Michael F. Henry

Abnormal grain growth and grain boundary faceting in a model Ni-base superalloy

Abstract Normal or abnormal grain growth in a model Ni-base superalloy is observed to depend on the grain boundary structure when heat-treated in a solid solution temperature range above the solvus temperature (1150°C) of the γ′ phase. When heat-treated at 1200°C abnormal grain growth occurs and most of the grain boundaries are observed to be faceted by optical microscopy, transmission electron microscopy, and scanning electron microscopy at the intergranular fracture surface. Some of the grain boundary facet planes are expected to be singular corresponding to the cusps in the polar plot of the boundary energy against the inclination angle, and it is proposed that if these boundary segments move by a boundary step mechanism, the abnormal grain growth can occur. When heat-treated at 1300°C normal grain growth occurs, the grain boundaries are defaceted, and hence atomically rough. Normal growth is expected if the migration rate of the rough grain boundaries increases linearly with the driving force arising from the grain size difference. The correlation between the grain boundary structural transition and the growth behavior thus appears to be general in pure metals and solid solution alloys.

The dendritic growth of γ′ precipitates and grain

The growth pattern of γ precipitates in the grains and at the grain boundaries has been investigated in a Ni-24Co-4Al-4Ti-5Cr-5Mo (weight percent) alloy of very small lattice misfit between the precipitate and the matrix phases under varying heat-treatment conditions. When aged at temperatures lower than the solvus temperature (Ts = 1150 °C) by more than 30 °C after direct cooling from the solution-treatment temperature, the nucleation density is high. In this condition, the supersaturation is quickly removed because of the overlapping diffusion fields and the precipitates undergo Ostwald ripening from the early stage. The precipitates then have an equilibrium shape of spheres in the grains and truncated spheres at nearly straight grain boundaries. The precipitates at the grain boundaries are coherent with one of the grains, and their number density is not much larger than that in the grains, apparently because of a large contact angle (about 150 deg) with the grain boundary. Quenching the alloy after the solution treatment and aging at any temperature also produce high precipitate number density and equilibrium shapes. When aged at temperatures just belowTs (above 1140 °C), the nucleation density is low, the precipitates grow dendritically in the grains, and the grain boundaries become serrated. The observed dendritic growth characteristics do not quantitatively agree with the predictions of Mullins and Sekerka theory, but the discrepancy may be due to the uncertainties in both the observed and calculated quantities. By deeply etching the matrix, it is shown that the grain boundary serration is produced by the precipitates growing preferentially in the direction of the incoherent boundary because of the rapid solute diffusion along the grain boundary. The dendritic growth and grain boundary serration can be obtained also by slowly cooling through the temperature range just belowTs.

The thermodynamic prediction of phase stability in multicomponent superalloys

Thermodynamic calculations, which are being performed on a daily basis in industrial laboratories such as the General Electric Corporate Research and Development Center, have proven to be of great assistance in the development of new alloys and the selection of processing conditions for existing or new alloys. This article presents an evaluation of the state-of-the-art ofthat capability for nickel-base superalloys.

High temperature phase chemistries and solidification mode prediction in nitrogen-strengthened austenitic stainless steels

Nitronic 50 and Nitronic 50W, two nitrogen-strengthened stainless steels, were heat treated over a wide range of temperatures, and the compositions of the ferrite and austenite at each temperature were measured with analytical electron microscopy techniques. The compositional data were used to generate the (γ + δ phase field on a 58 pct Fe vertical section. Volume fractions of ferrite and austenite were calculated from phase chemistries and compared with volume fractions determined from optical micrographs. Weld solidification modes were predicted by reference to the Cr and Ni contents of each alloy, and the results were compared with predictions based on the ratios of calculated Cr and Ni equivalents for the alloys. Nitronic 50, which contained ferrite and austenite at the solidus temperature of 1370 °C, solidified through the eutectic triangle, and the weld microstructure was similar to that of austenitic-ferritic solidification. Nitronic 50W was totally ferritic at 1340 °C and solidified as primary delta ferrite. During heat treatments, Nitronic 50 and Nitronic 50W precipitated secondary phases, notably Z-phase (NbCrN), sigma phase, and stringered phases rich in Mn and Cr.

