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Showing papers on "Inconel published in 1988"


Journal ArticleDOI
TL;DR: In this article, the precipitation of the metastable δ-Ni3Nb phase has been studied in two niobium bearing nickel base superalloys and the morphology and distribution of precipitates have been examined and the crystallographic orientation relationship between the austenite and the δ phases has been determined.
Abstract: The 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.

383 citations



Journal ArticleDOI
TL;DR: In this article, the effects of varying the strengthening alloying elements on the microstructural stability of the superalloy INCONEL 718 have been investigated using scanning electron microscopy and X-ray and synchrotron diffractometry.
Abstract: The effects of varying the strengthening alloying elements on the microstructural stability of the superalloy INCONEL 718 have been investigated using scanning electron microscopy, transmission electron microscopy, and X-ray and synchrotron diffractometry. Time-temperature-transformation curves were determined for the embrittling σ-phase using conventional and synchrotron X-ray diffractometry. Increasing the Al/Ti ratio and the Al + Ti concentration in the alloy decreases the driving force both to form σ phase and coarsen γ". Abbreviated mechanical tests also indicate that the alloys with the highest Al + Ti content and Al/Ti ratio are more stable than the commercial “SUPER” IN718 after long times at elevated temperatures.

118 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of electric discharge machining on the fatigue life of Inconel 718 alloy at room temperature was investigated, and the results were explained using the data produced employing microhardness measurements, profilometry, and optical and scanning microscopy.

60 citations


Proceedings ArticleDOI
TL;DR: A modified heat treatment for Inconel 718 has been developed in this article, which is a result of tailoring the microstructure to obtain the optimum combination of grain size, grain boundary structure and matrix precipitate morphology.
Abstract: A new modified heat treatment has been developed for Inconel 718. This heat treatment leads to substantial improvements in elevated temperature crack propagation resistance with apparently limited loss in resistance to LCF crack initiation as compared to the conventional heat treatment for this alloy. This is a result of tailoring the microstructure to obtain the optimum combination of grain size, grain boundary structure and matrix precipitate morphology.

39 citations


Journal ArticleDOI
TL;DR: In this paper, a test program to determine the optimum composition of chromium carbide based solid lubricant coatings for compliant gas bearing is described, and the friction and wear properties of the coatings are evaluated using a foil gas bearing test apparatus.
Abstract: A test program to determine the optimum composition of chromium carbide based solid lubricant coatings for compliant gas bearings is described. The friction and wear properties of the coatings are evaluated using a foil gas bearing test apparatus. The various coatings were prepared by powder blending, then plasma sprayed onto Inconel 718 test journals and diamond ground to the desired coating thickness and surface finish. The journals were operated against preoxidized nickel-chromium alloy foils. The test bearings were subjected to repeated start/stop cycles under a 14 kPa (2 psi) bearing unit load. The bearings were tested for 9000 start/stop cycles or until the specimen wear reached a predetermined failure level. In general, the addition of silver and eutectic to the chromium carbide base stock significantly reduced foil wear and increased journal coating wear. The optimum coating composition, PS212 (70 wt% metal bonded Cr3C2, 15 wt% Ag, 15% BaF2/CaF2 eutectic), reduced foil wear by a factor of two and displayed coating wear well within acceptable limits. The load capacity of the bearing using the plasma-sprayed coating prior to and after a run-in period was ascertained and compared to polished Inconel 718 specimens.

35 citations


Proceedings ArticleDOI
TL;DR: In this paper, seven new iron-nickel base superalloy compositions were designed to study the effect of systematically changing the Al, Ti, and Nb contents on the microstructural stability and long term elevated temperature mechanical properties.
Abstract: Recent advances in the jet engine industry are placing new demands for an alloy which exhibits the excellent fabricability and heat resistant properties of IN718, but which is not hampered by the alloy’s relatively low temperature ceiling. This ceiling is imposed by the microstructural instabilities that occur fairly rapidly at these higher temperatures. Seven new iron-nickel base superalloy compositions were designed to study the effect of systematically changing the Al, Ti, and Nb contents on the microstructural stability and long term elevated temperature mechanical properties. Rockwell hardness, tensile, and creep/stress rupture tests were performed on these alloys to determine their extended elevated temperature properties, while optical, scanning electron, and transmission electron microscopy, and x-ray diffractometry were used to analyze their phase stability. Increasing the Al + Nb content and the Al/% ratio in the alloy was found to (1) produce more of the stable y’ phase and less of the brittle 6 phase, and (2) enhance the mechanical properties of the alloy. Preliminary results indicate that the alloy containing a greater Al + Nb content and Al/Ti ratio is more stable than Inconel 718.

