Effects of Electric Field Treatment on Corrosion Behavior of a Ni-Cr-W-Mo Superalloy

Abstract

Electric field treatment was performed on a Ni-Cr-W-Mo superalloy to investigate the effects of electric field treatment on its corrosion behavior. The microstructure evolution and the elements distribution at grain boundaries of both annealing twins and high angle grains were examined. The results show that the corrosion resistance can be improved by the electric field treatment and both of the corrosion weight loss and corrosion rate are decreased with the increasing treating time. When the alloy is electric field treated at 1093 K for 600 min with 4 kVcm � 1 , the intergranular corrosion rate is 65.3 mmy � 1 with the decreasing ratio of 25.1% compared with the untreated one, and the immersion corrosion rate is 3.9 mmy � 1 with the decreasing ratio of 57.9% compared with the untreated one. The redistribution of elements between the original high angle grain boundaries and the annealing twins occurred by the formation and growth of the annealing twins during the electric field treatment, as well as the improvement of exhaustion of Cr and Mo elements at the grain boundaries. With the increasing treating time, a large amount of original high angle grain boundaries are replaced and the continuously distributed original grain boundaries are separated, which leads to the retardation the growth rate of the corrosion ditches. The corrosion resistance of the alloy is improved due to the changes of corrosion behavior of the grain boundary. Moreover, the promotion effect of electric field treatment on the atom diffusion rate decrease the exhaustion tendency of Cr and Mo elements on both sides of normal high angle grain boundary. Those can be considered as the reasons of improving the corrosion resistance after electric field treatment. (doi:10.2320/matertrans.MF200905) With the rapid development of the aerospace engineering, the alloying elements ratio of superalloy is increased to meet the demands for long-term service at elevated temperatures. Besides the excellent properties including the high-temper- ature strength and plasticity, the oxidation existence, high- temperature corrosion resistance and irradiation resistance of the alloy are also required. 1-3) The distribution and size of carbides at grain boundaries increase especially when the alloy was experienced long-term services. It is easy for the intergranular rupture to occur at the grain boundary under corrosive environment. Researches on nickel-base Ni-Cr- W-Mo and GH4586A superalloys after electric field treat- ment reveal that annealing twins occurred in alloys and the number of annealing twins increase with the increasing treated time. During the deformation, the crack propagation direction changes with the annealing twins, and this leads the increase of both the uniformity and work of plastic deformation. The plasticity can be improved greatly without strength changing. 4-7) The coherent twin boundary with low energy can be caused with electric field treatment on nickel-base super- alloys and the fraction of low energy boundary can be increased. This agrees with the Grain Boundary Design and Control-Grain Boundary Engineering (GBE). 8-12) It aims at preventing the intergranular rupture and intergranular corro- sion by obtaining low energy and coherent boundary with high fraction in polycrystalline. As conventional GBE techniques, such as deformation heat treatments including strain recrystallization and strain heat treatment can be used to increase the fraction of coherent grain boundary in austenite steel, nickel-base alloy, copper and aluminum alloys. In other words, it can be considered that the electric field treatment will become one of the GBE techniques for obtaining high fraction of coherent grain boundary with low energy. In the present study, the electric field was employed on a nickel-based Ni-Cr-W-Mo superalloy to investigate the effects of electric field on its corrosion resistance. The mechanisms were also discussed. The present work aims to study potential application for electric field treatment on the progress of research on superalloys. 2. Material and Experimental Procedures