Precipitation of Ni3Al in a nickel rich NiAl

A study has been made of precipitation processes occurring during ageing of NiAl martensite. The precipitation mechanism was found to be dependent on the ageing temperature, two mechanisms being observed: below 700° C (the G.P. zone solvus temperature), coherent nucleation and growth of a metastable form of Ni₃Al on the microtwins inside, and interfaces between, the martensite plates was observed. In the former case, the growth of precipitates in martensite involved particle-twin interactions, which could take two forms: either a particle pushed forward the twin interface, which could bulge forward sufficiently to cut up the twins, or a growing particle developed growth twins. A model is proposed to account for these observations. In the latter case the precipitates on the martensite interfaces coarsen preferentially, leading to the formation of an equilibrium precipitate, which has an irregular rod shaped morphology.     

Hydrogen-Induced Cracking of Two-Phase Alloy of Ni3Al＋NiAl

Hydrogen-induced cracking (HIC) of two-phase alloy of Ni_3Al+NiAl was in situ investigated using a WOL (wedgeopening-loading) constant deflection specimen in an optical microscopy and using a notched tensile specimen in scan-ning electron microscopy (SEM). Results showed that the threshold stress intensity of HIC was K_(IH)=15.7 MPa·m~(1/2),and (da/dt)Ⅱ=0.019 mm/h. For the uncharged specimen, microcrack initiated and propagated preferentially withinthe NiAl phase, resulting in cleavage fracture, but for the precharged specimen with hydrogen concentration of24.7×10~(-4)%, hydrogen-induced crack initiated and propagated preferentially along the Ni_3Al/NiAl interfaces, result-ing in interphase fracture.     

Simultaneous synthesis and densification of NiAl and Ni3Al by pressure-assisted combustion

Simultaneous synthesis and densification of NiAl and Ni₃Al from elemental powders of Ni and Al was investigated. Combustion synthesis was carried out under the uniaxial compressive pressure and preheating of die. It was shown that both the compressive pressure and preheating of die strongly affect the density, the grain size and the hardness of NiAl and Ni₃Al product. A high relative density of NiAl and Ni₃Al products as much as 99.8% was achieved under compressive pressure of 275 MPa and preheating temperature of 600°C.     

Selective leaching of NiAl3 and Ni2Al3 intermetallics to form Raney nickels

Annealed single-phase NiAl₃ and Ni₂Al₃ materials were leached with 20wt% aqueous NaOH solution to remove the aluminium. At temperatures of 274 to 323 K, NiAl₃ leached according to linear kinetics, yielding porous nickel which was friable and disintegrated. At these temperatures Ni₂Al₃ was unreactive, but at 343 to 380 K it leached according to parabolic kinetics, producing a strong, tightly adherent rim of residual material. The Ni₂Al₃ reaction proceeded in two steps, firstly to produce a two-phase mixture of Ni₂Al₃ plus nickel, and secondly to produce nickel alone. In both stages the detailed microstructure of the prior alloy was preserved, implying that the mechanism is selective dissolution. The surface adsorption properties of the nickel residues were obscured by reprecipitated aluminA. However, metal crystallite size measurements showed that a large nickel surface area was potentially available.     

The growth of single crystals of the intermediate phases NiAl and Ni3Al

Single crystals of stoichiometric NiAl, nickel-rich Ni₃Al, and aluminium-rich Ni₃(Al, Ti) have been grown by a modified Bridgman technique; a strain-anneal method has also been used to produce small, single crystals of non-stoichiometric NiAl.     

A study on the nickel-rich ternary Ti–Ni–Al shape memory alloys

The transformation sequence and hardening effects of 400 °C aged Ti47.5Ni50.65Al1.85 and Ti49.5Ni50.13Al0.37 shape memory alloys have been investigated by electrical resistivity tests, internal friction measurements, hardness tests and TEM observations. Both solution hardening and precipitation hardening are found to occur in these alloys. The hardening effects of Ti47.5Ni50.65Al1.85 alloy are obvious and much higher than those of Ti49.5Ni50.13Al0.37 alloy due to the former having the larger Ni/Ti ratio and a higher Al solute content in its matrix. The transformation sequence of 400 °C aged Ti47.5Ni50.65Al1.85 alloy shows B2↔R-phase only for an ageing time of more than 10 h and that of 400°C aged Ti49.5Ni50.13Al0.37 alloy shows the sequence B2↔R-phase↔B19′ or B2↔R-phase with different ageing times. All of these characteristics are associated with Ti11Ni14 precipitates during the ageing process. These aged Ti–Ni–Al alloys exhibit very good shape memory effects, in which the maximal shape recovery occurs at the peak of hardness. This revised version was published online in November 2006 with corrections to the Cover Date.     

