Nanoscale Cellular Structures at Phase Boundaries of Ni-Cr-Al-Ti and Ni-Cr-Mo-Al-Ti Superalloys
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FOR many decades, the pack cementation technique has been actively employed and versatilely adapted in the superalloy industry for creating wear- and corrosion-resistant coatings geared toward high-demanding service environments. In general, pack cementation process involves four components, i.e., substrate (Fe-, Co-, and Ni-base alloys), target alloying elements (Al, Cr, Si, etc.), activator (chloride and fluoride), and thermal ballast (inert Al2O3, ZrO2, MgO, etc.). Upon heating in a flowing or static inert atmosphere to the desired temperature, alloying elements and halide powders react to form volatile metallic halides, which subsequently deposit onto the substrate surface and diffuse into the substrate, leading to the formation of phases and microstructures that protect the substrate.[1–3] Previous studies have demonstrated the feasibility of alloying additions of major engineering elements, such as Al, known as c¢ inducer, as well as Cr, serving as oxidation resistance enhancer, for a variety of Ni-base superalloys.[4–8] In particular, co-deposition of two or more elements in a controlled manner, considered to be technically challenging due to the differences in halide vapor pressures, has been successfully practiced in CONG WANG, Professor, formerly with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, is now with the College of Materials and Metallurgy, Northeastern University, Shenyang 110819, P.R. China. Contact e-mail: [email protected] DAVID C. DUNAND, Professor, is with the Department of Materials Science and Engineering, Northwestern University. Manuscript submitted July 31, 2014. Article published online March 12, 2015 2680—VOLUME 46A, JUNE 2015
Al + Si,[9] Cr + Al,[10,11] and Al + Ti[12] systems. The addition of refractory metals, such as Mo, Ta, Re, and W, to Ni-base superalloys has been established to enhance solid solution strengthening, and, as a result, high-temperature creep resistance imparted by the slowed diffusion process of responsible element and the reduced kinetics of coarsening of c¢, the strengthening phase determining the elevated temperature properties of the alloys. However, to our knowledge, only one article applies the pack cementation technology to depositing refractory metals,[13] despite its potential for creating advanced coatings. In this article, Li et al.[13] successfully implemented NH4Cl-activated cementation to deposit Mo onto pure Ti substrate and form a Mo diffusion layer at temperatures from [1173 K to 1323 K (900 C to 1050 C)]. Cellular precipitation, often referred interchangeably as discontinuous precipitation or grain-boundary precipitation, has been extensively documented in various alloy systems,[14–16] and, in particular, in many nickelbase superalloys.[17–25] Reactions of such category generally result in the formation of multi-phase cellular structure. This type of phenomenon is of significance to the development of certain Ni-base superalloy grades, as the formation of cellular structures may be accompanie
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