PHASE-SEPARATION IN AL-RICH BETA'-NIAL AL LOY BY ANNEALING

Microscopic structures of aluminium-rich beta'-NiAl thin foils anneale d at 373-773 K have been studied by electron microscopy. The contrast of the bright field (BF) image shows a tweed pattern at the beginning stage of annealing and then turns gradually into a mottle pattern. The corresponding diffraction patterns of the [100] incidence are observe d at a temperature below 573 K and 573-673K. The former pattern contai ns the [110] streaks connecting the fundamental spots and the diffuse intensity maxima at the halfway of the streaks. In the latter one, fou r extra spots appear at the distance, one sixth of \g(110)\, from all spots of beta'-NiAl and the direction along the [110], streaks. The ch aracteristic in both the BF image contrast and the diffraction pattern s suggests that the structure change in beta'-NiAl by annealing occurs in the following two steps. The ordering of atom and constitutional v acancy occurs locally at the first step. In the new periodic structure , the Al and Ni sites of B2 type structure consist of Al, Al+Ni and Ni , Ni-vacancy respectively. At the second step, the structure of beta'- NiAl separates partially into the Al-rich and Ni-rich phases (Ni2Al3; trigonal and Ni3Al2; L1(0) structure). Thin phase separation is closly related to the modulation of Ni-vacancy concentration caused by the f ormation of a local ordering structure at the first step.