T. Kizuka et al., HIGH-RESOLUTION ELECTRON-MICROSCOPY OF NANOCRYSTALLINE NI-AL ALLOYS -INSTABILITY OF ORDERED STRUCTURE AND DYNAMIC BEHAVIOR OF GRAIN-BOUNDARIES, Journal of Materials Science, 29(21), 1994, pp. 5599-5606
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.