Phase In Inconel Research Articles

On the deformation induced heterogeneous precipitation of γ″ phase in Inconel 718 alloy

The unique coherency strain hardening mechanism and sluggish precipitation kinetics of γ″ phase have enabled Inconel 718 excellent elevated temperature serving performance and formability. However, the orienting character of γ″ phase determines its precipitation mutability and its morphology is susceptible to external stress and plastic deformation. Inconel 718 alloy is generally fabricated through plastic processing technologies like forging and rolling, where heterogeneous γ″ particles are commonly observed after aging. Therefore, it is vital to reveal the structural factor inducing the heterogeneous γ″ precipitation caused by deformation to lay fundamentals for particle configuration control. For this purpose, the present work aimed to define the heterogeneous γ″ configuration induced by prior deformation and explore the relevant structural mechanism. The γ″ particle morphologies in directly-aged cold-rolled Inconel 718 were characterized, prior rolling deformation resulted in heterogeneous γ″ precipitation during the aging process, which was defined as the inequivalence in their relative number fraction and particle size of perpendicular γ″-variants. Deformation led to the formation of β grains with lattice distortion in Inconel 718 alloy, which was the primary structural factor to induce the heterogeneous γ″ precipitation. These results explained that the FCC-γ lattice distortions caused by plastic deformation were responsible for the heterogeneous precipitation of second-particles.

Precipitation of δ phase in Inconel 718 superalloy: The role of grain boundary and plastic deformation

The control of δ precipitations is important during the thermal processing of Inconel 718 superalloy (IN718) because it can control the grain size and affect the mechanical properties of the material. The main purpose of this study was the effects of plastic deformation and Grain Boundary Misorientation (GBMO) on δ phase precipitation at grain boundaries (GBs). The content of the δ phase and the number of precipitated orientations at the GB were statistically analyzed. It was found that the δ phase grew into grains along the {111} plane of γ-matrix after nucleation at the GB. Plastic deformation can change the morphology and precipitate location of δ phase, with the increasing of plastic strain, the content of the δ phase and the number of precipitated orientations in GBs increased, which was related to the piling up of dislocations caused by deformation and the slip of {111} plane. For the undeformed samples, the δ phase precipitation at the GB was affected by the GBMO, which was consistent with the change of GBs energy with the GBMO. The influence of the structure and characteristics of the interfaces (GBs and twin boundaries) on the precipitation behavior of the δ phase is also analyzed in this study.

First-principles study of influence of Si on <i>γ</i> phase in Inconel 718 alloy

Inconel 718 (IN 718) is the most widely used nickel-based high-temperature alloy today. It is widely adopted in important fields such as aerospace, energy and chemicals, and is also one of the few high-temperature alloys, of which some can be fabricated by using additive manufacturing. There is a lack of research on the effect of Si on the structure and properties of IN 718 alloy on a microscopic scale. In this paper, the effect of Si doping on the γ phase in IN 718 alloy is investigated by first-principles calculations through using the CASTEP package. The lattice constants, total energy, defect formation energy, formation enthalpy, cohesive energy, density of states, and electron density difference of the γ phase are calculated before and after Si doping, and population analysis is performed. The calculation of the lattice constant reveals that the doping of Si atoms expands the cell volume of the γ phase supercell, which contributes to a certain solution strengthening effect, and is conducive to the improvement of the hardness of the alloy. The energy and electronic structure calculations show that the Si atoms prefer to occupy the Ni atomic positions in the γ phase. The number of valence electrons between the atoms, the distribution of the charge density, and the strength of the bonds between the atoms also change with Si doping, thus modifying the interaction of the atoms within the γ phase, reducing the stability of the γ phase, and favouring the precipitation of the second phase. Besides, uniform and dense IN 718 coatings with low-coat Si doping are successfully fabricated by using plasma cladding. The experimental results demonstrate that Si doping has no significant effect on the type of matrix structure of IN 718 coatings, but causes a slight expansion of the lattice of the alloy, which is consistent with the calculation result. The addition of Si can result in a transformation of the alloy coating from columnar crystal to equiaxed crystal, refining the grain size of the alloy, while reducing the volume fraction of the γ phase and increasing the volume fraction of the second phase. Moreover, the addition of Si exacerbates the segregation of Nb and Cr elements in the IN 718 coatings.

