Study on the Early Stage of Precipitation in a Ni-13 at%Al Alloy

The early stage of precipitation in a Ni-13 at%Al alloy was investigated by means of field ion microscopy (FIM) as well as transmission electron microscopy (TEM) and the small angle X-ray scattering (XSAS) technique. Fine γ′ precipitates with L12 crystal structure could be observed by using FIM. γ′ precipitates appeared even at the as-quenched state, and an average diameter of these precipitates was 3.3 nm. During the isothermal ageing at 823 K, their particle size increased and the interparticle distance also extended gradually with ageing time. After over 6 ks ageing, some precipitates showed a regular array in the {100} lattice plane and partially had a modulated structure accompanied with a shape change from ellipsoid to cuboid. TEM observation and XSAS measurements supported strongly the FIM results mentioned above. Especially the average radii of γ′ precipitates observed by FIM were in good agreement with Guinier ones obtained from the XSAS measurements.