Ni3Al-based Superalloy Research Articles

Study on Ni3Al-Based Single Crystal Superalloy Joints Brazed by Vacuum Brazing with Zr-Containing Filler

Melting point depressants (MPDs) are required to lower the melting point of filler for brazing. In this study, Zr was used as the MPD, and powder filler was prepared by adjusting the Zr and Mo content referring to Thermo-Calc calculations. The prepared filler was used to braze a high-Mo Ni3Al-based single crystal superalloy, IC21, for 1200 °C/30 min. The effects of adjusting the Zr and Mo content on the microstructure and tensile properties of the joint were investigated. The increase in Zr content promotes the formation of Ni7Zr2 in the joint, leading to a decrease in the tensile strength of the joint. The increase in Mo content forms diffusion barriers between the BM and filler, resulting in an enhancement in the tensile strength of the joint. However, continued increases in Mo content leads to an increase in the P-topologically close packed phase, causing a decline in the tensile strength of the joint. When the Zr content was (11.8–12.2) wt.% and the Mo content was (7.3–7.7) wt.%, the tensile strength of the joint at 980 °C reached a maximum of 550 MPa. This study provides a potential direction for the design of brazing filler composition for high-Mo Ni3Al-based superalloys.

Liberalizing the effects of Al and Cr in coatings for enhanced interface stability with Mo-rich Ni3Al-based superalloys

The interfacial behaviors between protective coatings and substrate single-crystal superalloys significantly impact their service performances. This work focuses on the elements interdiffusion and microstructural evolution at the interface between a NiCrAlY coating and a series of high-Mo Ni3Al-based single-crystal superalloys, through the coupled phase-field simulations and DICTRA kinetics calculations of their model systems. The critical roles of Al and Cr played in the microstructural evolution at coating/superalloy interface have been confirmed, which requires the appropriate content decreases of Al and Cr in the coating at 1100 °C. However, the addition of Mo reduces the driving force of element diffusion from coating to substrate by increasing the activities of Al and Cr in superalloy. Moreover, increasing the Mo content in superalloy from 8 wt% to 10 wt% could also counteract the promoting effect of Al on Ni mobility and mitigate the γ′ coarsening and detrimental TCP precipitation. Therefore, the effects of Mo enable the reasonable increase of Al content in coating to better balance the interfacial stability and oxidation resistance of coated superalloy, besides its advantage in reducing the anisotropy of TCP precipitate. The obtained interfacial evolution mechanisms resulting from the phase transformation thermodynamics and element interdiffusion kinetics are expected to aid the coating design for advanced single-crystal superalloys servicing at ultra-high temperature.

Creep failure mechanism of -oriented thin-wall Ni3Al-based single crystal superalloys

Currently, thin-wall structures are widely employed in aviation engine turbine blades to enhance cooling efficiency. However, the mechanical properties of thin-walled structures exhibit significant sensitivity to thickness. This study investigates the creep behavior of a <111>-oriented Ni3Al-based single-crystal superalloy with different thicknesses (0.3, 0.5, 0.8 mm) at 1100 °C/137 MPa. The results indicate a significant reduction in creep life with a decrease in wall thickness. The mechanism of various influencing factors on different creep stages was analyzed from both microscopic and macroscopic perspectives combined with finite element modeling. The alteration in stress state from thick-wall to thin-wall influences the number of slip systems that are activated, which mainly affects the primary and steady-state creep stages. Surface oxidation as time-dependent damage leads to non-invasive crack formation, somewhat increasing the steady-state creep strain rate of thin-wall samples. The accumulation and propagation in varying wall thicknesses of cleavage microcracks primarily influence the steady-state and tertiary creep stages, respectively, thereby exacerbating the thickness debit effect.

Oxidation Behavior of Pre-Strained Polycrystalline Ni3Al-Based Superalloy.

