Effect of ageing on the microstructure and mechanical behaviour of a directionally solidified Ni3Al-based alloy

The effect of ageing in the temperature range from 1023 to 1373 K on the micro-structure and mechanical behaviour of a directionally solidified (DS) Ni₃Al-based alloy modified with additions of chromium and iron was investigated. The microstructure of the as-grown alloy consisted of well-aligned and equally spaced lamellas composed of β(B2) intermetallic compound NiAl (Cr, Fe), some β′(L1₀) martensite and spherical -Cr precipitates. The matrix consisted of γ′(L1₂) intermetallic compound Ni₃Al (Cr, Fe), γ-phase (Ni-based solid solution) and lath-shaped -Cr precipitates. Ageing at 1123 and at 1173 K was found to be the most effective in transforming the unstable lamellae to γ′-phase and -Cr precipitates. The change of microstructural characteristics such as volume fraction of lamellae, size, morphology and distribution of γ′-phase, γ-phase and -Cr precipitates significantly influenced the room-temperature yield strength and elongation of DS alloy after ageing. The strain-hardening exponent varied with the ageing temperature between 0.30 and 0.46 and the quasi-steady work-hardening rate between 2710 and 5340 MPa. In the specimens with the lowest amount of disordered regions, the strain-hardening exponent was found to be 0.46 and the quasisteady work hardening rate was determined to be 3340 MPa.     

Evaluation of high temperature corrosion resistance of a Ni3Al (Mo) alloy

The sulfidation/oxidation and carburization resistances of a Ni₃Al(Mo) (IC-6) alloy at high temperatures were investigated in this work. The corrosion kinetics of the IC-6 alloy was found to follow parabolic rate law in an environment of high partial pressures of sulfur (10⁻⁵ atm) and low partial pressures of oxygen (<10⁻²⁰ atm) at 700 °C. Because the Ni sulfides are readily formed at the testing temperature, the sulfidation/oxidation resistance of the IC-6 alloy is similar to that of commercial Ni–Cr alloys in the current environments, although IC-6 is alloyed with Al. Compared with the HP heat resistant steel which is commonly used in the petrochemical industry, the IC-6 alloy possesses significantly improved resistance to carburization at 1100 °C. The mechanisms governing the corrosion attack in the environments used in this investigation were also discussed.     

Two-phase intermetallic alloy Ni3(Al, Si):microstructure and mechanical properties

Yield stress in compression (0.2% flow stress) from ambient temperature up to 800 °C has been studied on Ni₃(Al, Si) alloy with the atomic composition Ni₇₈Al₁₁Si₁₁. When annealed at 1000 °C, the alloy has a pure L1₂ (γ′) ordered structure. After subsequent annealing at 750 °C, the disordered solid solution of Al and Si in Ni (face centred cubic, γ) precipitates in fine coherent particles. Calorimetry helps to describe the various phase transformations necessary to obtain the last microstucture. Solute addition of Si, which replaces Al atoms, increases the 0.2% flow stress of Ni₃Al in the fully γ′ microstructure. The γ precipitation shifts the peak stress towards higher temperatures and stresses.     

Shear-induced chemical disordering in Ni3Al at different temperatures

Molecular dynamics simulations have been used to study shear-induced chemical disordering in Ni₃Al lattices at different temperatures and strain rates. Shearing determines the formation of an amorphous layer, the thickness of which increases linearly with the square root of time. The rate at which the amorphous layer grows is both shearing rate- and temperature-dependent. A linear correlation between the amorphous layer growth rate and the shear modulus is found. This suggests that mechanical properties could play a central role in shear-induced disordering processes.     

The oxidation of nanocrystalline Ni3Al fabricated by mechanical alloying and spark plasma sintering

Nanocrystalline Ni₃Al was fabricated through mechanical alloying of elemental powders and spark plasma sintering. The nanocrystalline Ni₃Al has a nearly full density after being sintered at 1223 K for 10 min under a pressure of 65 MPa. Isothermal and cyclic oxidations of nanocrystalline Ni₃Al were tested at 1173–1373 K with intervals of 100 K. The results indicate that nanocrystalline Ni₃Al exhibits excellent isothermal and cyclical oxidation resistance. The oxide scales consist primarily of dense and continuous -Al₂O₃. The grain refinement is beneficial for improving the oxidation resistance of Ni₃Al by providing more nucleation centers for the Al₂O₃ formation, promoting the selective formation of Al₂O₃ and improving the adhesion of oxide scales to the matrix.     

