NiAl Single Crystals Research Articles

Czochralski growth of NiAl single crystals:: effect of crucible support/insulation material

Single crystals of NiAl were grown by Czochralski pulling. The crystals were grown in the [1 0 0] direction up to 3 cm in diameter from alumina crucibles of 99.8% purity in a vacuum induction furnace under argon purging. EPMA analysis of the crystals indicated uniformity in composition. It was observed during crystal growth that an ashy material grew inward from the crucible wall to reduce the space available for crystal growth. It was found that the amount of the ashy material is affected significantly by the material supporting and/or insulating the crucible, and that it decreases in the order of graphite, zirconia and alumina. The ashy material was identified as mainly Al 4O 4C in the case of graphite, and Al 2O 3 in the cases of zirconia and alumina. The formation of the ashy material was explained.

Evolution of substructures in deformed NiAl single crystals oriented for single slip

Abstract We have examined the deformation behaviour of NiAl single crystals, oriented for single slip, at 77 and 300 K and the resulting substructures. The macroscopic yield strength decreases by a factor of two when the deformation temperature is increased from 77 to 300 K. At 77 K, the work-hardening rate after yielding is very high, and the deformation-induced substructures consist of uniformly distributed, nearly straight edge dislocations, jogged screw dislocations and prismatic loops. The substructure observed after deformation at 300 K is comprised of walls consisting of prismatic loops and zigzag screw dislocations. We have rationalized the preceding observations in terms of the non-conservative motion of jogged screw dislocations. We have also argued that the observed microstructures may limit the motion of dislocations during additional deformation, implying that the limited ductility in NiAl at low temperatures may be ‘acquired’.

Evolution of substructures in deformed NiAl single crystals oriented for single slip

Evolution of substructures in deformed NiAl single crystals oriented for single slip

Short Communication Wide gap transient liquid phase bonding of NiAl–Hf to a nickel base superalloy

Conventional transient liquid phase (TLP) bonds between the structural intermetallic compound NiAl and nickel base superalloys suffer from the formation, during bonding, of undesired stable second phases. The present paper investigates the extent to which this problem can be overcome by the use of a wide gap TLP bonding process, employing composite interlayers. The paper considers TLP bonding of single crystal NiAl–Hf to a polycrystalline superalloy Martin-Marietta (MM) 247 by means of an NiAl–Cu composite preform. The paper contrasts the relative influence of the TLP process and post-bond heat treatment on joint microstructure.

In-situstraining experiments in NiAl along soft orientations, and comparison with atomistic simulations

Abstract In-situ straining experiments have been carried out on NiAl single crystals, along soft orientations, at 143 and 300 K, in order to analyse the glide properties of (001) dislocations and the fracture process. The results show that dislocations slip either in {110} planes or in wavy slip surfaces as the result of intensive cross-slip, in agreement with atomistic calculations of the dislocation core structure along the screw orientation. Cross-slip also leads to the nucleation of high densities of prismatic loops. Dislocations exhibit kinked shapes with almost no screw portion, in agreement with results of atomistic and anisotropic elastic calculations. The local stress deduced from dislocation curvature is close to that measured in macroscopic experiments. Peierls frictional forces are low at 300 K, but higher at 144K on edge segments in planes close to {100}. The dislocations emitted at a crack tip are analysed and compared with those formed in atomistic simulations under similar conditions.

Study of the fracture behavior in soft and hard oriented NiAl single crystals by AFM

In order to understand the fracture behavior and room temperature brittleness of NiAl single crystals, atomic force microscopy (AFM) investigations were performed on in-situ loaded 4-point-bend specimens of hard and soft oriented crystals. A significant amount of dislocations are emitted from the crack tip on different slip systems. {1 0 0} slip planes are activated in soft crystals, whereas in hard crystals also slip planes, which are identified as {1 1 2} or {1 2 3} are observed. From evaluations of the AFM images, dislocation distributions ahead of crack tips and the plastic crack opening displacement (COD) from blunted crack tips are examined. The fracture toughness is calculated from the measured plastic COD and compared with the results obtained from load-displacement curves. First in-situ experiments on NiAl specimens heated up to 400 K are described, where a significant increase in plasticity is observed.

