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Journal ArticleDOI

Precipitation of the δ-Ni 3 Nb phase in two nickel base superalloys

01 Mar 1988-Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science (Springer-Verlag)-Vol. 19, Iss: 3, pp 453-465

AbstractThe precipitation of the equilibrium δ-Ni3Nb phase has been studied in two niobium bearing nickel base superalloys—INCONEL 718 and INCONEL* 625—both of which are hardenable by the precipitation of the metastableγ″-Ni3Nb phase. The morphology and the distribution of precipitates have been examined and the crystallographic orientation relationship between the austenite and theδ phases has been determined. The nucleation of theδ phase at stacking faults within pre-existing δ" precipitates has been discussed.

Topics: Inconel (53%), Nucleation (52%), Precipitation (chemistry) (51%), Phase (matter) (51%), Austenite (51%)

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Citations
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Journal ArticleDOI
Abstract: Direct metal deposition technology is an emerging laser aided manufacturing technology based on a new additive manufacturing principle, which combines laser cladding with rapid prototyping into a solid freeform fabrication process that can be used to manufacture near net shape components from their CAD files. In the present study, direct metal deposition technology was successfully used to fabricate a series of samples of the Ni-based superalloy Inconel 625. A high power CO2 laser was used to create a molten pool on the Inconel 625 substrate into which an Inconel 625 powder stream was delivered to create a 3D object. The structure and properties of the deposits were investigated using optical and scanning electron microscopy, X-ray diffraction and microhardness test. The microstructure has been found to be columnar dendritic in nature, which grew epitaxially from the substrate. The thermal stability of the dendritic morphology was investigated in the temperature range 800–1200 °C. These studies demonstrate that Inconel 625 is an attractive material for laser deposition as all samples produced in this study are free from relevant defects such as cracks, bonding error and porosity.

529 citations


Journal ArticleDOI
Abstract: Inconel 718 superalloy has been fabricated by selective laser melting technology (SLM). Its microstructure and mechanical properties were studied under solution+aging (SA) standard heat treatment, homogenization+solution+aging (HSA) standard heat treatment and as-fabricated conditions. Precipitated phases and microstructures were examined using OM, SEM, TEM and X-ray analysis methods. The fine dendrite structures with an average dendrite arm spacing of approximately 698 nm accompanying some interdendritic Laves phases and carbide particles can be observed in the as-fabricated materials. After standard heat treatments, dendrite microstructures are substituted by recrystallization grains, and Laves phases also dissolve into the matrix to precipitate strengthening phases and δ particles. The test values of all specimens meet Aerospace Material Specification for cast Inconel 718 alloy, and the transgranular ductile fracture mode exists for the three conditions. The strength and hardness of heat-treated SLM materials increase and are comparable with wrought Inconel 718 alloy, whereas their ductility decreases significantly compared with the as-fabricated material. This is because of the precipitation of fine γˊ and γ〞strengthening phases and needle-like δ phases. For the as-fabricated alloy, the formation of finer dislocated cellular structures that develop into a ductile dimple fracture shows excellent ductility. Due to dislocation pinning from γˊ and γ〞strengthening phases and the impediment of dislocation motion caused by the needle-like δ phases, the ductility of the SA materials decreases and causes a transgranular fracture, compared with the as-fabricated samples.

319 citations


Journal ArticleDOI
Abstract: The precipitation and dissolution kinetics of the δ-phase were studied in three wrought versions and one spray-formed (SF) version of the nickel alloy, Inconel 718. The precipitation in the spray-formed version and one wrought version was followed during isothermal ageing for up to 100 h between 700 and 1000 °C. The δ-phase precipitates in the form of nonuniformly distributed thin platelets and an alternative method of measuring volume fraction of this precipitate morphology is proposed. At and above 800 °C, the precipitation was measured quantitatively in terms of the volume fraction and platelet thickness distribution. The maximum rate of precipitation occurred at approximately 900 °C. The solvus temperature lay between 1005 and 1015 °C for Nb contents of 5.06 and 5.41 wt.%, respectively. A study was made of the dissolution of the δ-phase in all four alloys using isochronal treatments and this confirmed the above solvus temperature range. The effect of δ-phase and its dissolution on the grain growth of the alloys was also determined.

