On the Evolution of Primary Gamma Prime Precipitates During High Temperature and High Strain Rate Deformation and Subseq
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NI-BASED superalloys strengthened by the presence of a dispersion of coherent c¢ (L12) precipitates in an FCC-disordered solid solution matrix of c (A2) phase find wide applications in aero-engine and land-based power generation designed for high-temperature applications. Specifically, for turbine disc applications, the powder metallurgy (PM) route is followed. Superalloy powders synthesized by gas atomization, hot isostatic pressing (HIP) followed by blind die compaction (upset), and hot extrusion are used to produce the billet. The billet is subsequently isothermally forged and then heat treated. The evolution of the microstructure, i.e., specifically the c¢ precipitate morphology during these thermomechanical processes controls the grain N. D’SOUZA, W. LI, and C. ARGYRAKIS are with the RollsRoyce plc, PO Box 31, Derby, DE24 8BJ, UK. G.D. WEST and C.D. SLATER are with the Warwick Manufacturing Group, University of Warwick, Coventry CV4 7AL, UK. Contact e-mail: [email protected] Manuscript submitted April 11, 2019. Article published online June 26, 2019 METALLURGICAL AND MATERIALS TRANSACTIONS A
size.[1–4] The grain size has a significant effect on the evolution of the flow stresses during forging.[2,5–10] The evolution of the c¢ precipitate morphology is influenced by the stress, plastic strain, and most importantly the strain rate.[4,11,12] The 1st population termed as primary c¢ has the principal role of inhibiting grain growth during billet manufacture and forging by pinning the grain boundaries and are typically, 1 to 5 lm in size.[13] Thermomechanical operations carried out at temperatures where only these pinning particles are present in the microstructure (absence of secondary/tertiary populations, as they have lower solvus temperature compared to forging temperature) can be therefore carried out at decreased flow stresses. The evolution of flow stress under a range of thermomechanical treatments has been studied in Ni-based alloys encompassing total strain, strain rate and stress, and constitutive equations have been fitted to describe the behavior. Depending on the temperature/strain rates, either dynamic re-crystallization (DRX) occurs at high strain rates approaching ~ 1 s1, while at intermediate/lower strain rates, ~ 103 to 102 s1 super-plastic flow preceded by limited DRX occurs in the temperature range, 1273 K to 1423 K.[5–9,14] VOLUME 50A, SEPTEMBER 2019—4205
The motivation for this study stems from the fact that most prior studies have primarily focused on conditions of isothermal forging characterized by low strain rates, typically £ 0.1 s1. There are fewer investigations at high strain rates, typically ‡ 1 s1, which is akin to conditions associated with billet manufacture. At these high strain rates if DRX does not completely relax all the stress owing to the negligible deformation time (Dt ~ 5 seconds and depending on the final plastic strain), reduction in the remnant dislocation density leads to meta-dynamic (post-dynamic) re-crystallization during cooling.[4,15] Furthermore, unde
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