Superalloy Sheet Research Articles

Oxidation kinetics of microcrystalline Ni-11.5Cr-4.5Co-0.5Al superalloy sheet fabricated by Electron Beam Physical Vapor Deposition at 800 °C

Isothermal oxidation of microcrystalline Ni-11.5Cr-4.5Co-0.5Al superalloy sheet fabricated by Electron Beam Physical Vapor Deposition (EB-PVD) at 800 °C in air was investigated. The oxidation kinetics of microcrystalline Ni-11.5Cr-4.5Co-0.5Al superalloy sheet followed a parabolic power law at initial oxidation stage, and cubic power law for long oxidation terms. The fine oxide grains could be formed on microcrystalline superalloy surface. Taking the short circuit diffusion through grain boundaries into account, an oxidation kinetic with cubic power law was derived. The excellent results in agreement with the experimental data were obtained.

Oxidation kinetics of microcrystalline Ni–11.5Cr–4.5Co–0.5Al superalloy sheet fabricated by electron beam physical vapor deposition at 900 °C

Isothermal oxidation of microcrystalline Ni–11.5Cr–4.5Co–0.5Al (wt%) superalloy sheet fabricated by electron beam physical vapor deposition (EB -PVD) at 900 °C in air was investigated. Mass gain curves were obtained on testing the alloy in dry air. And characterization of the surface morphology and cross-sections was performed on samples isothermally oxidized for different exposure times. Surfaces and cross-sections of the oxidized specimens were studied by scanning electron microscopy (SEM). Phase identification of the oxide scale was performed by glancing angle X-ray diffraction (GAXRD) and energy dispersive X-ray microanalysis (EDX). The experiment results indicated that the oxidation kinetics of microcrystalline Ni–11.5Cr–4.5Co–0.5Al superalloy sheet follows a cubical power law at initial oxidation stage and fourth power law for long oxidation terms. The oxide scales consisted mainly of a mixed oxide at initial oxidation stage and a double-layer scale with an outer NiO oxide layer, and inner mixed oxides exposed for long terms at 900 °C. The fine oxide grains could be formed on microcrystalline superalloy surface. Taking the short circuit diffusion through grain boundaries into account, an oxidation kinetic with cubical power law in mixed oxide and fourth power law in single-phase oxide were derived. And the excellent fitting of the present model to the experimental data was obtained. The results suggested that the growing oxide layer of microcrystalline Ni–11.5Cr–4.5Co–0.5Al superalloy sheet, which controlled the ionic transportation, contained mainly mixed oxide at initial oxidation stage and the scale/substrate interface single-phase oxide for long terms.

Quality Analysis of Pulsed Laser Cutting of Superalloy Sheet

This paper presents the experiments of Nd:YAG pulsed laser cutting of GH3536 superalloy sheet and investigates the influences of different cutting parameters on laser cut quality factors including recast layer, kerf width and dross formation. The results show that the recast layer possesses finer granularity and higher hardness than those of the matrix, and the thickness of recast layer increases with increased pulse energy and decreases as the cutting speed and gas pressure increase. Oxygen-assisted cutting comes with thick recast layers and argon-assisted cutting acquires thin layers. The low-strength oxide layer worsens the kerf surfaces in oxygen-assisted cutting while argon-assisted cutting produces unaffected surface quality and is suitable for applications with subsequent welding.

ZrO 2-CeO 2 films as protective coatings against dry and wet corrosion of metallic alloys

ZrO 2-CeO 2 films were deposited on metals by sol-gel processing. The coatings were effective barriers against both corrosion in aggressive electrolytes and oxidation at high temperature. The protective action of the deposited coatings, was mainly dependent on their thickness and above all on the absence of any defects. Coatings withstood up to 80 h in acid environment and up to 18 h at 750°C in air. A strict control of the film thickness and quality could be performed by modifying the solutions of alkoxide precursors of ZrO 2-CeO 2 used for the deposition. The corrosion behaviour in 0.1 M H 3SO 4 and in 15 wt.% HCl was studied by electrochemical impedance spectroscopy. The electrochemical test on coated samples in different electrolytes, provided information about the nature and size of the defects with respect to the different preparation procedures. In order to evaluate the protective capacity of films independently of the substrate, superalloy sheets were coated by the same solutions and tested in the same acid environment, and the results were compared with those of 304 stainless steel-coated samples.

Laves phase in superalloy 718 weld metals

The important consequence of the solidification in cast or welded superalloy 718 is the segregation of Nb and the formation of laves phase. Laves phase is a brittle intermetallic topologically close-packed phase with hexagonal structure, known for its detrimental effect on mechanical properties at room temperature [1]. Although data available with regard to wrought materials is somewhat elaborate it is not true for welds in general and for electron beam welds in particular. In this letter the formation of laves phase in high heat input gas tungsten arc (GTA) and low heat input electron beam (EB) welds of 2 mm thick superalloy 718 in as-welded and post-weld heat treated (PWHT) conditions is evaluated. The results have a bearing on the tensile ductility and other properties of welds. Sheets of superalloy 718 of thickness 2 mm in solution treated condition (chemical composition in Table I) were autogenously welded by automatic GTA and EB welding processes, resulting in full penetration using the weld process parameters listed in Table II. The as-welded samples were subjected to two PWHT schedules: direct duplex ageing and solution treatment followed by ageing. Solution treatment was carried out at 980 °C for 20 rain with air cooling and the duplex ageing was carried out at 720 °C for 8 h with furnace cooling to 620 °C for 8 h with air cooling. The as-welded and heat treated samples were then subjected to scanning electron microscopic (SEM) examination and quantitative electron probe micro-analysis (EPMA) for the analysis of microsegregation of elements and determination of the formation of laves phase. Figs 1 and 2 show SEM micrographs of the aswelded microstructures of EB and GTA weld metals, respectively. Essentially, the solidified structure is of dendritic type. EB weld metal showed relatively finer

