1. J. Mater. Sci. Technol., Vol.25 No.4, 2009 433
Aluminizing Low Carbon Steel at Lower Temperatures
Xiao Si1)† , Bining Lu2) and Zhenbo Wang1)
1) Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences,
Shenyang 110016, China
2) The High School Affiliated to Renmin University of China, Beijing 100080, China
[Manuscript received March 9, 2009, in revised form March 16, 2009]
This study reports the significantly enhanced aluminizing behaviors of a low carbon steel at temperatures
far below the austenitizing temperature, with a nanostructured surface layer produced by surface mechanical
attrition treatment (SMAT). A much thicker iron aluminide compound layer with a much enhanced growth
kinetics of η-Fe2 Al5 in the SMAT sample has been observed relative to the coarse-grained steel sample.
Compared to the coarse-grained sample, a weakened texture is formed in the aluminide layer in the SMAT
sample. The aluminizing kinetics is analyzed in terms of promoted diffusivity and nucleation frequency in the
nanostructured surface layer.
KEY WORDS: Nanostructured materials; Surface mechanical attrition treatment; Aluminizing;
Diffusion; Nucleation
1. Introduction chromizing treatment[12] .
In the present work, we demonstrate the possibil-
Aluminizing is an effective surface modification ity of lowering aluminizing temperature by SMAT in
process for improving corrosion resistance of steels. a commercial low carbon steel plate, which is among
Various aluminizing processes have been developed the most broadly used steels. The microstructure,
by enriching the surface layer of steels with a high hardness, chemical and phase compositions of the alu-
concentration of Al to form iron aluminide diffu- minized SMAT surface layer were investigated in com-
sion coatings, so that the ability to form impervi- parison with those of the coarse-grained one after the
ous and tenacious alumina scale is enhanced in cor- same treatment.
rosive media[1–3] . Nevertheless, as limited by the in-
volved diffusion of Al and reaction kinetics between 2. Experimental
Al and Fe, effective aluminizing is normally performed
at high temperatures with austenite phase. An iron A commercial low carbon steel, with compositions
aluminide coating can only be achieved on steels at (wt pct) of Fe, 0.11C, 0.01Si, 0.39Mn, 0.024S (max),
temperatures above 900◦ C with a duration of several 0.01P (max), was used in the present work. The sam-
to dozens hours for the pack aluminizing process that ple was annealed at 950◦ C for 120 min in vacuum to
is most commonly used in industry. Holding at such eliminate the effect of mechanical deformation and to
high temperatures might induce serious distortion of obtain homogeneous coarse grains. A plate sample
workpieces, carbide precipitation and grain coarsen- (100 mm×100 mm×4.0 mm in size) was subjected to
ing of the steel matrix, hence, degradation of mechan- SMAT, of which the set-up and procedure have been
ical properties. Apparently, lowering the aluminizing described previously[6,12] . In brief, a large number of
temperatures of steels is of great significance for min- hardened steel balls (8 mm in diameter) were placed
imizing these negative effects and widening the appli- at the bottom of a cylinder-shaped vacuum chamber
cation of aluminizing techniques. More specifically, vibrated for 60 min by a generator at a frequency
aluminizing steels in the ferrite state at a tempera- of 50 Hz at ambient temperature. The as-annealed
ture below 700◦ C would be very much desired[4] . sample was fixed at the upper side of the chamber
and impacted by flying balls repeatedly and multi-
By means of a recently developed surface directionally. Because the sample surface was plasti-
nanocrystallization technique, surface mechanical at- cally deformed with high strains and high strain rates,
trition treatment (SMAT)[5,6] , lowering the aluminiz- grains in the surface layer were effectively refined.
ing temperature of steels becomes feasible. SMAT The SMAT sample and the coarse-grained one
enables to substantially refine grains in the surface were aluminized under same conditions, i.e., at two
layer of various steels into the nanometer scale via re- temperatures (500 and 600◦ C, respectively) for 8 h
peated and multidirectional plastic deformation[5–10] . in a packed powder mixture of 50Al, 2NH4 Cl and
Due to the significantly enhanced diffusion and chem- 48Al2 O3 (in wt pct) in a double container designed
ical reactivity of the nanostructured surface layer pro-
by Meier et al.[13] . After aluminizing treatment, the
duced by SMAT, gaseous nitriding has been suc-
samples were wire-brushed and ultrasonically cleaned
cessfully carried out on a Fe plate at 300◦ C[11] ,
to remove adhering packed materials.
evidently lower than conventional gaseous nitrid-
Cross-sectional observations of the as-SMAT and
ing temperatures (∼550◦ C). In addition, a much
the aluminized samples were performed on a Nova
thicker chromized surface layer has been obtained
Nano-SEM 430 scanning electron microscope (SEM).
on an SMAT low carbon steel sample than that on
Al distribution in the aluminized surface layer was
the coarse-grained counterpart after the same pack
monitored by using a fully quantitative (Oxford Pro-
grams) X-ray energy dispersive spectroscope (EDS).
