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Strengthening mechanisms in nanostructured interstitial free steel deformed to high strain
Materials Science & Engineering A 639 (2015) 656–662
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Strengthening mechanisms in nanostructured interstitial free steel deformed to high strain Roohollah Jamaati n, Mohammad Reza Toroghinejad, Sajjad Amirkhanlou, Hossein Edris a
Department of Mechanical Engineering, Babol Noshirvani University of Technology, Babol, Iran Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran c Young Researchers and Elite Club, Najafabad Branch, Islamic Azad University, Najafabad, Iran d Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran b
art ic l e i nf o
a b s t r a c t
Article history: Received 20 February 2015 Received in revised form 14 May 2015 Accepted 23 May 2015 Available online 2 June 2015
In this study, the strengthening mechanisms in nanostructured IF steel deformed to high strain by fourlayer accumulative roll bonding (ARB) process at room temperature in absence and presence of SiC particles were investigated. Microstructural observations were performed by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM). The results indicated that the average grain size of the pure steel, composite, and nanocomposite was 95, 73, and 55 nm, respectively and the microstructures consisted of equiaxed grains. Also, with increasing the number of ARB cycles, the dislocation density of samples increased. The first cycle of ARB process had remarkable effect on the dislocation density. The value of dislocation density first rapidly increased, then dwindled, and finally saturated by further ARB cycles. The presence of SiC microparticles and nanoparticles in the IF steel matrix increased the dislocation density during the ARB process. On the other hand, dislocation density of the nanocomposite was higher than that of the composite. After first cycle, a significant increase observed in the yield strength, from 84 MPa to 609, 682, and 689 MPa for pure steel, composite, and nanocomposite, respectively, which was almost 7.3, 8.1, and 8.2 times greater than that of the initial sample. There was no perfect saturation in yield strength of the pure IF steel with increasing the number of ARB cycles. Finally, the contribution of individual mechanisms such as the grain refinement, dislocation, second phase, and precipitation in strengthening of the IF steel was evaluated. The contribution of grain refinement and precipitation to the improvement in yield strength was maximum ( 67–72%) and minimum ( 3.1–3.7%), respectively. & 2015 Elsevier B.V. All rights reserved.
Keywords: Nanostructured materials Strengthening mechanisms Accumulative roll bonding
1. Introduction Strong materials are a classical goal for materials research and development. Today there is focus on nanostructured metals since they are found to have a very high strength as well as other excellent mechanical properties [1–5]. According to the well-known Hall Petch equation, the strength increases with a reduction in the grain size. Therefore, fabricating materials with a grain size in the nanorange (less than 100 nm) or ultrafine range (between 100 and 1000 nm) is an effective approach to increase the strength of materials [1–7]. Nanostructured metals can be processed by a number of different techniques and one promising method is to apply plastic deformation to very high strains [1–7]. The processing of metals through the application of severe plastic n
Corresponding author. E-mail addresses: [email protected], [email protected], [email protected] (R. Jamaati). http://dx.doi.org/10.1016/j.msea.2015.05.075 0921-5093/& 2015 Elsevier B.V. All rights reserved.