The harsh service environment of aeroengine hot-end components requires superalloys possessing excellent antioxidant properties. This study investigated the effect of pre-strain on the oxidation behavior of polycrystalline Ni3Al-based superalloys. The growth behaviors of oxidation products were analyzed by scanning electron microscope, transmission electron microscope, X-ray Photoelectron Spectroscopy and Raman spectroscopy. The results indicated that the 5% pre-strained alloys exhibited lower mass gain, shallower oxidation depth and more compact oxide film structures compared to the original alloy. This is mainly attributed to the formation of rapid diffusion paths for Al atoms diffusing to the surface under 5% pre-strain, which promotes the faster formation of protective Al2O3 film while continuing to increase the pre-strain to 25% results in less protective transient oxidation behavior being aggravated due to the increase in dislocation density within the alloy, which prevents the timely formation of the protective Al2O3 film, resulting in uneven oxidation behavior on the alloy.

Low temperature and rapid diffusion bonding of Ni3Al-based superalloy with Ni-based superalloy by spark plasma sintering

Conventional hot pressure diffusion bonding (HPDB) could result in undesired thermal and mechanical damages on base materials due to the high bonding temperature and long time. It has been a challenge to fabricate a reliable joint at a low bonding temperature and short time. In the present study, a spark plasma sintering (SPS) technique was used to resolve this challenge, and the microstructure and mechanical properties of the IC10/GH536 joints were systematically investigated. The results indicated that the pulsed current facilitated the micro-voids closure during the plasma activated diffusion bonding (PADB). At a relatively low temperature and short time (900 °C/30 min), a high joint shear strength of 541 MPa was obtained using the direct PADB, which was 4.74 times that of the direct HPDB joint. Increasing the bonding temperature to 1000 °C, a large number of M23C6 carbides formed at the grain boundaries and the bonding interface, which were the weak spots of the joint. In addition, by introducing a Ni foil to the PADB, the bonding temperature can be reduced to 800 °C and a joint shear strength of 424 MPa was obtained.

Regulating microstructure and mechanical property of dissimilar joints by pretreatments of S31042 steel and Ni3Al-based superalloy bonding substrates

Regulating microstructure and mechanical property of dissimilar joints by pretreatments of S31042 steel and Ni3Al-based superalloy bonding substrates

Vacuum diffusion bonding between Ni3Al-based superalloy and S31042 steel by surface self-nanocrystallization treatment

Vacuum diffusion bonding at a relatively low temperature is a recognized method for restraining intermetallics and optimizing joint microstructure. The surface self-nanocrystallization (SSN) treatment by shot peening technology is applied on the surface before bonding a couple of Ni3Al-based superalloy and S31042 steel, which lowers the bonding temperature for achieving a sound joint by 100 °C. Moreover, the joint strength of samples bonding at 1000 °C after SSN treatment is 178 MPa higher than that of the bonded samples at 1100 °C without SSN treatment. SEM, XRD and TEM analyses indicate SSN treatment improves bonding efficiency by increasing surface micro-deformation and introducing nanocrystalline and deformed layer. The formation of disordered Ni-solution on the surface makes γ’ strengthening phase re-precipitates in the joint and hard-brittle phase of AlN is restrained by diffusion bonding at 1000 °C. The good metallurgical bonding, re-precipitated γ’ phase and the formation of refining grains zone near the bonding interface contribute to the good performance of 1000 °C-bonded samples after SSN treatment.

The inhibiting effect of Re-rich layer on the interdiffusion between NiAl and Ni3Al-based superalloy and its degradation

A NiAl alloy/ Ni3Al-based superalloy diffusion couple with an electroplated NiRe layer was prepared. High-temperature interdiffusion performance of the diffusion couple was analyzed at 1100 ℃ under argon atmosphere and compared with that of the blank group without NiRe layer. A Re-based diffusion barrier formed during the diffusion. The microstructural instability of the superalloy was found to be alleviated by the Re-based diffusion barrier, and the detailed microstructure of the diffusion barrier was identified to be Re-rich σ phases and Mo-rich α phase detected by TEM. The chemical diffusion coefficient of Al was reduced by about two orders of magnitude with the modification of the Re-based diffusion barrier. A microstructural degradation of the Re-based barrier layer was observed after high-temperature diffusion of 300 h, which can be attributed to the internal diffusion of Mo.

