Evolution of Secondary Phases Formed upon Solidification of a Ni-Based Alloy
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AS a typical solid-solution-strengthened Ni-based alloy, UNS N08028 has good high-temperature strength, toughness, and excellent corrosion resistance and has been widely used in the chemical and petrochemical processing industry. Usually, substitutional alloying elements such as Fe, Cr, and Mo are added to Ni-based alloys to provide good corrosion resistance. However, the high concentration of alloying elements in such solid-solution alloys sometimes results in brittle secondary phases,[1–3] which will seriously affect the mechanical properties and degrade the corrosion resistance properties. Generally, such phases could form via both solid-state transformation and solidification.[4] Much effort was spent on the formation of secondary phases during solid-state transformation.[5–18] Upon high-temperature exposure, various deleterious secondary phases precipitated directly from the matrix,[5–7] and they can also appear during hot/cold deformation.[8–10] It is commonly believed that the precipitation of sigma phase, one of the common secondary phases in Ni-base alloys, is controlled by diffusion of Cr,[2,11] which can be accelerated by diffusion of other substitutional alloying elements (e.g., Mo).[12] Recently, the mechanism of eutectoid decomposition of d fi c + r[13–15] and precipitation of sigma phase attributed to the pre-existence of carbides[16,17] were studied. Furthermore, Schwind et al.[18] proposed that the precipitation of secondary QIANG ZUO and LEI WANG, Postdoctoral Candidates, and FENG LIU, Professor, are with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, P.R. China. Contact e-mail: [email protected] CHANGFENG CHEN, Professor, is with the Department of Materials Science and Engineering, China University of Petroleum, Beijing 102249, P.R. China. Manuscript submitted October 10, 2012. Article published online February 20, 2013 3014—VOLUME 44A, JULY 2013
phases in austenitic matrix hinges heavily on grain size and shape. Compared to the flourishing studies of secondary phases caused by solid-state transformation, the corresponding works by solidification are relatively few. In some nickel-based corrosion-resistant alloys, the deleterious secondary phases formed during the last stage of solidification, which is always observed in welding and casting processes.[4,19,20] Many works focus on the welding process with relative high cooling rate, such as the works of Dupont et al.[4,21,22] and Perricone et al.,[23,24] where the solidification behavior of several Ni-Cr-Mo, Fe-Ni-Cr–Mo alloys and some super austenitic stainless steels was studied. They emphasized that the solidification reaction should be primarily dependent on the segregation behavior of substitutional alloying elements, especially Mo. Besides, directional solidification[25] and differential thermal analysis methods[26,27] were performed to study solidifications with a range of cooling rates. Various reactions during the last stage of solidification, which reveal the forming mechanis
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