compression by using the Vienna Ab-initio Simulation Package (VASP) with the generalized gradient approximation (GGA) and ultrasoft pseudopotential. The elastic stiffness matrix is numerically evaluated at each point in the applied deformation pass, then the lattice stability is discussed based on the positiveness of the matrix. Both Ni and Ni3Al reach the Born’s stability criteria against the bifurcation to the anisotropic Poisson’s contraction in the [001] uniaxial tension, while they do the spinodal criteria against the structural transformation in the [001] uniaxial compression and hydrostatic tension. The hydrostatic compression increases the stability and shows no limit, however, it is also suggested that the spinodal instability appears when the ideal isotropy was broken. The “ideal strength” is evaluated with these stability limits and indicated as “yield curve” on the normal strain-lateral strain or normal stress-lateral stress planes.

Anisotropy in Cold Rolling of Single Crystalline Ni<sub>3</sub>-Al-Base Intermetallic Alloys

Texture and microstructure evolution during cold rolling of binary Ni3Al single crystals were studied as a function of the initial crystal orientations. Cold rolling behavior of the single crystals is strongly dependent on the initial crystal orientations, especially on the initial rolling direction (RD). Macroscopic texture evolutions for binary Ni3Al single crystals with the initial RD close to <001> are characterized as development of dual {110}<¯112> texture above 90% cold reduction and the banded structure formation. Microstructure observations reveal the extensive formation and accumulation of the planar defects including the stacking faults and the planes where the localized shear deformations occurs, which must be responsible for the observed anisotropic cold rolling behavior of Ni3Al single crystals.

A study of deformation and orientation dependent behavior in single crystals

Deformations of single crystals were studied using finite element analysis to investigate the localized modes and the orientation dependence of plastic deformation observed in single crystals Investigation of mechanical properties of single crystals is closely related with the understanding of deformation processes in single crystals Localized bands such as shear and kink were studied and the material and geometric characteristics that influence the formation of such localized bands were investigated. Orientation dependence of material behavior in NiAl single crystals was studied by rotating slip directions from ‘hard’ orientation The maximum nominal compressed stress in NiAl single ciystals was widely ranged depending on the misalignment from “hard” orientation As the compression axis was set closer to “hard” orientation, the maximum nominal compressed stress was rapidly increased and made slips difficult to activate Therefore, non- slips will be activated instead of slips for “hard” orientation

Transformation of Co–Ni–Al single crystals in tension

We present for the first time pseudoelasticity and shape memory results on Co–Ni–Al single crystals in tension. The experimental transformation strains approached 6% in tension in the [1 1 5] orientation. Despite the considerable volume change (0.0447), the transformation exhibits remarkable pseudoelasticity and shape memory recoverability in tension. A theoretical treatment is presented to explain the tension compression asymmetry in the stress–strain response, the change in transformation hysteresis with stress, and the role of secondary phase on the transformation temperatures.

Pyroelectric heat detector for measuring adsorption energies on thicker single crystals

The direct measurement of adsorption heats on the well-defined and highly ordered surfaces of single crystals (i.e., single crystal adsorption calorimetry, SCAC) has until now been performed only on samples with thickness of 0.2 or 1–8 μm, depending on the heat detection method. This paper reports a modification to a β-polyvinylidene fluoride (PVDF) pyroelectric heat detector, which allows measurements on much thicker samples (∼80 μm) with similar signal-to-noise. The improvement is achieved by increasing the compression between the single crystal and the 9 μm thick PVDF detector ribbon by pressing it with a thicker, insulating polymer, and by using a similarly-mounted reference detector to subtract away some of the voltage noise due to mechanical vibrations (also detected by the piezoelectric PVDF ribbons). For a 75 μm NiAl(1 1 0) single crystal, a sensitivity of 5 V per adsorbed joule and a pulse-to-pulse standard deviation of 4% in the measured heats are seen for adsorbed laser pulses of 3.7 μJ over a 0.138 cm 2 area. Application of this method to measure the heats of adsorption of Pb onto NiAl(1 1 0) is demonstrated. The pulse-to-pulse standard deviation of the heat of adsorption at high coverages, where the heat is independent of coverage, is about ±5.2%, or 10.2 kJ/mol. This broadens the range of surfaces that can be studied by calorimetry, since this thickness can be achieved with nearly any single crystal material by simple mechanical thinning, whereas that is not possible with samples thinner than ∼50 μm over the required surface area of ∼1 cm 2.

