Characterization of Recrystallization and Second-Phase Particles in Solution-Treated Additively Manufactured Alloy 718
- PDF / 2,712,291 Bytes
- 9 Pages / 593.972 x 792 pts Page_size
- 85 Downloads / 218 Views
I.
INTRODUCTION
ALLOY 718 is a Ni-based superalloy known for its combination of outstanding mechanical stability, fatigue, and corrosion resistance properties at elevated temperatures up to 650 C.[1,2] It is widely used as wrought alloy for high temperature components such as turbine disks in aircraft engines and power generation.[3] The excellent properties of the alloy 718 are derived from its microstructure, including fine matrix grain size and coherently precipitated second-phase particles. The microstructure of the alloy is formed by two types of standard heat treatments which consist of solution treatment and aging treatment: the first one, which is more standard and called ‘‘subsolvus solution treatment,’’ is to obtain fine grained materials for high strength and good fatigue properties. The second one, ACHMAD ARIASETA, SATORU KOBAYASHI, and MASAO TAKEYAMA are with the Department of Materials Science & Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama Meguro-ku, Tokyo, 152-8552 Japan. Contact e-mail: [email protected] YUTING WANG and SHINYA IMANO are with the Mitsubishi Hitachi Power Systems, Ltd, 3-1-1 Saiwai-cho, Hitachi-shi, Ibaraki, 317-8585 Japan. Manuscript submitted August 1, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
called ‘‘supersolvus solution treatment,’’ is to obtain a large grain size for good creep properties.[3–5] In the subsolvus heat treatment, the solution treatment is employed after a hot deformation process below the d-Ni3Nb phase solvus temperature followed by quenching to produce fine recrystallized and partially solutionized matrix grains with residual d-phase particles.[3] In the supersolvus condition, the solution treatment is employed above the d solvus temperature to have coarse recrystallized and completely solutionized matrix grains. The solution treatments are followed by two-step aging treatments of which conditions differ by the solution treatment condition. It has been found that the recrystallization kinetics is very slow in additively manufactured (AM) alloy 718 even in supersolvus solution treatment conditions at temperatures of 1000 C to 1100 C and above.[6–10] Tucho et al. assumed that the slow recrystallization kinetic is due to slow recovery of substructures in the as-built condition caused by undissolved fine second-phase particles which normally do not appear after the solution treatments in wrought alloy 718. MC type, Laves, and oxygen-containing particles were found to form in AM alloy 718 as unusual second-phase particles with respect to the wrought alloy 718.[7] The particle pinning is a possible reason but the size and density of
the fine particles have not been studied in a quantitative manner and therefore the mechanism is not still clear. Chlebus et al. assumed that the recrystallization occurs as a result of grain boundary migration induced by diffusion of solute atoms such as Nb, which leads to the dissolution of columnar arrayed phases and reduction of grain boundary area.[6] Liu et al.[11] stated three mechanisms are involved in
Data Loading...