Microstructural evolution and strengthening mechanisms of Inconel 718 alloy with different W addition fabricated by laser cladding

In this study, Inconel 718 alloys with various W additions (0, 6, 12, and 18 wt%) were fabricated by laser cladding. The influences of W addition on the microstructure and mechanical properties of the as-deposited Inconel 718 alloy were studied. The results show that with the rise of W addition, the fraction of Laves phase, the low-angle grain boundaries (LAGBs), and the dislocation density monotonically increase, whereas the grain size and interdendritic region of Inconel 718 alloy decrease, and the alloy with no interdendritic region can be obtained at a W addition of 18 wt%. Additionally, Fe2W phase is observed above the striped Laves phase in Inconel 718-18 W alloy. As the introduction of W increases from 0 to 18 wt%, the microhardness of alloys enhances from 273.1 ± 19.4 HV to 380.2 ± 12.6 HV. Moreover, the ultimate tensile strength (UTS) and yield strength tensile (YS) of the Inconel 718 are constantly enhanced by increasing W addition, while the elongation (El) decreases. The as-deposited alloy sample with 12 wt% W addition exhibits excellent comprehensive properties compared to the other samples. The YS, UTS, and El of Inconel 718-12 W alloy are 749.96 ± 16.25 MPa, 1106.71 ± 30.39 MPa, and 18.61 ± 1.47%, respectively. The excellent comprehensive mechanical properties of the Inconel 718-12 W alloy is mainly attributed to dislocation strengthening and solid solution strengthening. The present study provides a new way to achieve as-deposited Inconel 718 alloy with excellent synthetical mechanical properties.

Introduction of a New Method for Regulating Laves Phases in Inconel 718 Superalloy during a Laser-Repairing Process

• An in situ laser heat treatment method has been proposed and verified. • In situ regulation of Laves phase can be achieved during LAM process. • A flow diagram has been made based on the in situ laser heat treatment method. The morphology, size, and distribution of Laves phases have important influences on the mechanical properties of laser-repaired Inconel 718 (IN718) superalloy. Due to the deterioration of the substrate zone, the Laves phase in the laser cladding zone of IN718 superalloy cannot be optimized by a high-temperature solution treatment. In this study, an in situ laser heat-treatment method was proposed to regulate the morphology and size of the Laves phase in the laser cladding zone of IN718 superalloy without impacting the substrate zone. In the in situ laser heat-treatment process, a laser was used to heat previously deposited layers with optimized manufacturing parameters. A thermocouple and an infrared camera were used to analyze thermal cycles and real-time temperature fields, respectively. Microstructures and micro-segregations were observed by optical microscopy, scanning electron microscopy, electron probe microanalysis. It was found that the in situ laser heat treatment effectively changed the morphology and size of the Laves phase, which was transformed from a continuous strip-like shape to a discrete granular shape. The effective temperature range and duration were the two main factors influencing the Laves phase during the in situ laser heat-treatment process. The effective temperature range was determined by the laser linear energy density, and the peak temperature increased with the increase of the linear energy density. In addition, the temperature amplitude could be reduced by simultaneously increasing the laser power and the scanning velocity. Finally, a flow diagram was developed based on the in situ laser heat-treatment process, and the deposition of a single-walled sample with fine and granular Laves phases was detected.