Precipitation in an as-atomized nickel-based superalloy powder

The microstructure of argon-atomized René 95 powder has been characterized by a combination of transmission and scanning-transmission electron microscopy, and energy dispersive X-ray analysis. Specimen preparation techniques have been developed in order to obtain samples suitable for such analyses, and similar but complementary micro-structural features are revealed by each technique. Dendritic and cellular structures, both on the surface of the powder particles and in the interior, are delineated by fine precipitates. These have been identified as MC-type carbides, containing Nb and Ti, with some Cr, Ni, Mo and W.

Phase transformations during aging of a nitrogen-strengthened austenitic stainless steel

An analytical electron microscopy study was undertaken in order to characterize intergranular and matrix precipitation accompanying intermediate temperature aging in NITRONIC 50, a nitrogen-strengthened austenitic stainless steel. Extensive precipitation on most grain boundaries had occurred after aging for 24 hours at 675 °C. The primary intergranular phase at that time was Cr-rich M23C6, and energy dispersive spectra taken on grain boundary segments between these carbides indicated Cr-depletion and Fe- and Ni-enhancement relative to the matrix. After aging for 336 and 1008 hours at 675 °C, M6C (eta-carbide) precipitates were also present on grain boundaries. These precipitates were distinguished from M23C6 on the basis of their lattice parameters and chemistries, with M6C containing less Cr and Fe, and more Ni, Mo, and Si than M23C6. The differences in chemistry were clarified by a statistical treatment of the spectra. The statistical analysis also showed that precipitates with a range of chemistries between M23C6 and M6C coexisted with these phases on the grain boundaries. Associated with this shift in precipitate stoichiometry was an increase in the average concentration of Cr and a decrease in the average concentration of Ni at the grain boundaries. Intergranular sigma phase was also observed after times 24 hours at 675 °C, with sigma precipitating on grain boundaries containing carbides. Intragranular precipitates observed to be stable up to 1008 hours at 675 °C included Z-phase, a complex nitride which had formed during solution annealing; M7C3 carbides, which nucleated at Z-phase/austenite interfaces; M23C6 carbides, which precipitated on incoherent twin boundaries; and Cr-rich MN precipitates, which nucleated on dislocations.

Microstructure of a plasma-sprayed superalloy coating/substrate

Five different specimen preparation techniques were employed to characterize the microstructure of a cross-section of plasma-sprayed Rene′ 125 coating on a Rene′ 125 turbine blade substrate. These methods included optical microscopy, and transmission electron microscopy on three types of replicas and on thin foils. Interface and matrix precipitates were indentified using electron diffraction and energy dispersive spectrometry, with HfO2 and (Hf, Zr)O2 the predominant phases at the coating/blade interface, and both (Ti, Ta)C and HfO2 present in the coating. Those blade precipitates examined contained Hf and Ta, with some Ti and a little Co. A unique dendritic structure of γ′ was also found intermittently along the interface. The combination of the five techniques provides a wide variety of information, and is a strong tool for characterizing complex microstructures.

An investigation of switching stresses in bimetal disks

Abstract Bimetal disks used for thermostatic control and over-temperature protection can undergo a drift in switching temperature with repeated thermal cycling. The switching action involves a sudden snap-thru and inversion of curvature. A detailed stress analysis associated with this switching (based on work by W. H. Wittrick) and a solution procedure were developed for bimetal disks. Stresses, strains and deflections were then calculated. including changes on snap-thru at switching temperatures. Experimentally determined switching strains were found to be in good agreement with the analysis. The analysis shows high cyclic and mean stresses in the disk. These stresses are considered to be the major contributing factor in the models proposed for the drift in switching temperatures during prolonged cyclic operation. A parametric study was carried out in which several disk thicknesses, diameters and differences in switching temperatures were considered. As a result of this study a simple procedure was discovered for determining graphically the stresses developed during switching, from which a new design procedure is proposed.