27 citations


Proceedings ArticleDOI
TL;DR: In this article, the interrelationship between the physical metallurgy and properties of the alloy are explored in a two level factorial study of processing (two levels of Laves precipitation) and age hardening heat treatments (the alloy's standard heat treat cycle and a short time cycle).
Abstract: INCOLOY* alloy 909 is an iron-nickel-cobalt-niobium-titanium controlled low thermal expansion superalloy with an intentional silicon addition. Recommended for use at temperatures to 650°C, the alloy is now specified for major new gas turbine engines. The pseudo-equilibrium time temperature transformation behavior of alloy 909 is presented as well as a brief description of the major phases: Y’ 7 E” , E, and Laves. Gamma prime (r’), the major strengthening phase in the alloy, is shown to precipitate in the 538OC to 760°C rangy. The a” and E phases, which are similar to y” and 6 phase found in INCONEL alloy 718, precipitate at intermediate temperatures (about 700°C to 95OOC). Laves phase, which is used to control grain size, precipitates at the higher temperatures (800°C-1040°C) used during hot working and annealing. The alloy’s Si addition significantly affects the formation of these phases during processing, thereby affecting mechanical properties. The interrelationship between the physical metallurgy and properties of the alloy are explored in a two level factorial study of processing (two levels of Laves precipitation) and age hardening heat treatments (the alloy’s standard heat treat cycle and a short time cycle). Notched bar rupture tests and creep crack growth tests at 538OC demonstrate that resistance to stress accelerated grain boundary oxygen embrittlement (SAGBO) improves with precipitation of intergranular E phase. Excess Laves precipitation lowers residual Nb content, consequently reducing E abundance and reducing crack growth resistance. Combined with proper thermomechanical processing, the short aging cycle precipitates an E grain boundary structure which significantly improves SAGBO resistance.

26 citations


Journal ArticleDOI
TL;DR: In this article, the stress corrosion cracking susceptibilities of Alloy 600 and 690, AISI 316 NG stainless steel (SS), ASTM A508 carbon steel, and a number of compatible weld metals have been evaluated.
Abstract: The stress corrosion cracking (SCC) susceptibilities of Alloys 600 and 690, AISI 316 NG stainless steel (SS), ASTM A508 carbon steel, and a number of compatible weld metals have been evalu...

23 citations


Journal ArticleDOI
TL;DR: A transition metal joint between type 304 stainless steel and 2·25Cr-1Mo steel, with Alloy 800 as the transition piece, is being developed for application in the steam generator circuit of the 500 MW prototype fast breeder reactor.
Abstract: A transition metal joint between type 304 stainless steel and 2·25Cr–1Mo steel, with Alloy 800 as the transition piece, is being developed for application in the steam generator circuit of the 500 MW prototype fast breeder reactor. As part of this programme, the hot cracking susceptibility of Inconel 82/182 and of 16–8–2 welding consumables were compared and the microstructure and mechanical properties of butt welds between type 304 stainless steel and Alloy 800, welded by the two consumables, were studied to select the appropriate welding consumables for this joint. It is recommended that the 16–8–2 consumable should be used for welding this joint because of its lower microfissuring tendency and reduced mismatch in the coefficient of thermal expansion across the joint, although this would mean a slight adverse effect on the elevated temperature mechanical properties. Further, to select the optimum post-weld heat treatment (PWHT) of the joint between Alloy 800 and 2·25Cr–1Mo steel, welded with Inc...

22 citations


Journal ArticleDOI
TL;DR: The carbon solubility, stress corrosion cracking (SCC) resistance, and microstructure of Inconel Alloy 690 was studied and compared to Alloy 600 in this article, showing that Alloy690 exhibited lower carbon-solubility than Alloy 600 and, after a high temperature/short time thermal treatment, displayed greater SCC resistance in 10% NaOH than Alloy600 which had been thermally treated at 1300 F (704 C).
Abstract: The carbon solubility, stress corrosion cracking (SCC) resistance, and microstructure of Inconel Alloy 690 was studied and compared to Alloy 600. Alloy 690 exhibited lower carbon solubility than Alloy 600 and, after a high temperature/short time thermal treatment, displayed greater SCC resistance in 10% NaOH than Alloy 600 which had been thermally treated at 1300 F (704 C) for 15 h.

