In situ transmission electron microscopy study of ordering in a splat-cooled Ni-20 at.% 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 0.2nm. The cooling rate was estimated to be about 10⁶ 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 °C for observation, then reheated to a higher temperature, and the process repeated for increasingly higher temperatures. At 500 °C, the intensity of the SRO diffraction spots increased. At 665 °C, these SRO spots split into the long-range order (LRO) positions. At 765 °C, LRO was established with a fine (about 0.05 μm) domain structure. As the heating temperature increased, the cell structure became weak and disappeared at about 765 °C.