NiAl Phase Research Articles

Splitting of γ′ Precipitates in the Context of Phase Equilibrium

The splitting of γ′ (Ni3Al and Ni3Si) precipitates in five binary Ni-Al alloys and one Ni-Si alloy is reviewed in the context of phase equilibrium. Two mechanisms are considered: Purely Elastic (PE) splitting, driven solely by competition between elastic and interfacial free energies; Thermodynamically Driven (TD) splitting, involving precipitation of the Ni-Al or Ni-Si solid solution γ phases within supersaturated γ′ particles. The main assertion is that TD splitting is responsible for all the observations, with the possible exception of dendritic growth of Ni3Si precipitates; dendritic morphologies can mimic split configurations. In three of the six investigations splitting was reported for alloy compositions lying within the single-phase γ regions of the binary Ni-Al and Ni-Si phase diagrams wherein the γ′ phases are unstable. For the three others the aging temperatures were at or barely below the solvus temperatures, suggesting that five of the alloys were compositionally heterogeneous, “solution treatment” having failed to dissolve pre-existing γ′ particles. TD splitting was thus a byproduct of slow cooling to the aging temperatures, as in the formation of hierarchical microstructures. The nature of secondary γ′ precipitation in some of the alloys indicates that their compositions exceeded the authors’ quoted values, the enrichment enabling precipitation of γ′ during solution treatment followed by TD splitting on slow cooling. PE splitting is the only possible mechanism in solution-treated specimens that are quenched and subsequently isothermally aged. Splitting under such conditions has never been reported, lending further support to the viability of the TD mechanism.

High-temperature creep property deterioration of the alumina-forming austenitic steel: Effect of σ phase

The microstructural evolution of the alumina-forming austenitic (AFA) steel subjected to different cold rolling reductions during creep test at 973 K and 120 MPa was investigated by multiple characterization methods, and the high temperature creep property was also examined. It was found that the creep property of CR10 sample is not satisfactory although cold rolling truly improved it. The creep cavities were observed to nucleate at the interface between B2–NiAl phase and σ phase in the δ-ferrite. The serious coarsening of B2–NiAl phase and σ phase in δ-ferrite was considered as the main factors for the decline of creep property. In addition, the results also suggested that the shape of B2–NiAl phase changes from round to plate with increasing creep time, which resulted from the increase of elastic strain energy. Simultaneously, it was noteworthy that σ phase always grows adjacent to plate-shape B2–NiAl phase, which indicates the coarsening of σ phase is inseparable from that of B2–NiAl phase.

Effect of Cu content on the precipitation behavior of Cu-rich and NiAl phases in steel

The nanoscale precipitation and the mechanical properties of the two CuNiAl-containing steels with different Cu contents aged at 500 °C were investigated in this study. Results of mechanical property indicated that the co-precipitation of Cu-rich and NiAl phases during aging contributed to the precipitation strengthening effects. The influence of Cu content on the evolution of precipitates was studied by atom probe tomography (APT). The results demonstrated that the increasing of Cu content enhanced the number density of precipitates and accelerated the precipitation process. The two steels showed two kinds of precipitation sequences on account of different Cu contents. The steel 1 with low Cu content exhibited a precipitation sequence of “supersaturated solid solution → NiAl → NiAl + Cu”, while the steel 2 with high Cu content presented the process of “supersaturated solid solution → Cu → Cu + NiAl”. After further aging, the precipitates of both steels were transformed into Cu/NiAl co-precipitates without isolated Cu-rich phases and isolated NiAl phases. • Hardness of the CuNiAl-containing steel can be improved by increasing the Cu content. • The increasing of Cu content enhances the number density of precipitates and accelerates the precipitation process. • The precipitation sequences of two steels with different Cu contents are clarified.

