Degradation of High-temperature Oxidation Resistance for Ni-based Alloys by Re Addition and the Optimization of Re/Al Content.

Abstract

High-temperature oxidation tests were conducted with two groups of Ni-based superalloys in order to elucidate the Re effect on oxidation resistance. One group of alloys was the Ni-8Cr-2Ti-10Al-Re system, characterized by 10 mol% Al content. The other group of alloys was the Ni-8Cr-2Ti-15Al-Re system, characterized by 15 mol% Al content. The alloy compositions were chosen with the aid of the d-electrons concept, so that any undesirable phases would not appear in the microstructure. Both cyclic and isothermal oxidation tests were carried out at 1 373 K for a total time of 720 ks. The oxidation resistance decreased clearly with increasing Re content in the 10-Al series alloys, but did not in the 15-Al series alloys. The alloys having a Re/Al ratio (mol%) up to 0.1 exhibited very small mass change by oxidation, so the Re/Al ratio was indeed a good indication for the design of superalloys having a good oxidation resistance.KEY WORDS: Ni-based superalloy; high-temperature oxidation resistance; Re effect. Alloy PreparationButton ingots of the experimental alloys were prepared by arc-melting under the purified Ar gas atmosphere. Each button ingot was cut into pieces and remelted several times in order to get a compositional homogeneity. The ingots then underwent a solution heat treatment followed by aging heat treatments. The heat treatment conditions were determined on the basis of the experimental data obtained from the differential thermal analysis (DTA). As shown in Table 2, two-steps aging processes were adopted in this experiment. Microstructural ObservationThe heat-treated ingots were cut into small specimens. Those specimens were mounted with a conductive filled phenolic mounting compound and the specimen surfaces were polished with abrasive papers and then with alumina powders down to 0.05 mm mesh. Polished surfaces were etched in the 1 : 1 volume% HNO 3 and HCl solution for about 30 sec. The microstructural observation of the etched specimens of all the alloys was made using a scanning electron microscopy (SEM). The volume fraction of the gЈ phase in each alloy was measured by analyzing the SEM image in a conventional way. Oxidation TestsThe specimens for oxidation tests were prepared by cutting each alloy ingot into 10ϫ5ϫ1 mm size, and then the surfaces were polished mechanically using emery papers down to # 600 grits. Two kinds of oxidation tests were employed in this study. One was the cyclic oxidation test, in which the specimen was kept in a furnace at 1 373 K for 72 ks followed by air cooling in each cycle. This thermal cycle was repeated for 10 times, so the total exposure time at 1 373 K was 720 ks. The other was the isothermal oxidation test, in which the specimen was set in a furnace for the exposure to static air at 1 373 K for 720 ks followed by air cooling. This test temperature is much lower than the solution-heat-treatment temperature and is equal to the firststep-aging temperature (see Table 2), and therefore it is considered that the volume fraction and shape of the gЈ phase scarcely vary.The...

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