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Microstructure and texture evolution during the ultra grain refinement of the Armco iron deformed by accumulative roll bonding (ARB)
Materials Science & Engineering A 561 (2013) 60–66
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Microstructure and texture evolution during the ultra grain refinement of the Armco iron deformed by accumulative roll bonding (ARB) Erell Bonnot, Anne-Laure Helbert n, Franc- ois Brisset, Thierry Baudin Universite´ Paris-Sud, ICMMO, UMR CNRS 8182, 91405 Orsay Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 August 2012 Received in revised form 2 November 2012 Accepted 3 November 2012 Available online 10 November 2012
The Armco iron is one of the purest commercial iron with very low levels of carbon, oxygen and nitrogen. In order to improve the mechanical properties, it is worth applying severe plastic deformation to obtain ultrafine-grained bulk materials, with grain size o1 mm. In this study, samples of Armco iron were subjected to a technique of severe plastic deformation named Accumulative Roll Bonding. The important parameter of this process is the number of passes and the deduced von Mises equivalent strain. By means of the Electron BackScattered Diffraction and X-Ray diffraction, the evolution of microstructure and texture with the applied strain was studied. The microhardness was also measured as a function of the equivalent strain and reached 280 HV after 7 ARB cycles. The ARB technique applied to Armco iron resulted in a microstructure of ultrafine grains of about 580 nm after 7 cycles, with a high angle grain boundary fraction of 60%. The texture is mainly composed of a and g fibers with a reinforcement of both {001}/110S and {111}/110S components. & 2012 Elsevier B.V. All rights reserved.
Keywords: EBSD Ultrafine grained Armco iron Accumulative roll-bonding Crystallographic texture Hardness
1. Introduction Accumulative Roll Bonding (ARB) [1,2] is a widely used Severe Plastic Deformation (SPD) technique that consists in rolling 50% two sheets together, then in cutting and finally in stacking these sheets and in repeating the whole process. This method has been applied mainly to aluminum [3,4], IF-steels [5] and copper [6,7]. SPD techniques as ARB, Equal Channel Angular Extrusion (ECAE) [8–10] or High Pressure Torsion (HPT) [11,12] are used to obtain ultrafine-grained (UFG) structure of metallic materials. The formation of an UFG structure has been studied and described for Armco iron as the following. Applying HPT to Armco iron [13], the refinement process presents an evolution from a cellular structure, with cell walls with a very high dislocation density, then the cell size decreases with an increase of narrow walls (typical of subgrain structures) to an UFG structure with few dislocations and a grain size of about 100 nm after an equivalent strain of 181. Wetscher et al. [14] observed, after a small amount of HPT strain (e ¼ 30), a new structure with misoriented structural elements subdivided by dislocation walls, with a grain size that decreases down to 250 nm. Comparing ARB with other SPD techniques [15], it appears that the ARB processed materials have an elongated UFG structure and a more advantageous grain refinement due to the redundant
shear strain present when rolling dry. This redundant shear strain gives rise to an inhomogeneity of microstructure, texture and properties along the thickness of the sample [16–18]. It is demonstrated that shear strain greatly affects the grain refinement; this is why the shear strain distribution through thickness is correlated with grain size distribution [16]. As far as texture is concerned, a shear texture (/110SJND) appears in the region near the surface of the sample and rolling textures (/111SJND and /110SJRD) in the region near the center for an ARB processed IF-steel [17]. Finally, the hardness is found to be greater in the center, close to the bonding interface for the first 3 cycles and then constant in thickness [18]. Kamikawa et al. [17] observed grains of about 300 nm in a IF steel sheet processed by ARB up to 7 cycles (e ¼5.6) at 773 K, with a high angle grain boundary fraction of 80%.
Cutting Cleaning Wire brushing
Roll bonding
Welding and heating n
Corresponding author. Postal address : Bat 410, LPCES, Universite´ Paris-Sud, ICMMO, UMR CNRS 8182, 91405 Orsay Cedex, France. Tel.: þ 331 69 15 47 85; fax: þ 331 69 15 47 97. E-mail address: [email protected] (A.-L. Helbert). 0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.11.017
Fig. 1. Accumulative roll bonding process.
E. Bonnot et al. / Materials Science & Engineering A 561 (2013) 60–66
The Electron BackScattered Diffraction (EBSD) technique makes it possible to characterize the ultrafine-grain formation as well as local texture as a function of the equivalent strain corresponding to the number of ARB cycles. The present study aims to clarify the evolution of various microstructural parameters in an ARB processed Armco iron and to quantify the evolution of texture components during accumulative roll bonding.
350 300 Microhardness (HV)
61
250 200 150 100
2. Experimental details
50 0
0
0.8
1.6
2.4 3.2 Strain
4
4.8
Fig. 2. Microhardness evolution as a function of the strain.
5.6
Two 94% cold-rolled sheets of a 1 mm thick Armco iron, 15 mm width and 100 mm long were heat-treated in a secondary-vacuum furnace (10 4 Pa) for 2 h at 973 K to recrystallize the material. The initial grain size, measured by EBSD as the ‘‘Average Intercept
TD
300µm
10µm
ND
10µm
10µm
10µm
10µm
Fig. 3. Microstructure formation as a function of the number of ARB cycles, (a) n¼ 0, (b) n¼1, (c) n ¼2, (d) n¼3, (e) n ¼5 and (f) n¼ 7. The red lines represent LAGB and the black lines HAGB. (For interpretation of references to colour in this figure legend, the reader is referred to the web version of this article.)