Break the strength-ductility trade-off in a transformation-induced plasticity high-entropy alloy reinforced with precipitation strengthening

• A new TRIP high entropy alloy with precipitation strengthing was designed. • The yield strength is 700 MPa and the uniform elongation is close to 60%. • The nucleation of HCP phase from ∑3 TB and SFs was confirmed on the atomic scale. Transformation-induced plasticity (TRIP) endows material with continuous work hardening ability, which is considered as a powerful weapon to break the strength-ductility tradeoff. However, FCC based alloys with TRIP effect can not get rid of the “soft” feature of the structure entirely, resulting in insufficient yield strength. Here, a Co x Cr 25 (AlFeNi) 75- x high-entropy alloy is designed. NiAl phase is used as strengthening component to improve yield strength, while TRIP effect ensures plasticity. Compared with the previous TRIP high-entropy alloy, its yield strength is nearly doubled, and the uniform elongation is more than 55% at room temperature. Furthermore, the corresponding multiphase microstructure evolution and deformation mechanisms are investigated. Significantly, stacking faults and ∑3 twin boundaries are confirmed to be the nucleation sites of HCP phase by HAADF-STEM. Ingenious composition design and proper heat treatment process make it a perfect combination of precipitation strengthening and transformation-induced plasticity, and thus guide design in the high-performance alloy.

Investigation of the structure of an aluminum nitride film for a Bragg reflector obtained by magnetron sputtering

The phase composition of the aluminum nitride film obtained through magnetron sputtering was determined with the help of X-ray phase analysis. The analysis revealed the AlN, Al and V phases deposited during the synthesis of the film. The structural parameters of the primitive unit cell of the AlN and Al phases were determined. The effect of texturing the resulting film was detected.

Insight into the passivation and corrosion behavior of additive manufacturing repaired single crystal superalloy

The passivation and corrosion behaviors of additive manufacturing repaired Ni-based single crystal superalloy were analyzed by comparison with the substrate. We found that the cladding layer is comprised of fine cellular dendrites (FDs) and fine cellular interdendritic zones (FIDZs) with abundant nano particles of γ′(Ni3Al) phase. The cladding layer possesses thicker passive film which gives better protection to the cladded component. The FIDZs are more sensitive to corrosion because of the dissolution of Nb-rich and Cr-Mo depleted zones in FIDZs. The γ/γ′ couples in the substrate tend to form mesh-like corrosion morphologies due to the selective dissolution of γ′ phases.

Characterization of Al2O3 Matrix Composites Fabricated via the Slip Casting Method Using NiAl-Al2O3 Composite Powder.

This work aimed to characterize Al2O3 matrix composites fabricated by the slip casting method using NiAl-Al2O3 composite powder as the initial powder. The composite powder, consisting of NiAl + 30 wt.% Al2O3, was obtained by mechanical alloying of Al2O3, Al, and Ni powders. The composite powder was added to the Al2O3 powder to prepare the final powder for the slip casting method. The stained composite samples presented high density. EDX and XRD analyses showed that the sintering process of the samples in an air atmosphere caused the formation of the NiAl2O4 spinel phase. Finally, the phase composition of the composites changed from the initial phases of Al2O3 and NiAl to Al2O3, Ni, and NiAl2O4. However, in the area of Ni, fine Al2O3 particles remaining from the initial composite powder were visible. It can be concluded that after slip casting, after starting with Al2O3 and the composite powder (NiAl-Al2O3) and upon sintering in air, ceramic matrix composites with Ni and NiAl2O4 phases, complex structures, high-quality sintered samples, and favorable mechanical properties were obtained.

Al-based composites reinforced with ceramic particles formed by in situ reactions between Al and amorphous SiNxOy

This work combines two approaches to the creation of high-strength heat-resistant Al-based materials: dispersion hardening and stabilization of nanosized Al grains by grain-boundary precipitates. For this, the amorphous SiNxOy phase (1, 2, 3, 4, 5, and 10 wt%) was used as a reactive additive to the Al nanopowder. The Al-SiNxOy composites were obtained by a combination of high-energy ball milling and spark plasma sintering. Chemical reactions between a-SiNxOy and Al led to the formation of nanoscale AlN, Al2O3, and SiO2 phases located at the Al grain boundaries and inside the metal matrix with a bimodal size distribution: approximately 50–150 and 3–10 nm. Compared to unreinforced Al, the addition of 3% SiNxOy increased hardness by 464%, tensile strength by 103% (25 °C), 84% (300 °C), and 86% (500 °C), compressive strength by 200% (25 °C), 164% (300 °C), and 192% (500 °C), and impact wear resistance by 33–46%. TEM microstructure analysis after deformation revealed the types of defects and helped to elucidate the deformation and strengthening mechanisms. The obtained results are important for the development of Al-based composites capable of operating in an extended temperature range.

