THE NATURE OF UNUSUAL GAMMA' GAMMA' INTERFACES FORMED UPON QUENCHING DIRECTIONALLY SOLIDIFYING NI3AL ALLOYS/

Upon quenching directionally solidified Ni-Al alloys having compositio ns near the Ni3Al compound, two distinct and unusual interfaces (No. I and No. II) occur in the region of the solid-liquid growth front. The interfaces appear after etching in a variety of etchants or with an i on beam or after electro-polishing. Both interfaces are gamma'/gamma' interfaces. Experiments are presented which show that during the quenc h a band of gamma' phase forms upon the solid growth front, and interf ace No. II marks the top of the band, while interface No. I marks the bottom of the band where it overgrows freezing gamma'. It is also show n that both interfaces result from local planar shaped regions of comp osition change. Interface No. II is mainly an interesting experimental observation, but interface No. I should be very useful in experiments involving growth of gamma' because it marks the location of the solid -liquid growth front.