Ni3Nb Phase Research Articles

Effect of Nb content variations on the microstructure and mechanical properties of NiCrMo welded joints

The effects of varying the Nb content (1.04 wt%-1.55 wt%) in NiCrMo weld metals on the microstructure, tensile strength and cryogenic impact toughness were investigated. The microstructure of the three different weld metals was observed to be composed of austenite matrix and precipitates by characterization techniques such as SEM, EDS, EBSD, and TEM. The precipitates within the grains were NbC and Ni3Nb phases, and the precipitates on the grain boundaries were NbC phases. With the increase of Nb content, the average grain size of austenite decreased from 205.3 μm to 161.4 μm, and the area fraction of precipitates increased from 1.41% to 3.97%. The cryogenic impact value of the welded joints decreased from 97 J to 76.3 J, while the tensile strength increased from 682.5 MPa to 710 MPa, indicating that the increase in the area fraction of the Nb-rich phase had a deleterious effect on the cryogenic impact toughness and was conducive to the elevation of the tensile strength. When the Nb content in the weld metal was 1.25 wt%, the comprehensive mechanical properties of NiCrMo welded joints were the best, with a tensile strength of 695.5 MPa and a cryogenic impact value of 86.7 J.

Role of δ-phase on Mechanical Behaviors of Additive Manufactured Inconel 718: Detailed Microstructure Analysis and Crystal Plasticity Modelling

In this work, a multi-scale approach was applied to reveal the evolution mechanisms of the δ (Ni3Nb) phase and the resultant mechanical responses in wire-arc additive manufactured (WAAM) Inconel (IN) 718. Several heat treatment strategies, consist of homogenization, intermediate heat treatment and aging process, were firstly conducted to produce δ phases with different characteristics. The early evolution of the δ phase was then analyzed in detail via transmission electron microscopy. The nanoindentation and tensile tests were also used to verify the strengthen response of δ phase. Moreover, a crystal plasticity finite element (CPFE) model, taking into account the different configurations of δ phase, was developed to study and predict the micro deformation behavior. Results revealed that when the intermediate heat treatment time exceeded 1 h, the formation of the main strengthening phase γ" was constrained by the δ phase, accompanied by the coarsening of γ". The decrease of γ" phase and the destruction of coherent γ"/γ interface are the main causes of strength loss in the WAAMed IN718. The intergranular dislocation transmission was hindered by the δ phase, thereby increasing the grain boundary strength. However, the needle-like δ phase within the grain may cause premature fracture due to strain localization, thereby reducing the material properties. The evolution mechanisms of the δ phase obtained in this study provides theoretical guidance for the optimization of the microstructures and mechanical properties of WAAM Inconel 718.

Microstructure and Mechanical Properties of Zr‐4 Alloy and 316 Stainless Steel Diffusion Bonding Joint Using Nb/Ni Composite Interlayer

Diffusion bonding of Zr‐4 alloy and 316 stainless steel (SS) without and with the Nb/Ni composite interlayer has been performed. The maximum shear strength of the joint without interlayer is 131 MPa at 900 °C for 30 min under 10 MPa. The results show that the fracture occurs at the diffusion layer as a result of the limited mutual solubility between Zr and Fe and detrimental residual stress. Sound diffusion bonding joints without defects are obtained at lower bonding temperatures due to the addition of Nb/Ni interlayer, and the maximum shear strength is 380 MPa at 820 °C. The typical microstructure of the Zr‐4/Nb/Ni/316SS joint is Zr‐4/Zr(Nb)/Zr(Fe, Nb)2 /Zr(Nb)/Nb/Ni3Nb/Ni/(Fe, Cr, Ni)/316SS. The highest hardness value of the joint is 422 HV at the Ni3Nb phase, and cracks appear at the interface between Ni3Nb and the remaining Ni foil. The shear strength first increases and then drops as the Ni3Nb layer thickened. The peak residual stress of the joint reduces from 698 to 466 MPa due to the addition of the Nb/Ni composite interlayer. Finally, the growth kinetic model of the Ni3Nb layer is calculated, which provides a pathway for the regulation of joint properties.

Nickel-Based Superalloy Inconel 718, Relation between Non-Linear Ultrasonic and Microstructure Changes under the Creep Damages.