The formation and dissolution mechanisms of Laves phase in Inconel 718 fabricated by selective laser melting compared to directed energy deposition and cast

In this study, we explain why the amount of Laves phase in Inconel 718(IN718) manufactured by selective laser melting (SLM) unexpectedly increases under extremely rapid solidification and clarify its distinct dissolution behavior during heat treatment based on a comparative analysis of laser directed energy deposition (L-DED)-built and cast IN718. Compared with the cast and L-DED, SLM has much higher cooling rate, which greatly improves the dendrite growth rate and the solute trapping during the solidification process of molten pool. Thus, more Nb atoms are trapped in the matrix, and less Nb atoms are expelled to form inter-dendritic Laves phase in SLM-built IN718. However, the volume fraction of Laves phase is the highest in SLM-built IN718, which is attributed to the lower Nb concentration in the Laves phase and few Nb-rich region. During homogenization treatment, the dissolution of Laves phase is controlled by three-dimensional diffusion process at the early stage, and then is mainly controlled by the interface reaction process. After homogenization, there are still many tiny carbides, oxides, and nitrides undissolved both along the grain boundaries and inside the grains.

Early evolution of δ phase and coarse γ´´ phase in Inconel 718 alloy with high temperature ageing

The early evolution of the δ phase and coarse γ´´ phase in aged Inconel 718 alloy was studied by transmission electron microscope. The results show that the needle-like δ phase first nucleates at the grain boundary and then grows into the grain interior. It is found that two δ phases with different orientations were combined to form one δ phase as the ageing time extended to 4 h. In addition, the δ phase maintains a good orientation relationship with the matrix during ageing. The coarse γ´´ phase nucleates at the dislocation, and the increase in size leads to the loss of coherence with the matrix, in which the strengthening effect is not obvious.

Titanium Carbide Effects on the Fracture of Inconel 100—A Case Study

Carbides and borides appear as minor phases in Inconel 100, although the carbon content (0.18%) is relatively high in comparison to other nickel-base superalloys. The material properties of this alloy depend on a number of interrelated microstructural parameters, including the volume fraction of γ' to γ, grain size, elemental distribution, and precipitation of carbides and borides. This study presents a characterization and a failure analysis of Inconel 100 samples loaded to fracture at 760℃ and an examination of their fractography. Chemical analysis, optical metallography, SEM and EDS were used for the characterization of the unusual areas of fracture that were found on the samples. The thermodynamic stability of TiC led to the conclusion that the possibility of creating very large, stable mono carbides, especially TiC carbide, during the production process is the reason for the short time to failure obtained in this work.

Spheroidization Mechanism of Lamellar δ Phase in Inconel 718 during Delta Processing

The morphology evolution of lamellar δ phase in Inconel 718 during Delta Processing was characterized by scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. A spheroidization mechanism is proposed for transformation of the lamellar δ phase. The results showed that lamellar δ phase hindered the movement of dislocations, which led to a dislocation aggregation zone around the fracture location of the δ phase. Strain in the region thus increased, which promoted the occurrence of dynamic recrystallization and fracture of the lamellar δ phase. A diffusion channel for Nb atoms was simultaneously provided by the high-density dislocations, which led to dissolution and reduction in the amount of δ phase. Under the combined action of deformation, fracture, and dissolution, the lamellar δ phase gradually transformed into granular short rods and spheroids.

Determining volume fractions of γ, γ′, γ″, δ, and MC-carbide phases in Inconel 718 as a function of its processing history using an advanced neutron diffraction procedure

In this work, quantitative crystallographic and microstructural analyses of γ, γ′, γ″, δ, and MC carbide phases are performed on wrought samples, samples fabricated via additive manufacturing (AM), and samples that underwent hot isostatic pressing (HIP) after AM of alloy Inconel 718 (IN718). In doing so, an advanced neutron diffraction-based procedure is developed facilitating the determination of volume fractions of every detectable phase in the alloy. To supplement the diffraction procedure, precipitate sizes are measured by scanning electron microscopy. Moreover, semi-quantitative elemental analyses are performed by energy dispersive spectroscopy. Finally, image thresholding is carried out on micrographs of samples that underwent cathodic dissolution to create secondary electron contrast between phases to verify the phase fractions determined from the neutron diffraction datasets. The study reveals a significantly higher volume fraction of δ phase and a significantly lower volume fraction of γ″ phase governing a higher strength of the AM material relative to the lower strength AM + HIP and wrought materials. Furthermore, γ′ and MC volume fractions are found similar in the materials despite the differences in MC morphology, elemental composition and distribution controlling the dispersion strengthening. These results are presented and discussed in this paper along with the procedure developed for determining volume fractions of all detectable phases present in the alloy.