Divacancy reorientation in Al-rich B2 NiAl compounds

High-purity NiAl single crystals with different composition and different crystallographic orientation have been prepared by crucible-free inductive zone melting under Ar atmosphere. Internal friction measurements of undeformed Al-rich single crystals show a relaxation maximum with an activation enthalpy of (2.44 ± 0.05) eV and a very small pre-exponential factor of 3.3·10 −16±1 s. The relaxation strength of this maximum increases quadratically with the deviation from stoichiometry and shows an orientation dependence which is typical for atomic defects with tetragonal symmetry. In combination with the knowledge that structural defects in Al-rich NiAl are vacancies on the Ni-sublattice, these experimental results allow evidently to assign the relaxation maximum to the reorientation of divacancies on the Ni-sublattice. From the orientation dependence of the relaxation strength it is possible to determine the anisotropy of the elastic dipole tensor of a single divacancy to 0.2 ± 0.05.

Stoichiometry control and mechanical properties of NiAl single crystals

Stoichiometric NiAl crystals have been prepared by a modified Bridgman method. Chemical analysis methods have been elaborated for accurate composition analyses with a total error of 0.1 at.% Ni. The NiAl samples exhibit high room-temperature ductility in the as-grown state.

Micromechanisms of fracture in NiAl studied by in situ straining experiments in an HVEM

The room temperature brittleness of NiAl constitutes a major problem for technical applications. In order to investigate the micromechanisms of fracture in NiAl, we have carried out in situ tensile straining experiments on stoichiometric NiAl single crystals in a high-voltage electron microscope. According to our observations, crack propagation always involves dislocation activity around the crack tip, even in the hard orientation at room temperature. The Burgers vectors and the typical arrangements of the dislocations, as well as the extension of the corresponding plastic zone vary with the loading direction and the orientation of the microcrack versus potential glide systems. We observe that local concentrations of slip leads to irregular deviation of the cleavage plane from the {1 1 0} facets one usually observes at the macroscopic level. The results of our experiments help to understand why the mode I fracture toughness of NiAl is significantly larger for 〈1 0 0〉 loading directions than for non-〈1 0 0〉 directions.

Transient liquid phase bonding of a Hf bearing single crystal nickel aluminide to MM247 superalloy

An investigation of transient liquid phase bonding of single crystal NiAl to a conventional polycrystalline nickel base superalloy MM247 is presented here. The NiAl substrate material employed was cast single crystal NiAl (B2 compound, β phase) with 1·5 at.-% hafnium (Hf) as an alloying element. The interlayers employed in the bonds were 50 μm thick pure copper foils. Based on investigations employing transmission electron microscopy and scanning electron microscopy with energy dispersive X-ray spectrometry, the microstructural changes in the joint region and the adjacent substrates were examined as a function of bonding time at a temperature of 1150°C. The results indicated that hafnium as an alloying element in single crystal NiAl had a strong effect on the microstructure of joints. In particular, attention is focused on the formation of Hf bearing phases and the suppression of σ phase precipitation during bonding.

Mechanical Loss Associated with Stress Anomaly in Ni3Al and Ni3(Al, Ta) Single Crystals

AbstractThe mechanical loss and shear modulus behaviors of Ni3Al and Ni3(Al, Ta) single crystals have been investigated in the temperature range 100 K – 1300 K. The mechanical loss spectra exhibit two temperature regimes, which are separated by a relaxation peak at nearly 950 K for a frequency of 1 Hz. This relaxation peak has been interpreted by the stress re-orientation of Al-Al elastic dipoles in the (111) octahedral plane [1, 2]. In the low temperature regime, corresponding to the anomaly domain of the flow stress, the mechanical loss of pre-deformed specimens exhibit a strong positive dependence on both the oscillation amplitude and the amount of pre-strain.Pre-deformations, which were performed either at room temperature or at 100 K, yield a broad maximum in the mechanical loss that extends from nearly 100 K up to 550 K. This maximum is observable for only strain amplitudes larger than 10−4 and entirely vanishes after heating the specimens above 550 K. The increase in mechanical loss has been attributed to the bowing of the superkinks under the action of the applied stress. The gradual and irreversible decrease in damping above 300 K is interpreted in terms of pinning of the screw dislocation segments by a thermally activated process leading to the formation of Kear-Wilsdorf locks.

