Molecular-dynamics simulation on solid-solid phase transition

Free-energy calculation for many-body iron potentials

Cemal Engin

Trennlinie

Motivation

Using molecular-dynamics simulations we study the solid-solid phase transition in iron. As a first step we carefully characterize 2 available many-body interatomic potentials to identify the free-energy difference between the ground-state bcc and the metastabile fcc phase.

Method 1

By the method of metric scaling (Ref. [1]) we calculate the temperature dependence of the free-energy difference between the bcc and the fcc structure.
For both potentials studied, it turns out that the bcc structure has for all temperatures the lower energy. So the transition from bcc to fcc is not possible.

Method 2

It is interesting to know the size of the free-energy change per scaling step. The authors of publication [2] use the formula

&Delta Fstep = -p &Delta V

to determine this quantity. So in every scaling step in the Bain transformation from the bcc to the fcc structure, we equilibrate the system and measure the pressure p and the volume change &Delta V. We see an energy barrier which will prevent a phase transition for too small temperatures.

Literature

  1. M.A. Miller, W.P. Reinhardt:
    Efficient free energy calculations by variationally optimized metric scaling: Concepts and applications to the volume dependence of cluster free energies and to solid--solid phase transitions.
    J. Chem. Phys. 2000, 113(17): 7035--7046
  2. K. Kadau, P. Entel:
    Atomistic investigations of the thermodynamical stability and martensitic nucleation of Fe80Ni20 nanoparticles.
    Phase Transitions 2002,75: 59--65

Martensitic phase transformations induced by ion irradiation

Motivation

Some alloys (Fe1-xNix) and also stainless steels may exist in a metastable phase. Because of the technological relevance of this and related materials, it is very important to know how stable these alloys are under local disturbances. An example for this situation is the irradiation of the tank of a nuclear power plant.
It is well known that before failure, the material (Fe1-xNix) undergoes a martensitic phase transformation. We investigate the dynamical mechanisms underlying this microstructure change with the help of molecular-dynamics simulations.
Here we study a pure Fe sample. The phase transformation is induced a by primary-knock-on atom (PKA) with given energy and direction, which originates from the slowing-down of an ion or neutron.
The left picture is from my simulation and the right picture is taken with a microscope from a real stainless steel sample (M. Smaga, AG Eifler, FB MV, TU Kaiserslautern). The dark parts of the right picture are martensite needles in an austenite matrix. In both pictures the twin structure is clearly visible.