Materials Science and Engineering A 375–377 (2004) 899–904
Formation of nanocrystalline structure in carbon steels
by ball drop and particle impact techniques
M. Umemoto∗ , K. Todaka, K. Tsuchiya
Department of Production Systems Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan
Abstract
The formation of nanocrystalline structure on the surface of bulk steel samples was studied using ball drop and particle impact techniques.
Nanocrystalline layers with several microns thick were successfully fabricated by both methods. The microstructural observations, hardness
measurements and annealing experiments suggest that the nanocrystalline layers have similar characteristics with those produced by ball
milling (BM). The nanocrystalline regions have sharp boundaries with work-hardened neighboring regions and no intermediate regions were
observed. Pre-strain and low deformation temperature was found to enhance the formation of nanocrystalline regions. Nanocrystalline layers
with large shear deformation were sometimes observed. From the measured shear strain, the equivalent true strain necessary to produce
nanocrystalline region was estimated to be about 3.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Nanocrystallization; Plastic deformation; Ball mill; Steel; Work-hardening
1. Introduction
Nanocrystallization by severe plastic deformation in steels
has been a subject of many researches [1–5] in the last
decade. A popular method to produce nanocrystalline structure is ball milling (BM). From our previous BM experiments on steels [6–11], it was found that the nanocrystalline
regions produced by BM have the following characteristics:
(1) homogeneous structure with sharp boundaries between
the work-hardened regions as shown in Fig. 1; (2) ultrafine
grains of less than 100 nm with almost no dislocations; (3)
extremely high hardness (8–14 GPa); (4) include no cementite phase; and (5) no recrystallization and slow grain growth
by annealing. Although BM is a powerful method to produce nanocrystalline materials, it is not suitable to study the
nanocrystallization mechanism since the deformation mode
is extremely complicated and contamination is hard to be
avoided. To study nanocrystallization by severe plastic deformation, a method which produces simple deformation on
specimens without contamination is desired.
The purpose of the present study is to demonstrate new
severe plastic deformation methods to produce nanocrystal∗ Corresponding author. Tel.: +81-532-44-6709;
fax: +81-532-44-6690.
E-mail address: [email protected] (M. Umemoto).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2003.10.198
lization in steels, a ball drop [12,13] and a particle impact
methods. The nanocrystalline regions formed on the surface of steel plates by these methods were compared with
those in ball milled powder. From the observed shear bands
the amount of strain necessary to produce nanocrystalline
regions was estimated. The intermediate stage between
work-hardened and nanocrystalline regions is discussed
taking into account the observed sharp boundaries between
them.
2. Experimental procedures
The material used in this study was eutectoid carbon
steels of Fe–0.80C (Fe–0.80C–0.20Si–1.33Mn in wt.%) and
Fe–0.89C (Fe–0.89C–0.25Si–0.50Mn in wt.%) with either
pearlite or spheroidite structure. The pearlite structure was
obtained by the patenting treatment. Specimens were austenitized at 1223 K for 1.8 ks followed by an isothermal transformation to pearlite at 873 K for 0.3 ks in a lead bath. The
spheroidite structure was produced by a process of martensite tempering, in which specimens were austenitized at
1173 K for 3.6 ks and then quenched into water to obtain
martensite, and then tempered at 983 K for 79.2 ks. To study
the effect of pre-strain on the formation of nanocrystalline
structure, specimens were rolled to various reductions by
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M. Umemoto et al. / Materials Science and Engineering A 375–377 (2004) 899–904
Fig. 1. SEM micrographs of 360 ks ball milled Fe–0.89C powder showing the boundary between the nanocrystalline region (left-hand side) and
work-hardened region (right-hand side). (a) Pearlite and (b) spheroidite structures.
multipass rolling (with 10 or 20% reduction per pass). Annealing of nanocrystallized specimens were carried out at
873 K for 3.6 ks by sealing in a quartz tube under a pure
Ar protective atmosphere. Specimens were characterized by
SEM, TEM and Vickers microhardness tester (load of 0.98 N
for 10 s). Specimens for SEM observations were etched by
5% Nital.
In a ball drop experiment (Fig. 2(a)), a weight with a
ball attached on its bottom was dropped from a height of
1 or 2 m onto a bulk specimen with flat surface. The ball
of 6 mm in diameter, the weight of either 4 or 5 kg and the
specimens with 15 mm in diameter and 2–4 mm in thickness
were used. When the ball was dropped more than two times,
the specimen was systematically shifted by 2 mm step for
each drop test. All tests were carried out in air at either room
temperature or liquid nitrogen temperature. The details of
the ball drop test were described in our previous paper [12].
