The Mechanism of Grain Boundary Serration and Fan-Type Structure Formation in Ni-Based Superalloys
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NTRODUCTION
THE high-temperature mechanical properties of c/c¢ Ni-based superalloys strongly depend on the size, distribution and morphology of c¢-phase precipitates (Ni3Al-type ordered structure) that nucleate and grow during heat treatment.[1–3] The formation of the c/c¢ microstructure in Ni-based alloys during cooling or annealing has been extensively studied. The precipitation kinetics depend on the alloy composition, undercooling temperature below the solvus temperature and lattice mismatch between the c and c¢ phases.[1,2] In undercooled polycrystalline alloys (alloys cooled below the phase transformation temperature), the primary c¢ precipitation starts in triple junctions (TJs) and at GBs followed by secondary c¢
V.V. ATRAZHEV, D.V. DMITRIEV, and N.Y. KUZMINYH are with the Science for Technology LLC, Leninskiy pr-t 95, Moscow, Russia 119313 and also with the Institute of Biochemical Physics RAS, Kosygin str. 4, Moscow, Russia 119334. S.F. BURLATSKY and D.L. NOVIKOV are with the United Technologies Research Center, 411 Silver Lane, East Hartford, CT 06108. Contact e-mail: [email protected] D. FURRER, S. STOLZ, and P. REYNOLDS are with Pratt & Whitney, 400 Main Street, East Hartford, CT, 06108. I.L. LOMAEV is with the Science for Technology LLC and also with the M.N. Mikheev Institute of Metal Physics RAS, S. Kovalevskoy st. 18, Ekaterinburg, Russia 620108. Manuscript submitted July 3, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
precipitation in the grain bulk. This results in the formation of a microstructure with bi-modal size distribution of c¢ particles.[4–7] The GB c¢-phase precipitation at slow cooling rates results in either GB serration[8–11] or formation of ‘fan-type’ structures (FTSs).[8,9,11,12] A high cooling rate leads to many fine c¢ precipitates in the bulk. It is believed that the GB serration increases the fracture toughness and elevated temperature dwell fatigue properties of Ni-based alloys,[10,13–15] while the appearance of large fan-type c¢ particles increases alloy brittleness.[5–7,12] Understanding of the kinetics of c¢ precipitation is essential for designing novel Ni-based superalloys. However, the conditions of the FTS and GB serration formation are still debated.[1,8–10,12,13] The dependence of GB serration amplitude on the cooling rate from supersolvus temperature for the U720LI superalloy was studied in References 8 and 12. It was found that the serration amplitude increases with the decrease of the cooling rate. The same trend was observed for the GB serration amplitude in the RR1000 superalloy in Reference 4. These observations agree with the common trend of c¢ precipitate coarsening with the decrease of the cooling rate. The FTS term corresponds to the ‘‘fingers’’ of c¢ phase concentrically growing from an initial point of nucleation. Similar structures of GB precipitates were observed in Reference 5 for a model alloy with a small c-c¢ lattice misfit. Dependence of the size of FTS on the rate of cooling from the supersolvus temperature for
RR1000 was studied in Refere
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