Design and development of hot
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Fig. 3--Dependence of (a) melting range (AT) and (b) eutectic (3" + 3") volume fraction of the experimental alloys upon the Ta/(W + Mo) ratio.
be increased, more alloying elements could be added in the alloys. By comparing the parameters used in the new and the conventional PHACOMP methods (Mdt, Mdy, and Nv, as shown in Tables I and IV), it can be seen that the new PHACOMP method can give more satisfactory boundary conditions (Mdt = 0.991 or Mdy = 0.93) controlling the phase stability, while the Nv PHACOMP method cannot give definitive boundary conditions. As shown in Table IV, at the boundary separating the o-free and o-prone regions determined by the Md p_grameters (as shown by the dotted line in Figure 6), Nv values scattered between 2.182 and 2.208. Although the Nv values of the alloys studied were lower than the critical value suggested by Decker 1~71 (Nv = 2.26 - 2.41), o phase was observed in many of the alloys. This indicates that the Nv method is inadequate for this alloy design process compared to the Md method. It was found that when the quantity of eutectic (y + y') in the microstructure of an alloy is lower than a critical value (-
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element Fig. 7--Alloying features of two types of nickel--base superalloys (single-crystal and hot corrosion-resistant alloys) reported in the literature. The arrows indicate the positions of the designed alloys.
systems with high Cr contents is almost the same as those of the single-crystal superalloys with lower Cr and higher A1 contents (typically, Mdt = 0.985151). It indicates that through proper compositional adjustment (mainly the y' forming elements A1, Ti, Ta, and Nb), the new highperformance hot corrosion-resistant single-crystal nickelbase superalloys can be developed. The direction of the next study was determined based on the previous results, i.e., the effects of three refractory metals Ti, Ta, and Nb on the phase stability and other properties of the alloy system should be studied systematically to find the optimum alloy compositions and then develop the highperformance hot corrosion-resistant single-crystal superalloys. The results wilt be published in another article, u61 METALLURGICAL TRANSACTIONS A
1. A.D. Cetel and D.N. Duhl: Superalloys 1988, Proc. Int. Symp. on Superalloys, Seven Springs, Champion, PA, Sept. 1988, D.N. Duhl, G. Maurer, S. Antolovich, C. Lund, and S. Reichman, eds., TMS, Warrendale, PA, 1988, pp. 235-44. 2. K. Harris, G.L. Erickson, and R.E. Schwer: Proc. Conf. on High Temperature Alloys for Gas Turbines and Other Applications, Liege, Belgium, Oct. 1986, D. Reidel Publishing Company, Dordrecht, Holland, 1986, pp. 709-18. 3. D.A. Ford and R.P. Arthery: Superalloys 1984, Proe. Int. Symp. on Superalloys, Seven Springs, Champion, PA, 1984, M. Gell, C.S. Kortovich, R.H. Bricknell, W.B. Kent, and J.F. Radvich, eds., TMS, Warrendale, PA, 1984, pp. 115-24. 4. T. Yamagata, H. Harata, S. Nagazawa, M. Yamazaki, and Y.G. Nakazawa: Superalloys 1984, Proc. Int.
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