A particle impact experiment (Fig. 2(b)) was done by a
high-pressure light gas gun which can accelerate particles
in a desired speed. Helium was used as particle carrier gas.
The bore was 4.2 mm in inner diameter and the length of 4 m
to provide sufficient distance for acceleration. Bearing steel
(Fe–1Cr–1.5Cr) ball with 4 mm in diameter was chosen as
Fig. 2. Apparatus of ball drop and particle impact experiments. (a) Ball
drop and (b) particle impact.
projectile accelerated to a speed of 120 m/s. The number of
impacts studied was from 1 to 200. Specimens of 30 mm ×
30 mm × 3 mm were mounted at the end of bore. All the
experiments were done at room temperature in air.
When microhardness is converted to grain size, the following Hall-Petch-type relationship was used. This equation
was reported to hold down to 50 nm in various steels [14].
HV (GPa) = 0.363 + 1.90 d −0.5 (d, in ␮m)
(1)
3. Results
3.1. Ball drop test
The nanocrystalline regions formed by the ball drop technique usually appears at surface of specimens [13]. A typical nanocrystalline region formed in a pearlitic sample by
a ball drop test (eight times of ball drops with a weight of
4 kg from a height of 1 m) is shown in Fig. 3. A layer (dark
regions in SEM and bright regions in optical microscopy)
about 20 ␮m thick is seen near the bottom surface of a impact crater (indicated by an arrow).
TEM samples were prepared parallel to the specimen surface. Fig. 4 shows a dark field image and electron diffraction
pattern of a sample with pearlite structure after ball dropping
(25 times, 4 kg, 1 m). The dark field image of Fig. 4 (taken
from the (110) ring of bcc ferrite) shows that the ferrite grain
size is of the order of 100 nm. All the diffraction rings correspond to bcc ferrite, and rings corresponding to cementite
are hardly detected. This indicates that cementite is mostly
dissolved into the ferrite. The diffraction pattern taken from
the area of φ 2 ␮m shows nearly continuous rings, indicating
the random orientations of the ferrite grains.
Fig. 5 is a SEM micrograph of a cross-section of a pearlitic
specimen after eight times of ball drops (5 kg, 1 m) at liquid nitrogen temperature. In the nanocrystalline layer at the
M. Umemoto et al. / Materials Science and Engineering A 375–377 (2004) 899–904
901
Fig. 3. Nanocrystalline layer formed by ball drop test (eight times, 4 kg, 1 m) in Fe–0.89C pearlite.
surface of the specimen the lamellar structure of pearlite is
invisible, indicating that cementite lamellae are completely
dissolved. Under this layer, deformed pearlite structure is
clearly observed. The microhardness of nanocrystalline layer
is 11.0 GPa which corresponds to ferrite grain size of about
30 nm. This hardness is almost twice as high as the value for
the adjacent work-hardened region (5.9 GPa). It was noted
that the observed microstructures produced by a ball drop
test were similar to those observed in ball milled powders.
The number of ball drops necessary to produce nanocrystalline layer depends on the composition, microstructure and
temperature of specimens and ball drop conditions (weight
and height). The number of drops is less for harder sample and higher energy drop conditions (higher weight and
height). Low processing temperature also reduces the number of drops. This suggests that nanocrystallization by ball
drop is purely due to severe plastic deformation and not concerned with thermally produced martensite as a consequence
Fig. 5. Nanocrystalline layer formed by ball drop test (eight times, 5 kg,
1 m) in Fe–0.80C pearlitic at liquid nitrogen temperature.
Fig. 4. Dark field image and electron diffraction pattern of Fe–0.89C pearlite steel after ball drop test (25 times, 4 kg, 1 m).
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M. Umemoto et al. / Materials Science and Engineering A 375–377 (2004) 899–904
Fig. 6. Shear deformation produced by a ball drop (one time, 5 kg, 1 m) in the pre-strained (80% cold rolling) Fe–0.80C pearlite.
of adiabatic deformation. Pre-strain of specimens also reduces the number of ball drops. In the case of the pearlitic
specimen it was possible to produce the nanocrystalline region by one time of ball drop after cold rolling of 80% [13].
Shear bands with a large shear strain were often observed
in the nanocrystalline layers produced by a ball drop test as
shown in Fig. 6. The specimen was pre-strained to 80% by
cold rolling and ball dropped one time (5 kg, 1 m). Using
the stripes morphology crossing the nanocrystalline layer,
the amount of shear strain in the nanocrystalline layer was
estimated to be 8.1. Adding the pre-strain of 80% rolling
(1.9 in true strain) and the observed shear strain of 8.1 (1.2
in true strain), the total true strain necessary to produce
nanocrystalline layer is estimated to be 3.1.
