Use this URL to cite or link to this record in EThOS:
Title: Creep and microstructure evolution in nickel superalloys
Author: Coakley, James
ISNI:       0000 0004 2689 5690
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2010
Availability of Full Text:
Access from EThOS:
Access from Institution:
This thesis presents work carried out to develop the understanding of microstructural evolution and the corresponding macroscopic creep that occurs in nickel superalloys at gas-turbine operating conditions. In-situ time-of-flight (TOF) neutron diffraction creep experiments were performed in order to measure the change in lattice d-spacing of both [gamma] and [gamma]' in the CMSX-4 single crystal nickel superalloy. The loading responses of both phases are distinct. The d-spacing evolution of [gamma] and [gamma]'shows markedly different behaviour in the primary and tertiary creep regimes, suggesting different deformation mechanisms. The lattice strain evolution is interpreted in light of current dislocation theories. It is generally assumed that at gas-turbine operating temperatures, [gamma]coarsens according - R [is propotional to] 3[root]t, where -R is the mean radius and t is time. Heat-treatments were performed on samples of multimodal Ni115 to investigate this assumption. Electron microscopy was used to analyze the samples post heat-treatment, and the frequency distribution of radii was calculated. It is shown that a transient period can exist for thousands of hours, and the above coarsening rate is not valid. An existing LSW-based model is further developed to model the coarsening kinetics of a superalloy in real time and real radii for the first time, and model predictions are compared to experiment. The creep properties of different [gamma] distributions in the Ni115 nickel superalloy produced by heat-treatment were examined. At the stresses and temperatures employed it is shown that particle bypass cannot occur by cutting or bowing and so presumably occurs by a climb-glide motion. The Dyson creep model is a microstructure based climb-glide bypass model for unimodal distributions. It is developed further to account for bimodal distributions and the predictions compared to experiment. The fine [gamma], when present, controls dislocation motion, seen in both experiment and model predictions.
Supervisor: Reed, Roger ; Dye, David Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral