In this study, the sequence of the phase decomposition processes in rapidly-solidified (RS) Cu-rich Cu-Ti alloys has been investigated by TEM. Especially, short range ordering has been probed in depth.
Alloys of two compositions, Cu-12.3at.%Ti and Cu-15at.%Ti, were prepared by rapid solidification process(RSP). Owing to the rapid cooling rate during rapid solidification process, we were able to obtain the supersaturated single fcc phase which had not been spinodally-decomposed and track down the sepuence of initial stage of phase decomposition in Cu-rich Cu-Ti alloys. Hence, it is clear that short range ordering precedes spinodal decomposition.
Although an apparent single fcc phase was only deteccted in the X-ray diffraction pattern, the as-rapidly solidified microstructure was not homhgeneous. A grain was divided into solidification subcells. To examine the difference of the phase decomposition mode that occured at the cell boundary and the interior, we compared the microstructural changes of each constituent on ageing. It was found that cell boundary regions were decomposed first, and subsequently the decomposition penetrated into cell interior regions.
In as-RS state, the faint ${1 1/2 0}_m$ and ${1/2 1/2 0}_m$ superlattice spots were observed. Based on the static concentration wave model, it is shown that ${1 1/2 0}_m$ spots originate from a single {1 1/2 0} ordering wave, while hitherto unidentified ${1/2 1/2 0}_m$ spots originate from the superimposition of two perpendicular {1 1/2 0} ordering waves.
On 450℃ ageing, a modulated structure developed to form a two-phase mixture, matrix α and precipitate Dla β phases. In it, we also observed the $1/3{220}_m$ spots and the diffuse arcs. The presence of the latter type of diffraction spots indicate that metastable clusters of $Pt_2Mo$ crystal structure may exist in this alloys.
On 450℃ ageing, metastable β' phase formed concommitantly with spinodal decomposition, while on 750℃ ageing, β' phase are directly precipitates by necleation and growth mechanism.