Fig. 7 shows a specimen with spheroidite structure after a
ball drop test (eight times, 5 kg, 1 m) at liquid nitrogen temperature. Curved thin bands similar to those observed in the
ball milled powders are seen parallel to the specimen surface. This suggests that the formation process of nanocrystalline regions by a ball drop test is similar to that in ball
milling [6].
The annealing experiment of ball dropped specimens showed the similar results with those observed in
the ball milled samples [6,10,11]. After annealing, the
Fig. 7. Nanocrystalline region formed in Fe-0.80C spheroidite by a ball drop (eight times, 5 kg, 1 m) at liquid nitrogen temperature.
M. Umemoto et al. / Materials Science and Engineering A 375–377 (2004) 899–904
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Fig. 8. Cross-sectional SEM micrographs after various numbers of particle impacts on the pre-strained (82%, cold rolling) pearlitic specimens: (a) 1
time, (b) 8 times and (c) 200 times.
microstructures of prior work-hardened region and prior
nanocrystalline region are still quite different and there is
a sharp boundary between them. In the work-hardened region, recrystallization and grain growth of the ferrite took
place. In contrast, a much finer microstructure is seen in the
nanocrystalline region, where the fine cementite particles
are re-precipitated.
3.2. Particle impact experiment
The development of nanocrystalline layers with the number of particle impacts was studied using pre-strained (82%
cold rolling) pearlitic specimens. Fig. 8 shows the crosssectional SEM micrographs after various numbers of particle impacts. After one time of particle impact (Fig. 8(a)),
heavily deformed layers with reduced lamellar spacing were
formed. The structure is similar to that observed in heavily
cold rolled pearlitic samples. After eight times of particle
impacts (Fig. 8(b)), featureless shear bands were formed.
After 200 times of particle impacts (Fig. 8(c)), large area
of nanocrystalline regions were formed. High hardness of
10.4 GPa (corresponds to grain size of 36 nm) similar to that
observed in the ball milled or ball dropped specimens was
obtained in the nanocrystalline regions. Shear bands were
also often observed in the particle impacted specimens. The
shear bands observed are much thinner (less than 5 ␮m) than
those obtained in a ball drop test.
4. Discussion
In the present study, formation of nanocrystalline regions
was observed by ball drop and particle impact experiments.
The produced nanocrystalline regions have sharp boundaries
with the work-hardened regions similar to those produced by
ball milling. There are two possible explanations for the observed sharp boundaries. One is to assume that the degree of
deformation is a gradual function of distance. In this case, the
nanocrystallization by heavy deformation occurs by a drastic transition from work-hardened to nanocrystalline state
and a critical degree of deformation or a critical dislocation
density may exist corresponding to this transition. Another
is to assume that the degree of deformation also changes
sharply at the boundaries between nanocrystalline and workhardened regions. In this case, localized heavy deformation
like shear bands are considered to be responsible for the
formation of nanocrystalline regions. At present, the second
explanation sounds more reasonable since shear bands are
observed in most of the specimens. In any case, it is important to realize that the regions with intermediate properties between the nanocrystalline and work-hardened regions
were not observed in the specimens prepared by either ball
milling, ball drop or particle impact experiments. The microstructural observations, hardness measurements and annealing experiments suggest that there are no intermediate
stages between the nanocrystallization and work-hardened
states. It seems that the deformation conditions suitable for
the formation of nanocrystalline region prevent the formation of intermediate state, or intermediate states are unstable
and change easily to nanocrystalline state by further deformation. The deformation conditions and/or a degree of deformation to produce the intermediate state seem extremely
limited.
5. Summary
Nanocrystalline regions can be successively fabricated in
various carbon steel plates by means of ball drop and particle impact techniques. The formation of nanocrystalline regions was confirmed by TEM observations, microhardness
measurements and annealing experiments. The nanocrystalline regions formed were similar to those observed in the
ball milled powders. The nanocrystalline regions showed
hardness higher than 10 GPa. Sharp boundaries between the
nanocrystalline and work-hardened regions were observed.
Nanocrystalline layers with large shear deformation were
sometimes observed in ball dropped and particle impacted
samples although they were not observed in ball milled
powders. From the measured shear deformation, the necessary true strain to produce nanocrystalline region was estimated to be about three for the ball drop and about four for
the particle impact experiments. The observed shear bands
are thicker in ball dropped samples than particle impact
samples. By annealing, recrystallization did not take place
and slow grain growth was observed in the nanocrystalline
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M. Umemoto et al. / Materials Science and Engineering A 375–377 (2004) 899–904
regions. When the specimen contains cementite, it dissolves
completely when the matrix is nanocrystallized.
Acknowledgements
This study is financially supported in part by the
Grant-in-Aid by the Japan Society for the Promotion of
Science.
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