Abstract
Structural changes during ageing of Fe-13-15 and 20 at%Mo alloys were investigated by means of transmission electron microscopy and X-ray diffraction. The results obtained are as follows:
\
oindentIn an early stage of ageing, a typical ⟨100⟩ modulated structure, which had not been reported for the Fe-Mo alloy system in the liferature, was found in a Fe-20 at%Mo alloy aged at temperatures below 823 K. These modulated structures were theoretically proved to be produced through a mechanism of coherent spinodal-decomposition, and the critical temperature (spinodal temperature) was estimated at 860 K. With further ageing, coherency between the Mo rich and poor regions was broken down when the wavelength reached about 25 unit cells. Finally, a stable λ phase (Fe2Mo) was nucleated independently of the periodic structure formed spinodally. On the other hand, in a case where the alloys were held at temperatures over the spinodal line, plate-like precipitates were initially nucleated on the {100} plane instead of the modulated structure, and then the λ phase was produced. The latter structural changes is considered to be identical with the previously reported one for the Fe-Mo alloy system.
