ICAMS / Interdisciplinary Centre for Advanced Materials Simulation

Atomistic modelling of phase transitions in high-temperature shape memory alloys

Since the discovery of shape memory alloys (SMAs), much progress has been made both in the scientific understanding and application of these multifunctional materials. Because of their unique behaviour SMAs have become an important materials class for biomedical, technological and industrial applications and are extending into several other areas as well. The shape memory effect can be defined as a diffusionless solid state phase transformation mechanism that can be activated by temperature, stress and magnetic field.

Energy vs volume curves for BCC Ta calculated using density-functional theory with different exchange correlation functionals. The equilibrium lattice constant and bulk modulus are obtained by fitting Murnaghan's equation of state to the data.

Of particular interest are SMAs that exhibit martensitic transformation temperatures above 373 K. These high temperature SMAs can be used as advanced actuator and superelastic materials that can function in a hot environment, such as in home appliances (gas stove, heaters and many general appliances), transportation (automobile and aircrafts) and power generation systems (oil and gas energy plants). In addition to the high transformation temperatures, potential high temperature SMAs must also exhibit acceptable recoverable transformation strain levels, long term stability, resistance to plastic deformation and creep and adequate environmental resistance.

Promising results for the development of novel HTSMAs have been obtained for the Ti-Ta system. Within this project we will investigate the stability of different phases in the Ti-Ta system as a function of composition using density functional theory. Furthermore we will apply the solid state nudged elastic band method to determine the minimum energy path for the martensitic transformation between the β and α'' phase. A particular focus will be on the formation of the ω phase since large amounts of ω phase significantly reduce the stability and thus destroy the shape memory effect.

This project is part of the DFG FOR 1766 "High temperature shape memory alloy - From fundamental understanding to applications".

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