Biomedical Engineering Reference
In-Depth Information
13.11 Evolution of the grain size as a function of the annealing time at
three annealing temperatures for nanocrystalline iron. The grain size
was determined by the Scherrer equation.
Research has generally shown that after sintering, nanosized particles lose
nanoscale characteristics because grain size grows to greater than 100 nm.
Therefore, understanding and controlling grain growth is a critical scientific
and technical issue of sintering of nano particles.
In a systematic study of the stability of nanosized metal powders, Malow
and Koch
75-77
reported that the rate of grain growth of nanocrystalline iron
(Fe) powders made by ball milling is initially very rapid (
5 min) when
annealed at various temperatures. Grain growth then stabilizes during
extended isothermal holding (up to 142 hours). During isothermal holding,
grain growth follows a generalized parabolic grain growth law and is similar
to that found in bulk materials. It is noted, based on Fig. 13.11, that at the
first data point of the isothermal annealing curves at higher annealing
temperatures (825 and 875 K), the grain sizes are already several times
(3-6
<
8 nm) (Fig. 13.13). In
other words, grains grow rapidly during heat-up, prior to reaching the pre-
selected isothermal holding temperature.
In another study of the grain growth of nanocrystalline Fe using in-situ
synchrotron X-ray diffraction techniques, Krill et al.
66
further demonstrated
that grain growth of nano Fe particles comprises three steps: the 'initial
growth spurt,' a linear growth stage, and the normal parabolic stage, as
shown in Fig. 13.12. Once again, the normal parabolic stage can be modeled
using the classic grain growth parabolic law; however, the 'initial growth
spurt' of nanocrystalline Fe during annealing was not captured by
isothermal studies.
6
) greater than the original as-milled grain size (
~
