HIGH-RESOLUTION ELECTRON-MICROSCOPY OF NANOCRYSTALLINE NI-AL ALLOYS -INSTABILITY OF ORDERED STRUCTURE AND DYNAMIC BEHAVIOR OF GRAIN-BOUNDARIES

High-resolution transmission electron microscopy was performed on vacu um-deposited nanocrystalline nickel aluminide films. Several nickel al uminide ordered structures, i.e. L1(2)(Ni3Al)-, B2(NiAl)-, D5(13)(Ni2A l3)- and DO20(NiAl3)-type structures, were observed in the deposited f ilms. The L1(2) and B2 ordered structures became unstable with decreas ing grain sizes. The critical grain size on transformation from the L1 (2)- and B2-type ordered structures into disordered structures was ca. 5 nm at ambient temperatures. High atomic diffusion, sufficient for g rain growth, and an increase in the ordering occurred just above 400 d egrees C in the nanocrystalline Ni-Al films with L1(2)- and B2-type st ructures. The diffusion bonding process, at ambient temperatures, betw een Ni-Al nanocrystallites with an L1(2)-type structure was observed d ynamically at atomic resolution under strong electron irradiation. It was found that the nanocrystallites rotated and slid without crack gen eration, and neck-growth proceeded even at ambient temperatures.