Microstructure stability and evolution in CVD carbonyl Ni materials upon annealing – Grain growth and detwinning process

The evolution of microstructure upon different annealing conditions in the Ni-carbonyl chemical vapor deposited (CVD) Ni material was investigated systematically in the present study by differential scanning calorimetry (DSC), optical microscopy and transmission electron microscopy (TEM) with both ex situ and hot-stage in situ annealing approaches. TEM observations reveal that the as-deposited CVD nickel possesses a bi-modal grain structure, with large columnar grains embedded in a nano-grained matrix. The large columnar grains are found to be full of ultrafine- and nano-growth twins; there are also scattered fivefold symmetry grains as large as 3–5 μm. Microstructure observations upon annealing show that grain growth did not occur until annealing at 400 °C or higher, and the nano-grain matrix grew into large equaxial grains above 600 °C. A major microstructural evolution phenomenon, the detwinning process was observed at 400 °C or higher temperatures, and twin boundary receding in the detwinning process was found to have a number of mechanisms all involving dislocation activities. Upon annealing, the ultrafine- and nano-twins were found to transform into dislocation cell structures and this phenomenon is considered to be driven by the excess free energy associated with the high density of grown-in twin boundaries. The large sized, fivefold twinned grains found in the as-processed bulk deposited Ni are found to be thermally stable up to 600 °C. Surface dragging effect to twin and grain boundaries is detected during the hot-stage in situ TEM observation.