

Oliver Kastner: Molecular dynamic simulation of austeniticmartensitic phase transitions  entropic stabilisation
A 2dimensional moleculardynamic model for the investigation of
crystalline phase transitions is presented. The model is based on the
equations of motion, and Lennard Jones potential functions
are employed.
Two types of atoms may create a stable square lattice, which is called
the austenitic phase. It may transform into sheared variants, which
represent martensitic phases.
In numerical experiments  examples are presented on a
display screen during the talk  it is show, that the stability of
the austenitic phase depends on temperature. Once stability is lost,
the resultant phase transition exhibits strong similarities
to martensitic transformations as they are known from shape memory
alloys.
The temperature dependence of phase stability may be
explained by the principle of entropic stabilization: Martensite is
energetically more favorable, since it provides minimal inner energy
U. Austenite however is entropically more favorable, since it
provides maximal entropy S. The competition of both quantities is
reflected in the Gibbs free energy F = UTS , which has to be minimal
in phase equilibrium. Temperature T plays the role of a weight factor
which determines the influence of entropy.
The thermodynamic material properties of a small test body are
measured in numerical tensile experiments. The Gibbs free energy
of the body is determined from these data. Consequently the
thermodynamic criterium of phase stability may be investigated.