Study on behavior of NiAl coating with different Ni/Al ratios

β-NiAl coatings with different Ni/Al ratios were deposited on K403 superalloy substrates via magnetron sputtering. The phase transformation and diffusion phenomenon of the NiAl/Ni-based superalloy system after vacuum annealing at 900 and 1000 °C were analyzed using X-Ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and energy dispersive X-ray spectrometry (EDS). The effect of coating concentrations on the outward diffusion behavior of substrate elements was discussed. The high Cr concentrations in the Al-rich NiAl coatings were caused by the intense interdiffusion between Al and Cr. The Ti, W and Mo partitioned to γ′-Ni₃Al in the coatings. Several possible reasons for the formation of γ′-Ni₃Al at the surface of Ni-rich NiAl coating were identified, including: diffusion behavior of W and Mo in β-NiAl, destabilizing effect of substrate elements on β-NiAl, and diffusion rates of Ni and Al in β-NiAl. The volume change in β ⇛ γ′ transformation process shows Ni uphill diffused to the γ′-Ni₃Al islands at the surface of Ni-rich NiAl coatings. The IDZ (interdiffusion zone) thickness and precipitates in IDZ were related to the Al initial concentrations in the coatings.     

Metastable phase accompanying crystallisation of amorphous Al85Ni10Ce5 alloy

Abstract

Isochronal annealing and isothermal annealing of melt spun amorphous Al₈₅Ni₁₀Ce₅ alloy were carried out to investigate the crystallisation behaviour. It has been found that a metastable phase occurring in the crystallisation process, may exist for a certain time but in due course disappears either acting as a nucleus of another phase or transforming into another stable phase with an increase in either annealing temperature or annealing time. Transmission electron microscopy results reveal that Al₃Ce may be one of the metastable phases during the isochronal annealing process. Furthermore, microhardness measurements reveal that the specimens containing metastable phases exhibit higher microhardness than the as quenched and as crystallised alloy.     

Nanostructured Ni3Al layers and Ni3Al/Ni multilayers obtained by magnetron sputtering

Intermetallic Ni₃Al thin layers and Ni₃Al/Ni multilayers were deposited on a Si wafer by means of magnetron sputtering. The structure and morphology of the layers have been characterized by X-ray diffraction, transmission electron microscopy and atomic force microscopy. The polycrystalline films are textured in the (111) direction and have grain sizes below 20 nm. Superlattice reflections due to chemical order have been observed in the electron microscope. It is shown by x-ray diffraction that the multilayers grow coherently on the amorphous substrate.     

Effects of refined microstructures on ductility and toughness at room temperature in nickel-rich NiAl

Melt-spun Ni-34.6 at% Al ribbons with a minimum average grain size of 2.2 m were prepared and the microstructures at room temperature were investigated. Refining the sizes of grains and other substructures resulted in interfaces of different types with very high density and uniform distribution. The reaction between the interfaces and propagating microcracks led to plastic deformation ahead of the crack tips and in larger regions with very fine grains, about 0.2 m. Deflection, twisting and branching of the microcracks also resulted. The results indicate that the ductility and toughness of the nickel-rich NiAl all were considerably improved, which is consistent with previous test results. A simplified model based on the experimental results was used to analyse the mechanism of toughening NiAl. The important role of the interfaces in improving ductility and toughness of NiAl at room temperature is discussed.     

Precipitation behavior of Heusler phase (Ni2AlHf) in multiphase NiAl alloy

Precipitation behavior of Heusler phase (Ni₂AlHf) in a directionally solidified (DS) NiAl- 28Cr-5Mo-1Hf (at.%) alloy was examined using scanning electron microscope (SEM) and transmission electron microscope (TEM). In the as-cast alloy, the Ni₂AlHf phase generally appeared on the NiAl/Cr(Mo) interface, which degraded the NiAl/Cr(Mo) eutectic structure. In the heat-treated alloy, the density of the intercellular Ni₂AlHf phase was slightly reduced. In addition, the spherical Ni₂AlHf phase precipitated heterogeneously in the NiAl matrix, but the Ni₂AlHf phase did not precipitate in the lamellar Cr(Mo) phase. The precipitation behavior of the Ni₂AlHf phase could be explained in terms of the interfacial energy. A lattice model was also proposed to explain the NiAl↔Ni₂AlHf phase transformation.     

Zirconium-Induced Softening in Hyperstoichiometric Ni3Al

The room temperature compressive properties and microhardness of Ni3AI alloys doped with Zr were studied. For the hypostoichiometric Ni3AI alloys, the compressive strength and microhardness increased with an increase in Zr content, while softening behavior induced by doping with a certain amount of Zr was observed in hyperstoichiometric Ni3AI alloy. Possible mechanisms for the softening effect were suggested.     

Alloy softening in Ni3Al polycrystals

The alloying effect of boron on localized deformation in Ni₃Al polycrystals containing 24–26 at% AI was studied using microhardness indentation. Alloy softening was observed both along the grain boundaries and in the grain interior. The softening effect decreased as the aluminium concentration increased. For alloys of near-stoichiometric composition, the maximum effect occurred at about 0.23 at% (500 wt p.p.m.) boron. A softening mechanism based on cross slip of screw dislocations was proposed.