deformation (SPD) provides the potential for achieving exceptional grain refinement in bulk metal solids [1–7]. There are techniques for producing nanostructure and ultrafine grain (UFG) materials such as equal-channel angular pressing (ECAP), high-pressure torsion (HPT), and accumulative roll bonding (ARB) [1–7]. The ARB process is one of the SPD that was proposed by Saito et al. [8,9]. The ARB consists of multiple cycles of stacking, rolling and cutting in which large strains are imposed into the material without any change in the cross section [8,9]. During ARB, the sample is assumed to be deformed in a plain strain condition. Therefore, the equivalent strain εeq, can be calculated using the following equation [4]:
εeq =
h 2 2 1 n ln 0 = n ln h 1−r 3 3
(1)
where h0 is the initial thickness of the stacked sheets, h is the thickness after roll bonding, r is the reduction in thickness per cycle and n is the number of ARB cycles. The reduction per ARB
R. Jamaati et al. / Materials Science & Engineering A 639 (2015) 656–662
cycle is usually r ¼50%, which results in an equivalent strain of about 0.8 per cycle. The evolution of microstructures and the related mechanical properties during ARB process were studied for several metals such as commercial pure Al [4,8–12], Cu [13–15], Brass [16], Ti [17], Mg [18,19], and steel [20–23]. The ARB process is also applicable in the fabrication of multilayered composites [24–26] and metal matrix composites (MMCs) [27–33]. The density of dislocations is always difficult to determine experimentally. Traditionally there are some methods such as X-ray diffraction (XRD) [34–37], transmission electron microscopy (TEM) [38,39], electron backscatter diffraction (EBSD) [40,41], electron channeling contrast imaging [42], and hydrogen diffusivities [43]. Recently, the dislocation density has been estimated from hardness measurement [44,45]. Graca et al. [44] reported that it is possible to estimate the dislocation densities by the hardness indentation size using the Nix–Gao model [45]. In present study, the four-layer ARB process has been utilized at room temperature in presence of reinforcement particles to refine the grain size of interstitial free (IF) steel matrix. For a comparison, unreinforced pure IF steel was processed and characterized mechanically and microstructurally using the same methods. To quantify the microstructure, scanning transmission electron microscopy (STEM) is applied for fine-scale structures and scanning electron microscopy (SEM) is used for coarser-scale structures. Mechanical properties are determined by tensile testing at room temperature and are related to the structural parameters through the strength–structure relationship, which is discussed based on the operation of different strengthening mechanisms.
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Fig. 1. SEM micrograph of the SiC nanoparticles.
2. Experimental procedure The materials used in this study were fully annealed sheets of interstitial free steel (specifications are given in Table 1) and SiC microparticles (50 mm) and nanoparticles (50 nm) (Fig. 1). Four sheets of 150 mm 50 mm 0.7 mm were degreased via acetone and scratch brushed with a stainless steel wire brush 0.25 mm in diameter. For fabrication of steel-based composite (containing microparticles) and nanocomposite (containing nanoparticles), after the surface preparation, SiC micro/nano-particles were uniformly dispersed between the four sheets. To achieve a uniform dispersion of SiC particles between IF steel sheets, an acetonebased suspension was prepared. After surface preparation, the SiC particles in acetone were sprayed between the four sheets with an atomizer. Then, SiC particles were deposited and the acetone evaporated in air, so that the brushed surfaces of sheets were uniformly covered with SiC particles. The sheets were then stacked over each other and fastened at both ends by steel wires. Attention was also paid to proper alignment of the four sheets surfaces prior to rolling. The roll bonding process was carried out with no lubrication and with an amount of thickness reduction equal to 75% corresponding to a von Mises equivalent strain evM of 1.6 per cycle (first step). Then, the roll bonded sheets were cut into four pieces. Then, to achieve a uniform distribution of SiC particles in the IF steel matrix, the above procedure was repeated again up to fourth cycle without adding particles (second step). The schematic illustration of the ARB process for fabrication of steel-based composite and nanocomposite samples is shown in Fig. 2. Table 1 Chemical composition of IF steel (wt%). C
N
Si
Mn
Cu
Ni
Ti
Fe
0.002
0.004
0.01
0.14
0.01
0.018
0.055
Bal.
Fig. 2. Schematic illustration of ARB process.