Improving creep property and deformation mechanism of transient liquid phase bonded polycrystalline Ni3Al-based superalloy with post bond heat treatment

A polycrystalline Ni3Al-based superalloy was joined through transient liquid phase (TLP) bonding technique. A post bond heat treatment (PBHT) was successively conducted. Creep tests of the TLP and TLP + PBHT specimens were then carried out under 800 °C and 160 MPa. The creep deformation and fracture behaviors were further investigated through interrupted creep tests. During the PBHT procedure, microstructure evolution of the joint occurred with creep life prolonged from 64 h to 368 h. The superdislocations in TLP and TLP + PBHT specimens were determined to study the γ′ phase shearing modes. Both individual superdislocations and APB-coupled partial dislocation pairs are observed. The variation of dominate superdislocation configurations among different joint zones and the evolution of the superdislocations during the creep process are discovered. Besides, the formation of different γ′ rafting microstructures and the initiation of intergranular microcracks in the polycrystalline joint are both studied.

Microstructure and mechanical properties of Inconel713C Ni3Al-based superalloy joint welded by electron beam feeding wire welding technology

Owing to its excellent performance at high temperatures, the Inconel713C casting superalloy has been extensively used as a functional and structural material in the aviation and energy industries. Many Inconel713C superalloy components have to be manufactured by welding technologies, but joint cracks are often induced by fusion welding, deteriorating the performance of the components. Here, a modified Inconel713C casting superalloy joint without cracks was obtained using electron beam feeding wire welding with the Inconel718 alloy as the filler. The tensile strength of the joint reached 1070 MPa, exceeding that of the base metal (722 MPa) by 32.5 %. Smaller-size grains, smaller γ′ phase, and larger γ′ phase volume fraction were obtained by controlling the chemical composition and heat input of the welds. The microstructure of the fusion zone comprised dendrites, including a large number of γ′ phases (approximate size, 18 nm) and carbides (approximate size, 2 μm) distributed in the γ matrix. The microhardness of the fusion zone (approximately 420 HV) exceeded that of the base metal (approximately 370 HV), with no obvious softening in the heat-affected zone.

Thickness Debit Effect in Creep Performance of a Ni3Al-Based Single-Crystal Superalloy with [001] Orientation

With the complexity of the structure of aero-engine turbine blades, the blade wall thickness continues to decrease. It is found that when the blade wall thickness decreases to a certain extent, its mechanical properties will decline significantly. It is extremely important to study this phenomenon of a significant decline in mechanical properties caused by wall thickness. In this paper, the creep behavior of a second-generation Ni3Al-based single crystal superalloy with different wall thicknesses and [001] orientation at 980 °C/220 MPa has been studied and compared with the creep life of Φ4 round bar. The experimental results show that the second orientation and the surface affected zone are not the main reasons for the reduction of the life of thin-walled samples under this experimental condition. By analyzing the fracture morphology and deformed microstructure of thin-walled samples with different thicknesses, it is found that the thickness debit effect of the single crystal alloy occurs since the effective stress area of the alloy changes due to internal defects and surface affected zone during the creep process. For thicker samples, the creep life of the alloy can be extended by making the samples undergo certain plastic deformation through better plastic deformation coordination ability, while for thinner samples, the plastic coordination ability is poor, and the ability to extend the creep life through plastic deformation is also weaker when the effective stress area of the alloy changes, which leads to the thinner samples being more prone to fracture.

Interdiffusion and oxidation behaviour of Pt/Ru modified Ni–30Al–0.3Dy coating on Ni3Al-based superalloy

ABSTRACT A high purity of Ru or Pt was deliberately deposited as a middle layer between an alumina-forming Ni–30Al–0.3Dy coating and a Ni3Al-based single-crystal superalloy. It was found that the Ru modified Ni–30Al–0.3Dy coating effectively suppressed the interdiffusion between the coating and the substrate and inhibited the precipitation of topologically close-packed (TCP) phases which impaired the mechanical properties of the superalloy. The 1100°C annealing and oxidation tests suggested that, different from Pt, Ru restricted the inward diffusion of Al from the coating to the substrate, thereby hindering the interdiffusion. And the existence of Ru layer can simultaneously improve the oxidation resistance of the coating.

