The increase in the operating temperatures of the new generation of advanced gas turbine engines has focused attention on the time-dependent properties of powder-metallurgy nickel-based superalloys. At these higher temperatures, the ability of the alloys to resist crack growth under extended hold times becomes a design-limiting mechanical property. A considerable amount of research has been performed to explain and model hold-time crack-growth behavior in superalloys. Most of the research has focused on trying to identify the grain-boundary phases that are most susceptible to an environmental attack and that would reduce an alloys’ resistance to hold-time crack growth. Over the years, a number of grain-boundary species have been proposed to be susceptible to environmental attack, but none of these hypotheses has withstood the test of time.
A well-controlled study was performed at the NASA Glenn Research Center to determine the variables that influence the hold-time crack-growth resistance of two newly developed powder-metallurgy superalloys, Alloy 10 and ME3. The effects of both compositional changes and variation in heat treatments were investigated. The results indicate that significant changes in the alloy’s composition did not have an appreciable effect on hold-time crack-growth resistance, provided that the heat treatment remained constant.
Quantitative image analysis was performed to determine precipitate size distribution. Air-cooled; 1149 °C re-solution. Light areas indicate fine precipitates. Dark areas indicate coarse precipitates.
In contrast to the composition study, the heat treatments evaluated produced changes in the crack growth resistance of up to an order of magnitude. Quantitative image analysis was performed to analyze the microstructural features produced by each heat treatment.
We found that the cooling γ precipitate size distribution was the most important variable controlling hold-time crack-growth behavior. The larger the mean size of the cooling precipitates, the better was the resistance to hold-time crack growth. To gain further insight into the role of precipitate size distribution on time-dependent properties, we determined the alloys’ stress relaxation behavior as a function of precipitate size and volume fraction. An increase in the precipitate size corresponded directly to an increase in stress relaxation. On the basis of these observations, we proposed that the size and distribution of the precipitates play an important role in determining the extent of the crack-tip stress relaxation that occurs during hold times through creep-type processes. The different crack-tip stress relaxation rates have a significant effect on the crack-driving forces, producing a large variation in the measured hold-time crack-growth resistance. The microstructures that produced larger precipitates showed the most stress relaxation and, thus, the lowest crack-driving force, which in turn produced the slowest crack-growth rates.
A close relationship exists between precipitate size and hold-time crack-growth resistance; crack growth per second, da/dt, at 25 MPa-m2 = 1.87×10-7 (cooling γ’ area)-1.385; r2 = 0.98.
Long description of figure 2.
Last updated: October 11, 2006
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