Patent
Keh-Minn Chang1
05 Jul 1988
TL;DR: In this article, an alloy is disclosed which has been found to lend itself particularly well to thermomechanical processing and is strengthened by precipitates similar to those of Inconel 718.
Abstract: An alloy is disclosed which has been found to lend itself particularly well to thermomechanical processing. The alloy is strengthened by precipitates similar to those of Inconel 718 but the alloy matrix of the composition is a nickel-chromium-cobalt matrix rather than the nickel-chromium-iron matrix of the Inconel alloy. Also the alloy has grains of average diameter of 35 μm or larger. The fatigue resistance, tensile strength and the rupture strength of the alloy is improved to a remarkable degree as a result of the thermomechanical processing. The thermomechanical processing is carried out below the recrystallization temperature to prevent nucleation of fine grains. The residual strains from the thermomechanical processing or cold working provides the remarkably favorable combination of alloy properties which are found.

Journal ArticleDOI
TL;DR: Inconel 600 is a nickel-based superalloy which has been shown to exhibit stress corrosion cracking under certain conditions as mentioned in this paper, and the grain boundary interface was analysed using scanning Auger microscopy (SAM) with the grain boundaries being exposed, only after hydrogen embrittlement, using an in situ tensile stage.
Abstract: Inconel 600 is a nickel-based superalloy which has been shown to exhibit stress corrosion cracking under certain conditions. In general the stress corrosion cracking (SCC) is intergranular in nature and hence attention has been focused on the grain boundary microstructure. A series of model alloys was produced with a range of impurity levels. Initially a high-purity base alloy was prepared to which boron, carbon, silicon and phosphorus was added. Alloys were then examined in the mill-annealed state and after a standard thermal ageing treatment. The grain boundary interface was analysed using scanning Auger microscopy (SAM) with the grain boundaries being exposed, only after hydrogen embrittlement, using an in situ tensile stage. The segregation of phosphorus and boron to the grain boundary surface was studied and a number of particles identified at grain boundaries. The effect of the segregants on the mechanical properties of this alloy is discussed.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the near-threshold fatigue crack growth rate (FCGR) behavior of Inconel 706 at ambient (297K) and liquid helium (4.2 K) temperatures, respectively.

Proceedings ArticleDOI
TL;DR: In this article, the mechanisms leading to failure during high temperature, LCF and creep-fatigue are presented and discussed for two representative, commercial ODS (Oxide DispersionStrengthened) superalloys, Inconel MA 754 and MA 6000.
Abstract: The mechanisms leading to failure during high temperature, LCF and creep-fatigue are presented and discussed for two representative, commercial ODS (Oxide DispersionStrengthened) superalloys, Inconel MA 754 and MA 6000. The fatigue behavior of these alloys is compared with that of conventional (non-ODS) superalloys of nearly identical elemental composition. The behavioral differences may be understood on the basis of the two primary microstructural differences between ODS and non-ODS, namely, grain structure and the fine particle dispersion itself. It is found that recrystallization defects in the form of fine grains are the primary cause of crack initiation in the ODS materials. The results are discussed in light of previous creep and fatigue studies of dispersion-strengthened alloys.

Journal ArticleDOI
TL;DR: In this article, the characteristics of self-welding of fast breeder reactor structural materials in liquid sodium have been investigated and a maximum contact pressure of 98 MPa was loaded on the materials (Type 304, Type 321, Inconel 718, and 2.25 Cr-1 Mo steel) for 100 to 900 h in sodium at 505 to 550 0/C.
Abstract: The characteristics of self-welding of fast breeder reactor structural materials in liquid sodium have been investigated. A maximum contact pressure of 98 MPa was loaded on the materials (Type 304, Type 321, Inconel 718, and 2.25 Cr-1 Mo steel) for 100 to 900 h in sodium at 505 to 550/sup 0/C. Shear stress for breakaway between the bonded material couples is in proportion to contact pressure. The 2.25 Cr-1 Mo steel welded to itself is the strongest combination among couples of the same material. Each pair of two materials chosen from Type 304, Type 321, and Inconel 718 materials shows almost the same self-welding coefficient, which is defined as the ratio between shear stress for breakaway and contact pressure on testing. Self-welding coefficients are in proportion to the square root of the contact periods and increase as the temperature of sodium is elevated. The apparent activation energy obtained from the self-welding coefficient is -- 188 kJ/mol for the combination of 2.25 Cr-1 Mo/2.25 Cr-1 Mo, and 218 kJ/mol for Type 304 or Type 304/Type 321.