Preparation, microstructure, and mechanical behaviour of Ni3Al-based superalloy reinforced with carbide particles

The preparation, microstructure, and mechanical behaviour of Ni3Al-based superalloy have been studied. The superalloy reinforced with carbide particles forming in-situ in the melt was prepared by vacuum induction melting (VIM) followed by centrifugal casting. The as-cast alloy was subjected to heat treatment consisting of solid solution annealing, quenching, and ageing. The microstructure of the heat-treated superalloy consists of blocky MC carbides (6.9 vol%), which are uniformly distributed within the columnar grains containing cuboidal γ′(Ni3Al) precipitates (57.3 vol%) embedded in the γ (Ni-based solid solution) matrix and along γ′ grain boundaries. The equivalent plastic strain distribution in the plastic zone underneath Vickers macroindentation and within the compression specimens deformed to engineering strains ranging from 0.05 to 0.27 is calculated using finite element analysis (FEA). The correlation curve between experimentally measured Vickers microhardness values and calculated equivalent plastic strains has been proposed to quantify the level of local strain hardening state of the studied Ni3Al-based superalloy. The room temperature compressive strain hardening behaviour is evaluated and measured strain hardening exponent and strain hardening rate are discussed.

Microstructure evolution and mechanical properties of a Fe, Cr-rich multiphase Ni3Al-based superalloy during transient liquid phase bonding process

In this paper, a Fe, Cr-rich multiphase Ni3Al-based alloy is joined through transient liquid phase bonding (TLP) technique. Different from typical γ+γ′ dual phase Ni3Al-based alloys, the joint can be divided into three distinct zones: isothermal solidification zone (ISZ), gamma prime zone (GPZ) and diffusion affected zone (DAZ). Interestingly, the massive fine grains with γ+γ′ dual phase are observed inside the ISZ, and the GPZ consists of blocky γ′ phase. The precipitation of Cr-rich borides in DAZ provides an advantageous compositional condition for the local coarsening of γ′ phase. In addition, hardness value of the bonded samples fluctuates from the centerline to the BM and reaches the peak at the DAZ. The bonded specimens share equivalent room-temperature tensile strength and elongation to failure with the parent metal after the weld thermal cycle. Moreover, the fracture path propagates along the base metal (BM), which indicates excellent tensile properties of the joint.

Preparation and Enhanced Isothermal Oxidation Resistance of a Low Diffusivity NiRePtAl Single-Phase Coating

A low diffusivity NiRePtAl single-phase coating was formed on a Ni3Al-based SC superalloy by electroplating and aluminizing treatments, in which the electroplating consisted of depositing Ni-Re and Pt layer. The isothermal oxidation test of the sample was evaluated at 1100 and 1200 °C; the results indicated that the low diffusivity NiRePtAl single-phase sample promoted the oxidation resistance due to a greater β-NiAl phases-enriched outer layer. The Re-base diffusion barrier could effectively refrain the outward diffusion of Mo. Low diffusivity NiRePtAl single-phase coating has a lower inter-diffusion rate to the superalloy, where the thickness of the secondary reaction zone decreased by 35%. Mechanisms responsible for improved oxidation resistance and decreased extent for the formation of secondary reaction zones are discussed in the present study.

Short-term corrosion behavior of polycrystalline Ni3Al-based superalloy in sulfur-containing atmosphere

Harsh service environment especially in sulfur-containing atmosphere and complex structures of dominant γ' + γ dendrite, interdendritic region and interface region have significant effect on service life of polycrystalline Ni3Al-based superalloys. In this work, hot corrosion behavior of polycrystalline Ni3Al-based superalloy in sulfur-containing atmosphere was studied at 900 °C. Growth behaviors of corrosion products were analyzed by X-ray Photoelectron Spectroscopy, Raman spectrometer, Scanning Electron Microscope and Transmission Electron Microscope. The results showed that the growth behaviors of corrosion products were different in different regions. Corrosion products in interdendritic regions were Ni3S2 and Al2O3. While, as for dominant γ' + γ dendrite and interface regions, corrosion products were mainly composed of Ni3S2, Cr3S4 and mixed oxides. However, though the corrosion products are similar in composition, they have different growth mechanism. Spatial structure and precipitates were observed to analyze and explain the reasons for difference of corrosion behaviors in different regions.

