Microstructural Instabilities in Nickel-based Single Crystal Superalloys
Abstract
Single crystal superalloys, mostly Nickel-based, have been, so far, the material of choice for high-temperature applications, such as gas turbine blades. Single crystalline turbine blades are usually cast in the direction, providing the best combination of properties. While misorientations up to 15◦ are within accepted tolerance, the elastic/viscoplastic response of the bi-phased γ/γ0 microstructure in superalloys is highly sensitive towards the crystalline orientation. Further, multiaxial stress states create microstructural gradients and alter the performance of the material. As the microstructure stability dictates the blade’s structural integrity, it is necessary to understand the microstructural state as a function of the crystallographic orientation and multiaxial stress state to quantify the creep performance.
The lifetime of Ni-based single crystal superalloys is connected to the integrity of the strengthening phase, γ 0 . During high-temperature thermomechanical loading, the cuboidal shaped γ 0 phase coalesces, directionally coarsens(rafting), and is finally topologically inverted, viz. surrounds the γ phase and acts as the matrix. The topological inversion comes along with an increase in the plastic strain rate known as the tertiary creep. X-ray tomography experiment has recently revealed that the tertiary creep initiates before the expected increase in the volume fraction of pores. Thus, the initiation of the tertiary creep stage might also be due to the destabilization of the γ/γ0 interfacial dislocation network leading to the massive shearing of γ 0 rafts concomitantly resulting in topological inversion. Simultaneously, the strain-hardening and the void growth in these viscoplastic single crystals lead to the ductile fracture. Hence, for a fundamental understanding of the microstructural instability and to improve lifetime predictions, it is necessary to consider the interplay of orientation-specific microstructural evolutions, the destabilization of the interfacial network, and subsequent shearing of γ 0 particles, as well as dislocation, creep and void growth to fully explain what triggers the increase of the plastic strain rate in the tertiary creep stage leading to failure.
This dissertation aims to elucidate the microstructural instabilities in Ni-based single crystal superalloys through a multi-scale microstructure-sensitive thermo-mechanically coupled damage model in a finite-element crystal plasticity framework through the integration of high-temperature multiaxial creep experiments, advanced characterization techniques, and high fidelity computational tools. To that end, a phase-field model was extended to account for microstructural destabilization and damage. For the first time, topological inversion during creep was predicted using a phase-field model. Further, macroscale finite-element calculations were carried out using the realistic 3D microstructures derived from the phase-field model. A viscoplastic description of 3D rafting predicted the channel evolution irrespective of the loading conditions and crystallographic orientation. Results from this work will elucidate the effects of microstructural degradation, crystallography, and multiaxial stresses towards rupture in Ni-based single crystal superalloy subjected to high-temperature/low stress creep conditions.
Subject
Ni-based Single Crystal SuperalloyPhase-Field Modeling
Finite-Element Crystal Plasticity Modeling
High-Temperature Creep Experiments
Multiaxiality
Citation
Rajendran, Harikrishnan (2021). Microstructural Instabilities in Nickel-based Single Crystal Superalloys. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /195759.