The samples for scanning electron microscopy (SEM) observations were cut from the sheets and this was mounted in bakelite. Then, the samples were polished using 80–4000 grit water-proof SiC paper. Finally, the polishing was finished on a cloth using alumina paste of 3 mm. Scanning electron microscopy PHILIPS XL30 was used. The microstructural observations were performed using scanning transmission electron microscopy (STEM). Thin foils were prepared with electropolishing conducted at 30 °C using 60 V in a 5% perchloric acid/95% methanol solution. Multiple disc samples with 3 mm diameter were separated from thin foils. Then, the discs were prepared using a low angle Ion Milling System from Fischione Model 1010 with 5 kV operating voltage, 5 mA current, 2.5 h time duration, and angle of 10° conducted at 40 °C. A Hitachi S-4800 field emission scanning electron microscope was used. Vickers microhardness was measured according to the ASTM: E384-11e1 standard. The surfaces used for indentation testing were ground with SiC papers and polished with a sequence of alumina particles suspensions. Vickers indentation tests were performed using loads in the range 0.01– 2 N. The tensile test samples were machined from the ARB-processed sheets, according to the ASTM: E8M tensile sample,
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oriented along the rolling direction. The gauge width and length of the tensile test samples were 6 and 25 mm, respectively. The tensile tests were conducted at room temperature on a Hounsfield H50KS testing machine at an initial strain rate of 1.67 10 4 s 1.
3. Results and discussion One of the major microstructural features of influence in the strengthening of metals is grain size. The microstructure of the initial sample is shown in Fig. 3. This figure indicates that the initial grain size before deformation is about 25 μm. Fig. 4 demonstrates STEM microstructure of the pure steel, composite, and nanocomposite fabricated by ARB process after the final cycle in RD–TD plane. Average grain size of the pure steel, composite, and nanocomposite is 95, 73, 55 nm, respectively and the microstructures consist of equiaxed grains. Therefore, the grain size decreases into the nanostructured domain during ARB process, reaching grain sizes of below 100 nm. It should be noted that the final grain size of the ARB-processed samples decreases with the addition of silicon carbide microparticles and nanoparticles compared to the unreinforced IF steel. This matter can be attributed to the following two main reasons. Firstly, the particulates may induce recrystallization of the IF steel matrix through particle stimulation of nucleation (PSN) mechanism at the interfaces of particles and the IF steel matrix. Secondly, the added reinforcement particles could restrict the grain growth of new recrystallized grains formed during the ARB process. Another major microstructural features of effect in the strengthening of metals is dislocation density. In order to quantitative investigation of dislocation density variations during ARB process, a method based on the application of an ISE model [45] is used. This method developed by Graca et al. [44] which can determines the dislocation density of a metal from microhardness measurements. They deduced a simple expression to relate hardness with indentation depth [44]:
⎛ H ⎞2 ⎛ 1⎞ ⎜ ⎟ = 1 + h*⎜ ⎟ ⎝h⎠ ⎝ H0 ⎠
(2)
where H0 is the hardness in the limitation of infinite depth (bulk hardness) and h* is a characteristic length and H is the hardness value corresponding to indentation depth h. The correlation between the dislocation density stored in the sample and the h* is expressed as follows [45]:
Fig. 4. STEM micrographs from RD–TD plane after final cycle for: (a) pure steel, (b) composite, and (c) nanocomposite.
ρ=
Fig. 3. Microstructure of the initial sample.
3 1 tan2θ 2 f 3 bh*
(3)
where θ is the angle between the surface of the sheet and the indenter, b is the burgers vector of the dislocation and f is a correction factor for the size of the plastic zone. It should be noted that the indentation depth, h, was calculated by [46,47]
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Fig. 5. The hn values for ARB-processed samples calculated by Eq. (2).
