Non-destructive electrical characterization of controlled Waspaloy microstructures

In this research, controlled Waspaloy microstructures are produced with the objective of studying microstructural evolution in this alloy via non-destructive electrical measurements. Waspaloy is a precipitation-hardenable gamma-gamma' nickel-base superalloy that is used in turbine blade applications demanding superior strength retention capabilities at elevated temperatures. Overall, three different sets of microstructures were produced that varied systematically as a function of the matrix (gamma) grain size and gamma' precipitate size distribution or just the latter. Initial solutionizing treatments conducted at 1045°C, 1090°C and 1145°C resulted in average y grain size of 13, 52 and 89 mum respectively. A vacancy stabilization treatment at 1045°C followed the solutionizing treatments in Set I experiments, after which the specimens were aged at 800°C for durations ranging from 0.1 hrs to 100 hrs. In Sets 11 and III, the matrix grain size was kept unchanged by an initial solution-treatment at 1145°C. The stabilization treatment at 1045°C was only conducted in Set II after the solution-treatment. Aging experiments were then conducted at 700°C (or 725°C in Set III), 800°C and 875°C to study the growth kinetics of gamma' precipitate distribution. The specimens with controlled microstructures were investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), x-ray diffraction (XRD), ultra small-angle x-ray scattering (USAXS) and dc four-point probe resistivity. The applicability of two and four-probe ac impedance techniques was also investigated.;Characterization of heat-treated specimens. Solution-treated SEM microstructures showed the presence of polygonal etch-pit shapes, which was proposed to be due to the condensation of excess quenched-in vacancies along crystallographically soft-directions in the gamma phase. The etch-pits evolved upon subsequent aging into progressively irregular shapes, concurrent with gamma' growth inside the pits. SEM and USAXS observations clearly supported the progressive coarsening of the gamma' distribution with increasing aging time at 800°C. Complementary evidence from do resistivity measurements confirmed that gamma' nucleation was complete by 0.1 hrs of aging at 800°C. Coarsening studies indicated an LSW type volume diffusion mechanism of coarsening in Waspaloy, with an average coarsening rate constant of 3.25x10-29 [m3/sec] for Set I specimens aged at 800°C. This is an order of magnitude smaller compared to values quoted in the literature for binary Ni-Al systems. The resistivity decrease during initial stages of gamma' coarsening was attributed to the progressive decrease in the 'scattering power' of the gamma' distribution. The variations in the later stages were related to compositional changes during coarsening. Resistivity variations as a function of the aging time at 800°C did not show any dependence on the prior solution-treatment temperature in Set I specimens. Resistivity measurements also showed a drastic shortening in the first-stage coarsening with increase in the aging temperature from 725°C to 800°C, while this stage was completely absent at 875°C. In Set III specimens, the gamma-gamma' lattice misfit was found to increase with increasing aging duration at the same temperature or with increasing kinetics.