Nickel-based superalloy, Inconel 718, has high temperature strength effected by γ"(Ni3Nb) phase at approximately 973 K. It has been found that the creep damage of Inconel 718 was induced by γ"phase disappearance. In the study, we applied the nonlinear ultrasonic method, nonlinear three-wave interaction, for evaluation of the creep damages and the microstructural degradation in γ" phase for Inconel 718 at 973 K, 310MPa. In the nonlinear three-wave interaction method, two intersecting ultrasonic waves produced a scattered wave when the resonance condition was satisfied. The amplitude in resonant scattering wave was measured. We analyzed the relation between the microstructural change and nonlinear acoustic characterizations by the electromagnetic acoustic resonance (EMAR) throughout the creep life, and the nonlinearity corresponded to change of the dislocation density. This was supported with X-ray diffraction, EBSD and SEM observation.

The Elastic Effect of Evolving Precipitate Shapes on the Ripening Kinetics of Tetragonal Phases

Coherent tetragonal precipitates, such as the Ni3Nb phase γ″ found in Ni-base superalloys, appear as plate-shaped particles. These shapes are the result of anisotropic elastic misfit strains. We present 3D sharp phase-field simulations that capture this circumstance well due to the inclusion of the elastic effects from the misfit. These simulations reveal that the ripening behavior of γ″ precipitates deviates significantly from the classical LSW theory of Ostwald ripening. A ripening exponent of 2 rather than 3 describes the simulated γ″ size evolution at temperatures between 700 °C and 760 °C best. Employing a quantitative distinction argument, we show that 60 pct of this deviation is attributed to the elastically induced size dependence of the precipitate shapes. With increasing precipitate size, the minimization of elastic energy leads to steadily increasing plate aspect ratios. The precipitate ripening kinetics accelerate with increasing aspect ratio. Fitting the newly received square root time dependence to experimental data yields a physically conclusive activation energy of ripening close to the activation energy of Nb diffusion in the alloy.

Grain Boundary Precipitation Control of GCP Phase Using TCP or A2 Phase in Ni-Based Alloys

To cover the grain boundary (GB) of the Ni phase with precipitates, the GB precipitation behavior of both topologically close-packed (TCP) or A2 and geometrically close-packed (GCP) phases was investigated in two Ni–Nb–(Co, Cr) ternary systems. The Ni/TCP or A2/GCP three-phase region existed in both systems. In the Ni-Nb-Co ternary system, Nb was approximately equally partitioned into both Co7Nb2 (mC18 structure, TCP) and (Ni, Co)3Nb (D019 structure, GCP) phases. In the Ni–Nb-Cr ternary system, Nb and Cr were mainly partitioned into the Ni3Nb (D0a structure, GCP) and Cr (A2 structure) phases, respectively. In the Ni–Nb–Co ternary system, the Co7Nb2 phase grew along the GB, whereas the (Ni, Co)3Nb phase grew toward the grain interior (GI). However, the growth of the Ni3Nb phase toward the GI was suppressed in the Ni–Nb–Cr ternary system. The suppression of growth of the GCP phase and covering the GB using both the TCP or A2 and GCP phases might be possible in a system where the precipitation of the GCP phase nucleating on the GB prior to the TCP or A2 phase increases supersaturation for precipitation of the TCP or A2 phase.

Influence of powder-mixed EDM on surface morphology and metallurgical alterations of Inconel 625

ABSTRACT Current research work aims at improving the surface integrity characteristics of Inconel 625 (a nickel-based super alloy) during electric discharge machining (EDM) process. Powder particles such as graphite, aluminium and silicon were added to kerosene dielectric during the EDM process to enhance the surface integrity characteristics. A specially designed and fabricated dielectric recirculating system was attached to the EDM setup to carry the experiments. Design of experiments was carried out using response surface methodology. Influence of process variables such as powder material, particle concentration in dielectric, peak current, pulse on time, duty cycle and gap voltage was investigated on surface integrity characteristics of machined specimens. Mixing of powder particles with dielectric resulted in reduced surface cracking, recast layer thickness and surface microhardness. While EDM with conventional dielectric primarily demonstrated FCC structure of γ-phase of Inconel 625 along with some δ(Ni3Nb) phase, powder-mixed EDM has promoted formation of carbides of Ni, Cr and Mo along with Ni3Nb from Inconel 625. Addition of powder with dielectric also caused reduction in crystallite size as well as lattice strain due to enlargement of crater while maintaining uniform discharge over a large area.

