J. Mater. Sci. Technol., Vol.25 No.4, 2009 433Aluminizing Low Carbon Steel at Lower TemperaturesXiao Si1)† , Bining Lu2) and Zhenbo Wang1)1) Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China2) 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; Nucleation1. Introduction chromizing treatment . In the present work, we demonstrate the possibil- Aluminizing is an eﬀective surface modiﬁcation ity of lowering aluminizing temperature by SMAT inprocess for improving corrosion resistance of steels. a commercial low carbon steel plate, which is amongVarious 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 theous 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. ExperimentalAl and Fe, eﬀective aluminizing is normally performedat high temperatures with austenite phase. An iron A commercial low carbon steel, with compositionsaluminide 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 tois most commonly used in industry. Holding at such eliminate the eﬀect of mechanical deformation and tohigh temperatures might induce serious distortion of obtain homogeneous coarse grains. A plate sampleworkpieces, carbide precipitation and grain coarsen- (100 mm×100 mm×4.0 mm in size) was subjected toing of the steel matrix, hence, degradation of mechan- SMAT, of which the set-up and procedure have beenical properties. Apparently, lowering the aluminizing described previously[6,12] . In brief, a large number oftemperatures of steels is of great signiﬁcance for min- hardened steel balls (8 mm in diameter) were placedimizing these negative eﬀects and widening the appli- at the bottom of a cylinder-shaped vacuum chambercation of aluminizing techniques. More speciﬁcally, vibrated for 60 min by a generator at a frequencyaluminizing steels in the ferrite state at a tempera- of 50 Hz at ambient temperature. The as-annealedture below 700◦ C would be very much desired . 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 oneenables to substantially reﬁne grains in the surface were aluminized under same conditions, i.e., at twolayer of various steels into the nanometer scale via re- temperatures (500 and 600◦ C, respectively) for 8 hpeated and multidirectional plastic deformation[5–10] . in a packed powder mixture of 50Al, 2NH4 Cl andDue to the signiﬁcantly enhanced diﬀusion and chem- 48Al2 O3 (in wt pct) in a double container designedical reactivity of the nanostructured surface layer pro- by Meier et al. . After aluminizing treatment, theduced by SMAT, gaseous nitriding has been suc- samples were wire-brushed and ultrasonically cleanedcessfully carried out on a Fe plate at 300◦ C , to remove adhering packed materials.evidently lower than conventional gaseous nitrid- Cross-sectional observations of the as-SMAT anding temperatures (∼550◦ C). In addition, a much the aluminized samples were performed on a Novathicker 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 wasthe 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: email@example.com (X. Si).
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 patternA protective layer of pure Ni of ∼50 µm in thick- orientations, as indicated by the selected area electronness was electrodeposited onto the sample surface for diﬀraction (SAED) pattern (inset in Fig. 1(b)). Thepreparing 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 µmobserved by using a Philip EM-420 transmission elec- to ∼9 nm in the top surface layer by SMAT. Detailedtron 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 sizeto identify the phase information in the aluminized increases with increasing depth and it reaches 100 nmsurface layer, by using a Rigaku D/max 2400 X-ray at the depth of ∼18 µm .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, transformationaluminized 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 boundariesstruments) ﬁtted with a Berkovich indenter. The (GBs) separating the nanocrystallites[7,9,12] . Whenmaximum 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 theneighboring indentations was at least 10 µm. The steel are progressively reﬁned into smaller particlesload-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 isto determine hardness . observed in the top surface layer in Fig. 1(b).3. Results and Discussion 3.2 Aluminizing kinetics of the SMAT sample3.1 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 darkthickness, as shown in the cross-sectional SEM mor- layer) has been formed on both samples. Measuredphologies in Fig. 1(a). Grains in the surface layer Al concentration proﬁles (see Fig. 2(c)) indicate thatare signiﬁcantly reﬁned and the microstructure diﬀers the atomic concentration of Al is about 70% in themarkedly 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 ontreated surface, so that it is diﬃcult to distinguish the the SMAT sample (∼52 µm) is much thicker after themicrostructure in the top surface layer of ∼100 µm same aluminizing treatment. A similar diﬀerence hasby SEM. TEM observations in the top surface layer also been observed on the samples with and withoutof the SMAT sample (as shown in Fig. 1(b)) reveal SMAT after the aluminizing treatment at 500◦ C forthat the microstructure is characterized by ultraﬁne 8 h, as listed in Table 1. The thickness of the alu-equiaxed ferrite grains with random crystallographic