E. Bonnot et al. / Materials Science & Engineering A 561 (2013) 60–66
for n ¼0, n ¼1, n ¼2, n¼3, n ¼5 and n ¼7. For n ¼0, mostly composed of HAGB, few LAGB are present and divide big grains in smaller equiaxed ones. For n¼ 1, there are few HAGB that border coarse grains which are composed of lamellar subgrains delimited by LAGB. Then, when the strain increases, the fraction of HAGB increases and the grains become more equiaxed.
100
100 Grain size (µm) HAGB fraction (%)
70 60 50 40
1
30 20 10 0
0.8
1.6
The microstructure change can be studied by observing the evolution of grain shape together with the LAGB location (Fig. 3),
2.4
3.2 Strain
4
4.8
5.6
6.4
0
Fig. 4. Evolution of the mean grain size and the fraction of HAGB as a function of the number of ARB cycles.
60% n=1 n=2 n=3 n=5 n=7 Random
50% 40% 30% 20% 10% 0% 0
10
3.1. Microhardness
3.2. Microstructure
80
10
0.1
3. Results
20 30 40 50 Misorientation Angle (°)
60
50 45 Misorientation Angle (°)
The Vickers microhardness evolution as a function of the strain is shown in Fig. 2. It presents a great raise for e ¼0.8, corresponding to one ARB cycle, then a slight increase with strain until e ¼1.6 (n¼ 2) and finally reaches a saturated state. The higher hardness value is around 280 HV, that is 3 times larger than before the ARB process (n¼0). Similar results were obtained for IF steel, with a value 2 or 3 times greater than that of the initial state after 5 cycles and a maximum value of 215 HV [19], or 293 HV [2]. However, this increase is more progressive than in the case of Armco iron. From the hardness measurement that only shows a slight increase of microhardness after the first cycle, one could conclude that an ultrafine-grained structure has been directly created after the first ARB cycle. In order to really associate this mechanism with the formation of ultrafine grains, the microstructure has been studied by means of the EBSD technique.
90 HAGB fraction (%)
Mean Grain size (µ m)
Length’’, is 18 mm. Then, the ARB process was performed as shown in Fig. 1. To achieve good bonding, the contact surfaces were degreased, wire-brushed, cleaned and finally spot welded. The obtained strip was then pre-heated at 773 K for 10 min and finally rolled dry 50% at a rate of 0.11 m/s resulting in a single bonded sheet of 1 mm thickness. This sheet was cut in two halves and the process of cleaning and warm roll bonding repeated up to seven cycles. From the cycle number n, the number of layers in the resulting sheet, the thickness of each layer and the resulting equivalent strain can be calculated. In the case of seven cycles, for example, the sheet is made of 128 layers and the thickness of each layer is the following: t7 ¼t0/27 ¼7.8 mm, with t0 the initial thickness. The von Mises equivalent strain [3] is given by e7 ¼0.8 7¼5.6. To observe microstructure and texture, sheet cross-sections (ND– TD, normal and transverse direction respectively) were mechanically polished and then electropolished using the A2 Struers electrolyte at 40 V. The observations were carried out using a scanning electron microscope (SEM) equipped with a field emission gun (FEG) and EBSD, and the analysis performed using the Orientation Imaging Microscopy (OIMTM) software. The scanned area was 900 mm 800 mm with a 2 mm step size for the initial sample prior to ARB. After ARB, the orientation maps were performed with a 0.1 mm step size over an area of 250 mm 100 mm. The measured orientation maps were cleaned up using the Grain dilation method of OIM software with 5 pixels as a minimum grain size. Grain size was determined considering grain boundaries with a misorientation angle higher than 51 and using the linear intercept method. The misorientation profile was measured using as lower accepted value of 21. Boundaries types are defined as low-angle (LAGB) if the misorientation angle is y o151 and as high-angle (HAGB) for 151o y o62.81. The orientation distribution functions (ODF) were calculated by harmonic series expansion using truncation at L¼22 and the Bunge notation for Euler angles. Finally, the grains of orientations /110SJRD or /111SJND were isolated considering a tolerance angle of 151. The texture of the initial 94% cold-rolled state was characterized by X-ray diffraction in a Siemens goniometer system and the ODF was analyzed using the Labotex software. The Vickers microhardness (LECO, M400H) was determined using a load of 200 g during 25 s and averaged over five measurements.
Number Fraction
62
40 35 30 25 All HAGB LAGB
20 15 10 5 0 0
0.8
1.6
2.4
3.2 Strain
4
4.8
5.6
Fig. 5. (a) Distribution of boundary misorientation angle for n¼ 1, n¼ 2, n¼ 3, n¼5, n¼ 7 and random; (b) evolution of the mean boundary misorientation angle during the ARB process depending on the boundary nature (LAGB or HAGB).