Investigation of dissolution behavior of laves phase in inconel 718 fabricated by laser directed energy deposition

The mechanical properties of Inconel 718 are closely related to the morphology and size of the Laves phase, which must be quantitatively controlled to change the effect of the Laves phase from deleterious to beneficial. In this study, post-heat treatment was used to regulate the morphology and size of the Laves phase in Inconel 718 fabricated using laser directed energy deposition, and the dissolution behavior of the Laves phase during solution heat treatment was investigated. The results indicated that the sharp corners and grooves of the Laves phase preferentially dissolved, causing the morphology of the Laves phase to change from a long-striped to granular shape during dissolution. The dissolution kinetics of the Laves phase were also investigated using the Johnson–Mehl–Avrami–Kolmogorov and Singh–Flemings models. The initial stage of dissolution was controlled by both the long-range diffusion of Nb and the interfacial reaction. However, with decreasing degree of Nb segregation, the interfacial reaction became dominant.

Dendritic morphology evolution and microhardness enhancement of rapidly solidified Ni-based superalloys

The mechanisms of microstructure evolution and dendritic growth of Inconel alloys in a drop tube were investigated in this paper. The Vickers microhardness of the solidified alloy droplets was also measured to explore the effects of grain size and precipitated phase on the mechanical properties. From XRD results, the rapidly solidified Inconel 600, 617, 625, and 718 alloys are characterized by γ phase solid solution with fcc structure. Further analyses based on EDS and SEM reveal that Laves phase precipitates at the grain boundaries of γ phase in Inconel 718 alloy. With the decrease of droplet diameter (D), the dendritic morphology experiences a transformation of “orthogonal long dendrites→irregular stubby dendrites→equiaxed grains” in Inconel 600 alloy, “orthogonal long dendrites → stubby irregular dendrites” in Inconel 625 alloy and “orthogonal long dendrites→equiaxed grains” in Inconel 617 and 718 alloys. Although the dendrites become coarse locally, the dendritic grain size obviously reduces with the decrease of droplet diameter, leading to the increase of Vickers microhardness. For Inconel 718 alloy, the Vickers microhardness firstly decreases, when the droplet diameter decreases from 1000 to 900 µm, and then increases linearly with the decrease of droplet diameter. It is found that the Vickers microhardness of Inconel 625 alloy is the largest among the four alloys.

Micro-mechanisms and precipitation kinetics of delta (δ) phase in Inconel 718 superalloy during aging

The formation of δ phase in Inconel 718 superalloy during aging was quantitatively studied based on improved analyzing techniques to unambiguously determine the mechanism of δ phase precipitation and its kinetics. Based on the general transformation rate expressed in terms of the time corresponding to the transformed fraction of 0.5 or 50% and also the modified Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation, the average activation energy for the formation of δ phase was determined as 212.4 kJ/mol and 197.5 kJ/mol, respectively. These values are comparable to the activation energy for diffusion of Nb in Ni (202.59 ± 4.71 kJ/mol). Therefore, the micro-mechanism for the formation of δ phase was characterized as the diffusion of Nb in the matrix. Based on the kinetic studies, the temperature of 1055.7 K (782.7 °C) was found as the lower bound for the formation of δ phase, which was consistent with the precipitation-temperature-time (PTT) plot developed for this material. Finally, a formula was obtained to quantitatively show the effects of aging temperature and cold rolling reduction on the transformation rate for the precipitation of δ phase.

