Jan Philipp Liebig

Project description

Nanotwinned metals are a new class of nanostructured materials that exhibit an exceedingly high density of twins corresponding to an average twin boundary spacing of only a few tens of nanometers. When compared to their coarse-grained counterparts these materials exhibit outstanding mechanical strength levels while preserving a substantial ductility as well as a high electrical conductivity [1]. Therefore, they present an attractive material for structural applications in micro-electromechanical devices. Although the causes for these exceptional properties are not fully understood they are generally attributed to the unique interaction of dislocations with twin boundaries [2].

Despite current scientific interest in producing nanotwinned materials by deposition [3], deformation [4] or annealing, the reliable engineering of microstructures with appropriate twin densities remains a challenge. Irrespective of the twin formation mechanisms, one effective approach to increase a materials tendency to twin is the alteration of the energy barriers for the creation of intrinsic stacking faults (unstable stacking fault energy) and twin growth (unstable twin energy) [5]. This can be achieved by alloying with appropriate elements [3,4].

However, since fault energies also play an important role in the deformation mechanisms operating in nanotwinned metals [6,7], significant changes on the mechanical behavior are expected for very low stacking fault alloys. In this project we investigate these materials by using advanced micromechanical testing methods.

Compression testing of bicristalline micro-pillars is employed to study slip transmission and dislocation storage at individual twin boundaries. We utilize electron backscatter diffraction to orientate single twinned grains towards selected loading directions allowing for the comparison of alloys with variable fault energies.

In order to explore the impact of alloy composition and fault energies on the ductility of polycrystalline samples where regular grain boundaries impose additional constraints on the deformation of twins, these compression tests are complemented with micro-tensile tests on nanotwinned thin films monitored in situ by scanning transmission electron microscopy [8].

References

[1] L. Lu et al., Science 304 (2004) 422.
[2] T. Zhu and H. Gao, Scr. Mater. 66 (2012) 843.
[3] L. Velasco et al., Scr. Mater. 83 (2014) 33.
[4] F.K. Yan et al., Acta Mater. 60 (2012) 1059.
[5] E.B. Tadmor and S. Hai, J. Mech. Phys. Solids. 51 (2003) 765.
[6] Z.-H. Jin et al., Scr. Mater. 54 (2006) 1163.
[7] Z.-H. Jin et al., Acta Mater. 56 (2008) 1126.
[8] J.P. Liebig et al., Ultramicroscopy 171 (2016) 82.