• Journal of the Korean Society for Heat Treatment
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• v.16 no.6
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• pp.341-348
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• 2003

Superalloys which can be devided into three categohes, i.e. Ni-base, Co-base, and Fe-base alloys are widely used for high temperature applications. Since superalloys contain many alloying elements and precipitates, its chemistry and processing parameters need to be carefully designed. In this review, current state-of-the art in the superalloy technologies is described with special attention to the heat-treatment for the control of the microstructures and mechanical properties.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2009.10a
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• pp.89-92
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• 2009

As a class of materials, Ni-base superalloys are among the most difficult metal alloys to forge together with refractory metals and cobalt-base superalloys. The mechanical properties of Ni-base superalloys depend very much on grain size and the strengthening phases, $\gamma$' ($Ni_3$(Al,Ti)-type) and $\gamma$".($Ni_3$Nb-type). Especially, the control of grain size remains as a sole means for the control of mechanical properties. The grain size and distribution changes of the wrought superalloys during hot working and heat treatment are mainly controlled by the recrystallization and grain growth behaviors. In this presentation, prediction technology of grain size through the computer-aided process design, and numerical modeling for predicting the microstructure evolution of Ni-base superalloy during hot working were introduced. Also, some case studies were dealt with actual forming processes of Ni-base superalloys.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2004.05a
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• pp.139-142
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• 2004

Nickel base superalloys are widely used for high temperature applications due to heat resisting capability and corrosion resistance at high temperatures. Superalloys with many strengthening alloying elements are frequently used in powder form to alleviate harmful effects of alloy segregation. HIP (hot isostatic pressing) and DB (diffusion bonding) as a form of solid-state bonding process is used to make turbine components, such as integrated turbine rotors. HIP/DB process requires many technical overcomes related to dimensional changes as well as microstructural control. In this research, HIP/DB process for nickel base superalloys, Udimet 720 and MM 247, were investigated with a view to control the dimensional change during the consolidation process. Simple disc-shaped cans were used to select the conceptual die design for the control of the dimensional change especially in radial direction. The change in the shape of consolidated shape was investigated using commercial FE code with constitutive equations fur low temperature plasticity deformation.

• Advances in aircraft and spacecraft science
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• v.9 no.5
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• pp.367-375
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• 2022

Modeling the properties of complex alloys such as nickel superalloys is an extremely challenging scientific and engineering task. The model should take into account a large number of uncorrelated factors, for many of which information may be missing or vague. The individual contribution of one or another chemical element out of a dozen possible ligants cannot be determined by traditional methods. Moreover, there are no general analytical models describing the influence of elements on the characteristics of alloys. Artificial neural networks are one of the few statistical modeling tools that can account for many implicit correlations and establish correspondences that cannot be identified by other more familiar mathematical methods. However, such networks require careful tuning to achieve high performance, which is time-consuming. Data preprocessing can make model training much easier and faster. This article focuses on combining physics-based deep network configuration and input data engineering to simulate the solvus temperature of nickel superalloys. The used deep artificial neural network shows good simulation results. Thus, this method of numerical simulation can be easily applied to such problems.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.424-429
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• 2003

Hot gas path components, which are made of nickel based superalloys, are subject to periodic replacement due to degradation of thermomechanical properties that might bring catastrophic failure during normal operation of gas turbine units. In order to rejuvenate the metallurgical condition of the serviced components, heat treating techniques such as solution annealing and aging heat treatments have widely been employed. However, the effectiveness of those typical heat treatments is not apparent enough in terms of quantitative grounds. On the other hand the demand of the rejuvenation heat treatment and hot isostatic pressing (HIP) have constantly been raised by the end users. Therefore it is necessary to verify how the typical heat treating techniques affect to the aged and degraded material. As the result of experimental work in this study, GTD-111 and GTD-222 Ni-based superalloys were collected and analyzed quantitatively through microscopic observation, microhardness evaluation and creep test.

