INTERGRANULAR AND INTERPHASE BOUNDARIES IN MATERIALS
Geometrical and physical models of martensitic transformations
in ferrous alloys
Robert C. Pond Æ Xiao Ma Æ John P. Hirth
Received: 18 July 2007 / Accepted: 10 September 2007 / Published online: 6 March 2008
Ó Springer Science+Business Media, LLC 2008
Abstract The classical theory of the crystallography of
martensitic transformations developed in the 1950s is based
on the notion that the interface between the parent and
product phases is an invariant plane of the shape deforma-
tion. Underlying this hypothesis is the expectation that such
interfaces do not exhibit long-range strain, and the geometric
theory is an algorithm for finding invariant planes, the ori-
entation relationship and transformation displacement. In the
context of ferrous alloys, the classical theory has been
applied successfully to transformations with {295} habit
planes, but is less satisfactory for {575} for example. A new
model of martensitic transformations has been presented
recently based on dislocation theory, incorporating devel-
opments in the understanding of the topological properties of
interfacial defects. Topological arguments show that glissile
motion of transformation dislocations, or disconnections,
can only occur in coherent interphase interfaces. Hence, the
interface in the model comprises coherent terraces with a
superimposed network of disconnections and crystal dislo-
cations. It is demonstrated explicitly that this defect network
accommodates the coherency strains, and that lateral motion
of the disconnections across the interface effects transfor-
mation in a diffusionless manner. Moreover, it is shown that
a broader range of habit planes is predicted on the basis of the
semi-coherent interface model than the invariant plane
notion. In the case of ferrous alloys, it will be shown that a
range of viable solutions arise which include {575}.
Introduction
Martensitic transformations are diffusionless and displacive
[1]. They occur with isothermal, athermal or burst kinetics;
in most cases, nucleation is thought to be thermally activated,
and growth is not a rate-limiting process except for slow
isothermal instances [2]. The structure of the parent–mar-
tensite interface is a key issue in all these considerations, and
the object of the present article is to review recent progress in
the development of a model of the interface based on dislo-
cation theory. In the phenomenological model, developed by
Wechsler et al. [3] and Bowles and MacKenzie [4], the
interface is envisaged as an invariant plane of the shape
transformation. This notion is consistent with a large number
of experimental observations at the resolution of the optical
microscope, although certain martensitic morphologies, in
Fe alloys for example, are exceptions [5]. A dislocation
model can elucidate the structure of interfaces at the atomic
level and could therefore resolve the inapplicability of the
classical model to certain transformations. Furthermore,
dislocation modelling should relate directly to kinetic issues.
In the following sections, the dislocation model, referred to
as the topological model (TM) [6, 7], is outlined and com-
pared with the phenomenological model of martensite
crystallography (PTMC). The TM is then applied to the cases
of the b to a transformation in Ti and c to a in Fe-based alloys.
Topological model of martensitic interfaces
The ideal parent–martensite interface is required to be
glissile, i.e. to migrate readily without long-range diffu-
sion, and in a manner that produces the transformation
shear. In addition, unit cells of the two phases should only
be distorted in the vicinity of the interface. The former
R. C. Pond (&) Á X. Ma
Department of Engineering, University of Liverpool,
P.O. Box 147, Liverpool L69 3BX, UK
e-mail: r.c.pond@liv.ac.uk
J. P. Hirth
114 E. Ramsey Canyon Rd., Hereford, AZ 85615, USA
123
J Mater Sci (2008) 43:3881–3888
DOI 10.1007/s10853-007-2158-9