E. Bonnot et al. / Materials Science & Engineering A 561 (2013) 60–66
In order to characterize the formation of ultrafine grains, the mean grain size together with the boundary misorientation angles were calculated during the ARB process. The grain size was defined as the number of grain boundary intercepts per line length, in the normal direction, where a grain boundary is defined with a minimum misorientation angle of 51. Fig. 4 illustrates the evolution of the mean grain size and the HAGB fraction as a function of ARB strain. The initial average grain size is around 15 mm. Then, from the early first cycle, a reduction of grain size appears together with a high amount of LAGB (drop of HAGB). The initial grains are replaced by a large area of subgrained structure due to the accommodation of the large imposed strain [20]. In subsequent cycles, the average grain size decreases, reaching a value of 580 nm for n¼7, and the fraction of HAGB increases up to a saturated value of 60% after 5 cycles which is still less than that obtained for ARB processed IF-steel (80%) [17]. The distribution of misorientation angles for different numbers of ARB cycles is presented in Fig. 5a. It is clear from these distributions that there is a continuous evolution towards HAGB with increasing number of cycle. From n ¼5, the distribution tends to become constant, with a persistent large fraction of LAGB (40%). As compared to the dotted line in Fig. 5a, which corresponds to the statistical prediction of a set of random orientations, there remains an excess of low angle boundaries even after 7 cycles. One can note in Fig. 5b that after n ¼5, the
63
mean misorientation tends to saturate around 261–281. In the literature, it is assumed that the SPD microstructure is stable when the mean misorientation reaches about 351 [17]. In the present study, further ARB cycles may be needed to reach this value. Fig. 5b also shows that the mean misorientation of the LAGB is very quickly stabilized at 51 while the HAGB misorientation evolution gradually increases with strain. The increase in misorientation angle is the witness of dislocation accumulation in grain boundaries [21]. Comparing these results with the microhardness measurements in Fig. 2, it is obvious that the mechanism associated with the large increase of microhardness is the formation of sub-grains along with the augmentation of dislocation density (or LAGB fraction), straight observed after the first ARB cycle. This is consistent with previous works on other SPD techniques [9,10]. It remains to verify the metallurgical quality of the interfaces between the layers of the Armco iron sandwich processed by ARB. Fig. 6 illustrates that the welding-diffusion occurred by showing good grain continuity across interfaces between layers (Fig. 6c). However, locally, it remains defects (Fig. 6a-bottom) due to the sheet oxidation. 3.3. Texture Fig. 7 shows the evolution of microstructure during the ARB processing along with the grain orientations, for small zones
TD
ND
Fig. 6. (a) Welding-diffusion process after 4 ARB cycles through the mid thickness layer interface (dotted line); (b) and (c) Orientation maps of the {hkl}plane parallel to the rolling plane (color code in the stereographic triangle). (For interpretation of references to colour in this figure legend, the reader is referred to the web version of this article.)
n=0
n=2
n=5
n=7
ND TD
Fig. 7. Evolution of microstructure and crystallographic orientations for different ARB cycles: (a) and (e) n¼0, (b) and (f) n¼ 2, (c) and (g) n¼ 5, and (d) and (h) n¼ 7. Upper part: {hkl} maps (/hklSJND), lower part: /uvwS maps (/uvwSJRD). (For interpretation of references to colour in this figure, the reader is referred to the web version of this article.)
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of the measured EBSD maps in the ND–TD plane. For n ¼2–5, the grains show an elongated shape in the transverse direction which tends to be more equiaxed: the aspect ratio varies from 0.2 for n ¼2 to 0.47 for n¼ 5 ( 70.05). Then, the aspect ratio remains constant for n ¼7. From the analysis of these maps (Fig. 7a–d), qualitatively, it seems that, for the initial state and for n ¼2, the /hklS principal directions parallel to ND are /100S (red) and /111S (blue). For n ¼5 and n ¼7, the /100S direction clearly develops through the microstructure. On all these maps (Fig. 7e–h), the main crystallographic direction /uvwS is /110SJRD (green). Fig. 8 is a compilation of the ODF sections at j2 ¼ 451 for samples processed by ARB through 2, 3, 5 and 7 cycles measured at the mid-thickness of the specimen. The orthotropic sample symmetry was not imposed in order to check whether the shear
components were present in the mid-thickness of the samples. Fig. 8 also contains the ODF of the 94% cold-rolled state and shows a partial a-fiber between {100}/110S and {111}/110S with a strong rotated Cube component (Fig. 8a). After annealing at 973 K during 2 h, the ARB initial state n ¼0 is obtained, and a strong {311}/136S component, from the {h11}/1/h,1,2S fiber appears along with a g-fiber texture (/ 111SJND) (Fig. 8b). This {311}/136S component is characteristic of the recrystallized state of a highly cold-rolled BCC materials [22–24] due to an oriented nucleation from the strained rotated cube component that reveals internal orientation gradients at the deformed state. For the first cycles of ARB, the analyzed grain number is too low to calculate a statistical ODF (Fig. 8c and d, for n ¼2 and n¼3, respectively). However, a strong rotated Cube component ({001}
Fig. 8. j2 ¼ 451 section (01 o j1 o 3601, 01o F o901) of the ODFs for (a) the 94% cold-rolled state, before heat treatment, and (b) n¼ 0, (c) n ¼2, (d) n¼ 3, (e) n¼5 and (f) n¼7; (g) ideal components described in the ODF section assuming an orthotropic sample symmetry: j2 ¼ 451 (01 o j1 o901).
