Ordering Transformation and Age Hardening in a Ni-Cr-W Superalloy
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INTRODUCTION
NI-CR-MO-BASED superalloys have great potential in the field of nuclear and chemical industry by virtue of an excellent combination of mechanical and corrosion properties.[1,2] Neostructural ordering which is prevalent in several Ni-based alloys has been the subject of vigorous studies. The Ni-Cr-Mo system, because of it possessing a number of intermetallic compounds which precipitate through order-disorder transformation, e.g., long-range order (LRO) phases including the closed-related C11b (Pt2Mo-type), DO22, D1a, and short-range order (SRO) superlattices, has been the prime candidate for such investigations.[3–11] It has been proved that the particles of these LRO phases play a critical role in the improvement of mechanical properties of Ni-based superalloys.[9–13] It is somewhat surprising that few studies with the same vigor have been carried out for order-disorder transitions in the analogous Ni-Cr-W system. Ni-Cr-Wbased superalloys have attracted increasing interest in the nuclear and chemical industry due to their excellent corrosion resistance, creep resistance, and antioxidant activity.[14–17] It has been reported that Haynes 230 with the composition Ni-22Cr-14W (wt pct) is a promising XIANGYU GAO, Ph.D. Candidate, RUI HU, JINSHAN LI, and HENGZHI FU, Professors, and GONGLIAO LUO, Master, are with the Stake Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China. Contcat e-mail: [email protected] XIAOLIN LI, Ph.D. Candidate, is with the Stake Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, People’s Republic of China. Manuscript submitted April 6, 2016. Article published online October 13, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
candidate material as the heat exchange in the fourth-generation supercritical water reactor system (GEN IV SCWR).[17,18] The long-term service of the heat exchanger in the fourth GEN IV SCWR under harsh conditions makes researchers commence to pay more attention to the creep lasting performance of Haynes 230.[19,20] Kakehi[21] has reported that the superlattice precipitates have a significant effect on the creep strength and can improve the creep resistance of Ni-based superalloys dramatically. However, the ordering reaction in the Ni-Cr-W superalloy, leading to the precipitation of the LRO superlattices, has rarely been reported on. The limited research results about the ordering transformation occurring in the Ni-Cr-W alloy during long periods of aging have been published from our research group.[22] To date, it has never been reported that two different kinds of superlattices can precipitate in one kind of Ni-Cr-Mo alloy and do not coarsen and overage with further aging.[3–11] Nevertheless, C11b and DO22 phases have been obtained from the Ni-Cr-W alloy in this study aged at different temperatures, and no signs of coarsening and overaging have been observed with the increase of aging time. It is evident that more work on Ni-Cr-W alloy can help throw light on
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