Grain boundary deformation and damage mechanisms in intergranular crack growth of a nickel based superalloy
Aero engine disc failure has been one of the major concerns in the aircraft industries. The nature of failure is catastrophic and furthermore, the processing and manufacturing of turbine disc is expensive which makes is difficult to replace. The failure study of engine disc is concerned with long crack growth in stress concentrated areas which are subjected to various thermo mechanical loading. The objective of this work focuses on dwell time crack growth rates of ME3 (a nickel base superalloy used in aero engine disc) which can be expanded to the life predicting tools as well as understanding the nature of crack growth can further play a vital role consideration during processing to produce better materials. ^ Different experimental, analytical and numerical investigations were performed to explain the intergranular crack growth mechanism. The crack growth rates are experimentally obtained at different temperatures and dwell time in the Mechanics of Material Laboratory in air and vacuum. With the help of crack growth rates obtained and fractography images intergranular failure is observed in ME3 is explained. Furthermore, rate kinetics of crack growths in air and vacuum environment are discussed which are correlated with the various observations in the specimen after test to identify the failure mechanisms in front of the crack tip. The identified mechanisms in front of crack tip are related to grain boundary sliding and oxygen embrittlement at high temperature. The crack growth rates in ME3 are thus expressed in two models. The first model discussed is related to the kinetics of crack growth which identifies the role of stress, temperature and environment in relevance to activation energy for the fracture mechanism. A second model describes intergranular cracking which physical interpretation and critical grain boundary sliding displacement represent the failure criterion. The interaction of grain boundary sliding and oxygen atoms present results in higher crack growth rate in environment with oxygen atoms than in vacuum. ^ The nature of the crack growth above and below the observed transitional frequency is defined as cycle dependent and time dependent crack growth. Results indicate that cyclic crack growth increases with dwell time and temperature and the failure mode alters from transgranular to intergranular when the frequency of loading is below the transitional frequency. The time dependent crack growth with intergranular fracture mode in ME3 can be characterized by measuring slip band spacing. Furthermore, the observed energy of activation shows that dual mechanism of grain boundary sliding and embrittlement phenomena present in intergranular failure. The apparent energy of activation decreases with increase of crack length. The grain boundary sliding displacement increases with the temperature and the stress sensitivity decreases with the rise in temperature. The environmental effect can be characterized by concentration of oxygen atoms in the grain boundary. The concentration of oxygen atom in the grain boundary is proportional to stress and temperature. The embrittlement phenomena due to environment are the pinning of the grain boundary sliding dislocation which shortens the critical length of sliding displacements. The relaxation of stress due to grain boundary sliding alters the diffusion kinetics of oxygen in the grain boundary. ^ The major conclusions drawn are dwell time imposed at the maximum load increases the crack growth rate which switches the failure mode to intergranular. Intergranular failure has a larger slip band spacing than the transgranular failure which depends upon the loading frequency. Intergranular failure in air is due to the interaction of grain boundary sliding and oxygen atom interaction in the grain boundary. Furthermore, the intergranular failure increased with rise in temperature and duration of the loading frequency.^
"Grain boundary deformation and damage mechanisms in intergranular crack growth of a nickel based superalloy"
Dissertations and Master's Theses (Campus Access).