High-temperature oxidation performance of Inconel 625 superalloy fabricated by wire arc additive manufacturing

High-temperature oxidation behavior of Inconel 625 superalloy manufactured by wire arc additive manufacturing (WAAM) technique was investigated. Results showed that microstructure of the as-built alloy contained Ni-based solid-solution dendrites along with MC carbide, Laves, and δ phases. Holding of samples at 800–1000 °C for 20 h resulted in the formation of Cr2O3 and Ni3Nb phases on the surface of the alloy. However, at 1100 °C, oxidation of Ni3Nb led to the formation of CrNbO4 at the oxide/alloy interface. The growth kinetics of the oxide layer followed a parabolic law, and the diffusion activation energy was calculated as 167 kJ/mol.

Creep-induced Nonlinear Ultrasonic Change in Nickel-based Super alloy, Inconel 718.

We investigated the evolutions of nonlinear acoustic characterization: three-wave mixing, with electromagnetic acoustic resonance (EMAR) throughout the creep life in Nickel-based superalloy, Inconel 718, which has high temperature strength effected by γ″(Ni3Nb) phase at approximately 973 K and is determined that the γ″phase disappearance lead to the creep rupture. EMAR was a combination of the resonant acoustic technique with a non-contact electromagnetic acoustic transducer (EMAT).

Microstructure Evolution and Mechanical Properties of X6CrNiMoVNb11-2 Stainless Steel after Heat Treatment.

X6CrNiMoVNb11-2 supermartensitic stainless steel, a special type of stainless steel, is commonly used in the production of gas turbine discs in liquid rocket engines and compressor disks in aero engines. By optimizing the parameters of the heat-treatment process, its mechanical properties are specially adjusted to meet the performance requirement in that particular practical application during the advanced composite casting-rolling forming process. The relationship between the microstructure and mechanical properties after quenching from 1040 °C and tempering at 300–670 °C was studied, where the yield strength, tensile strength, elongation and impact toughness under different cooling conditions are obtained by means of mechanical property tests. A certain amount of high-density nanophase precipitation is found in the martensite phase transformation through the heat treatment involved in the quenching and tempering processes, where M23C6 carbides are dispersed in lamellar martensite, with the close-packed Ni3Mo and Ni3Nb phases of high-density co-lattice nanocrystalline precipitation created during the tempering process. The ideal process parameters are to quench at 1040 °C in an oil-cooling medium and to temper at 650 °C by air-cooling; final hardness is averaged about 313 HV, with an elongation of 17.9%, the cross-area reduction ratio is 52%, and the impact toughness is about 65 J, respectively. Moreover, the tempered hardness equation, considering various tempering temperatures, is precisely fitted. This investigation helps us to better understand the strengthening mechanism and performance controlling scheme of martensite stainless steel during the cast-rolling forming process in future applications.

Influence of heat treatment on the microstructure and corrosion properties of the Inconel-625 clad deposited by microwave heating

The present work explores the viability of using the microwave heating technique to apply Inconel-625 cladding to a stainless steel (SS-316) substrate. As-deposited clad specimens were subjected to various post-heat-treatments. These treatments included (1) a direct-aged (DA) treatment (2) solution at 981 °C followed by a DA treatment (981SHT) (3) solution at 1081 °C followed by DA treatment (1081SHT). Clad specimens displayed a gradual change in microstructure from planar to cellular to the dendritic depending upon the cooling rate experienced by the molten clad powder during the microwave cladding process. Due to precipitation of hardening particles (γ′), the heat-treated clad specimens displayed superior hardness than as-deposited clad specimen. In the interdendritic spaces/grain boundaries, heat-treatment at 981 °C induces a major reduction in the size of Laves process with a needle precipitation of δ (Ni3Nb) phase. Meanwhile, the heat-treatment at 1081 °C nearly dissolved the entire Laves phase in the microstructure. The corrosion test revealed that the coated samples at all conditions exhibit around 2 to 2.5 times more noble corrosion potential (corrosion resistance) than the uncoated samples. Moreover, the corrosion resistance of the post-treated sample decreases dramatically than that of the as-deposited clad sample, as heat-treatment improved the precipitate phase formation.

Effects of heat treatment on microstructure and mechanical properties of Inconel 625 alloy fabricated by wire arc additive manufacturing process

Abstract The microstructure and mechanical properties of Inconel 625 alloy fabricated by wire arc additive manufacturing process were evaluated under as-prepared and heat-treated conditions. A dendritic Ni-based solid solution phase along with (Nb, Ti)C carbide, Laves, and δ-Ni3Nb secondary phases were developed in the microstructure of the as-prepared alloy. Solution heat treatment led to the dissolution of Laves and Ni3Nb phases. In addition, dendrites were replaced with large columnar grains. Aging heat treatment resulted in the formation of grain boundary M23C6 carbide and nanometric γ″ precipitates. Hardness, yield and tensile strengths, as well as elongation of the as-prepared part, were close to those of the cast alloy and its fracture occurred in a transgranular ductile mode. Solution heat treatment improved hardness and yield strength and declined the elongation, but it did not have a considerable impact on the tensile strength. Furthermore, aging heat treatment caused the tensile properties to deteriorate and changed the fracture to a mixture of transgranular ductile and intergranular brittle mode.

СТРУКТУРА И СВОЙСТВА ДЕТАЛЕЙ, ПОЛУЧЕННЫХ СЕЛЕКТИВНЫМ ЛАЗЕРНЫМ СПЛАВЛЕНИЕМ ПОСЛЕ ИСПЫТАНИЯ НА МАЛОЦИКЛОВУЮ УСТАЛОСТЬ

Results of low cycle fatigue (LCF) testing, investigation of microstructure, and fractures of specimens, which was, obtain by selective laser melting process (SLM) of the powders from Inconel 718 alloy have been carried out in the present article. Chemical composition of the considered specimens, microstructure before and after testing, results at room and elevated temperatures, which were built in XY direction (horizontal), were carrying-out. The specimens in the as-build state were exposed by hot isostatic pressing (HIP) with subsequent inherent Inconel 718 heat treatment. It was established that in specimens in as-built state observe accurately expressed zones of layer-by-layer smelting with 100 μm height. After HIP with subsequent heat treatment strengthening of the considered alloy provides by intermetallic γ'' – Ni3Nb phase, γ'-phase and carbides, also was identified lamellar δ-phase in the microstructure. LCF-testing was conducted in a so-called «soft» cycle of loading with a predetermined interval of the strain for providing of fatigue life in 103, 5х103, 104 cycles. The obtained data was built logarithmic curve in coordinates «Strain σ – Number of cycles N», that allows, with sufficient reliability, determine a rational strain level at LCF-testing for providing a predetermined number of cycles. The following ratios of σ – N were established (Rσ=0, υ=1 Hz): for providing N=103, σ≤1020 МPа, N=5х103, σ=931…961 МPа, N= 104, σ≤941 МPа. Results of the fractures investigation after LCF-testing at 20°С and 550°С shows that during the cyclic elastoplastic deformation at room temperature multicentricity of crack initiations zones observes. Mainly fixed a viscous pattern of fracture with a fatigue grooves presence, which size increases in the process of crack advance to the zone of fracture area. At elevated temperature in the zones of fatigue crack initiation and spreading quasi-brittle character of destruction observes, and turns in viscous in the zone of fracture area. Authors should be pointed out that the application of additive technologies in the manufacture of aerospace parts requires extensive R&D works, and testing efforts to confirm the repeatability of alloy characteristics.

The effect of δ phase on the microplasticity evolution of a heat-treated nickel base superalloy

The micromechanical deformation behaviour of the commercial Ni-base polycrystalline Ni-base superalloy GH4169 has been investigated in terms of the volume fraction and grain boundary distribution of the δ (Ni3Nb) phase. The main objective of this work was to gain an understanding about the mechanisms of deformation of the superalloy at the grain scale. To that purpose, experimental and microstructural observations are relied upon to construct realistic models of the polycrystalline superalloy microstructure using crystal plasticity concepts. It was found that a volume fraction of the grain boundary δ phase in excess of approximately 3.0% results in a weakening of the room temperature strength, and the detrimental effect increases with increasing the δ phase volume fraction. This study also revealed that the grain boundary δ phase, being stronger than the surrounding grains, impedes the transmission of plastic slip through the grain boundaries, thus enhancing the localisation of plastic slip within the grains. However, the strengthening effect associated with such slip blocking mechanism by the δ phase is considerably lower than the weakening of the deformation resistance of the grains due to the depletion of the meta-stable γ″ precipitates within the grains resulting from the stable δ phase formation.

