Superalloys

Project Description

This project focuses on the microstructural evolution, phase stability, and grain boundary engineering in advanced Ni-based superalloys designed for high-temperature applications. Through a combination of thermo-mechanical processing, sub-solvus heat treatments, and multi-scale characterization techniques—including Atom Probe Tomography (APT), Transmission Electron Microscopy (TEM), Electron Backscatter Diffraction (EBSD), and in-situ X-ray diffraction—we investigate key phenomena such as γ/γ′ phase transformations, inverse precipitation, grain boundary engineering, and stress relaxation behavior. Particular emphasis is placed on solute segregation dynamics, the formation of heteroepitaxial interfaces, and the role of lattice misfit and crystallographic orientation relationships in influencing mechanical properties and long-term stability. These insights aim to support the design and optimization of next-generation superalloys with improved strength, corrosion resistance, and thermal stability for critical aerospace and energy applications.

Related Publications

• "Nucleation mechanism of hetero-epitaxial recrystallization in wrought nickel-based superalloys" - Scripta Materialia Jan 2021. View Publication
  This study examines the formation of γ-like shells around primary γ′ precipitates in the AD730™ Ni-based superalloy during sub-solvus heat treatment and controlled cooling. These shells form via inverse precipitation and are heteroepitaxial, showing no crystal misorientation with the γ′ core. Atom probe tomography revealed Cr, Co, and Fe segregation at dislocations within γ′ and enrichment in the γ-like shells. A mechanism is proposed to explain this process, shedding light on heteroepitaxial recrystallization in Ni-based superalloys.
• "Chemical redistribution and change in crystal lattice parameters during stress relaxation annealing of the AD730TM superalloy" - Acta Materialia Sept. 2022. View Publication
  This study investigates the macroscopic contraction and stress relaxation retardation in γ/γ′ Ni-based superalloy AD730™ during isothermal annealing at 760 °C. Using techniques like APT, XRD, TEM, and ECCI, it reveals that solute diffusion (Cr, Co, Fe) along dislocations alters the chemical composition of γ and γ′ phases. These changes reduce lattice parameters, which primarily drives the observed volume contraction.
• "Microstructural characterization of three different size of gamma prime precipitates in Rene 65" - Materials Characterization Nov. 2021. View Publication
  This study investigates phase evolution in a non-equiatomic nanocrystalline CoCuFeMnNi alloy using in-situ TEM heating, ACOM, EFTEM, and APT. It reveals a complex sequence of grain boundary segregation and depletion, leading to FeCo-rich B2 precipitate formation with specific orientation relationships. Cu, Ni, and Co segregate early, while Fe and Mn deplete, followed by Mn segregation after precipitation onset. The high grain boundary density in the nanocrystalline state plays a critical role in enabling these transformations, unlike in coarse-grained counterparts.
• "Role of carbide precipitates and process parameters on achieving grain boundary engineered microstructure in a Ni-based superalloy" - Metallurgical and Materials Transactions A Oct. 2015. View Publication
  This study explores thermo-mechanical processing routes to develop a grain boundary engineered (GBE) microstructure in alloy 617. A single-step process with 15% cold rolling and annealing at 1100 °C for 1 hour effectively increased the Σ3 boundary fraction, though with partial recrystallization. Iterative processing with adjusted parameters also achieved GBE by enhancing Σ3 boundaries and disrupting random high-angle boundary networks. The resulting microstructure exhibited improved intergranular corrosion resistance.