303 citations


Journal ArticleDOI
Abstract: Several technological applications demand materials able to have good mechanical performance at relatively high temperatures (as high as 650 °C). This performance must be kept constant during long periods at these high temperatures. Superalloys, and particularly Ni–Cr–Fe alloys (Inconel series) appear to be candidates to accomplish such requirements. In these types of alloys, mechanical properties are achieved by precipitation of second phase particles after adequate thermal aging treatments. The present work is focused on studying the aging characteristics of Inconel 718. The study is complemented by the characterization of the hot forming behaviour of this material, and the effect of the particles on the deformation mechanism, and particularly on the softening mechanisms.

300 citations


Journal ArticleDOI
Abstract: The service-exposed (∼60 000 h/873 K) Alloy 625 ammonia cracker tubes showed higher strength and lower ductility compared to the virgin material in the solution annealed state. Precipitation of intermetallic γ″ and Ni2(Cr,Mo) phases and the inter and intragranular carbides were found to be responsible for higher strength of the service-exposed alloy. Subjecting the service-exposed alloy to thermal aging treatments subsequently at 923 K and 1123 K (above the service temperature of the exposed alloy) led to the dissolution of the intermetallic phases that in turn increased the ductility of the alloy. Post-service aging of the alloy at 923 K for short durations resulted in the dissolution of the Ni2(Cr,Mo)-phase. The dissolution of the Ni2(Cr,Mo)-phase exhibited significant influence upon yield strength (YS) but negligible effect on ductility. Prolonged aging of the alloy for 500 h at 923 K resulted in the precipitation of intermetallic δ-phase. Post-service aging of the alloy at 1123 K promoted the dissolution of both Ni2(Cr,Mo) and γ″ formed during service. Longer duration aging at the same temperature led to the precipitation of the δ-phase with an associated increase in strength and loss in ductility. Re-solution annealing of the service-exposed alloy at 1423 K caused the dissolution of the strengthening phases. When the re-solution annealed alloy was subjected to prolonged exposure at 923 K, the yield stress was found to increase rapidly with aging time with attendent loss in ductility due to the precipitation of γ″.

289 citations


References
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Abstract: Publisher Summary This chapter reviews current theoretical models applicable to concentrated solid solutions. It focuses on concentrated solutions because dilute ones can be considered as special cases and because the interesting effects of clustering and ordering are most apparent at high solute concentrations. The extended ranges of solubility are often found in metallic systems, and for this reason most examples selected will refer to metals and alloys, although the models presented are equally applied to off-stoichiometric compounds, metallic or inorganic, and to amorphous solids. The chapter provides description of the state of the order of the system through suitable averaging procedures. It discusses the internal energy of solutions, from both electronic and elastic standpoints. The chapter presents free energy models, such as generalized Bragg- Williams's model, Landau theory, and cluster variation method. These models are applied to the study of phase equilibrium.

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Journal ArticleDOI
Abstract: Evaluation of a commercial heat treatment for 706 alloy indicated that it resulted in relatively low 1200° F stress rupture ductility. It was determined that this was caused by a solution treatment which dissolved all of the age-hardening phases in the alloy and caused a coarse grain size and supersaturated matrix condition. Based upon extensive fine structure study of the 706 alloy as well as previous experience with 718 alloy and other Fe−Ni-base superalloys, a heat treatment is developed which effectively optimizes the 1200°F stress-rupture properties of the alloy. The key to best properties was found to be the precipitation of globular to plate-like Ni3Cb/Ni3Ti at the grain boundaries in conjunction with maintaining a fine as-forged grain structure.

38 citations