Chemical reaction of Ni/Al interface associated with perturbation growth under shock compression

The exothermic reaction of Ni/Al laminates always starts from the interface, and the role of interfacial instability in the shock-induced chemical reaction has not been clarified. This work reports the Richtmyer–Meshkov (RM) instability growth, atomic diffusion, and chemical reaction of Ni/Al interface under shock compression based on atomistic simulations. For shocking from Al to Ni, the interface experiences finite collapse and exhibits weak localized reaction. The diffusion of solid Ni to molten Al will be inhibited due to the formation of NiAl phase, and continuous inter-diffusion occurs with the melting of Ni. For shocking from Ni to Al, a small amount of NiAl structure is formed due to the atomic residue during defect collapse. RM instability growth is observed at higher shock intensity, which significantly promotes the atomic mixing and results in a power-law increase in the number of diffusing atoms. Meanwhile, the chemical reaction propagates rapidly from the vortex to the head of the spike accompanied by the decomposition of many clusters, with the nonlinear development of RM instability. The number and the size of Ni clusters no more satisfy the simple power-law relationship for which we propose an improved power-law distribution. Interestingly, the growth of nanoscale perturbation approximately satisfies the logarithmic law with time, but the linear growth stage is inhibited due to significant inter-diffusion, especially for the small wavelength. Thus, the mixing width and the reaction degree are positively correlated with the initial wavelength in our simulation scale, which is contrary to the RM growth law of the free surface.

Microstructure and Properties of Novel Mg-Al-Zn-Mn-Ca-Ni Dissoluble Alloy Fabricated by Industrial Two-Step Extrusion Method

Dissoluble magnesium alloys for fabrication of fracturing tools have received increasing attention in recent years. However, most of the existing research is focused on the small-sized samples prepared in the laboratory, and there is almost no report on the industrial dissoluble magnesium alloys. In this study, large-scale Mg-Al-Zn-Mn-Ca-Ni alloys with a diameter of 110 mm were prepared by a semi-continuous casting and two-step extrusion method, and the corresponding microstructure and mechanical and corrosion properties were also investigated. It was found that after two-step extrusion, the mainly precipitate phases in the Mg-Al-Zn-Mn-Ca-Ni alloy are bulk-like AlMnNi, strip-like Al3Ni, and granular-like and lamellar-like Mg17Al12 phases. Due to the combined effects of grain refinement and precipitation strengthening, the Mg-Al-Zn-Mn-Ca-Ni alloy obtained excellent mechanical properties after two-step extrusion, and its ultimate tensile strength, yield strength, and elongation were 314.6 MPa, 191.2 MPa, and 13.1%, respectively. Moreover, the corrosion rate of the alloy in 3 wt.% KCl at 93 °C was as high as 97.61 mg·cm−2·h−1. This work provides a high-performance, low-cost, and large-scale alloy product for the fabrication of dissoluble fracturing tools.

Mechanical properties of as-cast and wrought Mg–5Ni-xAl magnesium alloys

Microstructural development and mechanical properties of the newly developed Mg–5Ni-xAl (x = 2.5, 5, and 7.5 wt%) alloys were investigated. The microstructures of all alloys were composed of α-Mg, Mg17Al12, and Al3Ni2 phases. The increase in Al content (from 2.5% to 7.5%) extremely modified the as-cast grain size from 507 μm to 62 μm with the consequent enhancement of mechanical properties. While elevated temperature homogenization treatment led to the dissolution of the brittle Mg17Al12 eutectics, the Al3Ni2 intermetallics showed high thermal stability. Moreover, hot extrusion led to a remarkable structural refinement via dynamic recrystallization (DRX), as well as fragmentation and dispersion of particles during thermomechanical processing. Grain refinement via Al addition and hot extrusion (investigated by the Hall-Petch law), as well as the dissolution of unfavorable intergranular Mg17Al12 intermetallics and solid solution hardening (via increasing the Al content of the α-Mg matrix) during homogenization were found to be the main contributing factors to the enhancement of tensile properties. As a result, the best combination of the ultimate tensile strength (∼ 354 MPa) and total elongation (∼ 23%) was obtained for the hot extruded alloy containing 7.5 wt% Al with the average grain size of 7.9 μm.