h=
d 2 2 tan
θ 2
(4)
where d is the indentation diameter. In this research θ ¼22°, f¼ 1.9, and b¼ 0.248 nm [48,49]. Introducing these values in Eq. (3) a dislocation density is obtained. Fig. 5 shows the value of h* in the ARB-processed samples through 0, 1, 2, 3, and 4 cycles which are determined by fitting the Eq. (2) to experimental data. The dislocation density of samples versus number of cycles calculated from Eq. (3) is presented in Fig. 6. It is clear that with increasing the number of ARB cycles, the dislocation density of samples increases. The dislocation density increased from 2.02 109 cm 2 (for the initial sample) to 6.53 109 cm 2 for pure steel, 6.91 109 cm 2 for composite, and 7.01 109 cm 2 for nanocomposite, respectively, registering 223%, 242%, and 247% increase. It can be concluded that the first cycle of ARB process has significant effect on the dislocation density. The dislocation density increased continually until the dislocation density of the pure steel, composite and nanocomposite after fourth cycle was about 4.7 times (9.47 109 cm 2), 5.3 times (10.73 109 cm 2),
Fig. 6. The dislocation densities for ARB-processed samples calculated by Eq. (3) based on hn values of Fig. 5.
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and 6.1 times (12.29 109 cm 2) higher than that of the initial sample, respectively. Regarding Fig. 6, the value of dislocation density first rapidly increased, then dwindled, and finally saturated by further ARB cycles. The saturation of dislocation density occurs because the material reached to the steady-state density of dislocation. The steady-state density of dislocations is attributed to dynamic balance between dislocation generation during ARB process and annihilation by occurrence of dynamic restoration phenomena. It can be concluded that, the ARB process leads to a remarkable increase in the dislocation density of the material. In addition, a further enhancement in dislocation density can be achieved by adding the second phase particles in the material. In fact, the presence of SiC microparticles and nanoparticles in the IF steel matrix increases the dislocation density during the ARB process because these particles act as obstacles for dislocation movement. On the other hand, dislocation density of the nanocomposite is higher than that of the composite. The enhanced dislocation density with decreasing particle size is a result of the larger amount of particle/ matrix interfacial area, which results in a higher amount of processing-induced dislocation during the ARB process. During the ARB process, the nanoparticles increase dislocation density in the interfaces much more than microparticles due to strain incompatibility between matrix and reinforcement, and difference in thermal expansion between IF steel and SiC particles after the ARB process and during cooling. In addition, the presence of the SiC nanoparticles in the IF steel matrix during the ARB process can also activate other strengthening mechanisms such as the Orowan strengthening mechanism in the matrix (Fig. 7). The variations of yield strength for pure steel, composite and nanocomposite versus the number of cycles are shown in Fig. 8. After first cycle, a significant increase can be observed in the yield strength, from 84 MPa to 609, 682, and 689 MPa for pure steel, composite, and nanocomposite, respectively, which is almost 7.3, 8.1, and 8.2 times greater than that of the initial sample. The significant increase in yield strength at the first cycle can be attributed to strain hardening and formation of subgrains. After final cycle, the yield strength value of pure steel, composite, and nanocomposite increased to 909, 1061, and 1189 MPa, respectively. It can be said that that yield strength variations in the ARB-processed materials are governed by two main strengthening mechanisms: strain hardening and grain refinement. In the early stages of ARB process, strain hardening plays a main role in increasing the yield strength, while the formation of subgrains or dislocation cells also contributes to strength. But at higher cycles, higher yield strength is achieved by grain refinement as the ARB
Fig. 7. The Orowan loops in the IF steel microstructure.
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estimated on the basis of a simple rule-of-mixtures [50–52]:
σ = σm(1 − f ) + σrf
Fig. 8. Variation of yield strength in ARB-processed samples for different cycles.
