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Nickel-based superalloys are widely used in manufacturing of several important components of aeronautics and land based gas turbines. The good high temperature resistance of these materials is due to their particular microstructure consisting of a FCC lattice nickel matrix, called γ, strengthened by precipitation of a coherent second phase Ni3(Ti,Al) known as γ′. During service at very high temperatures, γ′ precipitates tend to lose their initial configuration and change their size, shape and distribution, so decreasing material properties, in particular creep resistance. In this work the evolution of microstructure in a serviced first stage bucket from a GE machine has been examined. Several cross sections taken at different heights along the airfoil have been observed by SEM, obtaining a detailed map of the microstructure in the component. A quantitative evaluation of service temperature in the different areas has then been achieved by γ′ precipitates size determination by means of image analysis, combined with a γ′ coarsening model optimized for the particular alloy in use. Temperature values so obtained have been compared with those resulting from the finite element method (FEM) temperature distribution performed during design stage.

This paper outlines a microstructure-based model relating gamma prime microstructure and grain size of Ni-base alloys to their creep behavior. The ability of the model to explain creep of multiple superalloys with a single equation and parameter set is demonstrated. The only parameters that are changed from alloy to alloy are related to the gamma prime characteristics and grain size. This model also allows prediction of creep performance as a function of heat treatment and explains some apparently contradictory data from the literature.

Nickel-based superalloys are widely used in applications requiring strength at high temperature, and in particular in manufacturing of several important components of both aeronautics and land based gas turbines. The main property of these materials is due to their particular microstructure consisting of a fcc lattice nickel matrix (γ phase), strengthened by precipitation of a second phase Ni3(Ti,Al) (γ′ phase), having fcc lattice. During aging at high temperatures, γ′ precipitates increase their size, following a kinetic law described by the classical LSW theory. In this work the growth kinetic of γ′ precipitates for the superalloy GTD 111 has been investigated by SEM. Samples of the alloy have been aged in the typical range of service temperatures for times up to 8000 hours. For each sample a large number of images has been acquired and the size and distribution of γ′ particles have been evaluated. Plotting the average size values, corresponding to the different times and temperatures analyzed, it was possible to obtain the growth kinetic of these particles, finding that the above mentioned theory gives a good description of the observed behaviour. The data obtained for GTD 111 have been also compared with other data referring to INCONEL 738, obtained from samples aged in the same conditions and analyzed in the same way. GTD 111, when compared to IN738, resulted to have a much slower growth kinetic, resulting in a much higher creep resistance. From the collected data it was possible to calculate also the activation energy for the diffusion process for both alloys, finding out values in agreement with those obtained by other authors and very close to the activation energies of Ti and Al in Ni matrix.