Effect of interlayer on microstructure and mechanical properties of diffusional-bonded Ni3Al-based superalloy/S31042 steel joint

The combination of Ni3Al-based superalloy and S31042 steel could meet service requirement of different parts in nuclear power units. To broaden the application of Ni3Al-based alloy, vacuum diffusion bonding with S31042 austenitic heat-resistant steel was studied in this work. Different thicknesses (2, 5, 10, 15, 30 μm) of pure Ni interlayer were adopted to obtain a sound bonded joint. Result shows that the formed AlN phase in bonding process hindered atom diffusion and deteriorated joint properties. When interlayer thickness is 15 μm, the joint strength reached 96% of Ni3Al-based alloy, meanwhile elongation was 320% of Ni3Al-based alloy and 62% of S31042 alloy. The less AlN phase, uniform precipitation of γ′ phase and grain boundary intermigration behavior in two substrates contribute to the high strength and elongation of joint. The prevailing mechanism of the dissimilar joint bonded by 15 μm interlayer is ductile fracture. However, the fracture with low elongation showed the equiaxial pot and intergranular torn edges in other bonded samples due to the typical dual-phase structure in interlayer.

Effects of heat treatment on the microstructure and mechanical properties of Ni3Al-based superalloys: A review

Ni3Al-based alloys have drawn much attention as candidates for high-temperature structural materials due to their excellent comprehensive properties. The microstructure and corresponding mechanical properties of Ni3Al-based alloys are known to be susceptible to heat treatment. Thus, a significant step is to employ various heat treatments to derive the desirable mechanical properties of the alloys. This paper briefly summarizes the recent advances in the microstructure evolution that occurs during the heat treatment of Ni3Al-based alloys. Aside from γ′ phase and γ phase, the precipitations of β phase, α-Cr precipitates, and carbides are also found in Ni3Al-based alloys with the addition of various alloying elements. The evolution in morphology, size, and volume fraction of various types of secondary phases during heat treatment are reviewed, involving γ′ phase, β phase, α-Cr precipitate, and carbides. The kinetics of the growth of precipitates are also analyzed. Furthermore, the influences of heat treatment on the mechanical properties of Ni3Al-based alloys are discussed.

Directional coarsening behavior of primary γ′ phase in Ni3Al-based superalloy during aging heat treatment

Polycrystalline Ni3Al-based superalloy with up to 70% γ′ phase owns excellent strength at around 1100 °C. It has been successfully used as a substitute for turbine blade crown, which is one of the aircraft components working in the severest environment (i.e., sometimes nearly overheating). However, few studies concentrate on the coarsening resistance and phase thermal stability in this newly designed polycrystalline superalloy at the working temperature, hence, directional coarsening behavior of primary γ′(γI′) has remained restricted. This paper investigated directional coarsening behavior of γI′ during aging treatment (1100 °C) after overheating at 1270 °C with furnace cooling (SFA), air cooling (SAA) and water cooling (SWA) respectively. It clearly showed that γI′ grew into lamellar and gradually became irregular during the directional coarsening process. Moreover, its morphological combination mode was illustrated. Since parallel short lamellar γI′ connected each other, long lamellar formed. When perpendicular long lamellar γI′ connected each other, it became L-type or T-type, i.e., precursor of irregular γI′. The kinetics of directional coarsening regime of γI′ was in line with the matrix-diffusion-controlled (MDC) theory according to the evaluation of γI′ size. This was caused by higher migration of γI′ interfaces than the diffusion of solute elements. It was also confirmed that driving force of directional coarsening mainly depended on the competition of elemental diffusion, lattice misfit distribution and γ channel distance.

Isothermal oxidation behavior of micro-regions in multiphase Ni3Al-based superalloys

The isothermal oxidation behavior of micro-regions in multiphase Ni3Al-based superalloys (high Fe and Cr contents) was investigated at 600 °C in air. The oxide scale is mainly composed of the outer mixed oxide layer (NiO, NiFe2O4 and Al2O3) and the inner single Al2O3 layer. In particular, Cr2O3 is formed in the outer oxide scale of γ' + γ dendrite with enrichment in Cr element. The relative dense continuous Al2O3 layer is formed in the interdendritic region (β phase with high Al content). In addition, γ' envelope (connecting the γ' + γ dendrite and interdendritic β phase) shows preferential oxidation featuring significant cellular bulge (NiO and NiFe2O4 mixed oxides).