TD ND
Fig. 9. (a) Microstructure and (b) j2 ¼451 ODF section of sample surface after 3 ARB cycles.
E. Bonnot et al. / Materials Science & Engineering A 561 (2013) 60–66
<110>//RD ± 15° (alpha-fiber) <111>//ND ± 15° (gamma-fiber)
60
HAGB α - HAGB γ (%)
HAGB Fraction (%)
70
50 40 30 20 10 0
0
0.8
1.6
2.4
3.2
4
4.8
5.6
10 9 8 7 6 5 4 3 2 1 0
0
0.8
1.6
Strain
2.4 3.2 Strain
65
4
4.8
5.6
Fig. 10. Evolution as a function of strain (a) HAGB fraction in a-fiber grains (/110SJRD) and g-fiber grains (/111SJND) and (b) difference of HAGB fraction between the a and the g fiber.
/110S), coming from the initial 94% cold-rolled state, clearly appears along with the a-fiber (/110SJRD) that replaces the {h11}/1/h,1,2S fiber. After 7 ARB cycles (e7 ¼5.8, Fig. 8f), the texture is mainly composed of fibers /001SJND, /111SJND and /110SJRD with a strengthening of the rotated Cube and the {111}/110S components which is actually particularly interesting for magnetic applications [23]. Let us note that after hot rolling in the ferrite region of an ultra-low carbon steel, Re´gle´ et al. [25] have shown also that the texture is characterized by a partial a fiber from {100}/110S to {111}/110S, with a strengthening of these two components, a complete g fiber and a Goss component which is not found in the present work. No shear texture (/110SJND) is observed in the specimen mid-thickness even for highly ARB strained samples. Nevertheless, a shear texture can be measured at the sample surface as shown in Fig. 9 for n¼3, as already reported by Kamikawa et al. [17].
4. Discussion The miscrostructure of the ARB processed ARMCO iron is composed of misoriented ultrafine grains of 580 nm after 7 cycles. As described by Gazder et al. [20] for IF-steel during ECAE, the microstructural evolution can be divided into two stages. The first one, for n o5 (e5 ¼4), corresponds to a rapid refinement of grain size together with an increase of the fraction of HAGB. The second one, for n 45, presents a flattening of the fraction of HAGB and of the grain refinement. During the first stage, the increase in misorientation is due to the production of deformation-induced HAGB by grain splitting into primary elongated grains, as well as by accumulation of dislocations in boundaries. During the second stage, the subgrain walls act as further barriers to dislocation propagation and as deformation proceeds, subgrains rotate to the nearest stable end-orientation which leads to a flattening of the HAGB fraction. ARB technique allows the formation of an almost equiaxed structure with more than 60% of HAGB. In comparison, results obtained by Ivanisenko et al. [26], for High Pressure Torsion in Armco iron show also an equiaxed structure but with only 50% of HAGB after applying a very high strain (g ¼210). The HAGB fraction of 60% is still lower than that found for ARB processed IF-steel (80%) [17], and the mean misorientation saturates at 281 while 351 is obtained in the IF-steel. This suggests that a more misoriented structure could be obtained by increasing the number of ARB cycles, even if the grain size remains constant. Such an ultra-fine grained microstructure could then be stable during further annealing since a dominant HAGB network inhibits the discontinuous recrystallization upon annealing [27]. As far as texture is concerned, the absence of the shear components suggests that the shear texture developed at the
sample surface at the ARB cycle n is destroyed by the subsequent rolling cycle (plane-strain deformation) in the sample center, this is in agreement with what was reported in ARB processed aluminum alloys [28]. To study the effect of crystallographic orientation on the formation of HAGB, the evolution of HAGB fraction has been calculated both for /110SJRD and /111SJND grains orientation, respectively a and g fibers classically observed in BCC materials. The results are presented in Fig. 10a. It appears that as strain proceeds, the HAGB are produced in higher fraction in the a-fiber than in the g-fiber. It is interesting to note that the gap between the HAGB fraction in the a-fiber and in the g-fiber increases with strain (Fig. 10b). This is due to the gradual rotation of the a-fiber components towards two specific components: the rotated Cube and the {111}/110S orientation. This is at the origin of HAGB formation inside the a-fiber that becomes mainly composed, as strain increases, of two components: {111}/011S and {100}/ 011S misorientated by 551. On the contrary, in the g-fiber, even if the {111}/112S component strengthens, a large range of orientations persists, which leads to higher LAGB fraction compared to the a-fiber.
5. Conclusion Applying 7 ARB cycles to an Armco iron sample results in an almost equiaxed structure with a large saturated fraction of HAGB (60%) and a grain size of about 580 nm. The early formation of a subgrained structure allows a great raise of the microhardness, of around 3 times the initial value, after only one ARB cycle. Then, the hardness value quickly saturates around 280 HV. The study of the texture evolution as a number of ARB cycles shows the replacement of the {h11}/1/h,1,2S annealing fiber by the a-fiber. After n ¼2, the rotated Cube and {111}/110S components develop to the detriment of the other orientations of the a-fiber and this provides the misorentation increase inside the a-fiber in comparison with the g fiber. The enhancement of the rotated Cube component is particularly helpful for magnetic applications. No shear texture is formed at the sample center due to the alternated plane strain and shear strain deformations.
