TEM In-Situ Annealing Study of the Abnormal Grain Growth in
Electrodeposited Nanocrystalline Nickel
P. Cizek1, A. Sankaran1 and M.R. Barnett1
1Institute for Frontier Materials, Deakin University, Waurn Ponds, VIC 3216, Australia
e: pavel.cizek@deakin.edu.au
The aim of the current study was to elucidate the mechanism of abnormal grain
growth and the accompanying fibre texture transition that occur during annealing in
nanocrystalline nickel. The investigation was performed using transmission electron
microscopy (TEM) for both the as-received material and that subjected to in-situ
heating in the temperature range from 150 to 400 C. The recently introduced
ASTAR automated crystal orientation mapping technique was employed to determine
individual crystallite orientations. The data acquisition was performed using a
NanoMEGAS Digistar device, attached to a JEM 2100F microscope. The above
device performs scanning and simultaneous precession of the focused primary
electron beam and the obtained nanobeam spot (disc) diffraction patterns are
recorded by an external fast optical camera. The crystal orientation determination is
performed off-line through template matching of experimental electron diffraction spot
patterns to their pre-calculated theoretical counterparts with the angular precision of
about 1. The JEM 2100F microscope was operated at 200 kV in a nanobeam mode.
The step size was ranging from 2 to 10 nm and the beam precession angle used was
0.7. The orientation data obtained were exported to the electron back-scattered
diffraction (EBSD) Channel 5 software for post-processing and visualization. In the
in-situ heating experiments, the sequence of microstructure changes within a given
area of the thin foil was monitored during the incremental temperature increase,
through performing a number of repeated heating, cooling and ASTAR map
acquisition cycles while keeping the location fixed. A Gatan TEM single-tilt straining
and heating holder was employed with the heating being controlled by a Gatan
SmartSet controller.
The as-deposited nanocrystalline nickel contained, apart from nanograins, also
coarse (sub)grain clusters having complex shapes and large internal misorientation
gradients. Contrary to some previous suggestions, during annealing these clusters
neither served as nuclei for the observed abnormal grain growth nor displayed a
tendency for (sub)grain coalescence. Instead, they remained almost intact and
gradually became consumed by the growing abnormal grains. This might be related
to their observed large internal cumulative misorientation gradients, that would make
the coalescence extremely difficult, and the increased density of geometrically
necessary dislocations, producing higher stored energy compared to the surrounding
nanograins. The abnormal grain growth appeared to originate from randomly
distributed nanograin nuclei and involved the profuse formation of annealing twins on
the growth front. The twinning reorientation appeared to be primarily responsible for
the observed <001> to <111> fibre texture transition accompanying the abnormal
grain growth.
The financial support provided by the Australian Research Council is gratefully
acknowledged.