† Corresponding author. Senior Engineer; Tel.: +86 24
23971882; E-mail address: xsi@imr.ac.cn (X. Si).

2. 434 J. Mater. Sci. Technol., Vol.25 No.4, 2009
Fig. 1 (a) Cross-sectional SEM morphology and (b) a typical bright-field TEM image of the top surface layer
of the SMAT low carbon steel sample. The insert in (b) shows the corresponding selected area electron
diffraction pattern
A protective layer of pure Ni of ∼50 µm in thick- orientations, as indicated by the selected area electron
ness was electrodeposited onto the sample surface for diffraction (SAED) pattern (inset in Fig. 1(b)). The
preparing the cross-sectional samples. Microstructure mean grain size obtained from a number of TEM im-
of the top surface layer of the SMAT sample was also ages indicates that it has been refined from ∼50 µm
observed by using a Philip EM-420 transmission elec- to ∼9 nm in the top surface layer by SMAT. Detailed
tron microscope (TEM). In addition, X-ray diffraction microstructural characterizations of the SMAT sur-
(XRD) analysis of the surface layer was carried out face layer by XRD and TEM show that the grain size
to identify the phase information in the aluminized increases with increasing depth and it reaches 100 nm
surface layer, by using a Rigaku D/max 2400 X-ray at the depth of ∼18 µm[12] .
diffractometer with Cu Kα radiation. Ferrite grains are refined via sequential formation
The microhardness variation along depth from the of dislocation cells in original grains, transformation
aluminized surface was measured on cross-sectional of cell walls into subboundaries, and evolution of sub-
samples by using a Nano Indenter XPTM (Nano in- boundaries into highly misoriented grain boundaries
struments) fitted with a Berkovich indenter. The (GBs) separating the nanocrystallites[7,9,12] . When
maximum load for each measurement was 9 mN with ferrite grains are refined to a critical size, plastic de-
duration of 5 s, and the distance between any two formation occurs in carbide phase. Carbides in the
neighboring indentations was at least 10 µm. The steel are progressively refined into smaller particles
load-displacement data obtained during the first un- and/or dissolved into the ferrite phase with increas-
loading were analyzed using the Oliver-Pharr method ing strain and strain rate[9,12] , so that no cementite is
to determine hardness[14] . observed in the top surface layer in Fig. 1(b).
3. Results and Discussion 3.2 Aluminizing kinetics of the SMAT sample
3.1 Microstructures of the SMAT surface layer The cross-sectional SEM observations for the
SMAT and the coarse-grained samples after the alu-
minizing treatment at 600◦ C for 8 h are shown in
Clear evidences of plastic deformation have been
Fig. 2(a) and (b), respectively. It is clear that a con-
observed in the SMAT surface layer of ∼200 µm in
tinuous and dense aluminide surface layer (the dark
thickness, as shown in the cross-sectional SEM mor-
layer) has been formed on both samples. Measured
phologies in Fig. 1(a). Grains in the surface layer
Al concentration profiles (see Fig. 2(c)) indicate that
are significantly refined and the microstructure differs
the atomic concentration of Al is about 70% in the
markedly from that in the coarse-grained matrix (see
aluminide surface layers on both samples. In compar-
the bottom part in Fig. 1(a)). The degree of deforma-
ison with the aluminide coating formed on the coarse-
tion increases with decreasing depth from the topmost
grained sample (∼16 µm in thickness), the coating on
treated surface, so that it is difficult to distinguish the
the SMAT sample (∼52 µm) is much thicker after the
microstructure in the top surface layer of ∼100 µm
same aluminizing treatment. A similar difference has
by SEM. TEM observations in the top surface layer
also been observed on the samples with and without
of the SMAT sample (as shown in Fig. 1(b)) reveal
SMAT after the aluminizing treatment at 500◦ C for
that the microstructure is characterized by ultrafine
8 h, as listed in Table 1. The thickness of the alu-
equiaxed ferrite grains with random crystallographic

3. J. Mater. Sci. Technol., Vol.25 No.4, 2009 435
Fig. 2 Cross-sectional SEM morphologies of the SMAT (a) and the coarse-grained (b) low carbon steel samples
after the aluminizing treatment at 600◦ C for 8 h. (c) and (d) show variations of Al concentration and
hardness with the depth from the topmost surface, respectively
Table 1 Comparisons of the average thicknesses (in formation of a nanostructured surface layer, in which
µm) of aluminide surface layers on the a considerable volume fraction of GBs (∼30 vol. pct
SMAT and the coarse-grained (CG) low for an average grain size of 10 nm[16] ) act as numer-
carbon steel samples after the aluminizing ous fast diffusion “channels” for Al. In addition, a
treatments at 500 and 600◦ C for 8 h, re-
higher energy state of GBs induced by SMAT rela-
spectively. m is the ratio of k (see Eq. (1))
on the SMAT sample to that on the CG
tive to the conventional GBs is expected to further
sample increase the diffusivity of Al in the nanostructured
surface layer[17,18] . The lower m value at 600◦ C than
Temp./◦ C SMAT sample CG sample m at 500◦ C in Table 1 might be induced by a faster grain
500 10.9±1.8 2.4±0.5 20.6 growth at the higher temperature. This is because the
600 52.5±9.3 16.3±4.3 10.4 fraction and the excess energy of GBs may decrease
and result in a reduction of growth kinetics of the alu-
minide coating on the SMAT sample aluminized at
minide coating on SMAT sample, accompanying the
500◦ C is comparable with that on the coarse-grained
grain growth at temperatures above 500◦ C[12] .