Characterization, corrosion resistance and hardness of rapidly solidified Ni–Nb alloys

In this research various Ni(-x)Nb (x = 10, 15, 20, 30, 40, 45, 52 and 57 wt %) alloys samples were rapidly solidified (RS) by using copper mold casting. High purity elements were used to produce the RS Ni–Nb samples. Even though Ni–Nb alloys have been extensively investigated over the past decades regarding thermodynamics, phase diagram and heat treatment; as-solidified microstructures and properties generated under non-equilibrium conditions remain undetermined for this alloy system. The Ni–Nb system is characterized by two eutectic reactions (termed E1: 22.5 wt % Nb and E2: 52 wt % Nb) and three Ni3Nb, Ni6Nb7 and Ni8Nb intermetallics, being the later barely reported in literature. The E1 group of alloys (i.e., x = 10, 15, 20 wt%) showed the microstructures constituted by Ni-fcc cellular/dendritic matrix enveloped by Ni + Ni3Nb eutectic. In contrast, the higher niobium (Nb) containing compositions (E2 group: x = 40, 45, 52 and 57 wt%) exhibited plate-like morphologies associated with either Ni3Nb or Ni6Nb7 phases. Characterization of the microstructure (cell/dendrites) spacing associated with the E1 group of alloys allowed the cooling rates to be estimated. A mixture of irregular γ″-Ni3Nb and needle-like δ-Ni3Nb morphologies is present in the Ni–20%Nb alloy solidified under a cooling rate of 1.7 × 103 K/s. The passive range of the Ni–30%Nb or higher Nb content alloys was demonstrated to be at least two-fold higher when compared to that of the commercial 625 Ni-based superalloy. The concurrent evaluation of key requirements for Ni–Nb alloys such as Vickers microhardness and passivability were determined as function of Nb content. They are both strongly affected by the Nb containing.

Investigation on γ′ to γ″ transition in an IN718G superalloy during heat treatment, using first principle

By means of microstructure observation and XRD analysis, the influence of heat treatment on phases' constitution of IN718G superalloy was investigated. The results show that microstructure of IN718G alloy consists of γ matrix, particle-like γ′, disc-like γ″ phases and needle-like δ phases. As the aging time prolongs, the Nb atoms dissolve into the lattice of γ′ phase to promote the γ′ phase being transformed into γ″-Ni3Nb phase, and the coherence interfaces are kept between the phases. The calculation of First-principles indicated that in the γ′-Ni3Al and γ″-Ni3Nb phases may be co-existed in the γ matrix, the 15.625(at.%) is thought to be the critical concentration of doping Nb for promoting the transition of γ′-Ni3Al → γ″-Ni3Nb. Wherein, the close parameters of two phases in the Nb doping range of 15.625–18.75(at.%) are thought to be the essential condition of the phase transition of γ′-Ni3Al → γ″-Ni3Nb under coherence. During heat treatment, the Nb atoms of dissolved into γ″ phase may promote its growth, while the Nb atoms of dissolved into γ′ phase may promote the transition of γ′-Ni3Al → γ″-Ni3Nb phases. And the decomposition of γ′-Ni3Al phase may occur with the precipitation and growth of γ″-Ni3Nb phase, which is thought to be the main reason of increasing the volume fraction of γ″-Ni3Nb and diminishing the one of γ′-Ni3Al phase in the alloy.

Effects of the γ″-Ni3Nb Phase on Mechanical Properties of Inconel 718 Superalloys with Different Heat Treatments.