Effect of Long-Term 1093 K Exposure to Air or Vacuum on the Structure of Several Wrought Superalloys

Long-term 1093 K heat treatments of three commercial superalloy sheet materials were undertaken in air and vacuum. With either exposure, significant precipitation of second phases occurred in the Co-base Haynes® Alloy 188 (HA 188) and the Ni-base Haynes® Alloy 230 (HA 230); however, much less precipitation was found in the exposed Ni-base alloy Inconel® 617 (IN 617). Although some grain growth occurred in HA 188, no changes in the grain size of either HA 230 or IN 617 were observed after 22,500 h at temperature. Oxidation during air heat treatments led to weight gain due to the formation of chromia + spinel scales and surface-connected grain boundary pits/oxides in all three superalloys. Both the weight gain and depth of intergranular attack were dependent on the square root of time, which is indicative of diffusion-controlled phenomena. Because many alloy samples had neighbors in close proximity, dmost vacuum heat treated specimens did not suffer significant loss of volatile elements. However, some exposed samples were subjected to unrestricted vacuum heat treatments, allowing estimates of volatilization to be made. Based on the data for HA 188, the weight loss during 1093 K vacuum exposure was diffusion controlled once the inhibiting effects of surface films on the as-received alloys were broken down.

Small angle neutron scattering from steels

By small angle neutron scattering from steels valuable quantitative parameters of the microstructure can be obtained in a non-destructive way. Various features of this method are discussed. Microstructural results are presented which were obtained from neutron scattering experiments with neutron-irradiated pressure vessel steel, with a creep-tested super alloy and transformer sheets.

Powder Metallurgical Innovations for Improved Hot-Section Alloys in Aeroengine Applications

Throughout the past decade a number of new powder metallurgy processes have appeared which offer con siderable promise for superior aeroengine combustor, blade, and disc alloys. Furthermore, several features of these processes can be exploited for improved material utilization such that total manufacturing costs are contained to combat steep increases in basic alloying element prices. The processes include gas atomization of superalloy powder for critical rotating parts, rotating electrode atomization for titanium powder in similar components, controlled thermomechanical processing of attrited powders to produce oxide dispersion strengthened superalloy sheet and airfoil parts, and finally plasma-sprayed gas-atomized powder for advanced hot-section overlay and thermal barrier coatings. Considerable alloy and process development work has already been undertaken on all these systems by aeroengine material suppliers and advantages accruing in terms of superior properties and/or lower processing costs demonstrated at laboratory level and for some cases also in engine service. PM/0186

The creep fracture characteristics of nickel-base superalloy sheet samples

Center notched sheet samples of INCONEL alloy 718 with thicknesses from 0.5 to 3.1 mm were creep tested in the 595–704°C temperature range. No significant effect of thickness was observed in terms of failure times, minimum stress intensities needed to initiate crack growth, or fracture mode. Flat, intergranular fractures with no shear lips and no detectable change in thickness were observed in all cases. The results indicate that with materials such as nickel-base superalloys tested at half their melting point, creep fractures can be analyzed by fracture mechanics techniques.

Effect of simulated earth reentry exposure on mechanical properties of several oxide dispersion strengthened and superalloy sheet materials

The effects of simulated multiple reentry into the earth's atmosphere on the mechanical properties of several high temperature metallic sheet materials were evaluated. The materials included five tin-gage (nominally 0.025- or 0.037-cm) oxide dispersion strengthened (ODS) alloys and two thin-gage (nominally 0.037-cm) superalloys. Multiple reentry conditions were simulated through cyclic Plasma Arc Tunnel (PAT) exposure. PAT exposure generally consisted of 100 cycles of 600 second duration at 1255, 1366, or 1477 K in a Mach 4.6 airstream with an impact pressure of nominally 800 N/m2. PAT exposure generally produced a uniform oxide scale, oxide pits or intergranular oxidation, Kirkendall porosity, and alloy depletion zones except for the aluminum-containing ODS alloys. Only a uniform oxide scale was formed on the aluminum-containing ODS alloys. PAT exposure did not significantly affect the mechanical properties of the thin-gage (nominally 0.025- or 0.037-cm) alloys evaluated. Thus it appears that the microstructural changes produced by Plasma Arc Tunnel exposure has little influence on mechanical properties.

Fatigue of nickel-base superalloy sheets containing various diameter small holes

The effects of small (0·007–0·020 in.) holes on the fatigue strength of thin sheets of a wrough nickel-base superalloy loaded in pulsating tension have been investigated. Fatigue crack initiation, propagetion, and critical crack length were separately analyzed using surface replicas taken periodically throughout each test. The effect of hole size on the stress intensity range ( ΔK) for various crack lengths was theoretically calculated. Analyzed in this way, the experimentally-measured crack growth rates ( da/dN) showed the same dependence on ΔK [da/dN = A (ΔK) B] for all hole sizes. Using the experimental materials constants A and B, this crack propagation law was integrated from the hole perimeter to the critical crack length to predict the number of cycles spent in propagation for various hole size and stress range combinations. The analysis predicts, as observed, that the time spent in crack propagation decreases rapidly with increasing hole size as well as nominal stress range. Larger holes also produce earlier crack initiation even though the stress concentration factor is no higher. The microscopically planar slip character of the alloy and the various strain gradients adjacent to the holes are believed responsible.