Acknowledgments This study has been realized in the framework of the ANRs ‘program blanc’, ‘MAEL’ project, ref. BLAN08-2_367373. The authors thank P. Franciosi and M.H. Chavanne (Laboratoire des Proprie´te´s Me´caniques et Thermodynamique des Mate´riaux de Villetaneuse, LPMTM, Universite´ Paris 13) for the elaboration of the cold-rolled Armco iron.
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Microstructure and texture evolution during the ultra grain refinement of the Armco iron deformed by accumulative roll bonding (ARB) Erell Bonnot, Anne-Laure Helbert n, Franc- ois Brisset, Thierry Baudin Universite´ Paris-Sud, ICMMO, UMR CNRS 8182, 91405 Orsay Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 August 2012 Received in revised form 2 November 2012 Accepted 3 November 2012 Available online 10 November 2012
The Armco iron is one of the purest commercial iron with very low levels of carbon, oxygen and nitrogen. In order to improve the mechanical properties, it is worth applying severe plastic deformation to obtain ultrafine-grained bulk materials, with grain size o1 mm. In this study, samples of Armco iron were subjected to a technique of severe plastic deformation named Accumulative Roll Bonding. The important parameter of this process is the number of passes and the deduced von Mises equivalent strain. By means of the Electron BackScattered Diffraction and X-Ray diffraction, the evolution of microstructure and texture with the applied strain was studied. The microhardness was also measured as a function of the equivalent strain and reached 280 HV after 7 ARB cycles. The ARB technique applied to Armco iron resulted in a microstructure of ultrafine grains of about 580 nm after 7 cycles, with a high angle grain boundary fraction of 60%. The texture is mainly composed of a and g fibers with a reinforcement of both {001}/110S and {111}/110S components. & 2012 Elsevier B.V. All rights reserved.
Keywords: EBSD Ultrafine grained Armco iron Accumulative roll-bonding Crystallographic texture Hardness
1. Introduction Accumulative Roll Bonding (ARB) [1,2] is a widely used Severe Plastic Deformation (SPD) technique that consists in rolling 50% two sheets together, then in cutting and finally in stacking these sheets and in repeating the whole process. This method has been applied mainly to aluminum [3,4], IF-steels [5] and copper [6,7]. SPD techniques as ARB, Equal Channel Angular Extrusion (ECAE) [8–10] or High Pressure Torsion (HPT) [11,12] are used to obtain ultrafine-grained (UFG) structure of metallic materials. The formation of an UFG structure has been studied and described for Armco iron as the following. Applying HPT to Armco iron [13], the refinement process presents an evolution from a cellular structure, with cell walls with a very high dislocation density, then the cell size decreases with an increase of narrow walls (typical of subgrain structures) to an UFG structure with few dislocations and a grain size of about 100 nm after an equivalent strain of 181. Wetscher et al. [14] observed, after a small amount of HPT strain (e ¼ 30), a new structure with misoriented structural elements subdivided by dislocation walls, with a grain size that decreases down to 250 nm. Comparing ARB with other SPD techniques [15], it appears that the ARB processed materials have an elongated UFG structure and a more advantageous grain refinement due to the redundant
shear strain present when rolling dry. This redundant shear strain gives rise to an inhomogeneity of microstructure, texture and properties along the thickness of the sample [16–18]. It is demonstrated that shear strain greatly affects the grain refinement; this is why the shear strain distribution through thickness is correlated with grain size distribution [16]. As far as texture is concerned, a shear texture (/110SJND) appears in the region near the surface of the sample and rolling textures (/111SJND and /110SJRD) in the region near the center for an ARB processed IF-steel [17]. Finally, the hardness is found to be greater in the center, close to the bonding interface for the first 3 cycles and then constant in thickness [18]. Kamikawa et al. [17] observed grains of about 300 nm in a IF steel sheet processed by ARB up to 7 cycles (e ¼5.6) at 773 K, with a high angle grain boundary fraction of 80%.
Cutting Cleaning Wire brushing
Roll bonding
Welding and heating n
Corresponding author. Postal address : Bat 410, LPCES, Universite´ Paris-Sud, ICMMO, UMR CNRS 8182, 91405 Orsay Cedex, France. Tel.: þ 331 69 15 47 85; fax: þ 331 69 15 47 97. E-mail address: [email protected] (A.-L. Helbert). 0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.11.017
Fig. 1. Accumulative roll bonding process.
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The Electron BackScattered Diffraction (EBSD) technique makes it possible to characterize the ultrafine-grain formation as well as local texture as a function of the equivalent strain corresponding to the number of ARB cycles. The present study aims to clarify the evolution of various microstructural parameters in an ARB processed Armco iron and to quantify the evolution of texture components during accumulative roll bonding.
350 300 Microhardness (HV)
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250 200 150 100
2. Experimental details
50 0
0
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1.6
2.4 3.2 Strain
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4.8
Fig. 2. Microhardness evolution as a function of the strain.
5.6
Two 94% cold-rolled sheets of a 1 mm thick Armco iron, 15 mm width and 100 mm long were heat-treated in a secondary-vacuum furnace (10 4 Pa) for 2 h at 973 K to recrystallize the material. The initial grain size, measured by EBSD as the ‘‘Average Intercept
TD
300µm
10µm
ND
10µm
10µm
10µm
10µm
Fig. 3. Microstructure formation as a function of the number of ARB cycles, (a) n¼ 0, (b) n¼1, (c) n ¼2, (d) n¼3, (e) n ¼5 and (f) n¼ 7. The red lines represent LAGB and the black lines HAGB. (For interpretation of references to colour in this figure legend, the reader is referred to the web version of this article.)