Application of the electron channeling contrast imaging technique to the study of dislocations associated with cracks in bulk specimens

The application of the electron channeling contrast imaging (ECCI) technique to the study of dislocation behavior in the region of crack tips in high-purity stoichiometric single crystal NiAl is presented. Because the ECCI technique, which is based on the dependence of the intensity of backscattered electrons on the incident beam orientation with respect to the crystal lattice and defects near the specimen surface, allows near-surface defects to be imaged in bulk specimens, it has specific advantages over transmission electron microscopy approaches. The technique eliminates the tedious task of thin foil preparation and avoids many of the constraints imposed by the thin foils. Dislocations associated with crack propagation were observed by examining bulk 4-point bend specimens deformed in situ in a field emission gun scanning electron microscope. Dislocations were directly imaged in the crack tip region as well as along the crack paths. Crack deflection and blunting were observed to occur primarily by plastic zone formation in the immediate region of the crack tip.

Decomposition of a〈111〉 and a〈101〉 dislocations in hard-oriented NiAl single crystals

The B2 intermetallic compound NiAl and its alloys have been subjects of extensive study for potential use as high temperature structural materials, particularly in the aerospace industry. The factors that make them attractive candidates are their low density (5.85 g/cm{sup 3}), excellent oxidation and corrosion resistance and high melting points. But they suffer from lack of ductility at low temperatures and poor strength at higher temperatures. There are still several issues regarding deformation behavior in NiAl and other B2 alloys (e.g., FeAl) that merit further research. Study of the deformation behavior in NiAl should provide further insight into some of the outstanding issues in B2 alloys: chiefly, the nature of slip transition and the effect of various parameters such as temperature, strain rate and stoichiometry on the deformation behavior. The preferred slip system in NiAl involves a slip. But on stressing along the cube axes, also known as the hard orientation, single crystals of NiAl deform at much higher stresses through the motion of a dislocations at lower temperatures. Non-a slip dominates at higher temperatures, with an associated increase in ductility and dramatic decrease in strength. There is also some confusion regarding the mode of deformation beyond the slipmore » transition temperature. The exact process of this slip transition needs to be addressed in its entirety to completely understand the mechanical response of NiAl.« less

Residual resistivity ratio and plasticity of high-purity NiAl single crystals

High-purity NiAl single crystals have been prepared by crucible-free inductive zone melting under an Ar atmosphere. The influence of an annealing treatment (1073 K for 24 h) on the residual resistivity ratio and on the plasticity was studied for these samples. For stoichiometric single crystals the residual resistivity ratio increases a factor of about two to three during the annealing treatment and the annealed specimens show a higher ductility. Both improvements can be explained by the formation of vacancy clusters during the annealing treatment as observed by transmission electron microscopy investigations.

Solute diffusion of Al-substituting elements in Ni 3Al and the diffusion mechanism of the minority component

Solute diffusion of Al-substituting elements (Ga, Ge, Ti, and Nb) has been investigated in Ni 3Al single crystals with the composition Ni 75.9Al 24.1 in a wide temperature range. The concentration profiles were determined by secondary ion mass spectrometry (SIMS). The experimental data were analyzed to establish the governing diffusion mechanism in this compound. A model for the minority component diffusion by anti-structure defects has been suggested for the L1 2 structure. It involves both nearest-neighbor jumps of the solute atoms on the Ni sublattice as anti-structure atoms and the formation of so-called anti-structure bridges which correspond to jumps between different sublattices. The model under consideration was shown to agree with the experimental data on the Al-substituting solute diffusion (X=Ga, Ge, Ti, Nb) in Ni 3Al and it allows the observed ratio of the diffusivities D Ni/ D X to be explained, which may be higher and/or lower than unity depending on the solute and temperature.

