Kinetics and Grain Boundary Selectivity of Discontinuous Precipitation in Binary Ni-Cr Alloy
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TRODUCTION
NICKEL-based superalloys are a special class of alloys that are known for excellent mechanical strength, resistance to thermal creep, and resistance to oxidation.[1–3] These alloys derive their special properties from the various alloying additions.[1,4] While Cr (1-20 pct by weight) imparts oxidation resistance to conventional Ni-based superalloys, Mo and W are added for solid solution strengthening and high temperature stability. The additions of Al and Ti form c¢ precipitates which impart precipitation hardening, and B, Zr, and C strengthen the grain boundaries.[1,5,6] The oxidation resistance provided by Cr addition is exceedingly important in aggressive environments such as gas turbine engines.[5,7–15] In fact, in alloys such as IN 713 LC, wherein the matrix is depleted of Cr, the decreased oxidation resistance has to be compensated by higher Al content or thermal barrier coatings.[16–18] In the light of the importance of Cr in the oxidation resistance of Ni-based superalloys, a series of high Cr alloys have been developed, which contain 32-40 pct Cr. Some well-known alloys of this type are Inconel 617, 625, 671, and 690 along with Haynes G35 and VDM33. These alloys are prime candidates in aggressive environments in chemical and petrochemical industries, nuclear industry, and fertilizer manufacturing. In the Ni-Cr phase diagram,[19,20] the Cr solubility in Ni is seen to decline sharply with decreasing temperature, leading to the precipitation of a-Cr in the
two-phase region. Depending on the Cr content and the heat treatment cycles, different Cr-based precipitates with various morphologies such as cellular or discontinuous precipitation (DP) and continuous precipitation (CP)[1,19,21–30] can form in high Cr Ni-based superalloys. While M23C6 type of carbides, in the form of DP, are known to form in Alloy 690 (30 pct Cr),[31] M6C carbides and d phase are known to occur, respectively, in GH169 (18 pct Cr) and GH625 (22 pct Cr). While a-Cr precipitation can occur in GH169 and GH625 after long-term aging, it is the major precipitate phase that can form in GH648 (33 pct Cr), GH837 (37 pct Cr), and GH840 (40 pct Cr) as DP lamellae or short-plates or star shapes depending on the heat treatment temperature and cycle.[32] The relative fractions of the different morphologies of precipitates can have serious implications on the mechanical properties, the ease of processing, and the corrosion resistance of the alloy.[12,32–36] The localized nature of DP can have relatively more adverse effects on the performance of the high Cr Ni-based alloys. Hence, it would be beneficial to determine the kinetics and mechanisms of DP and the temperature ranges of its dominance in isolation from the effects of other alloying elements. Therefore, the present study focuses on the kinetics, mechanisms, and the microstructural sensitivity of DP reaction in a high Cr (42 wt pct) Ni-based binary alloy system.
II. N. KESKAR, A.K. PATTANAIK, K.V. MANI KRISHNA, D. SRIVASTAVA, and G.K. DEY are with the Mechanical Metallurgy Division, Material
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