DEFORMATION MECHANISMS AT DIFFERENT GRAIN SIZES IN A CRYOGENICALLY BALL-MILLED Al-Mg ALLOY Page: 3 of 9
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vial and balls (6.4 mm in diameter) at a rate of 180 rpm for 8 hours. The ball-to-powder
weight ratio was 36:1. During the milling operation, liquid nitrogen was added into the
mill to maintain complete immersion of the milling media, which keeps the temperature
at -190 C. Prior to milling, about 0.25 wt.% of stearic acid (CH3(CH2)16CO2H) was
added to the powders as a process control agent to moderate the cold welding process.
The resulting powder was pressed under a pressure of 1.1 GPa to form a small
pellet, which was then mechanically ground to a thickness of about 50 pm. Further
thinning to a thickness of electron transparency was carried out using Gatan precision ion
polishing system with Ar+ accelerating voltage of 3.5 kV. HREM investigation was
carried out using a JEOL 3000F transmission electron microscope (TEM) working at 300
Results and Discussion
TEM investigation shows that the as-milled powder was not deformed
homogenously and different stages of the deformation process co-existed during the
cryogenic BM. Three kinds of nanostructures with different grain size ranges have been
found in the sample. Figure 1(a) shows a typical image of the largest crystallites having a
lamellate structure with a length range of 100 - 200 nm and a width of around 30 nm.
Figure 1(b) shows a typical structure of the most commonly seen equiaxed crystallites
having diameters of around 10 - 30 nm and Fig. 1(c) presents the smallest crystallites
with a dimension of less then 10 nm. As will be discussed later, these three kinds of
structures were produced by three different deformation mechanisms.
a -0 m10n
Figure 1 Nanostructures of three different sizes of crystallites. (a) Large crystallites; (b)
medium-sized crystallites; (c) the smallest crystallites; (d) an electron diffraction pattern taken
from the area shown in (a).
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LIAO, X.; HUANG, J. & AL, ET. DEFORMATION MECHANISMS AT DIFFERENT GRAIN SIZES IN A CRYOGENICALLY BALL-MILLED Al-Mg ALLOY, article, September 1, 2001; New Mexico. (digital.library.unt.edu/ark:/67531/metadc723447/m1/3/: accessed October 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.