cycles increase. This result is consistent with the discussion of Figs. 4 and 6. It can be also found that with the addition of SiC microparticles and nanoparticles, the yield strength of the composite and nanocomposite are enhanced compared with the pure steel. The grains of IF steel matrix in the composite and nanocomposite samples were finer than that of pure steel sample after ARB process as shown in Fig. 4. Therefore, according to Hall–Petch equation, the yield strength of composite and nanocomposite increases. In addition, the grain size of matrix in the nanocomposite is smaller than that of composite with the same ARB process resulting in higher yield strength. There is one important point that should be considered. In the present work, unlike the dislocation density, there is no perfect saturation in yield strength of the pure IF steel with increasing the number of ARB cycles. This is attributed to pinning effect of oxide particles which forms after every ARB cycle. According to Fig. 2, after each cycle, a large number of interfaces including oxide film are introduced. These films break and fragment during the ARB process and transform to oxide particles. The oxide particles act as obstacles for grain boundary and dislocation motion and therefore lead to dislocations accumulation and increasing the yield strength even at high number of ARB cycles. It should be noted that the SiC microparticles and nanoparticles also have an outstanding effect on the tensile behavior of samples. The onset of plastic deformation in the composite and nanocomposite represents a deformation mechanism of dislocation activity, such as the dislocation accumulation, the dislocation multi-plication and the interactions of dislocations in the primary slip system. Thus, the SiC microparticles and nanoparticles would hinder the dislocations slip during tensile process. There are two different strengthening mechanisms that are typically associated with composite and nanocomposite. First, direct strengthening resulting from load transfer from the IF steel matrix to the reinforcing particles and indirect strengthening resulting from the influence of reinforcement particles on the IF steel matrix microstructure or deformation mode, such as dislocation strengthening induced by the deformation mismatch between the reinforcement and the matrix. The steel/SiC interface plays an important role in determining the mechanical response of the composite and nanocomposite, and its effect can be examined in the context of a load transfer mechanism. Under these conditions, the contribution of the microstructure to the mechanical properties can be
(5)
where sm is the yield strength of the metal matrix, sr is the yield strength of the reinforcement phase, and f is the volume fraction of the reinforcement phase. In the present work s¼ 1061 MPa (for composite) and 1189 MPa (for nanocomposite), f ¼0.02, and sr ¼3440 MPa [53,54]. Therefore, the yield strength of the IF steel matrix (sm) is calculated as 1012.5 MPa (for composite) and 1143.1 MPa (for nanocomposite). It should be noted that the contribution of reinforcement microparticles and nanoparticles to the improvement in yield strength of the IF steel matrix through its direct strengthening effect is equal to 68.8 MPa. On the other hand the IF steel matrix can be strengthened by three different mechanisms including grain refinement strengthening, dislocation strengthening, and precipitation strengthening. It is clear that sm must be a direct consequence of the aforementioned strengthening effects. With this in mind, an attempt has been made to separate the influence of certain parameters on strengthening of composites. The contribution of various strengthening mechanisms to the improvement in sm is discussed as follows. sm can be estimated by the following equation:
σm = σ0 + Δσg + Δσdis + Δσp
(6)
where s0 is the yield strength of undeformed pure single-crystal IF steel (s0 ¼ 30 MPa [48]), and Δsg, Δsdis, and Δsp are the increases in the yield strength caused by grain refinement, dislocation, and precipitation, respectively. The contribution of the grain refinement Δsg is first evaluated. Δsg can be calculated by the following equation [48,49]: 1
Δσg = kd− 2
(7)
where k is the Hall–Petch slope (k¼ 200 MPa μm0.5 for IF steel [49]) and d is the average grain size (95 nm for pure steel, 73 nm for composite, and 55 nm for nanocomposite). The contribution of grain refinement to the improvement in yield strength of the samples is then calculated as 649 MPa for pure steel, 707.1 MPa for composite, and 852.8 MPa for nanocomposite. Next, the contribution of dislocation strengthening is analyzed. Δsdis can be calculated by the following equation [48,49]: 1
