IN-SITU TRANSMISSION ELECTRON-MICROSCOPY STUDY OF ORDERING IN A SPLAT-COOLED NI-20 AT-PERCENT MO ALLOY

Using transmission electron microscopy, the microstructure of splat-cooled (through hammer and anvil) Ni-20 at.% Mo specimens has been examined for the as-cooled condition and for heated samples. In the as-cooled condition, the structure consisted of columnar grains with a cell structure of about 0.2 mu m. The cooling rate was estimated to be about 10(6) K/s; but even for this rapid cooling, weak short-range order (SRO) was present. Specimens were step-heated in situ in the transmission electron microscope to a specific temperature, held for 5 minutes, cooled to 25 degrees C for observation, then reheated to a higher temperature, and the process repeated for increasingly higher temperatures. At 500 degrees C, the intensity of the SRO diffraction spots increased. At 665 degrees C, these SRO spots split into the long-range order (LRO) positions. At 765 degrees C, LRO was established with a fine (about 0.05 mu m) domain structure. As the heating temperature increased, the cell structure became weak and disappeared at about 765 degrees C.