The effects of the γ″-Ni3Nb phase on the mechanical properties of Inconel 718 superalloys, with standard heat treatment, hot isostatic pressing + solution treatment + aging, and hot isostatic pressing + direct aging, were characterized by morphological observation, X-ray diffraction, and tensile experiments. The results of the morphological observation revealed that many fine γ″ precipitates of ~26.49 ± 1.82 nm in mean size were formed in all samples. However, the relatively coarser γ″ precipitates formed in the grain boundaries were only observed in the sample treated with hot isostatic pressing + direct aging. The yield strengths of the hot isostatic pressing + direct aging sample at room temperature and at 650 °C both exhibited the maximum values about 993 ± 5.7 and 811 ± 12.6 MPa, respectively. The γ″ precipitate was considered to be the dominant strengthening phase in the sample according to the lattice misfits (ε) of γ/γ″. The strengthening mechanism of the samples can be explained as the coherency strain strengthening of fine γ″ precipitates. Moreover, due to the coarser γ″ precipitates in the grain boundaries, dislocation-cut ordered particle strengthening also occurred in the sample after hot isostatic pressing + direct aging treatment.

Effects of the γ″-Ni3Nb Phase on Fatigue Behavior of Nickel-Based 718 Superalloys with Different Heat Treatments.

The effects of the γ″-Ni3Nb phase on fatigue behavior of nickel-based 718 superalloys with standard heat treatment, hot isostatic pressing + solution treatment + aging, and hot isostatic pressing + direct aging were investigated by scanning electron microscope, transmission electron microscopy, and fatigue experiments. The standard heat treatment, hot isostatic pressing + solution treatment + aging, and hot isostatic pressing + direct aging resulted in the formation of more and smaller γ″ phases in the matrix in the nickel-based 718 superalloys. However, the grain boundaries of the hot isostatic pressing + direct aging sample showed many relatively coarse disk-like γ″ phases with major axes of ~80 nm and minor axes of ~40 nm. The hot isostatic pressing + direct aging sample with a stress amplitude of 380 MPa showed the longest high cycle fatigue life of 5.16 × 105 cycles. Laves phases and carbide inclusions were observed in the crack initiation zone, and the cracks propagated along the acicular δ phases in the nickel-based 718 superalloys. The precipitation of fine γ″ phases in the matrix and relatively coarse γ″ phases in the grain boundaries of the hot isostatic pressing + direct aging sample can hinder the movement of dislocation.

Microstructures and mechanical properties of Ti3Al/Ni-based superalloy joints brazed with AuNi filler metal

For the purpose of high-temperature service and the weight reduction in aviation engineering applications, the dissimilar joining of Ti3Al-based alloy to Ni-based superalloy (GH536) was conducted using Au-17.5Ni (wt%) brazing filler metal. The microstructure and chemical composition at the interfaces were investigated by scanning electron microscope, X-ray diffraction and transmission electron microscope. The diffusion behaviors of elements were analyzed as well. The results indicated that the Ti3Al/GH536 joint microstructure was characterized by multiple layer structures. Element Ni from Au-Ni filler metal reacted with Ti3Al base metal, leading to the formation of AlNi2Ti and NiTi compounds. Element Ni from Ti3Al base metal reacted with Ni and thus Ni3Nb phase was detected in the joint central area. Due to the dissolution of Ni-based superalloy, (Ni,Au) solid solution ((Ni,Au)ss) and Ni-rich phase were visible adjacent to the superalloy side. The average tensile strength of all the joints brazed at 1253 K for 5–20 min was above 356 MPa at room-temperature. In particular, the joints brazed at 1253 K/15 min presented the maximum tensile strength of 434 MPa at room-temperature, and the strength of 314 MPa was maintained at 923 K. AlNi2Ti compound resulted in the highest hardness area and the fracture of the samples subjected to the tensile test mainly occurred in this zone.

Influences of solution cooling on microstructures, mechanical properties and hot corrosion resistance of a nickel-based superalloy

The influences of solution cooling on the microstructures, mechanical properties and hot corrosion resistance of a nickel-based superalloy are researched. It is found that the mechanical properties and hot corrosion resistance are greatly influenced by the solution cooling methods including furnace-cooling (FC), water-cooling (WC) and air-cooling (AC). For the FC superalloy, large amount of coarse γ′′ (Ni3Nb) and γ′ (Ni3Al) phases precipitate from the matrix, and δ (Ni3Nb) phase preferentially precipitates on grain boundaries. Besides, compared with the WC and AC superalloys, the ultimate tensile strength, yield strength and micro-hardness of the FC superalloy are higher, but the elongation to fracture is lower. Additionally, the hot corrosion behavior can be well described by the sulfidation-oxidation model and the alkaline-base melting model. Furthermore, the FC superalloy has the best hot corrosion resistance among the three solution cooled superalloys,