Proceedings ArticleDOI
TL;DR: In this paper, the rotating bending fatigue tests were conducted for nickel-base Inconel 751, nickel-iron-based InConel 718 and Fe-42Ni-15Cr-3Mo alloy, at 800°C in the NazSOb-NaCl molten salt environment as well as in air.
Abstract: In order to clarify an effect of hot corrosive environment both on the fatigue strength and the fatigue fracture behavior of superalloys, the rotating-bending fatigue tests were conducted for nickel-base Inconel 751, nickel-iron-base Inconel 718 and Fe-42Ni-15Cr-3Mo alloy, at 800°C in the NazSOb-NaCl molten salt environment as well as in air. A grain size was controlled in a wide range from the viewpoint of its practical importance. It was revealed that hot corrosive environment brings about a significant fatigue strength degradation for all the alloys, in particular with reduced grain size, by affecting the initiation and propagation processes of the fatigue cracks. A grain size was confirmed to concern strongly in the corrosion fatigue crack propagation behavior. In particular, the detrimental effect of reducing a grain size was emphasized on the enhanced intergranular crack propagation. A fundamental criterion of the grain size control for the improved corrosion fatigue strength properties of superalloys was presented in connection with a Ni content.


Book ChapterDOI
01 Jan 1988
TL;DR: In this article, high temperature fatigue crack growth rates in air and in vacuum were measured at different frequencies with triangular waveshape (R=0.05) in the following nickel base superalloys: the wrought alloy Inconel X-750, the cast alloys IN 792+Hf and IN 713 LC, and the ODS alloy INconel MA 6000.
Abstract: High temperature fatigue crack growth rates in air and in vacuum were measured at different frequencies with triangular waveshape (R=0.05) in the following nickel base superalloys: the wrought alloy Inconel X-750, the cast alloys IN 792+Hf and IN 713 LC, and the ODS alloy Inconel MA 6000. The experimental results have shown that, in general for a given ΔK, fatigue crack growth rates (FCGR) of all the alloys increase when frequency is decreased in both air and vacuum tests. Different environmental behaviours were observed in the various materials. The Inconel X-750 alloy presents a strong oxidation sensitivity and FCGR in vacuum are much lower than in air. The effect of oxidation on FCGR of cast alloys is less remarkable than in Inconel X-750. In the ODS alloy, however, FCGR in vacuum are higher than in air in all the conditions investigated.

Journal ArticleDOI
TL;DR: Inconel X750 stress relaxation data have been reported in both thermal and fast reactors as mentioned in this paper, and the results of these stress relaxation tests confirm the expectation that more stress relaxation should occur for a thermal reactor bend test relative to a fast reactor torsion test.

01 Apr 1988
TL;DR: In this paper, the authors used the Chaboche theory of viscoplasticity to model the viscopolynesian behavior of Inconel 718 at 1200 F, the operating temperature for turbine blades.
Abstract: A large number of tests, including tensile, creep, fatigue, and creep-fatigue were performed to characterize the mechanical properties of Inconel 718 (a nickel based superalloy) at 1200 F, the operating temperature for turbine blades. In addition, a few attempts were made to model the behavior of Inconel 718 at 1200 F using viscoplastic theories. The Chaboche theory of viscoplasticity can model a wide variety of mechanical behavior, including monotonic, sustained, and cyclic responses of homogeneous, initially-isotropic, strain hardening (or softening) materials. It is shown how the Chaboche theory can be used to model the viscoplastic behavior of Inconel 718 at 1200 F. First, an algorithm was developed to systematically determine the material parameters of the Chaboche theory from uniaxial tensile, creep, and cyclic data. The algorithm is general and can be used in conjunction with similar high temperature materials. A sensitivity study was then performed and an optimal set of Chaboche's parameters were obtained. This study has also indicated the role of each parameter in modeling the response to different loading conditions.