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 ) 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 the500◦ C is comparable with that on the coarse-grained grain growth at temperatures above 500◦ C .sample aluminized at 600◦ C. Formation of an obviousaluminide coating is diﬃcult at temperatures below A previous work revealed that the growth ki-500◦ C, due to the limited deposition rate of Al onto netics of aluminide diﬀusion coating on an alloyedthe 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 progressivelytained in the pack powder mixture, a constant Al con- diminished as temperature increased. This work alsocentration in the source might be expected during the suggests a positive eﬀect of microstructure reﬁnementaluminizing 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 thesented 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 isTherefore, the ratios (m) of k on the SMAT sample to much thicker than that on the coarse-grained samplethe one on the coarse-grained sample are derived for after the same aluminizing treatment. This diﬀerencethe 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 thatgrowth kinetics of the aluminide layer on the SMAT surface hardness of the aluminized samples has beensteel 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 doubledat 500◦ C. 3.3 Phase evolution in the aluminide layer The much enhanced aluminizing kinetics in theSMAT low carbon steel is expected to result from the XRD patterns (as shown in Fig. 3) demonstrate
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 . (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. AcknowledgementsFig. 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) REFERENCESthat aluminide coatings formed on the SMAT and [1 ] R. M´vrel, C. Duret and R. Pichoir: Mater. Sci. Tech- ecoarse-grained samples aluminized at 600◦ C for 8 h nol., 1986, 2, 201.consist almost exclusively of η-Fe2 Al5 phase. Com- [2 ] K. Murakami, N. Nishida, K. Osamura, Y. Tomotaparing with the XRD pattern for a standard powder and T. Suzuki: Acta Mater., 2004, 52, 2173.specimen of η-Fe2 Al5 (Fig. 3(c), JCPD card No. 29- [3 ] R.W. Richards, R.D. Jones, P.D. Clements and H.0043), where (002) and (130) Brag diﬀraction peaks Clarke: Int. Mater. Rev., 1994, 39, 191.show the highest diﬀraction intensities, it is appar- [4 ] Z.D. Xiang and P.K. Datta: Acta Mater., 2006, 54,ent that the iron aluminide coating on the aluminized 4453.coarse-grained sample is strongly textured because a [5 ] K. Lu and J. Lu: J. Mater. Sci. Technol., 1999, 15,much higher intensity of (002) peak is detected, while 193.no obvious texture forms in the aluminized SMAT [6 ] K. Lu and J. Lu: Mater. Sci. Eng. A, 2004, 375-377, 38.surface layer. According to the results of pole ﬁgure [7 ] N.R. Tao, Z.B. Wang, W.P. Tong, M.L. Sui, J. Lu andanalyses in literature , where a similar XRD pat- K. Lu: Acta Mater., 2002, 50, 4603.tern was obtained, a ﬁbrous texture is expected in the [8 ] H.W. Zhang, Z.K. Hei, G. Liu, J. Lu and K. Lu: Actacoarse-grained sample. Mater., 2003, 51, 1871. From thermodynamic considerations, θ-FeAl3 [9 ] L. Zhou, G. Liu, X.L. Ma and K. Lu: Acta Mater.,phase possesses the lowest free energy of formation 2008, 56, 78.and it is expected to form preferentially in Fe-Al sys-  W.L. Li, N.R. Tao and K. Lu: Scripta Mater., 2008,tem. However, it is the η-Fe2 Al5 phase that forms 59, 546.in most cases due to the higher growth rate and  W.P. Tong, N.R. Tao, Z.B. Wang, J. Lu and K. Lu:favored crystallographic orientation (c axis)[2,3,15] . Science, 2003, 299, 686.For example, the growth rates of Fe2 Al5 and FeAl3  Z.B. Wang, J. Lu and K. Lu: Acta Mater., 2005, 53,at 715◦ C were reported to be 220 and 21 µm2 /s, 2081.  G.H. Meier, C. Cheng, R.A. Perkins and W. Bakker:respectively . The (00l) planes of Fe2 Al5 phase are Surf. Coat. Technol., 1989, 39-40, 53.thought to be the most densely packed and smooth,  W.C. Oliver and G.M. Pharr: J. Mater. Res., 1992,giving the lowest surface energy . In the coarse- 7, 1564.grained sample, few Fe2 Al5 particles are expected to  P.N. Bindumadhavan, S. Makesh, N. Gowrishankar,nucleate at the GBs at the early stage of aluminiz- H.K. Wah and O. Prabhakar: Surf. Coat. Technol.,ing process and grow up with c axis aligned along 2000, 127, 252.the direction perpendicular to the interface between  C. Suryanarayana: Int. Mater. Rev., 1995, 40, 41.coating and matrix to minimize the surface energy.  Z.B. Wang, N.R. Tao, W.P. Tong, J. Lu and K. Lu:Therefore, a strong ﬁbrous texture is developed after Acta Mater., 2003, 51, 4319.the aluminizing process. While a plenty of GBs in the  Z.B. Wang, K. Lu, G. Wilde and S. Divinski: Appl.nanostructured surface layer produced by SMAT sig- Phys. Lett., 2008, 93, 131904-1.  Z.D. Xiang and P.K. Datta: Scripta Mater., 2006, 55,niﬁcantly increase the nucleation frequency of Fe2 Al5 , 1151.the texture is more diﬃcult to be developed because