In situ transmission electron microscopy observations of the fcc-hcp phase transformation in Co nanowhiskers

Metallic nanowhiskers have shown in the past extraordinary physical properties. In tensile testing experiments, the theoretical strength was reached for various metals. Ferromagnetic materials show single magnetic domain structure. Typical diameters of the nanowhiskers are 100 nm and lengths of up to 50 µm are observed. Therefore these structures are perfect model systems to study materials properties in confined geometries.

In the presentation, we focus on the phase transformation of Co nanowhiskers. Co shows two allotropes, a room temperature hcp crystal structure and above 420°C a stable high temperature phase of hcp structure. For nanoparticles the fcc phase is seen repeatedly. This can be explained by the increasing effect of the lower surface energy for the fcc phase compared to the possible hcp low indexed surface crystal planes.

We investigated the growth of Co nanowhiskers and the onset of the fcc-hcp phase transformation in those nanostructures as a function of temperature as observed in situ a high resolution, high voltage transmission electron microscope. Compared to the perfect crystal structure of fcc nanowhiskers, Co exhibit stacking faults parallel to its axis when observed at room temperature. However, the overall crystal structure is fcc. During thermal cycling partial dislocations are nucleated and propagate parallel to the nanowire axis, forming only a local hcp structure, the overall fcc crystal structure however remains.