Structural Transformations and Their Precursors

Abstract

For a number of materials which exhibit a change of structure on being cooled below a certain temperature Tm, some physical properties display anomalous behaviour at temperatures above Tm. The particular structural transformations in mind have been broadly classified as 'martensitic' and the anomalous physical properties as 'precursive phenomena'. Some debate exists regarding the role of the precursive phenomenon in the kinetics of the structural transformation. The most direct evidence for 'martensite precursors' is obtained from electron diffraction, although various indirect evidence is contained in X-ray, neutron and y-ray diffraction and various physical properties, for example, elastic constants and thermal expansion. In this paper current understanding of 'martensite precursors' is reviewed and examples of data from the A15 structure compounds V 3Si and Nb3Sn,. In-TI and TiNi alloys are discussed.