NiAl Samples Research Articles

Atomic and electronic structure of the NiAl(111) surface

There are two possible terminations for the ideal NiAl(111) surface, i.e., all Al or all Ni on the surface atomic layer. We investigate which termination occurs on the NiAl(111) surface by comparing the pseudopotential electronic band structure of a Ni-terminated and an Al-terminated NiAl(111) surface with the angle-resolved photoemission data on a NiAl(111) sample. The measured surface band structure shares common features partly with that of the Ni termination and also partly with that of the Al termination, which supports the theory that the real NiAl(111) surface is composed of both Ni- and Al-terminated (111) domains, as suggested by a recent low-energy electron diffraction (LEED) study. We also determine the relaxation of the two outermost atomic layers for both terminations by the pseudopotential total-energy calculations and compare them with the LEED results. We find that the present results are in good agreement with the LEED analysis for Ni termination, but in qualitative disagreement for Al termination.

Miniaturized Disk-Bend Testing of Ni3 Al: Effect of Stoichiometry and Boron Content

ABSTRACTThe results of miniaturized disk-bend tests on samples of Ni 3Al of different stoichiometry and boron content are presented. The yield strengths and ductilities of alloys containing 24, 25 or 26 %Al. either boron-free or doped with 0.3 or 0.35 %B, were measured. Specimens 3 mm in diameter and approximately 200 μm thick were tested, some of these having been cut from the grip sections of previously tested tensile bars. The yield strengths were in excellent agreement with the results obtained from the uniaxial tensile tests. The load-displacement curves for the brittle alloys (all but the boron-doped 24 %Al alloy) exhibited a maximum load corresponding to crack initiation. The shapes of the deformed specimens confirmed the assumption that they deform as if they were clamped even though they are not. The fracture surfaces of the brittle alloys are consistent with intergranular failure. Nevertheless, the ductility of the alloys increases with decreasing Al content and decreasing grain size, even for the boron-free alloys which are all brittle. The fracture stress of the boron-doped 26 %Al alloy is about 30% greater than that of the boron-free alloy. It is argued that this is most likely a consequence of the depletion of aluminum at grain boundaries, coupled with boron segregation. Independent evidence suggests that this should increase the cohesive strength of grain boundaries in the boron-doped 26 %Al alloy.

Defect structures in cold worked and small grain pure and boron-doped Ni3Al alloys

Positron lifetime spectroscopy was used to study the isochronal annealing of cold worked Ni3Al samples. In pure Ni76Al24, Ni74Al26, and boron-doped Ni74Al26 three annealing stages were observed. Boron-doped Ni76Al24 showed only two annealing stages. Vacancy annealing (stage III) was identified in all cases to start at ∼250 °C, somewhat higher than previously reported. The discrepancy is suggested to be due to carbon-vacancy interactions, because carbon (impurities) was observed to diffuse out of all samples at or above ∼350 °C. The high-temperature annealing stage in boron-doped Ni76Al24 (which is ductile) starts at 700–750 °C and is complete at 1000 °C. This stage was attributed to migration of dislocations to various sinks. In pure Ni76Al24, Ni74Al26, and boron-doped Ni74Al26 (which are brittle) the intermediate and high-temperature annealing stages occur at ∼750–800 and 1000 °C, respectively. These stages were attributed to the migration of dislocations (750–800 °C) and recrystallization (∼1000 °C) with incomplete annealing of dislocations at 1000 °C. This being the case, it is clear that dislocations interact with grain boundaries far more strongly in brittle than in ductile Ni3Al alloys. Thus, these data strongly suggest that the ductilization of Ni76Al24 by boron is largely due to a change in grain boundary structure which inhibits the pinning of dislocations at grain boundaries that occurs in pure Ni76Al24 (i.e., boron increases the susceptibility of grain boundaries to slip). Similar results for small-grain samples support this interpretation.

Grain boundary segregation of boron and sulfur and its effect on ductility in rapidly solidified Ni-base L1 2 compounds

This paper reports a study of boron segregation in Ni 3Al, Ni 3Si, Ni 3Ge, and Ni 3Ga. Sulfur segregation in Ni 3Al is also considered. The results show that boron segregates in Ni 3Al and that the amount of segregation tends to increase as the bulk concentration of boron in the alloy increases. However, because the samples had not reached equilibrium during the heat treatment, there was some scatter in this correlation. Boron also segregated in Ni 3Si and Ni 3Ge, but there appeared to be no dependence of the amount of segregation on the bulk concentration. This result is not surprising because in these alloys the boron concentration was above the solubility limit. Boron segregation also occurred in Ni 3Ga and it appeared to increase with increasing bulk concentration. However, only a small amount of data was obtained for this system. Sulfur segregated in Ni 3Al and its concentration on the grain boundaries increased with increasing bulk concentration. It did not appear to compete with boron for grain boundary sites. Aluminum also segregated in Ni 3Al, but there was a large amount of scatter in the data. The plastic strain to failure measured for the samples of Ni 3Al did not correlate with the amount of boron segregation. In particular, we could not explain the fact that boron additions enhance grain boundary cohesion more effectively in Ni-rich alloys by an increase in boron segregation in these alloys. Stoichiometric alloys and Ni-poor alloys that were very brittle had boron segregation in equivalent amounts to that found in ductile Ni-rich alloys.

