Workshop on

numerical methods for multiscale problems

 

     
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  Oliver Kastner: Molecular dynamic simulation of austenitic-martensitic phase transitions - entropic stabilisation

A 2-dimensional molecular-dynamic 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 = U-TS , 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.
Impressum
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