Effect of structure, phase, and elemental composition of AlN, CrAlN, and ZrAlN coatings on their electrochemical behavior in 3% NaCl solution

AbstractThin coatings AlN, AlZrN, and AlCrN were deposited by pulsed magnetron sputtering. The high‐speed steel Т1, structural alloy steel 5140, and structural carbon steel 1017 were used as substrates. The magnetron current, nitrogen content in the gas mixture, and bias voltage on the substrate were changed to obtain nanostructured and amorphous layers of coatings with different elemental compositions. The voltammetry and impedance spectroscopy were performed on the coated samples in 3% NaCl solution. The corrosion behavior of the coatings was characterized by the corrosion current density icorr, the polarization resistance Rp (at the corrosion potential), the ratios icorr,s/icorr, and Rp/Rp,s, where subscript s refers to the substrate. It was shown that the coatings under study (except AlN) are electrochemically active, and the corrosion processes occur not only on the substrate in the coating discontinuity but on the coating surface as well. The coatings AlN/T1, AlZrN/5140, AlCrN/T1, and AlCrN/5140 with icorr ~ 10−7 A cm−2 are found to be the most corrosion‐resistant in 3% NaCl. The paper discusses factors affecting the corrosion behavior of the investigated coatings.

Effect of Pt-doping on the oxidation behaviors of the γ'-Ni3Al and β-NiAl phases in the NiSiAlY alloy

The oxidation behaviors of the NiSiAlY+xPt (x = 0, 1, 5 and 10 wt%) alloys were investigated at 900 ℃, with the focus on the Pt effect, phase and oxidation stage. The doped Pt improved the oxidation resistance but caused different oxidation behaviors in β-NiAl and γ'-Ni3Al phases. During the initial oxidation process, the Pt-doped NiSiAlY alloys showed a faster increase in mass gain, which was mainly contributed by the β-NiAl phase. However, at the stable oxidation stage, the undoped NiSiAlY alloy had the highest oxide growth rate, it was dominated by the oxidation behavior of the γ'-Ni3Al phase.

Effect of substrate roughness and PVD deposition temperatures on hardness and wear performance of AlCrN-coated WC-Co

Elevated temperature experienced in the cutting zone is the main issue contributing to severe wear and responsible in reducing the lifespan of cutting tools'. This study aims to improve the performance of the cutting tool by applying AlCrN coating on the WC-Co substrate using physical vapor deposition (PVD). Prior to the coating process, the WC-Co surface was chemically roughened by Murakami's solution, followed by Caro's solution for 10s. The etched substrates were coated with AlCrN at various PVD deposition temperatures. The scanning electron microscopy (SEM) showed the coating thickness increased from 1.24 μm to 2.78 μm. X-ray diffraction (XRD) pattern confirmed that both AlN and CrN phases dominated the coating layer indicated the presence of AlCrN coating. These XRD peaks were broader, indicating a smaller grain size (11.5 nm) produced at higher deposition temperatures. Apart from that, a minute amount of the Cr2N phase is also detected. The average compressive residual stresses ranged from −5.409 GPa to −1.707 GPa attributed to increased coating thickness associated with Cr2N+N2 → CrN phase transformation. There are no cracking and coating delamination observed at this state. The mechanical properties of the coated structure were further evaluated via Vickers microhardness and linear reciprocating tests. The hardness obtained ranged from 600 to 1617 HV0.2. While the coefficient of friction (CoF) measured was 0.21 after 240 m of sliding distance. The calculated wear rate was 2.283×10−4 mm3/N·m which exhibited a significant reduction in wear when coated at 450 °C compared to 4.550×10−4 mm3/N·m at a lower temperature (250 °C). Therefore, it can be concluded that the effect of roughened substrate and various PVD deposition temperatures have significantly influenced the coating phase formation, which subsequently effects the hardness and wear performance of AlCrN-coated tungsten carbide.