Microstructure and Texture Evolution during Cold Rolling and Recrystallization of Ni<sub>3</sub>Al Single Crystals

We have studied the texture and microstructure evolution during cold rolling of Ni3Al single crystals as a function of the initial crystal orientations and revealed that the cold rolling behavior of the single crystals are strongly dependent on the initial crystal orientations, especially on the initial rolling direction (RD). An optimum condition for thin foil fabrication is determined that the initial RD is close to <001>. According to the conditions we have successfully fabricated the wide and thin foils of binary Ni3Al by cold rolling the single crystalline ingots. The thinnest foils obtained so far are about 20µm in thickness and 50mm in width. This document reviews the current status of our research on the thin foils of intermetallic compound Ni3Al.

Microstructures and mechanical properties of a directionally solidified NiAl–Mo eutectic alloy

A NiAl–Mo ternary eutectic alloy, having the nominal composition Ni–45.5Al–9Mo (at.%), was directionally solidified in a high-temperature optical floating zone furnace. Well-aligned rod-like microstructures were obtained, consisting of NiAl matrix and 14% (by volume) continuous Mo fibers having a square cross-section. With increasing growth rate (from 20 to 80 mm/h), the spacing and size of the Mo fibers decreased, from ≈2 to 1 μm (fiber spacing) and 800 to 400 nm (fiber size). X-ray microprobe analyses revealed that the NiAl matrix contained essentially no Mo (<0.1 at.%) and had the composition Ni–45.2Al (at.%), while the composition of the Mo fibers was Mo–10.1Al–3.9Ni (at.%). From electron backscatter diffraction patterns, the following orientation relationships were obtained: 〈1 0 0〉 NiAl//〈1 0 0〉 Mo, and {0 1 1} NiAl//{0 1 1} Mo. The growth directions and Mo/NiAl interface boundaries were found to be parallel to 〈1 0 0〉 and {0 1 1}, respectively, in both Mo and NiAl. Nanoindentation was used to probe the mechanical properties of the individual phases in the eutectic microstructure and the modulus and hardness of the NiAl matrix were determined to be 180 and 2.9 GPa, respectively, and those of the Mo fibers 270 and 4.8 GPa. Tensile tests were used to investigate the temperature dependence of the strength and ductility of the composite. Its ductile-to-brittle transition temperature was found to be ∼675 °C, and its yield strength about 25–30% higher than that of 〈1 0 0〉 NiAl single crystals over the temperature range 600–1000 °C.

Cyclic Oxidation Kinetics Modeling of NiAl Single Crystal

For alumina forming nickel-based alloys, sulfur content is a key element as it modifies the onset and extent of spalling of the oxide scale. To get a better understanding of that effect, isothermal and cyclic oxidation of two NiAl single crystals (with two S contents) have been carried out at 1050 and 1150°C on (100) and (110) orientated surfaces. Continuous thermogravimetry in cyclic conditions allows to follow isothermal oxidation kinetics and spalling at each cycle. Oxidation kinetics are compared between the two compositions and the two crystallographic orientations. Low sulfur NiAl oxidizes with slightly lower kinetics for both (100) and (110) surface orientations, and, in both isothermal as well as in cyclic conditions at 1150°C in O2 . Experimental results of cyclic oxidation are compared to those obtained with simulations using a previously published simple statistical model and using a more complex Monte-Carlo code. Spalling is found to be an increasing function of the average oxide thickness and seems to depend on the square of the average oxide thickness.

High-temperature oxidation kinetics of NiAl single crystal and oxide spallation as a function of crystallographic orientation

Isothermal and cyclic high-temperature oxidation of NiAl single crystal samples are presented. Oxidations have been carried out at 900, 1050, 1100 and 1150 ◦ C on (1 0 0) and (1 1 0) oriented surface. Continuous thermogravimetry in cyclic conditions allows isothermal oxidation kinetics and spalling at each cycle to be followed. Oxidation kinetics are compared between (1 0 0) surface and (1 1 0) surface. (1 0 0) oriented surfaces appeared to oxidize slightly faster than (1 1 0) oriented surfaces. Experimental results of cyclic oxidation are compared to simulated results using a previously published statistical model. Spalling increases when the average oxide scale thickness increases with the number of cycles. Longer tests are necessary to study this evolution during the ’steady-state’ but no critical oxide thickness was found.

Substructure and texture formation during extrusion of NiAl single crystals

and oriented NiAl single crystals have been extruded at 1000 °C through a round die. The crystals subdivide into two types of deformation bands producing fibre texture components parallel and normal to the extrusion axis. These rotations as well as shape changes under partially constrained deformation conditions yield informations on the slip system activity. The relevance of deformation banding on texture formation in polycrystals is discussed.