Δσdis = MαGbρ 2
(8)
where M ¼3.2 [49] is the Taylor factor, α ¼0.33 [48] is a constant, G ¼77 GPa [49] is shear modulus, b ¼0.248 nm [48] is Burgers vector, and ρ is the dislocation density (9.47 109 cm 2 for pure steel, 10.73 109 cm 2 for composite, and 12.29 109 cm 2 for nanocomposite). The contribution of dislocation strengthening to the improvement in yield strength of the IF steel matrix is then calculated as 196 MPa for pure steel, 208.9 MPa for composite, and 223.6 MPa for nanocomposite. It should be noted that the Δsdis of samples depends on thermal expansion mismatch, Orowan mechanism, and other mechanisms:
Δσdis = ΔσCTE + ΔσOr + Δσother
(9)
Thermal mismatch between silicon carbide microparticles and nanoparticles and the IF steel matrix can result in internal stresses. These internal stresses are the generator of dislocations leading to increase of dislocation density in the IF steel matrix. Arsenault and Shi [55] developed the model describing the density of dislocations formed due to thermal mismatch between reinforcement particles and the matrix. Dislocation density generated due to thermal mismatch (ρCTE) is given by [55,56]:
R. Jamaati et al. / Materials Science & Engineering A 639 (2015) 656–662
ρCTE =
12ΔT ΔαVp bdp
(10)
where b is Burgers vector (b¼0.248), dp is diameter of SiC microparticles (50 mm) and nanoparticles (50 nm), Vp is the volume fraction of reinforcement particles (Vp ¼0.02), ΔT is the temperature difference (ΔT¼80 K, considering the temperature rising during ARB process was 80 K), and Δα is the difference in coefficient of thermal expansion between the IF steel matrix and the SiC particles (Δα ¼ αSteel αSiC ¼11 10 6 K 1 4.5 10 6 K 1 ¼6.5 10 6 K 1). The ρCTE is then calculated as 1.02 106 cm 2 for composite and 1.02 109 cm 2 for nanocomposite. According to Eq. (8), the contribution of thermal expansion mismatch to the improvement in dislocation strengthening of the IF steel matrix is then determined as 2.1 MPa for composite and 64.4 MPa for nanocomposite. The interaction of dislocations with the non-sharable nanoprecipitates increases the dislocation density of steel-based nanocomposite according to the Orowan mechanism. Owing to the presence of the dispersed nanocarbides and nanonitrides in the IF steel matrix, dislocation loops form as dislocation lines and bypass the particles (Fig. 7). In fact, since Orowan loops are present around the nanoprecipitates, there would be contributions from the Orowan strengthening effect. ΔsOr can be calculated as [56,57]
ΔσOr =
661
dpr 0.13Gb ln λ 2b
(11)
where G ¼77 GPa, b¼ 0.248 nm, dpr is diameter of nanoprecipitates (dpr ¼20 nm), and is the inter-particle spacing that is expressed as [56] 1 ⎤ ⎡ ⎥ ⎢⎛ 1 ⎞ 3 ⎟⎟ − 1⎥ λ ≈ dpr ⎢⎜⎜ 2Vpr ⎠ ⎝ ⎥⎦ ⎢⎣
(12)
where dpr ¼ 20 nm and Vpr is the volume fraction of nanoprecipitates (Vpr ¼10 4). The λ is then calculated as 342 nm. According to Eq. (11), the contribution of Orowan mechanism to the improvement in dislocation strengthening of the IF steel matrix is then determined as 26.8 MPa for pure steel, composite and nanocomposite. Based on 196, 208.9, and 223.6 MPa of the total contributions of dislocation strengthening for pure steel, composite and nanocomposite, respectively, and according to Eq. (9), the contribution of other mechanisms to the improvement in yield strength of samples is then calculated as 169.2 MPa (for pure steel), 180 MPa (for composite), and 132.4 MPa (for nanocomposite). Finally, the contribution of precipitation strengthening of nanocarbides and nanonitrides is determined. Based on 909 MPa (for pure steel), 1012.5 MPa (for composite), and 1143.1 MPa (for nanocomposite) of the total yield strength of the IF steel matrix and according to Eq. (6), the contribution of precipitation to the improvement is then calculated as 34, 33.4, and 37.3 MPa for pure steel, composite, and nanocomposite, respectively. Fig. 9 shows the contribution of various strengthening mechanisms to the improvement in yield strength of ARB-processed samples. As seen, the contributions of different strengthening methods are different. The contribution of grain refinement and precipitation to the improvement in yield strength is maximum ( 67–72%) and minimum ( 3.1–3.7%), respectively. It can be concluded that the predominant method for the improvement of IF steel strength is the grain refinement.
