Abstract

Bob Pond
Department of Engineering
University of Liverpool, U.K.


Wednesday November 28, 2007
4:00- 5:00pm
WIL Rm. 101

"Geometrical and Physical Models of Martensitic Transformations"

 

Martensitic Transformations underlie the properties of a wide range of engineering materials. These transformations are usually discussed in the framework of the Phenomenological Theory developed in the 1950s. This theory is a geometrical algorithm for finding a plane left invariant by the shape deformation. The crystallography of numerous transformations have been successfully analysed using this procedure. However, no satisfactory explanation of the crystallography observed in certain transformations, notably in ferrous alloys, can be obtained.

Recently, a model of martensitic transformations based on dislocation theory has been developed in which the interface structure is semi-coherent, as observed using transmission electron microscopy. This model explicitly describes the transformation mechanism, demonstrating that it is diffusionlesss and also that strains only arise at short-range from the interface. Moreover, this approach shows that the invariant-plane notion is a sufficient, but not necessary, condition for the absence of long-range strains.

To illustrate the dislocation model and contrast it with the phenomenological theory, transformations in Ti alloys and ZrO 2 are discussed as examples of invariant-plane transformations, and the (575) lath martensite in ferrous alloys is analysed as a non invariant-plane case.

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