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for n ¼0, n ¼1, n ¼2, n¼3, n ¼5 and n ¼7. For n ¼0, mostly composed of HAGB, few LAGB are present and divide big grains in smaller equiaxed ones. For n¼ 1, there are few HAGB that border coarse grains which are composed of lamellar subgrains delimited by LAGB. Then, when the strain increases, the fraction of HAGB increases and the grains become more equiaxed.
100
100 Grain size (µm) HAGB fraction (%)
70 60 50 40
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30 20 10 0
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The microstructure change can be studied by observing the evolution of grain shape together with the LAGB location (Fig. 3),
2.4
3.2 Strain
4
4.8
5.6
6.4
0
Fig. 4. Evolution of the mean grain size and the fraction of HAGB as a function of the number of ARB cycles.
60% n=1 n=2 n=3 n=5 n=7 Random
50% 40% 30% 20% 10% 0% 0
10
3.1. Microhardness
3.2. Microstructure
80
10
0.1
3. Results
20 30 40 50 Misorientation Angle (°)
60
50 45 Misorientation Angle (°)
The Vickers microhardness evolution as a function of the strain is shown in Fig. 2. It presents a great raise for e ¼0.8, corresponding to one ARB cycle, then a slight increase with strain until e ¼1.6 (n¼ 2) and finally reaches a saturated state. The higher hardness value is around 280 HV, that is 3 times larger than before the ARB process (n¼0). Similar results were obtained for IF steel, with a value 2 or 3 times greater than that of the initial state after 5 cycles and a maximum value of 215 HV [19], or 293 HV [2]. However, this increase is more progressive than in the case of Armco iron. From the hardness measurement that only shows a slight increase of microhardness after the first cycle, one could conclude that an ultrafine-grained structure has been directly created after the first ARB cycle. In order to really associate this mechanism with the formation of ultrafine grains, the microstructure has been studied by means of the EBSD technique.
90 HAGB fraction (%)
Mean Grain size (µ m)
Length’’, is 18 mm. Then, the ARB process was performed as shown in Fig. 1. To achieve good bonding, the contact surfaces were degreased, wire-brushed, cleaned and finally spot welded. The obtained strip was then pre-heated at 773 K for 10 min and finally rolled dry 50% at a rate of 0.11 m/s resulting in a single bonded sheet of 1 mm thickness. This sheet was cut in two halves and the process of cleaning and warm roll bonding repeated up to seven cycles. From the cycle number n, the number of layers in the resulting sheet, the thickness of each layer and the resulting equivalent strain can be calculated. In the case of seven cycles, for example, the sheet is made of 128 layers and the thickness of each layer is the following: t7 ¼t0/27 ¼7.8 mm, with t0 the initial thickness. The von Mises equivalent strain [3] is given by e7 ¼0.8 7¼5.6. To observe microstructure and texture, sheet cross-sections (ND– TD, normal and transverse direction respectively) were mechanically polished and then electropolished using the A2 Struers electrolyte at 40 V. The observations were carried out using a scanning electron microscope (SEM) equipped with a field emission gun (FEG) and EBSD, and the analysis performed using the Orientation Imaging Microscopy (OIMTM) software. The scanned area was 900 mm 800 mm with a 2 mm step size for the initial sample prior to ARB. After ARB, the orientation maps were performed with a 0.1 mm step size over an area of 250 mm 100 mm. The measured orientation maps were cleaned up using the Grain dilation method of OIM software with 5 pixels as a minimum grain size. Grain size was determined considering grain boundaries with a misorientation angle higher than 51 and using the linear intercept method. The misorientation profile was measured using as lower accepted value of 21. Boundaries types are defined as low-angle (LAGB) if the misorientation angle is y o151 and as high-angle (HAGB) for 151o y o62.81. The orientation distribution functions (ODF) were calculated by harmonic series expansion using truncation at L¼22 and the Bunge notation for Euler angles. Finally, the grains of orientations /110SJRD or /111SJND were isolated considering a tolerance angle of 151. The texture of the initial 94% cold-rolled state was characterized by X-ray diffraction in a Siemens goniometer system and the ODF was analyzed using the Labotex software. The Vickers microhardness (LECO, M400H) was determined using a load of 200 g during 25 s and averaged over five measurements.
Number Fraction
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40 35 30 25 All HAGB LAGB
20 15 10 5 0 0
0.8
1.6
2.4
3.2 Strain
4
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5.6
Fig. 5. (a) Distribution of boundary misorientation angle for n¼ 1, n¼ 2, n¼ 3, n¼5, n¼ 7 and random; (b) evolution of the mean boundary misorientation angle during the ARB process depending on the boundary nature (LAGB or HAGB).