Dislocation velocities in Ni3Al single crystals

The average velocities of dislocations moving on octahedral and cube planes in Ni3Al single crystals have been measured as a function of the resolved shear stress (RSS) in the temperature range from 293 to 1133 K by the etch-pit technique. The RSS dependence of the velocities can be expressed by an equation of the form v = v0(T/T 0)m in all cases. Some of the parameters associated with the dislocation mobility are deduced from the experimental data. For the octahedral slip, the exponent m lies between 20 and 31 and the activation area A* = (81-99)b2. For cube slip, m = 12- 16 and A* = (56-84)b2. Three temperature domains have been defined where different glide systems operate. In the first domain the dislocation can move only on the octahedral plane, in the second domain it can move on both octahedral and cube planes, and in the third domain it can move only on the cube plane. It is found that the velocity of dislocations moving on the octahedral plane exhibits anomalous behaviour, consisting of a positive temperature dependence of the RSS as required for a constant dislocation velocity and a tension-compression velocity asymmetry. No obvious anomalous velocity behaviour for the cube glide system was observed. All these results suggest that positive temperature dependence of the critical resolved shear stress and the tension-compression flow asymmetry in L12Ni3Al can be directly related to the behaviour of individual dislocation velocities.

Investigation of Localized Deformation in NiAl Single Crystals

Deformation of NiAl single crystals was studied using finite element analysis to investigate the modes of localized deformation. Constitutive parameters and hardening characteristics of the active slip systems were estimated by comparing numerical simulation results with experimental results. Deformation of tensile specimens of NiAl single crystal was simulated when loaded along different crystal orientations to understand the deformation mechanism that results in various localized modes of deformation. In particular, the formation of shear bands and kink bands was studied and the material and geometric characteristics that influence the formation of such localization were investigated.

Orientation dependence of creep of Ni 3Al at intermediate temperature

The orientation dependence of creep of single crystal Ni 3Al (1 at.% Ta) has been investigated at an intermediate temperature of 520°C and at a stress of 550 MPa. Steady-state creep was observed for all orientations. The higher the resolved shear stress on the cube cross-slip plane, the larger the steady-state creep rate. Surface slip line and TEM analysis confirmed that for most orientations the dislocations glide on the primary octahedral plane(s) during the primary stage but glide on the cube cross-slip plane(s) during the secondary stage. For the [ 11 ] orientation, however, the dislocations remain on the octahedral planes.

On the primary creep of Ni3Al

The primary creep stage in single-crystal Ni3Al (1 at.% Ta) at an intermediate temperature of 520°C and at a stress of 550 MPa has been investigated. It is found that edge superdislocations in the [123] orientation dissociate on the octahedral plane while their counterparts in the [1 1 12]orientation, which is about 7° off the [001]orientation, climb-dissociate subject to the same duration of creep (7 h). It is concluded that the climb-dissociation leads to the end of primary creep and that it occurs after the edge dislocations are halted by forest interactions.

Effect of Al-rich off-stoichiometry on the yield stress of binary Ni 3Al single crystals

Stoichiometric binary single-phase Ni 3Al single crystals (Ni–25 at.%Al) and Al-rich single crystal-like Ni 3Al alloys (Ni–26 at.%Al, Ni–27 at.%Al) have been successfully produced using the floating zone method (FZ). The Al-rich crystals consisted of single crystalline Ni 3Al matrix and NiAl precipitated phase. The compressive yield stress of the as-grown crystals was measured at 293–1273 K for loading axes between [001] and [011]. All the crystals exhibited the yield stress anomaly. Compared to Al-rich crystals, stoichiometric Ni 3Al exhibited higher peak temperature of the yield stress anomaly, lower yield stress below the peak temperature, but higher yield stress above the peak temperature. These features are thought to result from the combined effect of Ni 3Al off-stoichiometry and NiAl second phase on the yield stress. The critical resolved shear stress (CRSS) on primary octahedral (111)[1̄01] slip system in the yield stress anomaly region was found to be orientation-independent for stoichiometric Ni 3Al, but strongly orientation-dependent in Al-rich crystals. Thus, the Schmid law is obeyed in stoichiometric Ni 3Al single crystals, whereas it is violated in off-stoichiometric crystals.