The approach of this work is to explore, whether the shape-memory effect and the underlying martensitic phase transformation is suited for the purpose of designing and constructing functional switching devices at the nano-scale. During a martensitic phase transformation, one crystal structure converts to another at a certain temperature or external stress. This effect is observed for various alloys, but also for some pure materials, for example iron, which shows the "alpha-gamma-transition" at 1184 K between a low-temperature bcc (called martensite) and a high-temperature fcc (austenite) structure.
We simulate nano-systems of martensitic transforming materials with a number of particles between 1000 and 200 000 with molecular dynamics. This is done either with free boundary conditions in a NVT ensemble or with periodic boundary conditions in a NPT ensemble with a fully flexible simulation box [d1]. In order to achieve simulation results comparable to experiments or ab initio calculations with a lower number of particles, we use sophisticated model potentials like EAM ("embedded-atom method") [d2] and tight-binding in the second moment approximation [d3].
In this way, many different alloy systems or pure materials can be simulated and comparisons with experiments as well as predictions can be done in analyzing transition temperatures, crystal structures, free energies during the phase transitions and phonon dispersion relations.
Besides Fe and FeNi alloys with different Ni concentrations, which show a bcc-fcc transition at high temperatures, we mainly focus on NiTi in equiatomic and nearly equiatomic composition. The occurring B19' to B2 transformation is analyzed in bulk and nano systems. Furthermore, we study the shape memory effect of nano model systems by applying stress and temperature changes.
b) Old publications
c) Completed work
- H. Knoth, diploma thesis: Finite-Size-Studien zu "Phasenumwandlungen" in Modell-Nanostrukturen mittels Computer-Simulationen (2002)
- D. Mutter, diploma thesis: Modellierung und Simulation der Magnetisierungsdynamik auf Kugelkappen und verwandten Geometrien (2007)
- D. Mutter, dissertation: Computersimulationen zu strukturellen Phasenübergängen und Formgedächtnisverhalten auf Nanometerskala (2012)
d) References
- G. J. Martyna et al., J. Chem. Phys. 101, 4177 (1994).
- R. Meyer and P. Entel, Phys. Rev. B 57, 5140 (1998).
- W. S. Lai, B. X. Liu, J. Phys. Cond. Mat. 12, L53-L60 (2000).
e) External links (programs, potentials ...)
f) Books
- D. Mutter, dissertation, Verlag Shaker (2012)