Session: 16-01-01: Government Agency Student Poster Competition

Paper Number: 150162

150162 - Engineering the Precipitate Growth Patterns in Single Crystal Superalloys 

Laser peening (LP) significantly boosts the durability of Ni-based superalloy components by inducing compressive residual stresses through plastic deformation at the specimen's surface and subsurface areas, thus mitigating surface damages. Recent findings indicate that these methods may also precipitate coarsening behavior at elevated temperatures, resulting in rafting phenomena. This study investigates how LP and subsequent heat treatment affect γ' precipitate coarsening and rafting in the single crystal Ni-based superalloy CMSX-4. It was demonstrated that LP influences the directional coarsening behavior of γ' precipitates, depending on the magnitude and depth of compressive residual stresses and the density of induced dislocations when followed by a heat treatment process. X-ray diffraction results revealed compressive surface stresses (-400 MPa to -800 MPa) that transitioned to tensile stress at greater depths, eventually stabilizing in an unstressed state. These contrasting stress states led to horizontal and vertical rafting of γ' precipitates up to a depth of 1 mm, transitioning back to a non-directionally coarsened microstructure at depths exceeding 5 mm. Mechanisms governing rafting thermodynamics were elucidated, including dislocation-aided interface growth, solute diffusion, and stress superposition. Electron microscopy showed horizontal coarsening in compressive regions and vertical coarsening in tensile regions due to stress-aided diffusion. The top surface of the LPed material exhibited the highest deformation level, leading to coarsening driven by an overall reduction in plastic strain energy, resulting in misshapen, globular γ' phases. Increased plastic strain near the LPed surface exhibited a lattice misorientation of about 5°, which normalized post-heat treatment due to rafting-aided recovery and defect reorganization. Energy dispersive x-ray spectroscopy revealed notable depletion of γ'-forming elements around primary rafted precipitates, underscoring preferential solute diffusion. Additionally, precipitate-free zones surrounding the primary rafted precipitates were formed due to solute depletion following coarsening. LP-induced plastic deformation influenced the coarsening behavior of the γ' phase by generating interfacial dislocations, destabilizing the misfit strain, and lowering the coarsening energy barrier. These findings demonstrate the potential of LP and heat treatment to modify the microstructure and enhance the performance of CMSX-4 at high temperatures. Understanding the effects of surface hardening processes on the rafting behavior of single-crystal Ni-based superalloys is crucial for advancing this robust class of materials. The potential for localized surface strengthening to enhance the resistance of turbine blades and other engine components against surface-initiated damage mechanisms is highly promising. However, in engineering these processes, it is essential to consider how the residual stress state and defect environment affect the microstructural response to thermal input.

Presenting Author: Alireza Doroudi The university of Alabama

Presenting Author Biography: My name is Alireza Doroudi, and I am a PhD student in the Department of Mechanical Engineering at the University of Alabama. My research focuses on advanced manufacturing, with a particular emphasis on additive manufacturing and laser peening techniques.

Authors:

Alireza Doroudi The university of Alabama
Noah Holtham The University of Alabama
Keivan Davami The University of Alabama