Patent
21 Sep 1988
TL;DR: In this paper, an oxalate film is formed on the surface of high-nickel alloy steel such as incoloy, inconel or stainless steel containing >=20wt.% of nickel, followed by treatment with, as the lubricating agent, an aqueous solution made up of (A) 10-35pts. of acrylic resin with the glass transition point regulated to -50-10 deg.
Abstract: PURPOSE:To afford outstanding coefficient of friction and wear-resistant baking characteristics by forming an oxalate film on the surface of high-nickel alloy steel followed by treatment with an aqueous solution containing acrylic polymer and wax, etc., and then heating and baking. CONSTITUTION:An oxalate film is formed on the surface of high-nickel alloy steel such as incoloy, inconel or stainless steel containing >=20wt.% of nickel, followed by treatment with, as the lubricating agent, an aqueous solution made up of (A) 10-35pts.wt. of an acrylic resin with the glass transition point regulated to -50-10 deg.C, (B) 3-15pts.wt. of wax, (C) 0.5-5pts.wt. of a surfactant and (D) the rest of water with the weight ratio A/B regulated to 2-12, and the drying at normal temperature, heating and baking, thus accomplishing the objective lubricating treatment.

Journal ArticleDOI
TL;DR: A single step aging treatment of 200 hours at 811 °C followed by furnace cooling after solution treating for 2 hours at 1075 °C has been found to provide an improved combination of strength, ductility, and resistance to SCC under simulated PWR test conditions.
Abstract: Unfavorable morphology of precipitates and inclusions has been thought to be the cause of severe intergranular stress corrosion cracking (IGSCC) in double aged INCONEL* X-750 alloy used in reactor water environments. A single step aging treatment of 200 hours at 811 °C followed by furnace cooling after solution treating for 2 hours at 1075 °C has been found to provide an improved combination of strength, ductility, and resistance to SCC under simulated PWR test conditions. In this single aged condition a reprecipitated secondary carbide, together with γ′ was produced at the grain boundary which resulted in a mixed fracture mode comprising dimple rupture and microvoid coalescence compared with a predominantly intergranular mode for the fully age hardened specimens. This improvement has been explained in terms of the morphology of the second phase precipitates which are produced in these heat treatment regimes.

Patent
24 Aug 1988
TL;DR: In this paper, a highly repetitive excimer laser device is obtained in which the reduction of the laser output due to repeated oscillations is difficult to occur by constructing the surface of the parts of a high repetition excimer device to be brought into contract with a halogen gas of a metal or an alloy having a good corrosion resistance.
Abstract: PURPOSE:To obtain a highly economical highly repetitive excimer laser device in which the reduction of the laser output due to repeated oscillations is difficult to occur by constructing the surface of the parts of a high repetition excimer laser device to be brought into contract with a halogen gas of a metal or an alloy having a good corrosion resistance to the halogen gas. CONSTITUTION:In a highly repetitive excimer laser device, electrodes 2, 3, a preliminary condenser, a heat exchanger, a line flow fan, a halogen gas piping and a laser case 4 which are brought into contact with a medium gas are constructed of a metal such as Mo or W or of an alloy such as 'Hastelloy(R)' or 'Inconel(R)', or the surface of these parts is coated with a metal such as Mo, W, Pt or Au or with an alloy such as Hastelloy or Inconel. With this, a highly economical highly repetitive excimer laser can be obtained in which the laser output reduction is suppressed as much as possible.

Journal ArticleDOI
TL;DR: Inconel X-750 was loaded at 700°C and crept to increasing fractions of its creep life as mentioned in this paper, which indicates that possible deleterious changes in fracture toughness may be the governing factor which determines the useful and safe lives of materials which serve while loaded at high temperatures.
Abstract: Precipitation-hardened polycrystalline nickel-base superalloy type Inconel X-750 was loaded at 700° C and crept to increasing fractions of its creep life Secondary small fracture-toughness specimens and standard tensile specimens were machined from the initial rather large creep specimens The secondary specimens were used to study the resultant resistance of the crept alloy to the extension of sharp cracks and its resultant tensile properties as functions of the amount of prior creep Intergranular formation and coalescence of voids, which took place during the creep process, were responsible for a drastic reduction in the fracture toughness of the alloy long before it reached the end of its creep life This behaviour indicates that possible deleterious changes in fracture toughness may be the governing factor which determines the useful and safe lives of materials which serve while loaded at high temperatures