Bond strength measurement for additively manufactured Inconel 718- GRCop84 copper alloy bimetallic joints

To benefit from the fascinating properties of multi-material structures, the interfacial joint should exhibit good mechanical strength. Evaluating the shear strength of a bimetallic joint via conventional methods is usually complex, and in most cases produces unreliable data due to induced bending stress among others. In this work, a novel single-shear test device was designed and fabricated to measure shear strength of bimetallic joints. The device was first standardized by shearing standard materials, and the results were in good agreement with published data. Subsequently, the shear strength of Inconel 718/copper alloy (GRCop-84) bimetallic joint built via laser engineered net shaping (LENS™) was evaluated. Compression test on the bimetallic joint was carried out as well for more mechanical characterization. Both shear and compressive yield strengths of the bimetallic joints were compared with the base materials in addition to influence of thermal cycling on the joint strength. Inconel 718/GRCop-84 bimetallic-joint shear strength was 220 ± 18 MPa and 231 ± 27 MPa for as-printed sample and after thermal cycling, respectively. Likewise, the bimetallic yield strength after compression test was 232 ± 3 MPa and 337 ± 15 MPa. No cracking through or along the interface was observed even after thermal cycling, which indicates no thermal degradation at the bimetallic interfacial joint. Increase in compressive yield strength after thermal cycling could be attributed to precipitation of Cr2Nb particles in GRCop-84 matrix along with strengthening due to gamma phases in Inconel 718. Scanning electron microscopy (SEM) and backscatter electron imaging were used to examine the interfacial microstructures and failure modes. EDS was used as well to analyze the interface elemental composition. The development of the single-shear test device can provide an added opportunity to effectively evaluate mechanical behavior, reliability and performance of additively manufactured multi-material structures through bond strength analysis.

Effect of Strain Hardening on Precipitation Kinetics in ATI 718Plus

ATI 718Plus components are manufactured by forging a wrought billet in stages to obtain the desired geometry and microstructure. Parts are then heat treated to optimized proportions of γ’ and η phases. η phase is a plate-like phase that precipitates on the grain boundaries of ATI 718Plus, similar to δ phase in Inconel 718. However, the complete kinetic behaviour of η phase precipitation during forging and heat treatment is still not fully understood. This paper investigates the effects of strain hardening on η phase precipitation kinetics in ATI 718Plus. This is achieved through the use of isothermal hot compression tests and heat treatment. Strain hardening was found to affect the η precipitation kinetics considerably. The results reported are a contribution to a fuller understanding of this important process

Effect of hot deformation on γ″ and δ phase precipitation of Inconel 718 alloy during deformation&isothermal treatment

A deformation&isothermal treatment is applied to investigate the effect of hot deformation on precipitation of γ″ and δ phase in Inconel 718 alloy. The results show the precipitation behaviors are tailored by the deformation temperatures that γ″ phase precipitation is accelerated at 800 °C in place of δ phase as for 900 °C. Deformation at 800 °C and 900 °C makes the precipitation peaks for γ″ phase shift towards lower temperatures to be 742 °C and 735 °C, while the precipitation peaks for δ phase moving towards higher temperatures to be 941 °C and 943 °C, respectively. When subjected to the deformation&isothermal treatment at 800 °C, the thus formed structure is composed of ellipsoidal, disc and irregular-shaped γ″ precipitates with sizes varying from 80 nm to 440 nm in the interior of broadened GBs, as well as dispersively ultrafine intragranular γ″ particles. As for the specimen deformed at 900 °C, the δ-phase precipitation is promoted under the effect of static recrystallization occurring at the initial aging stage, that granular, disc and needle-shaped δ precipitates with 7.5 vol % are generated in the limited duration of 1 h.

Dimensionless numbers in additive manufacturing

The effects of many process variables and alloy properties on the structure and properties of additively manufactured parts are examined using four dimensionless numbers. The structure and properties of components made from 316 Stainless steel, Ti-6Al-4V, and Inconel 718 powders for various dimensionless heat inputs, Peclet numbers, Marangoni numbers, and Fourier numbers are studied. Temperature fields, cooling rates, solidification parameters, lack of fusion defects, and thermal strains are examined using a well-tested three-dimensional transient heat transfer and fluid flow model. The results show that lack of fusion defects in the fabricated parts can be minimized by strengthening interlayer bonding using high values of dimensionless heat input. The formation of harmful intermetallics such as laves phases in Inconel 718 can be suppressed using low heat input that results in a small molten pool, a steep temperature gradient, and a fast cooling rate. Improved interlayer bonding can be achieved at high Marangoni numbers, which results in vigorous circulation of liquid metal, larger pool dimensions, and greater depth of penetration. A high Fourier number ensures rapid cooling, low thermal distortion, and a high ratio of temperature gradient to the solidification growth rate with a greater tendency of plane front solidification.