The Interaction Pathway in the Mechano-Ultrasonically Assisted and Carbon-Nanotubes Augmented Nickel–Aluminum System

The influence of mechano-ultrasonic activation (MUA) and nano-additives (carbon nanotubes-CNT) on the interaction pathway of nickel–aluminum powder mixture at high heating rates was investigated. The optimum conditions of the mechano-ultrasonic activation, along with the phase and structure formation peculiarities of nickel–aluminum and nickel–aluminum–carbon nanotubes mixtures by thermal analysis method, the so-called high-speed temperature scanner (HSTS), were found out. The optimum duration of mechanical and ultrasonic activation aiming to achieve homogeneous distribution in the agitated mixtures was determined. A shift in characteristic temperatures of MUA mixtures by the influence of both heating rate and ultrasound on the Ni + Al interaction pathway for the mechano-activated (1, 3, 5 min) and 1 wt% CNT containing mixtures was observed. The formation patterns of NiAl + Ni3Al mixture or pure NiAl phase was manifested according to the interaction mechanism (depending on solid–liquid or solid–solid state of intermediates). The effective activation energy values for the Ni + Al exothermic reactions of all studied systems were determined by the isoconversional method of Kissinger.

A combinatorial assessment of microstructure and mechanical properties in AlCrCuFeNi2Vx concentrated alloys

This study developed a novel series of AlCrCuFeNi2Vx (x = 0, 0.3 and 0.6) high-entropy alloys (HEAs), and systemically assessed the relationship between composition, processing, microstructure and mechanical properties. The alloys were produced by arc melting and studied in as-solidified and homogenized (annealing at 650 ℃ for 4 h) conditions. In the base alloy of AlCrCuFeNi2, the microstructure consists of disordered bcc and fcc phases, ordered B2 and L12 phases, and the B2 phase and bcc phase are coherent. With the increase of V content, the volume fraction of bcc/B2 phases increase, accompanied with a peculiar phenomenon that the shape of B2 phase has transferred from irregular matrix to spherical precipitates, which caused by the increase of lattice misfit after V addition. Moreover, a large number of Cu particles are randomly distributed in the NiAl (B2) phase, and the crystal structure of Cu particles has changed from 9 R structure to fcc structure after annealing treatment. Mechanical tests of these alloys reveal that the 0.6 at% V addition has doubled the yield strength (V0, 780 MPa; V0.6, 1673 MPa) for the increase in volume fraction of bcc/B2 phase and the solid solution strengthening produces by V addition. The yield strength is increased by 124 MPa after annealing treatment in V0.6 alloy without sacrificing the ductility due to the promotion of L12 phase after annealing treatment.

Microstructure, hardness, thermal and wear behaviours in Al–10Ni/TiO2 composites fabricated by mechanical alloying

In this study, the effects of milling time and sintering temperature on the microstructural evolutions and thermal properties of Al–10Ni/TiO2 composites fabricated by mechanical alloying were investigated. X-ray diffraction revealed that Al3Ni2 intermetallic phase appeared after 20 h of milling. Also, X-ray diffraction and scanning electron microscopy results revealed that as the milling time increased, a more homogeneous structure and particle size reduction occurred. The differential thermal analysis results showed a series of endothermic and exothermic peaks indicating phase transformation and crystallization during continuous heating. After the sintering, polishing and etching of the pressed composite samples, the surface analysis was examined with an optical microscope and the microhardness was also measured. It was observed that as the sintering temperature increased, the formation of Al3Ni and Al3Ni2 intermetallic phases and the microhardness increased. The maximum microhardness value of the pressed composites was found as 541 ± 20 HV1 in the Al–10Ni/TiO2 composite sample produced with 20 h milling and sintered at 500 °C. In addition, the wear behaviour of the composites was investigated using pin-on-disk wear test under a certain load (5–15 N). It was determined that the mass loss decreased significantly as the grinding time and sintering temperature increased.

