Abstract
The early stages of decomposition in Ni-9.3 and 14.7 at.% Ti alloys are studied by means of high resolution electron microscopy. The microstructure of the as-quenched Ni-9.3 at.% Ti alloy shows contrast of broad circular regions, and local lattice images show some difference in lattice spacing suggesting compositional modulations. Electron and optical diffraction patterns indicate faintly ordered regions. After ageing for 15 min at 570°C, the ordered regions, approx. 50 Å in diameter, appear as clear lattice images surrounded by dark areas of the f.c.c. γ-solid solution. The Ni-14.7at.% Ti alloy aged for 15 min at 570°C shows ordered lattice images in wide areas of the matrix, apart from some large needle-like precipitates of the equilibrium η-phase formed during the homogenization at 1200°C. As the atomic arrangement in the ordered regions corresponds to the (110) projection of the L1 2 structure, the image is interpreted as the structure image of ordered L1 2, Ni 3(Ti x Ni 1− x ). The lattice spacings locally change in the ordered regions, suggesting some modulation in composition and degree of order.
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