On the peritectoid formation of Ni5Al3

The peritectoid formation of Ni₅Al₃ from the two phases NiAl and Ni₃Al was studied in a Ni---Al alloy containing 66 at% Ni by means of transmission electron microscopy. The product phase does not form as a uniform layer between the two initial phases as expected and already observed in a few systems. In the system studied here there are only very few nucleation sites located at the NiAl/Ni₃Al interface. The further growth of Ni₅Al₃ takes place only into one of the initial phases which is NiAl. A strict orientation relationship between NiAl and Ni₅Al₃ was observed; the growth direction was [221]. The transformation is presumably diffusion controlled; it is very sluggish and it can be described by a nucleation and growth process. From the study presented here we conclude that the formation of Ni₅Al₃ proceeds by a micromechanism which differs from that normally assumed for peritectoid reactions.     

Sigmoidal creep of Ni3(Al, Ta)

Incremental creep tests have been used to explore the time-dependent plastic behavior of single-slip oriented Ni₃(Al, Ta) at low temperatures in the anomalous flow regime. For selected incremental creep experiments at 20 and 100 °C, it was discovered that Ni₃Al exhibited sigmoidal creep, where there is a significant time delay before the plastic strain rate accelerates to a maximum value during a creep experiment. Several of the factors that affect the sigmoidal creep response have been identified. The origin of sigmoidal creep is accounted for using a simple model of work hardening in Ni₃Al, where the acceleration of the creep rate is a direct result of the annihilation of the existing dislocation substructure.     

Heat affected zone microfissuring in a laser beam welded directionally solidified Ni3Al-base alloy

The laser beam weld heat affected zone (HAZ) microstructure of a newly developed aerospace alloy, IC 6, was examined. HAZ microfissuring was observed and found to be associated with grain boundary liquation facilitated by subsolidus eutectic-type transformation of the alloy’s major phase, γ′ precipitates, and interfacial melting of M₆C-type carbide and (Mo₂Ni)B₂-type boride particles.     

Formation of interfacial voids in cast and micro-grained γ′-Ni3Al during high temperature oxidation

Specimens of cast and micro-grained γ′-Ni₃Al, which were produced with vacuum casting and unbalanced magnetron sputter deposition, respectively, were isothermally oxidised in air at 1473 K for different periods of time. The formation of interfacial voids at the alloy/oxide interface was observed with SEM, which indicated that there were more interfacial voids formed in the cast Ni₃Al than in the micro-grained alloy under the same oxidation conditions. A phenomenological equation describing the fraction of the void projected areas was established, in which the impingement and coalescence between voids during their growth was taken into consideration. It was elucidated that low vacancy density in the micro-grained Ni₃Al due to the high creep, re-crystallisation and the enhanced Al diffusion reduced the void percentage. Also, it was confirmed that aluminium evaporation, perhaps supplemented by surface diffusion, supplied most Al to the oxide scales formed above the interfacial voids.     

Grain boundary self-diffusion of Ni in pure and boron-doped Ni3Al

Grain boundary (gb) self-diffusion in pure Ni-rich Ni₃Al was measured between 882 and 1374 K using the radiotracer ⁶³Ni, a serial sectioning technique and sensitive liquid scintillation counting. The results of the gb diffusivity P = δD_gb (δ : gb width, D_gb : gb diffusion coefficient) can be represented by the Arrhenius parameters P₀ = 3.27 · 10¹³and Q_gb = 168 kJ/mol. Additionally gb diffusion was investigated in boron-doped (0.24 at%) Ni-rich Ni₃Al in the range from 882 to 1352 K yielding P₀ = 1.24 · 10⁻¹² m³/s and Q_gb = 187 kJ/mol. The increase in the activation enthalpy Q_gb and the decrease of P upon boron-doping is explained by the segregation of B in Ni₃Al gbs, which may lead to an increase in the vacancy formation enthalpy and to a blocking of energetically favourable diffusion paths in the gbs. For comparison gb self-diffusion in pure Ni was remeasured yielding Q_gb = 112 kJ/mol. Ordering of the lattice and the preservation of ordering up to the gb planes, as predicted in Ni₃Al, therefore has a pronounced decelerating influence on gb diffusion, stronger than on bulk diffusion. Applying the semi-empirical relation of Borisov et al. (Phys. Met. Metallogr., 17 (1964) 80) gb energies γ_gb were determined for arbitrary high angle gbs in pure and B-doped Ni₃Al, resulting in 915 and 870 mJ/m², respectively, at 1100 K.     