The preferential formation of Ni2Al3, Fe2Al5, and Ti2Al5 phases in aluminide systems

The preferential formation of phases in Ni–Al, Fe–Al, and Ti–Al systems was observed at the interface of solid-solid and solid-liquid phases. It was found that phases already formed at temperatures lower than the melting point of aluminum. The Ni2Al3, Fe2Al5 and Ti2Al5 phases formed preferentially in the Ni–Al, Fe–Al and Ti–Al systems, respectively. The favored formation was not associated with thermodynamic assumptions, but was caused by crystal structures of arisen phases. The mentioned phases could form due to the two reasons, (i) similarities of their lattice parameters to lattice parameters of the individual elements (Ni, Fe, Ti, or Al) and (ii) the total volume of its lattice. The lattice volumes of Ni2Al3, Fe2Al5, and Ti2Al5 phases are always smaller than that of NiAl3, FeAl3, and TiAl3 phases. The last listed phases are ordinarily accepted by other authors as the phases, which form preferentially based on thermodynamic data only. This work disproves such claims and shows the reason for preferential phases' formation.

Mechanical properties of NiAl–Cr alloys in relation to microstructure and atomic defects

AbstractThe quasi-binary eutectic NiAl–Cr of the ternary Ni–Al–Cr system is composed of B2 type NiAl(Cr) solid solutions and the b.c.c. transition metal chromium as second phase. This constitution enables a systematic alloy design in dependence on the Cr content for improving elastic stiffness, high temperature strength, creep resistance, and fracture toughness of intermetallic NiAl. Elasticity and mechanical properties of NiAl(Cr) reinforced by nano-sized Cr particles or quasi-continuous Cr fibres of several microns in diameter are presented and discussed in respect to single phase NiAl and literature data. The microstructural characterization and discussion of mechanical properties of NiAl–Cr alloys are supplemented by atom probe field ion microscopy analysis (APFIM). APFIM was used to determine the atomic defect structures, the Cr site preference and reveals Cr segregations at antiphase boundaries.

Dense, single-phase, hard, and stress-free Ti0.32Al0.63W0.05N films grown by magnetron sputtering with dramatically reduced energy consumption

The quest for lowering energy consumption during thin film growth, as by magnetron sputtering, becomes of particular importance in view of sustainable development goals. A recently proposed solution combining high power impulse and direct current magnetron sputtering (HiPIMS/DCMS) relies on the use of heavy metal-ion irradiation, instead of conventionally employed resistive heating, to provide sufficient adatom mobility, in order to obtain high-quality dense films. The major fraction of process energy is used at the sputtering sources rather than for heating the entire vacuum vessel. The present study aims to investigate the W+ densification effects as a function of increasing Al content in (Ti1-yAly)1-xWxN films covering the entire range up to the practical solubility limits (y ~ 0.67). Layers with high Al content are attractive to industrial applications as the high temperature oxidation resistance increases with increasing Al concentration. The challenge is, however, to avoid precipitation of the hexagonal wurtzite AlN phase, which is softer. We report here that (Ti1-yAly)1-xWxN layers with y = 0.66 and x = 0.05 grown by a combination of W-HiPIMS and TiAl-DCMS with the substrate bias Vs synchronized to the W+-rich fluxes (to provide mobility in the absence of substrate heating) possess single-phase NaCl-structure, as confirmed by XRD and SAED patterns. The evidence provided by XTEM images and the residual oxygen content obtained from ERDA analyses reveals that the alloy films are dense without discernable porosity. The nanoindentation hardness is comparable to that of TiAlN films grown at 400–500 °C, while the residual stresses are very low. We established that the adatom mobility due to the heavy ion W+ irradiation (in place of resistive heating) enables the growth of high-quality coatings at substrate temperatures not exceeding 130 °C provided that the W+ momentum transfer per deposited metal atom is sufficiently high. The benefit of this novel film growth approach is not only the reduction of the process energy consumption by 83%, but also the possibility to coat temperature-sensitive substrates.