Influence of Iron Oxide Particles on the Strength of Ball-Milled Iron Page: 4 of 26
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Data are analyzed113 over the range of grain sizes from 6 nm to 20 pm and volume
fraction of oxide particles from trace amounts to 8 volume percent.
2. Materials and Processes Evaluated in This Study
The microstructure and resulting properties of ball-milled materials are dependent on
many variables. Among these are the initial and final compositions of the iron-base
material, the powder size, the composition and size of the milling balls, and the
time/temperature history of ball milling and of consolidation. Table 1 is a summary of a
number of investigations that are pertinent to the present objectives of interpreting the
strength of ball-milled iron. Investigations are listed from 1990 to the present time. Ball
milling time typically ranged from 20 to 500 hours. Initial composition of the powders
ranged from commercial pure iron powder (99.2% Fe)'3 to 99.9% Fe.4 Powder sizes
were from 5 to 100 microns, and in one instance high purity iron chips, one mm thick,
were used.4 The ferrite grain size range studied was from 6 nm to 20 m. Specific
studies of the hardness, microstructural changes and contamination of powders as a
function of ball milling time were made ranging from a few hours to 500 hours. The
oxygen content was generally given, and a column is devoted to this variable in Table 1.
The oxygen content was noted to increase from 0.2 mass% to about 0.45 mass% oxygen
after 200 hours of ball-milling.13 Examples of the change in composition of the powders
with milling time were noted by Sakai et al.6 ; the chromium content increased from zero
to about 0.4%, and the carbon content from 0.02 to 0.07 % after 100 hours.
Figure 1 shows the yield strength-grain size data plotted as logarithm of the strength
as a function of logarithm of the grain size. This method avoids the commitment to the
minus one-half exponent that is made by the typical approach of plotting the linear
function of yield strength (6y) as a function of the grain size term (L)1 /2, according to the
Hall-Petch relation ay = 60 + ky-L12 where 6o and ky are material constants. Hardness
numbers were converted to yield strength by H = 3 ay for hardness values below H =
6.50 and H = 2.5.ay for hardness values above 6.5.2) The line at the bottom of Fig. 1 is
the predicted Hall-Petch relation for data on interstitial-free iron and steel evaluated by
Armstrong et al.14 and Tsuji et al.15 The value of 60 = 28 MPa is from Cracknell and
Petch.16 As can be seen, the ball-milled iron data are not compatible with the Hall-Petch
prediction and are located much above the interstitial-free iron line. It will be shown that
the higher strength arises principally from oxide particle strengthening.
The data shown in Fig. 1 reveal three regions of dissimilar pattern. At ultra-fine grain
sizes from 6 nm to about 60 nm, the strength is seen to decrease gradually with increase
in grain size. These data reveal the high hardness obtained in powders before
consolidation. In the intermediate grain size range, from 0.12 jm to about 1 m, a steep
dependence of strength with grain size is observed with a grain size exponent of about 0.7
rather than 0.5. These data are mostly from consolidated powders. The temperature of
consolidation varied widely, from as low as 400 C 2) to as high as 780 C.13 In the coarse
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Lesuer, D R; Syn, C K & Sherby, O D. Influence of Iron Oxide Particles on the Strength of Ball-Milled Iron, article, December 7, 2005; Livermore, California. (digital.library.unt.edu/ark:/67531/metadc878000/m1/4/: accessed April 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.