• 한국기계연구소 소보
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• s.20
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• pp.57-63
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• 1990

The morphologies of MC carbides (M stands for metal) and creep-rupture properties in directionally solidified Rene 80 having standard and Hf-modified chemistries were studied. It was found that Hf depressed the melting point, $\gamma$- $\gamma$’ eutectic temperature and $\gamma$’ solvus of nickel-base superalloy Rene 80, but did not depress MC carbide forming temperature. The morphologies of MC carbides depended upon solidification sequence, which led to blocky type in the early stage and script type in the late stage of solidification. Creep failure occurs through the crack initiation at the transverse components of longitudinal grain boundaries or interdendritic carbides in directionally solidified superalloys. It could be concluded MC carbide morphologies played an important role in creep properties of DS superalloys, that is, Hf additions increased the creep ductilities and lives of Rene 80.

• Corrosion Science and Technology
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• v.15 no.3
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• pp.129-134
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• 2016

The effects of Ti on the high temperature oxidation of Ni-based superalloys were investigated by cyclic oxidation at $850^{\circ}C$ and $1000^{\circ}C$. The oxide scale formed at $850^{\circ}C$ consists of $Cr_2O_3$, $Al_2O_3$, and $NiCr_2O_4$ layers, while a continuous $Al_2O_3$ layer was formed at $1000^{\circ}C$. The oxidation rate of the alloy with higher Ti content was higher than the alloy with less Ti content at $850^{\circ}C$, possibly due to the increase in the metal vacancy concentration in the $Cr_2O_3$ layer involved by incorporation of $Ti^{4+}$. However, Ti improved the oxidation resistance of the superalloy at $1000^{\circ}C$ by reducing oxygen vacancy concentration in $Al_2O_3$ layer.

• Korean Journal of Materials Research
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• v.26 no.5
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• pp.235-240
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• 2016

Appropriate thermo-mechanical properties of nickel-based superalloys are achieved by heat treatment, which induces precipitation and solid solution hardening; thus, information on the temperature ranges of precipitation and dissolution of the precipitates is essential for the determination of the heat treatment condition. In this study, thermal analyses of nickel-based superalloys were performed by differential scanning calorimetry method under conditions of various heating rates of 5, 10, 20, or 40K/min in a temperature range of 298~1573K. Precipitation and dissolution temperatures were determined by measuring peak temperatures, constructing trend lines, and extrapolating those lines to the zero heating rate to find the exact temperature under isothermal condition. Determined temperatures for the precipitation reactions were 813, 952, and 1062K. Determined onset, peak, and offset temperatures of the first dissolution reaction were 1302, 1388, and 1406K, respectively, and those values of the second dissolution reaction were 1405, 1414, and 1462K. Determined solvus temperature was 1462K. The study showed that it was possible to use a simple method to obtain accurate phase transition temperatures under isothermal condition.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.24 no.10 s.181
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• pp.2496-2501
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• 2000

High speed ball end milling is attracting interest in the aerospace industry for the machining of complex 31) airfoil surfaces in nickel based superalloys, Experimental work is detailed on the effect of cutter orientation on tool life, cutting forces, chip formation, specific force and workpiece surface roughness, when high speed ball end milling nickel based supperalloy(lnconel 718). Dry cutting was performed using 8min diameter solid carbide cutters coated with either TiA1N or CrN for the workpiece mounted at an angle of 45˚ from the cutter axis. A horizontal downwards cutting orientation provided the best tool life with cut lengths~50% longer than for all other directions. Evaluation of cutting forces and associated spectrum analysis of results indicated that cutters employed in a horizontal downwards direction produced the least vibration.

• Corrosion Science and Technology
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• v.18 no.1
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• pp.8-15
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• 2019

A high-temperature gas-cooled reactor (HTGR) is recognized as the best candidate reactor for next generation nuclear reactors. Helium is used to be the coolant in the core of the HTGR with temperature expected to exceed $900^{\circ}C$ at the core outlet. Several iron- and nickel-based superalloys, including Alloy 800H, Hastelloy X, and Alloy 617, are potential structural materials for intermediate heat exchanger (IHX) in an HTGR. Oxidation behaviors of three selected alloys (Hastelloy X, Alloy 800H, and Alloy 617) were investigated at four different temperatures from $650^{\circ}C$ to $950^{\circ}C$ under helium environments with various concentrations of $O_2$ and $H_2O$. Preliminary results showed that chromium oxide as the primary protective layer was observed on surfaces of the three tested alloys. Based on results of mass gain and SEM analyses, Hastelloy X alloy exhibited the best corrosion resistance in all corrosion tests. Further details on the oxidation mechanism of these alloys are presented in this study.