Influence of Elastic Energies on the Changes in Precipitation Structures in Fe-Base Ordering Alloys.

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(1) In weakly constrained systems such as D0_3 in Fe-Al-Ge, particles coarsen as the cube root of the ageing time t, i.e. the so-called 't^<1/3>-law' holds, and the size distribution is scaled with respect to the mean particle size. The coarsening obeys the conventional coarsening theory such as the LSW theory of Ostwald ripening.(2) In strongly constrained systems such as D0_3 in Fe-Al-V, D0_3 in Fe-Si-V and B2 in Fe-Al-Co, the particle coarsening proceeds more slowly than the prediction of the LSW theory, and the scaling rule does not hold. Furthermore, the splitting of single particle into a doublet or to the octet is sometimes observed.(3) Such unusual coarsening behavior in strongly constrained systems can only be explained by the new coarsening theory named Bifurcation theory in which not only the surface energy but also the elastic energies are considered as the driving force for coarsening.(4) Coarsening behavior of coherent precipitates in Fe-base alloys is quite similar to that of gamma'-precipitates in Ni-base alloys. It is very likely that unusual behavior peculiar to the elastically constrained systems should be universal regardless of crystal structure.(5) It is very likely that the two-phase alloys having highly stable structure can be obtained with controlling the elastic constraint, i.e. the ratio of lattice misfit to surface energy.