Preliminary investigation of inertia friction welding B2 aluminides

An attempt is made to achieve inertia friction-welding in FeAl and NiAl samples, taking into account their intermetallics' compositions, extrusion parameters, and microstructural data. The energy required for the weld is stored in a rotating flywheel mass attached to one of the two pieces to be joined; when enough energy is introduced, the flywheel is disconnected and an axial load is applied which forces the spinning piece against the stationary one, converting the energy into heat by means of friction. Due to the inherent brittleness of the aluminides, a step-load program was used in which an initial, low-pressure heat buildup increased the work pieces' ductility.

Low-temperature heat capacities of Ni3Al

Heat capacities of two single-crystal Ni3Al samples have been measured between 2 and 14 K. The data can be well fitted to the sum of a lattice and an electronic term, without a magnetic cluster contribution as previously reported by de Dood and de Chatel. The resulting Debye temperature values are comparable to those derived from elastic constant measurements, but the calorimetrically determined density of states at the Fermi level differs appreciably from that based on band-structure calculations.

The de Haas-van Alphen effect, exchange splitting and Curie temperature in the weak itinerant ferromagnetic Ni3Al

High-sensitivity de Haas-van Alphen (DHVA) measurements are reported for pure samples of Ni3Al in the temperature and magnetic field ranges 0.35<or=T<or=8K and 10<or=H<or=100 kG, respectively. Complex DHVA spectra consisting of up to 20 or more fundamental, harmonic and magnetic breakdown components are interpreted starting with two light and one very heavy sheet of the nonmagnetic Fermi surface. The complex and unusual character of the DHVA spectra associated with the light sheets is shown to arise from the spin-orbit and exchange interactions which are found to yield comparable splittings of the quasidegenerate bands. The ratio of the observed and band-calculated cyclotron masses are close to 2 for the light sheets and as high as 3.4 for the heavy sheet. In addition the ratio of the experimental Curie temperature to that calculated in mean-field band theory in terms of the best available band model is found to be as low as approximately 1/10. The large mass enhancement and strong suppression of Tc from the mean-field value suggest that enhanced magnetic fluctuations are important in the ground state and thermal properties. A simple description of the effect of both transverse and longitudinal magnetic fluctuations of long wavelength is presented which can account for the observed value of Tc in this material.

Effect of impurities (graphite) on the high-temperature creep properties of NiAl

Previous high-temperature compression creep studies of NiAl have shown peculiar behaviour in the temperature range 700 to 900° C, which was perhaps due to precipitation of impurities in the matrix. To isolate the “impurity” effect, high-purity NiAl samples with 0.15 at% and 0.20 at% carbon (graphite) additions have been creep tested at four temperatures between 700 and 850° C. Addition of graphite has been shown to produce a significant reduction in the creep strength of the alloyS. However, alloys with higher graphite concentrations have shown better creep resistance than those with lower graphite concentrations. Transmission electron microscopy indicates the presence of competing softening and hardening mechanisms in the alloys. Softening is due to the graphite particles acting as a dislocation source. Hardening results from a grain-boundary hardening mechanism due to the graphite particles segregating at grain boundaries and a dislocation-impurity (fine graphite) interaction, developing a Cottrell-like atmosphere.

The intrinsic low temperature magnetic and transport properties of NiAl

The magnetic susceptibility, resistivity, magnetoresistance, and NMR properties of very pure stoichiometric NiAl samples have been measured. The Kondo-like phenomena previously observed in this material are absent suggesting that the intrinsic properties agree with a simple Fermi surface as predicted by APW calculations.

Recovery Processes and Ordering in Ni3Al

Isothermal changes in long-range order S, x-ray line shifts due to stacking faults Δ, and x-ray particle size from Fourier analysis of line shapes D were measured for filed samples of Ni3Al. Recovery temperatures between 200° and 270°C were investigated. The recovery of all three measured properties appeared to depend upon the mobility of vacancies in this temperature range. The evaporation of vacancies from faulted loops appeared to supply the vacancy flux for ordering and particle-size changes. Fault shrinkage was found to cease quite abruptly between 215° and 200°C; at 200°C faulting initially increased, during which ordering did not occur, but after some 20 h the faulting began to anneal out and somewhat later ordering proceeded. An activation energy of 1.65 eV was found for the ordering reaction; this compares with a value of 1.5 eV for Emv in nickel.