Martensitic transformation of Ni64Al34Re2 shape memory alloy

As-rolled and annealed Ni₆₄Al₃₄Re₂ shape memory alloy (SMA) exhibits B2 → L1₀ (3R) martensitic transformation with M_s temperature up to about 210 °C. Experimental results indicate that the annealing temperature is the major factor that affects the M_s temperature. It is found that adding 2 at.% Re to replace Al in Ni₆₄Al₃₆ binary SMA can significantly refine the alloy's grain size and enhance the softening behavior during transformation. Meanwhile, Re has the same trend as Ni to affect the M_s temperature, but it has a less effect than Ni. The lattice constants and microstructures of NiAl-B2 phase, NiAl-L1₀ (3R) martensite and Ni₃Al-L1₂ phase are almost similar to those of Ni–Al binary SMAs.     

Hot deformation behavior of a Fe3Al-binary alloy in the A2 and B2-order regimes

A binary Fe₃Al alloy is investigated with respect to hot deformation behavior and microstructural as well as microtextural modifications. Applying the hot deformation simulator (WUMSI) to hot rolling conditions in the A2 and B2-order regimes in combination with data analysis, significant changes in deformation behavior are identified. These conditions are selected for performing hot rolling experiments. The differences in microstructure are investigated. On the basis of microtexture investigations by means of electron backscatter diffraction (EBSD) differences concerning orientation gradients and sub-grain structures are found. A model of combined order-related and non-order related effects is proposed explaining the observed material behavior. The information gained is the basis for the optimization of the thermomechanical treatment to produce ductile Fe₃Al sheet material.     

HREM observation of grown-in antiphase boundaries in an Fe3Al based alloy

Atomic arrangements of grown-in antiphase boundaries (APBs) in Fe₃Al based alloy were observed by means of high resolution electron microscopy. According to the continuity of Fe and Al sublattices across the APBs in different crystallographic planes, all observed APBs were identified as next-nearest neighbour types. When observed edge-on, the grown-in APBs are very thin. The comparison of the contrast of the atomic columns in the close vicinity of the APBs and the interior of the domains indicates sharp, un-relaxed grown-in domain walls.     

Grain refinement of a Fe3Al-based alloy using κ-Fe3AlC precipitate particles stimulating nucleation of recrystallization

Effects of particle distribution level on recrystallization were investigated in a Fe₃Al-based alloy containing coarse κ-Fe₃AlC precipitate particles. Volume fraction of 10–12% of rod-like κ particles with different size and interparticle spacing was introduced within the Fe₃Al matrix by changing the cooling rate from 1200 °C, which is above the precipitation temperature of the κ phase. These samples were warm rolled at 700 °C to a total reduction of 75%. Annealing of the warm rolled samples produced complete recrystallized structures. The average recrystallized grain size against interparticle spacing showed a valley-shaped curve with a minimum size of 20 μm. Orientation analyses of the warm rolled samples with high resolution EBSD method revealed that the valley shape of the curve may be explained by the particle stimulated nucleation density of recrystallization around κ particles, dependent on the particle distribution.     

Effect of Dy on the microstructures of directionally solidified NiAl–Cr(Mo) hypereutectic alloy at different withdrawal rates

The effect of various Dy content on the microstructure of Ni–31Al–32Cr–6Mo hypereutectic alloy was studied at the withdrawal rates of 6, 30 and 90 μm/s. The results show that the solid–liquid interface morphology has an evolutionary process of planar → cellular → dendritic interface with the increasing withdrawal rate. The primary Cr(Mo) dendrites are gradually weeded out through competitive growth between the primary phase and the eutectic phase. The volume fraction of primary Cr(Mo) dendrites decreases with the modest addition of Dy (0.05 wt.%) at 6 μm/s. When the withdrawal rate increases to 30 μm/s, the appropriate addition of Dy (0.1 wt.%) refines the microstructure, such as the width of intercellular zone and the lamellar thickness in the intercellular zone. With the increase of withdrawal rate to 90 μm/s, the addition of Dy has no significant effect on the microstructure. In addition, the white Dy-containing phase can occur in the boundary of eutectic cells when the Dy content is no less than 0.1 wt.%.     

