Aseries of experiments is carriedout with six nickel based single crystal superalloys for use in industrial gas turbines. Each alloy contains lZmass%Cr, but no Re. The alloy compositions are chosen with the aid of the d-electrons concept, so that any undesirable phases do not appear in the alloy. The creep rupture life is measured at a stress of 196MPa and a temperature of 1193K which is close to the service temperature, 115OK of industrial gas turbines in power plants. The measured life of two alloys in this condition is found to be comparable to that of the 2nd generation nickel based single crystal superalloy containing 3%Re alloy. Also, it is shown that the amount of lZmass%Cr is not necessary to get good hot-corrosion resistance, as long as both the compositional ratios of 'H/Al andTa/(W+Mo) are high in the alloy. In addition, a dense 'liOz-rich layer is found on the surface of the alloy with good hot-corrosion resistance. In contrast to the hot-corrosion resistance, the oxidation resistance is lowered if the Tl/Al compositional ratio is high in the alloy. The possible mechanisms for hot corrosion and oxidation are discussed in this study.
There is a clear trend of lowering Cr and increasing Re content in the evolution from the 1st-to the 2nd-and to the 3rd-generation Ni-based single crystal (SC) superalloys. Following this trend, two groups of superalloys were made with the aid of the d-electrons concept. One is the A-group which is characterized by the lower Cr and the higher Re contents in the alloys than in the 3rd-generation SC alloys. A homogeneous microstructure was obtained in these A-group superalloys containing 0, 1 and 2 mass% Cr, while their Re and Co contents were set at about 8.5 mass% and 13.5 mass% respectively. The other is the B-group which is characterized by the lower Re content than the A-group. Three alloys in the B-group containing 2.5, 4.2 and 12.1 mass% Cr were studied for the comparison with the A-group. According to a hot corrosion test, weight gain occurred in a Cr-free alloy instead of observing weight loss in Cr containing alloys. Hot corrosion resistance of 2 mass% Cr alloy was quite good as long as the alloy contains about 8.5 mass% Re. The oxidation resistance was improved remarkably in 1 mass% Cr alloy than in the Cr-free alloy. It was found that the addition of at least 2 mass% Cr into superalloys is necessary in order to keep good surface stability at high temperatures. Also, it was seen that Re is a harmful element for oxidation resistance.KEY WORDS: single crystal; Ni-based superalloy; hot-corrosion; oxidation resistance.hand, the B-group alloys had Cr and Re content varying in a wide range of 2.5-12.1 and 0-5.4 mass%, respectively.
Experimental Procedure
Alloy PreparationButton ingots of the designed alloys were prepared by arc-melting under a purified Ar gas atmosphere. Each button ingot was cut into pieces and remelted three times in order to get compositional homogeneity. The conditions for the solution-and the aging-heat treatments were determined on the basis of the data obtained from the Differential Thermal Analysis (DTA) of the alloys. The heat treatment conditions are given in Table 2. The ingots first underwent the solution heat treatment and then followed by the aging heat treatment. The volume fraction of the gЈ phase in each alloy was measured after the aging heat treatment from the SEM images following a conventional image analyzing method.
Hot Corrosion and Oxidation TestsBoth gravimetric and metallographic methods were employed to evaluate the extent of hot corrosion and oxidation resistance of the experimental alloys.
Gravimetric MeasurementSpecimens for hot corrosion test were prepared by cutting each alloy ingot and the surface was polished mechanically using emery papers up to 600 grits. A coating method 4) was employed for the hot corrosion test using the specimens of 10ϫ5ϫ1 mm in size. The specimen surface was coated with a Na 2 SO 4 -25wt%NaCl salt by 0.2 kg/m 2 , and then the specimen was put into a furnace for the exposure to static air at 1 173 K for 72 ks. Corrosion products formed on the specimen surface were descaled with a brass-wire brush after boiling in a 18wt%NaOH-5wt%KMnO 4 ...
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