Effect of cold rolling and first precipitates on the coarsening behavior of γ″-phases in Inconel 718 alloy

The coarsening behaviors of γ″-phase particles in Inconel 718 alloy aged at 750, 800, and 850°C were investigated by scanning electron microscopy (SEM). Detailed observations and quantitative measurements were conducted to characterize the coarsening behavior of the γ?-phase under various aging conditions. The experimental results indicate that the existence of the δ-phase retards the formation and coarsening of the γ″-phase, without influencing its final particle size or amount. Moreover, when cold rolled with a reduction of 50%, the dimensions of the γ″ particles in Inconel 718 alloy decrease with increasing aging time. Furthermore, the coarsening behavior of the γ″-phase in the Inconel 718 alloy after a normal aging treatment (sample A) and that of the primary δ-phase (sample B) follow the Lifshitz–Slyozov–Wagner (LSW) diffusion-controlled growth theory; the thus-obtained activation energies for the γ″-phase are 292 kJ·mol-1 and 302 kJ·mol-1, respectively.

Effects of cold rolling on the precipitation kinetics and the morphology evolution of intermediate phases in Inconel 718 alloy

Isochronal DSC measurements of various degrees cold-rolled Inconel 718 alloy samples were implemented to investigate the effect of cold rolling on the precipitation kinetics of intermediate phases in Inconel 718 alloy. The result shows that the values of the onset temperature ( TOnset1), the peak temperature (TPeak1) and the end temperature (TEnd1) of γ″-phase precipitation decrease as the degree of cold rolling increases. In contrast, all values of the onset temperature (TOnset2), the peak temperature (TPeak2) and the end temperature (TEnd2) of δ-phase precipitation first increase and then decrease with the degree of cold rolling increasing as a consequence of a combined effect of lack of Nb atoms and recrystallization. Two sets of isothermal aging experiments were also carried out to study the effect of cold rolling on the morphology evolution of γ″-phase and δ-phase in Inconel 718 alloy. It is found that cold rolling promotes γ″→δ transformation and the morphology of γ″-phase turns more globular as the degree of cold rolling increases for the cold-rolled Inconel 718 alloy samples with isothermal aging at 800 °C for 8 h (procedure A). Moreover, for the cold-rolled Inconel 718 alloy samples with isothermal aging at 950 °C for 6 h (procedure B), cold rolling facilitates δ-phase precipitation and the δ-phase particles gradually sphericalized with increase in the degree of cold rolling.

Strain-induced Precipitation Behavior of δ Phase in Inconel 718 Alloy

To study the precipitation dynamics of δ phase in Inconel 718 alloy, two-stage interrupted compression method was used in the region of cold deformation temperatures and the temperatures range from 875 to 975 °C. The precipitation-time-temperature (PTT) curve of δ phase was obtained by analyzing the softening kinetics curves. For verifying the type of the precipitates and confirming the validity of the test, the transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy dispersion spectrum (EDS) were employed. Experimental results indicated that the PTT curve for δ precipitation exhibited a typical “C” shape and the nose points of start and finish precipitation were about 5 s at 920 °C and 2815 s at 940 °C, respectively. In addition, the nucleation of δ was heterogeneous. The nucleation sites varied with temperatures, including dislocation, grain boundary and stacking fault within γ′ phase. And δ particles grew quickly at higher temperature with lower density. Moreover, the driving force of nucleation was mainly including chemical free energy, interfacial energy and dislocation distorted energy. And the dislocation distorted energy could decide the density of nucleation in the strain-induced process.