Effect of directional solidification and heat treatments on the microstructure and mechanical properties of multiphase intermetallic Zr-doped Ni–Al–Cr–Ta–Mo alloy

The effect of directional solidification and heat treatments on the microstructure and mechanical properties of intermetallic Ni–21.29Al–7.04Cr–1.46Ta–0.64Mo–0.57Zr (at.%) alloy was studied. Increasing growth rate is found to decrease primary dendrite arm spacing and to increase volume fraction of β(NiAl)-based dendrites and low melting point γ′(Ni₃Al)/Ni₅Zr eutectic. Room-temperature tensile yield strength and ultimate tensile strength increase and plastic elongation to fracture decreases with the increasing growth rate. Two types of heat treatments of directionally solidified (DS) specimens including two-step ageing at temperatures of 1273 and 1123 K and two-step solution annealing at 1373 and 1493 K were performed. Ageing at 1273 and 1123 K decreases volume fractions of the dendrites and eutectic regions and leads to a coarsening of spherical -Cr and needle-like γ′ precipitates within the β-phase. Annealing at 1373 K for 100 h is shown to be sufficiently long to completely dissolve the eutectic regions. Compressive yield strength increases with increasing temperature reaching a peak value at about 1023 K and then decreases at higher temperatures. Minimum creep rate is found to depend strongly on the applied stress and temperature according to a power law. The power law stress exponent n is determined to be 5.1 and apparent activation for creep Q_a is measured to be 326 kJ/mol.     

Si-modified aluminide coatings deposited on Ti46Al7Nb alloy by slurry method

Ti46Al7Nb alloy has been used as the research substrate material for the deposition of water-based slurries containing Al and Si powders. The diffusion treatment has been carried out at 950 °C for 4 h in Ar atmosphere. The structure of the silicon-modified aluminide coatings 40 μm thick is as follows: (a) an outer zone consisting of TiAl₃ phase and titanium silicides formed on the matrix grain boundaries composed of TiAl₃–type Ti₅Si₃; (b) a middle zone containing the same phase components with the matrix TiAl₃ and the silicides Ti₅Si₃, which formed columnar grains; (c) an inner zone, 2 μm thick, consisting of TiAl₂ phase. Cyclic oxidation tests were conducted in 30 cycles (690 h at high temperature) and showed a remarkably higher oxidation resistance of the Ti46Al7Nb alloy with the protective coating in comparison with the uncoated sample.     

In-situ precipitation of Al2O3 and κ-Fe3AlC0.5 in iron aluminides through spark plasma sintering: Microstructures and mechanical properties

Iron aluminides are ordered intermetallic alloys which offer good resistance to corrosion and sulfidation. At the same time, their Achilles' heel is low ductility at room temperature and sometimes they have poor mechanical properties. By means of mechanical alloying and spark plasma sintering (MA–SPS) it is possible to obtain bulk nanostructured iron aluminides which show high hardness and high yield stress.

In this work we present the production of nanostructured powders and their consolidation through spark plasma sintering. The inevitable use of methanol as processing control agent (PCA) leads to a supersaturation in carbon and oxygen of the milled powder and a consequent in-situ precipitation of carbides and oxides during SPS. The presence of carbides, oxides and a nanostructured matrix leads to high mechanical properties with hardness 5.20 ± 0.05 GPa and a yield stress of 1305 MPa.     

Processing of high carbon Fe3Al based intermetallic alloy

A melting procedure for air induction melting (AIM) of an Fe₃Al based intermetallic alloy Fe-15.38 wt%Al-1.1 wt%C is described. Use of an appropriate slag cover during AIM results in elimination of hydrogen gas porosity in cast AIM ingots. Criteria for slag selection and slag to metal ratio are discussed. Refining by slag-metal reactions results in significant reduction in impurity levels (S, O, N) during AIM. Consequently, low cost raw materials such as mild steel scrap and commercial aluminium were used for melting the alloy. The AIM ingot exhibited excellent tensile properties. The ductility and hot workability of the ingot may be further improved by subsequent processing through electroslag remelting. It is also argued that the presence of carbon may be necessary to get AIM castings with desirable mechanical properties.     

Strong bonding of infrared brazed α2-Ti3Al and Ti–6Al–4V using Ti–Cu–Ni fillers

Infrared dissimilar brazing of α₂-Ti₃Al and Ti–6Al–4V using Ti–15Cu–25Ni and Ti–15Cu–15Ni filler metals has been performed in this study. The brazed joint consists primarily of Ti-rich and Ti₂Ni phases, and there is no interfacial phase among the braze alloy, α₂-Ti₃Al and Ti–6Al–4V substrates. The existence of the Ti₂Ni intermetallic compound is detrimental to the bonding strength of the joint. The amount of Ti₂Ni decreases with increasing brazing temperature and/or time due to the depletion of Ni content from the braze alloy into the Ti–6Al–4V substrate during brazing. The shear strength of the brazed joint free of the blocky Ti₂Ni phase is comparable with that of the α₂-Ti₃Al substrate, and strong bonding can thus be obtained.