Dynamic Strain Aging and Embrittlement Behavior of IN718 During High-Temperature Deformation
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INTRODUCTION
IN718* is a Ni-Cr-Fe based superalloy extensively *IN718 is a trademark of Special Metals Corporation, New Hartford, NY.
used in the gas turbine, rocket engine, space craft, and nuclear reactor industries as a high-temperature structural material due to its good combination of strength at elevated temperature, good creep resistance, and excellent weldability. The matrix has a face-centered-cubic (fcc) austenitic structure known as Gamma (c). The alloy contains Cr, Fe, Nb, and Mo as major alloying elements with a minor amount of Ti and Al. Cr
K. SARAVANAN, V.S.K. CHAKRAVADHANULA, SUSHANT K MANWATKAR, S.V.S. NARAYANA MURTY, and P. RAMESH NARAYANAN are with the Materials and Mechanical Entity, Vikram Sarabhai Space Centre, Indian Space Research Organisation, Trivandrum 695022, India. Contact email: [email protected] Manuscript submitted January 27, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
contributes to the increase of the corrosion resistance of the alloy, whereas Mo plays the role of solid solution strengthener.[1] Ti and Al are added to form Ni3(Al, Ti) intermetallics, which act as strengthening precipitates in the c matrix.[1] Nb with Ni forms Ni3Nb intermetallic precipitates, which also act as strengthening precipitates. Nb is present both in the matrix and at grain boundaries (GBs). A minor amount of C is added to this alloy, forming MC-type carbides such as (Ti, Nb)C. These carbides are present at GBs, which inhibit GB sliding, thereby improving the high-temperature creep properties. C content is kept to a minimum to facilitate Nb and Ti precipitation in the c matrix. The alloy is strengthened by precipitation of Gamma prime (c¢) together with Gamma double prime (c¢¢) within the c matrix. The c¢ made up of Ni3(Al, Ti) intermetallics having simple cubic with ordered L12 structure with spherical morphology is coherently precipitated in the c matrix. This c¢ phase improves high-temperature strength and creep resistance. The effective strengthening precipitate is c¢¢, which is made up of Ni3Nb intermetallics and has a body-centered-tetragonal structure. Though c¢¢ is coherent with the c matrix, it induces large mismatch strains; hence, the c¢¢ phase provides very high strength at low to intermediate temperature ranges. The c¢¢ precipitate is
stable up to 650 C. Above 650 C, the c¢¢ precipitate size increases rapidly and subsequently dissolves in the matrix.[2] In addition to c¢ and c¢¢ strengthening phases, an equilibrium phase Ni3Nb (d) having orthorhombic structure is also present at GBs.[3] Apart from these precipitates, some secondary carbides will form on prolonged aging. Thus, the c matrix, c¢, c¢¢, d, and MC-type carbides all together form the microstructure of IN718. The mechanical properties of IN718 depend on the microstructure, especially the grain size and morphology, size and volume fraction of c¢, c¢¢, and d precipitates. The precipitation behavior of this alloy has been extensively studied by many researchers,[4–7] and it primarily depends on the quantity of alloying eleme
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