Project Details
Projekt
Experimental and theoretical investigations of the structure and the properties of Ti-Ta-X-Y and Co-Ni-Ga-X high-temperature shape Memory alloys: Syncrotron and Neutron diffraction and atomistic modelling
Subject Area
Mechanical Properties of Metallic Materials and their Microstructural Origins
Term
from 2012 to 2020
Project identifier
Deutsche Forschungsgemeinschaft (DFG) - Project number 200999873
In Subproject 3 the structure and stability of Ti-Ta based HT-SMA is investigated on an atomistic level and combined with the study of crystallographic mechanisms of the shape memory effect in Ti-Ta- and Co-Ni-Ga alloys. The obtained insights are used to identify i) Ti-Ta-based alloys and appropriate thermomechanical treatments providing stable shape memory behaviour at high Ms temperatures and ii) Co-Ni-Ga alloys with cyclic stability of superelastic strain at high temperatures. For this we combine density-functional theory calculations with high-resolution diffraction methods. Essential aspects of the 2nd funding period are the extension of the theoretical methods to ternary systems Ti-Ta-X (X=Al, Sn, Zr), a consistent treatment of finite temperatures and disordered systems, and the investigation of diffusion processes. The envisaged approaches to disordered systems and diffusion are the Special Quasirandom Structures method and the Nudged Elastic Band method, respectively. Phonon contributions to the free energy will be calculated - depending on the system - by ab initio quasiharmonic approximation or molecular dynamics simulation. The diffraction experiments determine structural parameters and phase formation processes with neutron- and synchrotron-radiation. In the 2nd funding period the experiments are expanded to cover ternary and quaternary alloys Ti-Ta-(X-Y), with X,Y = Al, Sn, Zr, according to alloy development in TP1. Combining simulations and experiment the function of alloying elements in modifying the structure and thermodynamic properties of stable or metastable phases and the kinetics of phase formation (or inhibition, respectively) is addressed. In the Ti-Ta-(X-Y)-system this relates to (i) the newly discovered nano-beta-phase and its relationship to the other beta-phases forming in the system, (ii) the omega-phase and the influence of alloying elements X,Y on its structure, stability and kinetics of its formation, and (iii) the formation of alpha-Ti coexisting with several beta-phases of different Ta-contents, which are observed in Ti-rich systems (Ti-20Ta-5Al) and in the creep experiments of TP2. Furthermore, investigations of the crystallographic characteristics of the martensitic twinning and transformation behaviour in the Ti-Ta and Co-Ni-Ga-system are performed. By diffraction we investigate the effect of advantageous microstructures for improved shape memory behaviour on the martensite variant formation and residual strain. Here, the Bamboo-structure of Co-Ni-Ga bicrystals (produced in TP5) and thermomechanical treatments of the Ti-Ta-X-Y alloys (from TP1/TP4) and Co-Ni-Ga systems (from TP5) are of special interest. TP2 carries out complementary TEM-investigations on phases and microstructures and will be supported by the theoretical insights and diffraction experiments.
DFG Programme
Research Units
Co-Investigator
Professor Dr. Ralf Drautz