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In order to characterize the formation of ultrafine grains, the mean grain size together with the boundary misorientation angles were calculated during the ARB process. The grain size was defined as the number of grain boundary intercepts per line length, in the normal direction, where a grain boundary is defined with a minimum misorientation angle of 51. Fig. 4 illustrates the evolution of the mean grain size and the HAGB fraction as a function of ARB strain. The initial average grain size is around 15 mm. Then, from the early first cycle, a reduction of grain size appears together with a high amount of LAGB (drop of HAGB). The initial grains are replaced by a large area of subgrained structure due to the accommodation of the large imposed strain [20]. In subsequent cycles, the average grain size decreases, reaching a value of 580 nm for n¼7, and the fraction of HAGB increases up to a saturated value of 60% after 5 cycles which is still less than that obtained for ARB processed IF-steel (80%) [17]. The distribution of misorientation angles for different numbers of ARB cycles is presented in Fig. 5a. It is clear from these distributions that there is a continuous evolution towards HAGB with increasing number of cycle. From n ¼5, the distribution tends to become constant, with a persistent large fraction of LAGB (40%). As compared to the dotted line in Fig. 5a, which corresponds to the statistical prediction of a set of random orientations, there remains an excess of low angle boundaries even after 7 cycles. One can note in Fig. 5b that after n ¼5, the
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mean misorientation tends to saturate around 261–281. In the literature, it is assumed that the SPD microstructure is stable when the mean misorientation reaches about 351 [17]. In the present study, further ARB cycles may be needed to reach this value. Fig. 5b also shows that the mean misorientation of the LAGB is very quickly stabilized at 51 while the HAGB misorientation evolution gradually increases with strain. The increase in misorientation angle is the witness of dislocation accumulation in grain boundaries [21]. Comparing these results with the microhardness measurements in Fig. 2, it is obvious that the mechanism associated with the large increase of microhardness is the formation of sub-grains along with the augmentation of dislocation density (or LAGB fraction), straight observed after the first ARB cycle. This is consistent with previous works on other SPD techniques [9,10]. It remains to verify the metallurgical quality of the interfaces between the layers of the Armco iron sandwich processed by ARB. Fig. 6 illustrates that the welding-diffusion occurred by showing good grain continuity across interfaces between layers (Fig. 6c). However, locally, it remains defects (Fig. 6a-bottom) due to the sheet oxidation. 3.3. Texture Fig. 7 shows the evolution of microstructure during the ARB processing along with the grain orientations, for small zones
TD
ND
Fig. 6. (a) Welding-diffusion process after 4 ARB cycles through the mid thickness layer interface (dotted line); (b) and (c) Orientation maps of the {hkl}plane parallel to the rolling plane (color code in the stereographic triangle). (For interpretation of references to colour in this figure legend, the reader is referred to the web version of this article.)
n=0
n=2
n=5
n=7
ND TD
Fig. 7. Evolution of microstructure and crystallographic orientations for different ARB cycles: (a) and (e) n¼0, (b) and (f) n¼ 2, (c) and (g) n¼ 5, and (d) and (h) n¼ 7. Upper part: {hkl} maps (/hklSJND), lower part: /uvwS maps (/uvwSJRD). (For interpretation of references to colour in this figure, the reader is referred to the web version of this article.)
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of the measured EBSD maps in the ND–TD plane. For n ¼2–5, the grains show an elongated shape in the transverse direction which tends to be more equiaxed: the aspect ratio varies from 0.2 for n ¼2 to 0.47 for n¼ 5 ( 70.05). Then, the aspect ratio remains constant for n ¼7. From the analysis of these maps (Fig. 7a–d), qualitatively, it seems that, for the initial state and for n ¼2, the /hklS principal directions parallel to ND are /100S (red) and /111S (blue). For n ¼5 and n ¼7, the /100S direction clearly develops through the microstructure. On all these maps (Fig. 7e–h), the main crystallographic direction /uvwS is /110SJRD (green). Fig. 8 is a compilation of the ODF sections at j2 ¼ 451 for samples processed by ARB through 2, 3, 5 and 7 cycles measured at the mid-thickness of the specimen. The orthotropic sample symmetry was not imposed in order to check whether the shear
components were present in the mid-thickness of the samples. Fig. 8 also contains the ODF of the 94% cold-rolled state and shows a partial a-fiber between {100}/110S and {111}/110S with a strong rotated Cube component (Fig. 8a). After annealing at 973 K during 2 h, the ARB initial state n ¼0 is obtained, and a strong {311}/136S component, from the {h11}/1/h,1,2S fiber appears along with a g-fiber texture (/ 111SJND) (Fig. 8b). This {311}/136S component is characteristic of the recrystallized state of a highly cold-rolled BCC materials [22–24] due to an oriented nucleation from the strained rotated cube component that reveals internal orientation gradients at the deformed state. For the first cycles of ARB, the analyzed grain number is too low to calculate a statistical ODF (Fig. 8c and d, for n ¼2 and n¼3, respectively). However, a strong rotated Cube component ({001}
Fig. 8. j2 ¼ 451 section (01 o j1 o 3601, 01o F o901) of the ODFs for (a) the 94% cold-rolled state, before heat treatment, and (b) n¼ 0, (c) n ¼2, (d) n¼ 3, (e) n¼5 and (f) n¼7; (g) ideal components described in the ODF section assuming an orthotropic sample symmetry: j2 ¼ 451 (01 o j1 o901).
TD ND
Fig. 9. (a) Microstructure and (b) j2 ¼451 ODF section of sample surface after 3 ARB cycles.