Journal Article
TL;DR: In this article, an accelerated laboratory test method was developed to evaluate the stress corrosion cracking (SCC) of kraft continuous digesters using circular patch test welds made from 38mm-thick ASTM A516 Grade 70 steel plate.
Abstract: An accelerated laboratory test method was developed to evaluate the stress corrosion cracking (SCC) of kraft continuous digesters. The method uses circular patch test welds made from 38-mm-thick ASTM A516 Grade 70 steel plate. The specimens were exposed to a 110 C solution containing 40 gLNaOH and 20 gLNa/sub 2/S at a controlled electrochemical potential. Several different welding procedures were evaluated for resistance to SCC, along with stress relief, shotpeening, sealed thermal spray coatings, and weld overlays of Inconel/sup (1)/ 82 and AISI 309L stainless steel (SS). Stress relief, shotpeening, and sealed thermal spray coatings prevented SCC during the test. Compared to a control specimen, SCC was less severe after temper-bead welding and after welding with an E6010 capping pass. Severe SCC occurred in a specimen welded with the worst-case welding procedure. Deep, circumferential SCC occurred in the carbon steel at the edge of the Inconel 82 weld overlay. SCC was also observed at the interface between the AISI 309L SS weld overlay and the base plate.

Proceedings ArticleDOI
TL;DR: The use of Electron Beam Cold Hearth Refining (EBCHR) to produce very clean nickel based superalloys has been the subject of a number of investigations in the past, aimed at providing material with enhanced properties for components such as aeroengine gas turbine discs as mentioned in this paper.
Abstract: The use of Electron Beam Cold Hearth Refining (EBCHR) to produce very clean nickel based superalloys has been the subject of a number of investigations in the past, aimed at providing material with enhanced properties for components such as aero-engine gas turbine discs. In these, the useful working life is largely limited by the presence of certain defects which would ultimately initiate fracture. Such undesirable features can include nitrides, carbonitrides and oxides, the quantity and morphology of which is generally referred to as the cleanness of an alloy. This paper describes the EB refining of two established disc alloys melted in a 250 kw Leybold EB furnace equipped with two KSR 250 guns, INCONEL alloy 718 and NIMONIC alloy PK50 (similar composition to Waspaloy**) and describes the EB melting process performance and resulting metallurgical structures, chemistry, forgeability, cleanness levels and mechanical behaviour . * Trademark of the Into family of Companies. ** Trademark of the United Technology Corporation. Superalloys 1988 Edited by S. Reichman, D.N. DUN, G. Maurer, S. Antolovich and C. Lund The Metallurgical Society, 1988 397

Book ChapterDOI
01 Jan 1988
TL;DR: In this paper, the Ni-Fe base disk superalloy Inconel 718 under normal heat treatment and thermal mechanical treatment conditions has been studied in order to find that the segregation of alloying elements and the detrimental effect of δ-Ni 3 Nb phase could be reduced notably after the present thermal mechanical treatments.
Abstract: The Ni-Fe base disk superalloy Inconel 718 under normal heat treatment and thermal mechanical treatment conditions has been studied in this investigation. It was found that the segregation of alloying elements and the detrimental effect of δ-Ni 3 Nb phase could be reduced notably after the present thermal mechanical treatments. The experimental results indicated that the tensile strength and fatigue properties as well as the ductility of the alloy were increased substantially with its stress rupture lives comparable to that under normal heat treatment condition. The improvement in these mechanical properties of the alloy after thermal mechanical treatment can be ascribed to the refinement of microstructures and the strengthening of grain boundary regions by dispersed granular δ-Ni 3 Nb particals, which, in turn, alternate the fracture process of the alloy at elevated temperature, i.e. a change from an intergranular cleavage rupture to a ductile “dimple” fracture.

Book ChapterDOI
Keh-Minn Chang1
01 Jan 1988
TL;DR: In this article, a commercial high-strength superalloy, Inconel 718, has been studied in the temperature range from 400 to 650°C, and the authors have shown that the crack growth can be divided into two regimes: cycle-dependent and time-dependent.
Abstract: Fatigue crack propagation (FCP) behavior of a commercial high-strength superalloy, Inconel 718, has been studied in the temperature range from 400 to 650°C. Phenomenologically, the crack growth can be divided into two regimes: cycle-dependence and time-dependence. At low temperatures or under fast fatigue frequencies, crack growth rate (da/dN) is only determined by the cyclic stress intensity, ΔK, and the crack propagates transgranularly. An increase in the temperature and cycle period would cause the acceleration of crack growth, and the fracture mode becomes intergranular. The mechanism of time-dependent FCP is attributed to the crack tip embrittlement by the air environment. The improvement on the time-dependent FCP, i.e., reducing the crack growth acceleration at low fatigue frequencies or retarding the transition from the cycle-dependent to the time-dependent regime, has been successfully achieved through two methods: chemistry modification and thermomechanical processing (TMP).