Micromechanisms of Thermomechanical Fatigue-A Comparison With Isothermal Fatigue.
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION CLEVELAND OH LEWIS RESEARCH CENTER
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Thermomechanical Fatigue TMF experiments were conducted on Mar-M 200, B-1900, and PWA-1480 single crystal over temperature ranges representative of gas turbine airfoil environments. The results were examined from both a phenomenological basis and a micromechanistic basis. Depending on constituents present in the superalloy system, certain micromechanisms dominated the crack initiation process and significantly influenced the TMF lives as well as sensitivity of the material to the type of TMF cycle imposed. For instance, high temperature cracking around grain boundary carbides in Mar-M 200 resulted in short in-phase TMF lives compared to either out-of-phase or isothermal lives. In single crystal PWA-1480, the type of coating applied was seen to be the controlling factor in determining sensitivity to the type of TMF cycle imposed, out-fo-phase lives being shortened compared to in-phase pr isothermal lives if a coating that is brittle at the low temperature end of the cycle were applied. Micromechanisms of deformation were observed over the temperature range of interest to the TMF cycles, and provided some insight as to the differences between TMF damage mechanisms and isothermal damage mechanisms. Finally, the applicability of various life prediction models to TMF results was reviewed. It was concluded that current life prediction models based on isothermal data must be modified before being generally applicable to TMF. Doc.
- Jet and Gas Turbine Engines