sample aluminized at 600◦ C. Formation of an obvious
aluminide coating is difficult at temperatures below A previous work[19] revealed that the growth ki-
500◦ C, due to the limited deposition rate of Al onto netics of aluminide diffusion coating on an alloyed
the sample surface. steel was enhanced by shot peening at temperatures
Because a large content (50 wt pct) of Al is con- below 667◦ C and the enhancement effect progressively
tained in the pack powder mixture, a constant Al con- diminished as temperature increased. This work also
centration in the source might be expected during the suggests a positive effect of microstructure refinement
aluminizing procedure at a fixed temperature and the on the aluminizing kinetics at lower temperatures.
growth kinetics of the aluminide layer can be repre- The variation of hardness along depth in the
sented by a parabolic rate equation of the form[3,15] , SMAT sample aluminized at 600◦ C was compared
with that in the aluminized coarse-grained counter-
y 2 = kt (1) part in Fig. 2(d). The hardness values of both sur-
face layers are ∼15 GPa after the aluminizing treat-
where y is the thickness of the aluminide layer after ment, while the matrix is about 3 GPa. However,
treating duration of t and k is the mean growth rate. the hardened surface layer on the SMAT sample is
Therefore, the ratios (m) of k on the SMAT sample to much thicker than that on the coarse-grained sample
the one on the coarse-grained sample are derived for after the same aluminizing treatment. This difference
the aluminizing treatments at 500 and 600◦ C, respec- is consistent with the measured thicknesses of alu-
tively, as shown in Table 1. It is indicated that the minide surface layers on the samples. It is clear that
growth kinetics of the aluminide layer on the SMAT surface hardness of the aluminized samples has been
steel is about 10 times higher than that on the coarse- promoted by the formation of iron aluminide coatings.
grained sample at 600◦ C, and the m value is doubled
at 500◦ C. 3.3 Phase evolution in the aluminide layer
The much enhanced aluminizing kinetics in the
SMAT low carbon steel is expected to result from the XRD patterns (as shown in Fig. 3) demonstrate

4. 436 J. Mater. Sci. Technol., Vol.25 No.4, 2009
grains might catch each other and stop growing at an
earlier stage. It was discussed that the nucleation fre-
quency might be increased by an order of about 106
with a reduction of grain size from 40 µm to 40 nm[12] .
(a)
4. Summary
In conclusion, it has been demonstrated that alu-
minizing low carbon steels at a temperature far below
the austenitizing temperature is possible by the for-
mation of a nanostructured surface layer by SMAT. A
Intensity / a.u.
surface layer consisted of η-Fe2 Al5 phase, of ∼52 µm
(b) in thickness, is formed on the SMAT sample after a
pack aluminizing treatment at 600◦ C for 8 h, more
than 3 times thicker than that on the aluminized
coarse-grained counterpart. And the enhancement ef-
fect is doubled at 500◦ C. The enhanced aluminizing
(002)
(130)
(c)
-Fe Al
2 5 kinetics is expected to result from a much increased
GB diffusivity in the nanostructured surface layer. In
(020)
(400)
addition, no obvious texture is detected in the Fe2 Al5
(200)
(240)
(112)
surface layer on the aluminized SMAT sample, due to
(310)
(331)
the significantly increased nucleation frequency of the
Fe2 Al5 phase in the nanostructured surface layer.
20 30 40 50 60
2 / deg.
Acknowledgements
Fig. 3 XRD patterns of the SMAT (a) and the coarse- This work was financially supported by the Na-
grained (b) samples after the aluminizing treat- tional Science Foundation of China (Nos. 50701044 and
ment at 600◦ C for 8 h. (c) shows an XRD pat- 50890171), and the Ministry of Science and Technology of
tern obtained from the reported powder diffrac- China (No. 2005CB623604).
tion data (JCPD card No. 29-0043)
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