Experimental Studies on Precursor Phenomena in Displacive Phase Transformations

Instead of listing results of all these different experiments on the large variety of possible materials we will focus on some case studies which might serve as model systems for a given 

In the case of Ni-Al the martensitic transformation occurs in a composition range between 62 and 67 at.% Ni where the excess Ni is accommodated randomly on the A1 sublattice. The resulting c/a ratio of the Llo structure is around 0.85, depending on composition. Below 63 at.% Ni the martensite structure has a (52) sequence of close packed planes (Zhdanov notation) which is currently denoted as 14M (formerly as 7R). At higher Ni contents this typical sequence is lost and the martensite plates are simply internally twinned without a specific periodicity. 

Although the above results again point towards a relation between the precursing distortions and the martensite shuffles, detailed analysis of the neutron scattering data in the temperature region close to Ms indicate that an intermediate phase may exist with an effective 6-layer periodicity. In this view the ISD modulations could rather be precursors to this intermediate phase than to the final martensite structure. 

For this particular system, the phonon branches are not investigated as yet but, based on the accumulated knowledge on other B2 materials transforming to close packed structures, one would expect a low lying [110] TA2 branch in the B2 range, possibly with a dip at 1/2 

Some of the above discussed precursor phenomena are also observed prior to diffusion driven phase transformations. A typical example are the conventional EM tweed images obtained in the tetragonal parent phase in high Tc superconductors and other ceramics. In a recent survey by Putnis Sj e of such observations it was concluded that in these cases the tweed contrast resulted from underlying microstructures formed by symmetry changes driven by cation ordering27. These symmetry changes yield a fine patchwork of twin related domains which coarsen when the transformation proceeds. However, in view of the diffusion driven character of the latter examples, these cases should be clearly sq)arated from those in the field of the martensites. 

EXPERIMENTAL STUDIES ON PRECURSOR PHENOMENA IN DISPLACIVE PHASE TRANSFORMATIONS 

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