Superalloy microstructural variations induced by gravity level during directional solidification
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ample is monitored by three accelerometers mounted to the furnace assembly. The accelerometers are mounted on orthogonal axes, one of which is parallel to the longitudinal axis (aircraft yaw axis) of the sample. The remaining are parallel to the pitch and roll axes of the aircraft. For a typical maneuver during lo-g, the acceleration on all axes averages below 10-2 g. During pullout and climb, the hi-g acceleration parallel to the sample longitudinal axis reaches 1.75 g while the accelerations on the other two axes are less than 0.15 g. Growth rates from 0.25 cm per minute to 1.0 cm per minute were selected for the alloy since these produced excellent dendritic structure in the laboratory and would enable sufficient sample to be solidified for analysis. The advantage to processing alloys on the KC-135 flights is the ability to produce low-gravity and high-gravity effects alternately in the same sample. The disadvantages are the short solidification times and the inability to achieve steady state. Before metallographic examination, each sample was first mounted longitudinally, then polished using successively smaller grit paper. For the volume fraction of carbide measurements, the sample was analyzed unetched since this revealed the carbides without the other phase structures. The samples were then etched in Adler's reagent. Secondary dendrite arm spacing measurements and volume fraction carbide measurements were taken along each ingot length parallel to the growth axis. Lateral crosssections were also examined at the end of the hi-g and lo-g regions (where the greatest effect should be evident). The secondary arm spacing data for the 0.6 cm per minute, 0.8 cm per minute, and the 1.0 cm per minute samples are shown in Figure 1. The secondary arm spacings increase with solidification time in lo-g, then decrease with time when the gravity force returns. Primary arm spacings were also found to be larger in the lo-g than the hi-g crosssections, but these data will be treated in a subsequent paper. The ground-based samples did not show any significant variations in dendrite arm spacings as a function of distance along the axis. Figure 2 shows macrostructure of lo-g and hi-g crosssections in the 1.0 cm per minute sample. The lo-g sections have significantly less interdendritic phase than the hi-g sections. A similar trend is also apparent in the volume fraction of carbide phase (Figure 3). The carbide volume fraction decreases with solidification time in low gravity, and increases again with time after the gravity increases. In addition, the carbides in the lo-g cross section adopted a smaller, blockier microstructure than the carbides in the hi-g cross section. Again, the ground-based samples did not show such variation in segregation, volume fraction, or carbide morphology as a function of distance along the axis. It should be noted that the 0.25 cm per minute sample analyzed had the same arm spacings trends with gravity as did the higher growth rate samples. The data are not given because it was possible to obtain only o
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