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<110>//RD ± 15° (alpha-fiber) <111>//ND ± 15° (gamma-fiber)
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HAGB α - HAGB γ (%)
HAGB Fraction (%)
70
50 40 30 20 10 0
0
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1.6
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2.4 3.2 Strain
65
4
4.8
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Fig. 10. Evolution as a function of strain (a) HAGB fraction in a-fiber grains (/110SJRD) and g-fiber grains (/111SJND) and (b) difference of HAGB fraction between the a and the g fiber.
/110S), coming from the initial 94% cold-rolled state, clearly appears along with the a-fiber (/110SJRD) that replaces the {h11}/1/h,1,2S fiber. After 7 ARB cycles (e7 ¼5.8, Fig. 8f), the texture is mainly composed of fibers /001SJND, /111SJND and /110SJRD with a strengthening of the rotated Cube and the {111}/110S components which is actually particularly interesting for magnetic applications [23]. Let us note that after hot rolling in the ferrite region of an ultra-low carbon steel, Re´gle´ et al. [25] have shown also that the texture is characterized by a partial a fiber from {100}/110S to {111}/110S, with a strengthening of these two components, a complete g fiber and a Goss component which is not found in the present work. No shear texture (/110SJND) is observed in the specimen mid-thickness even for highly ARB strained samples. Nevertheless, a shear texture can be measured at the sample surface as shown in Fig. 9 for n¼3, as already reported by Kamikawa et al. [17].
4. Discussion The miscrostructure of the ARB processed ARMCO iron is composed of misoriented ultrafine grains of 580 nm after 7 cycles. As described by Gazder et al. [20] for IF-steel during ECAE, the microstructural evolution can be divided into two stages. The first one, for n o5 (e5 ¼4), corresponds to a rapid refinement of grain size together with an increase of the fraction of HAGB. The second one, for n 45, presents a flattening of the fraction of HAGB and of the grain refinement. During the first stage, the increase in misorientation is due to the production of deformation-induced HAGB by grain splitting into primary elongated grains, as well as by accumulation of dislocations in boundaries. During the second stage, the subgrain walls act as further barriers to dislocation propagation and as deformation proceeds, subgrains rotate to the nearest stable end-orientation which leads to a flattening of the HAGB fraction. ARB technique allows the formation of an almost equiaxed structure with more than 60% of HAGB. In comparison, results obtained by Ivanisenko et al. [26], for High Pressure Torsion in Armco iron show also an equiaxed structure but with only 50% of HAGB after applying a very high strain (g ¼210). The HAGB fraction of 60% is still lower than that found for ARB processed IF-steel (80%) [17], and the mean misorientation saturates at 281 while 351 is obtained in the IF-steel. This suggests that a more misoriented structure could be obtained by increasing the number of ARB cycles, even if the grain size remains constant. Such an ultra-fine grained microstructure could then be stable during further annealing since a dominant HAGB network inhibits the discontinuous recrystallization upon annealing [27]. As far as texture is concerned, the absence of the shear components suggests that the shear texture developed at the
sample surface at the ARB cycle n is destroyed by the subsequent rolling cycle (plane-strain deformation) in the sample center, this is in agreement with what was reported in ARB processed aluminum alloys [28]. To study the effect of crystallographic orientation on the formation of HAGB, the evolution of HAGB fraction has been calculated both for /110SJRD and /111SJND grains orientation, respectively a and g fibers classically observed in BCC materials. The results are presented in Fig. 10a. It appears that as strain proceeds, the HAGB are produced in higher fraction in the a-fiber than in the g-fiber. It is interesting to note that the gap between the HAGB fraction in the a-fiber and in the g-fiber increases with strain (Fig. 10b). This is due to the gradual rotation of the a-fiber components towards two specific components: the rotated Cube and the {111}/110S orientation. This is at the origin of HAGB formation inside the a-fiber that becomes mainly composed, as strain increases, of two components: {111}/011S and {100}/ 011S misorientated by 551. On the contrary, in the g-fiber, even if the {111}/112S component strengthens, a large range of orientations persists, which leads to higher LAGB fraction compared to the a-fiber.
5. Conclusion Applying 7 ARB cycles to an Armco iron sample results in an almost equiaxed structure with a large saturated fraction of HAGB (60%) and a grain size of about 580 nm. The early formation of a subgrained structure allows a great raise of the microhardness, of around 3 times the initial value, after only one ARB cycle. Then, the hardness value quickly saturates around 280 HV. The study of the texture evolution as a number of ARB cycles shows the replacement of the {h11}/1/h,1,2S annealing fiber by the a-fiber. After n ¼2, the rotated Cube and {111}/110S components develop to the detriment of the other orientations of the a-fiber and this provides the misorentation increase inside the a-fiber in comparison with the g fiber. The enhancement of the rotated Cube component is particularly helpful for magnetic applications. No shear texture is formed at the sample center due to the alternated plane strain and shear strain deformations.
Acknowledgments This study has been realized in the framework of the ANRs ‘program blanc’, ‘MAEL’ project, ref. BLAN08-2_367373. The authors thank P. Franciosi and M.H. Chavanne (Laboratoire des Proprie´te´s Me´caniques et Thermodynamique des Mate´riaux de Villetaneuse, LPMTM, Universite´ Paris 13) for the elaboration of the cold-rolled Armco iron.
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