24 July 2021 High Temperature Alloys
GH4169 High Temperature Alloy Steel Bar
GH4169 alloy is a nickel-based superalloy extensively used in the aircraft engine industry because of its excellent mechanical properties and good fabrication ability. The mechanical properties of the GH4169 at high temperature, rupture stress under severe condition deserves a close attention. In this paper, the creep rupture of the GH4169 alloy under constant load and different temperatures from 550 °C to 700 °C conditions is systematically evaluated and major impact factors in the stress rupture behavior are analyzed. Furthermore, an improving method for the alloy stress rupture is proposed.
GH4169 alloy is a nickel-based superalloy extensively used in the aircraft engine industry because of its excellent mechanical properties and good fabrication ability. The mechanical properties of the GH4169 at high temperature, rupture stress under severe condition deserves a close attention. In this paper, the creep rupture of the GH4169 alloy under constant load and different temperatures from 550 °C to 700 °C conditions is systematically evaluated and major impact factors in the stress rupture behavior are analyzed. Furthermore, an improving method for the alloy stress rupture is proposed.
GH4169 alloy is a precipitation hardened, nickel-based superalloy that is extensively used in the aircraft engine industry. Rotor components, such as compressor disks, spools as well as turbine disks, compressor blades and power drive-shafts are typically fabricated from this alloy. The GH4169 alloy could provide operational conditions up to a maximum useful service temperature of about 650 °C to meet the requirements of the rotor components. For instance, a high tensile strength, fracture toughness, oxidation resistance and excellent stress rupture resistance are attained.
Especially, the stress rupture should be primarily considered in the design of rotor components operating at elevated temperature. Stress rupture generally involves the time-dependent deformation and fracture of materials, and it is accelerated by an increase in stress or temperature. The creep stress rupture is associated with the slow deformation of a material under constant stress that leads to a permanent change in shape of components. This type of fracture is mainly characterized by the nucleation, growth and coalescence of microscopic internal cavities. Up to now, the stress rupture characteristics of GH4169 alloy have been studied on conditions within maximum service temperature. Although the accelerated stress rupture behavior of GH4169 alloy is one of major considerations in aerospace structures applications, the studies related to accelerated stress rupture at a very high temperature environment have not been deeply explored.
In this study, the creep rupture under constant load and different temperatures from 550 °C to 700 °C conditions behavior of GH4169 alloy are investigated. The stress rupture life is finally evaluated by using the Larson–Miller parameter (LMP). In the deformation process of stress rupture, examples of crack initiation and propagation, as well as coarsen of strengthening phase, were carefully investigated.
Hot rolling bar of GH4169 superalloy, which was produced from double melt (VIM/VAR) ingots with a diameter of 18 mm, was used in this study. The chemical composition was obtained by Plasma 2000 type inductively coupled plasma atomic emission spectrometry (ICP-AES) and HCS-500 type high frequency-infrared absorption spectroscopy. The chemical composition of GH4169 alloy was as follows: Ni 53.44, Cr 18.56, Mo 3.02, Nb 5.3, Al 0.44, Ti 1.04, C 0.026, P 0.005, S 0.001, B 0.002 and Fe bal (wt.%). GH4169 alloy is usually used in the solution and aging condition, and the exact conditions of temperature, time and cooling rate depend on the application need. When applied in the aerospace, a high tensile and fatigue strength, as well as good stress-rupture properties are necessary. Therefore, a solution treatment below delta-solves and a two-step aging treatment are needed. The heat-treatment of the alloy is based on the AMS 5596 standard, and the stress rupture tests are controlled by using constant loading conditions based on ASTM E139 standard. The stress rupture tests were performed using an ATS (Applied Test Systems Inc.) Series 2330 lever arm creep tester. According to the dimensions of ASTM E 8M-94a standard, rod specimens with gage length 25.4 mm were machined to the diameter of 6.35 mm. Strain rate range is 2 × 10−5–10−3. Stress rupture tests were performed at 550, 600, 650, 680 and 700 °C. Stress levels were selected from 30% to 90% of the yield strength.
Metallographic samples were prepared using standard mechanical polishing procedures and are electrolytically etched in the mixture of 150 ml H3PO4 + 10 ml H2SO4 + 15 g chrome anhydride. The study of microstructure characteristics is conducted by using optical microscopy (OM) and scanning electron microscopy (SEM). JSM-6480LV type scanning electron microscopy (SEM) is used to observe the morphology and distribution of phases in the alloy. Substructure characteristics, involving γ″ morphology and size, are analyzed by field emission scanning electron microscopy (FESEM). ZEISS SUPRA 55 type field emission scanning electron microscopy is used to observe the morphology and distribution of all precipitates.
For preparing specimens of transmission electron microscopy, samples are first mechanically processed to a thickness of about 100 μm. Discs are subsequently punched from these ground samples and these discs are thinned by the dual jet electropolishing technique, which is carried out at about 20 volts. The electrolyte contained 70% butanol, 20% ethanol, and 10% perchloric acid. JEOL JEM-2010 high-resolution transmission electron microscopy (TEM) and selected area electron diffraction (SAED) are used to identify and confirm the phases.
GH4169 is a precipitation-strengthened nickel-iron-base superalloy. The precipitation within the fcc γ-matrix of the coherent, disk-shaped, bct (DO22 type superlattice) ordered γ″ precipitate (Ni3Nb) having an average diameter and thickness of about 30 and 10 nm, respectively, is responsible for the contributions of coherency strain hardening and order strengthening. The incoherent equilibrium orthorhombic (Ni3Nb) δ-phase precipitate is stable up to about 1010 °C, above this temperature δ-phase will be dissolved. δ-Phase plays an important role during deformation processing for improved stress rupture resistance.
During aging, the precipitation of a coherent, quasi-spherical, fcc (Ni3(Al,Ti)) L12 γ′ precipitation also occurs and has an average particle diameter between 10 and 40 nm. The contribution of γ′ to the strength of GH4169 is considered to be minimal. Primarily because the precipitate possesses a low anti phase boundary energy (APB γ″ ≈ 25APB γ′) . Previous studies found that the precipitation of γ″ was favored by the increased Nb/(Ti + Al) atomic concentration ratio, and because the volume of γ″ and γ′ contained within the matrix were about 0.13 and 0.04, respectively. At long time of service above 650 °C, γ″ precipitates could transform to δ phase.
The microstructure of GH4169 alloy after solution-annealed at 960 °C and two-step aging treatment. Grain size of the alloy is in the range of 5–20 μm and the average grain size is about 10 μm. The method of determining the grain size followed the ASTM E112-96. OM micrograph of the alloy indicates that carbides precipitate in matrix and at grain boundaries. The precipitates at grain boundary are δ phase.