Burner Rig Hot Corrosion of a Single Crystal Ni-48Al-Ti-Hf-Ga Alloy
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SAll concentrations are given in atomic percent. KK9.5.1 Mat. Res. Soc. Symp. Proc. Vol. 552 0 1999 Materials Research Society
hrs at 1371°C (2500'F) and 138 Mpa (20 Ksi), aged for 30 min at 13710 C (25000 F), cooled to 982°C (1800 0 F) at 550 C/min (100°F/min), and finally held for 6 hrs at 982 0 C (1800 0 F) before cooling to room temperature. This processing produced a microstructure containing small oxide, carbide and Hfcontaining intermetallic precipitates [10]. The carbides were identified as either HfC or (HfTi)C and the oxide was identified as HfO2 . The oxides and carbides were often agglomerated. The intermetallic precipitates were relatively large (-1 m) and were likely the Heusler Ni2A1Hf phase. Samples were machined into hollow right circular cylinders with approximate dimensions of 1.27cm (0.5 in) outer diameter, 0.635cm (0.25 in) inner diameter by 1.27cm (0.5 in) height. All machining was accomplished by wire EDM. Due to limited material, samples were cut from singlecrystal bars without regard to crystal orientation. All samples underwent a chemical milling treatment for 30 minutes at GEAE in order to remove the EDM recast layer. The samples were tested in a Mach 0.3 burner rig at the Lewis Research Center. Jet A fuel was burned and a synthetic sea salt solution was injected into the burner to yield a concentration of 2 ppmw in the hot combustor exhaust. The samples and individual holders were fixtured on a twelve position carousel which rotated at 250-300 RPM in the flame of the burner. The samples underwent testing for 300 1-hr cycles. After each 1-hr exposure, the burner was shifted away exposing the samples to a high velocity, ambient temperature air stream to cool the samples for a minimum of 6 minutes. The fuel to air ratio in the burner was maintained at approximately 0.062 to yield average sample temperatures of 900±115C (1652F) as monitored using both a two-color and a laser pyrometer aimed at the rotating samples. Further details of the testing are given elsewhere [11 . RESULTS AND DISCUSSION After 300 hrs of testing, a number of corrosion mounds were present on the ends and curved surface of the sample (Fig 1). Increased attack at the ID and OD edges resulted in the formation of a corrosion product "lip". X-ray diffraction of the ends and curved surface indicated the presence of aA120 3, NiO, a spinel phase (ao=8.30A), TiO 2 and HfO2. The P-NiAl substrate peak was also observed. A cross-sectional SEM micrograph of the smooth oxide scale away from any corrosion mounds after 300 cycles of testing is shown in Fig 2a. The scale has a distinct inner and outer region with the inner region containing bright particles. EDS spectra (Fig 2b,c) reveal that both regions are Al rich, i.e., Al20 3. The outer region contains significant amounts of Ca and Mg from the sea salt, although Na, the major constituent of the salt, was not detected. The Fe and Cr detected in the outer region were commonly observed and result from deposition of Fe, Ni, Cr, Si and Mn from the corrosion of the burner and nozzle. Ti
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