Phase Transformation and Segregation to Lattice Defects in Ni-Base Superalloys

Nanostructural features of nickel-base superalloys as revealed by atom probe field ion microscopy (APFIM) and atom probe tomography (APT) are reviewed. The more salient information provided by these techniques is discussed through an almost exhaustive analysis of literature over the last 30 years. Atom probe techniques are shown to be able to measure the composition of tiny γ′ precipitates, a few nanometers in size, and to reveal chemical order within these precipitates. Phase separation kinetics in model NiCrAl alloys was investigated with both 3DAP and Monte-Carlo simulation. Results are shown to be in good agreement. Plane by plane analysis of {001} planes of Ni3Al-type γ′ phase makes it possible to estimate the degree of order as well as the preferential sites of various addition elements (Ti, Cr, Co, W, Ta, Re, Ru, etc.) included in superalloys. Clustering effects of Re in the γ solid solution were also exhibited. Due to its ultrahigh depth resolution, the microchemistry of interfaces and grain boundaries can be characterized on an atomic scale. Grain boundaries in Astroloy or N18 superalloys were found to be enriched in B, Mo, and Cr and Al depleted.

Interfacial Segregation in Oxide Scales on Nicrai-Based Alloys

Previous studies addressing the segregation of reactive elements in protective oxide scales and their beneficial effect on scale adhesion have primarily concentrated on primary alumina-formers (e.g. β-NiAl + FeCrAl).In our study the oxidation behaviour of three NiCrAl alloys, which form complex scales was studied in air at 1423 K and at 1473 K, both in isothermal (100 h) and in cyclic oxidation (100 x lh). The composition (in at.-%) of these alloys is the following: General Electric alloy René N5 (64.9 Ni, 7.8 Cr, 13.9 Al, 0.1 Fe, 2.1 Ta, 0.05 Hf, 1.6 W, 1.0 Re, 0.15 Si, 7.3 Co, 0.9 Mo, 0.003 Y, 0.003 Zr, 4 ppm S, 0.25 C), Ni-7Cr-6.5Al+Y (80.1 Ni, 7.2 Cr, 12.5 Al, 0.01 Fe, 0.14 Si, 0.012 Y, 18 ppm S, 0.05 C) and Ni-10Cr-10Al+Y (71.2 Ni, 9.9 Cr, 18.8 Al, 0.01 Fe, 0.02 Si, 0.041 Y, 16 ppm S, 0.04 C).

Microstructural development of protective A12O3 scales

The most severe demands of a jet turbine engine in terms of high temperature (1000-1100°C) oxidation of superalloys have been successfully met with the use of NiCrAlY or NiAl types of coating materials. The success of these coatings is due to their ability to form an adherent slow-growing scale of α-Al2O3. The growth mechanism of these scales is generally accepted to be short-circuit grain boundary diffusion of oxygen resulting in an inward growing scale. Oxide-metal adherence is ultimately due to a fine dispersion of oxygen active elements (Y, Zr, Hf, Th, etc.) in the coating, whose effect appears to be intricately involved with subtle changes in scale growth processes. The present paper documents the scale microstructures developed on NiCrAl alloys, and emphasizes features relevant to growth mechanisms.