4. Conclusions In the present work, the strengthening mechanisms in nanostructured IF steel deformed to high strain by four-layer
Fig. 9. Contribution of strengthening mechanisms to the improvement in yield strength of ARB-processed samples.
accumulative roll bonding (ARB) process at room temperature in absence and presence of SiC particles were investigated. The conclusions drawn from the results can be summarized as follows: (1) The average grain size of the pure steel, composite, and nanocomposite was 95, 73, and 55 nm, respectively and the microstructures consisted of equiaxed grains. (2) With increasing the number of ARB cycles, the dislocation density of samples increased. (3) After first cycle, the dislocation density increased from 2.02 109 cm 2 (for the initial sample) to 6.53 109 cm 2 for pure steel, 6.91 109 cm 2 for composite, and 7.01 109 cm 2 for nanocomposite, respectively, registering 223%, 242%, and 247% increase. The first cycle of ARB process had significant effect on the dislocation density. (4) The dislocation density increased continually until the dislocation density of the pure steel, composite and nanocomposite after fourth cycle was about 4.7 times (9.47 109 cm 2), 5.3 times (10.73 109 cm 2), and 6.1 times (12.29 109 cm 2) higher than that of the initial sample, respectively. (5) The value of dislocation density first rapidly increased, then dwindled, and finally saturated by further ARB cycles. (6) The presence of SiC microparticles and nanoparticles in the IF steel matrix increased the dislocation density during the ARB process because these particles acted as obstacles for dislocation movement. On the other hand, dislocation density of the nanocomposite was higher than that of the composite. (7) After first cycle, a remarkable increase observed in the yield strength, from 84 MPa to 609, 682, and 689 MPa for pure steel, composite, and nanocomposite, respectively, which was almost 7.3, 8.1, and 8.2 times greater than that of the initial sample. After final cycle, the yield strength value of pure steel, composite, and nanocomposite increased to 909, 1061, and 1189 MPa, respectively. (8) There was no perfect saturation in yield strength of the pure IF steel with increasing the number of ARB cycles. (9) The contribution of grain refinement and precipitation to the improvement in yield strength was maximum (67–72%) and minimum ( 3.1–3.7%), respectively. The predominant method for the improvement of IF steel strength was the grain refinement.
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Strengthening mechanisms in nanostructured interstitial free steel deformed to high strain Roohollah Jamaati n, Mohammad Reza Toroghinejad, Sajjad Amirkhanlou, Hossein Edris a
Department of Mechanical Engineering, Babol Noshirvani University of Technology, Babol, Iran Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran c Young Researchers and Elite Club, Najafabad Branch, Islamic Azad University, Najafabad, Iran d Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran b
art ic l e i nf o
a b s t r a c t
Article history: Received 20 February 2015 Received in revised form 14 May 2015 Accepted 23 May 2015 Available online 2 June 2015
In this study, the strengthening mechanisms in nanostructured IF steel deformed to high strain by fourlayer accumulative roll bonding (ARB) process at room temperature in absence and presence of SiC particles were investigated. Microstructural observations were performed by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM). The results indicated that the average grain size of the pure steel, composite, and nanocomposite was 95, 73, and 55 nm, respectively and the microstructures consisted of equiaxed grains. Also, with increasing the number of ARB cycles, the dislocation density of samples increased. The first cycle of ARB process had remarkable effect on the dislocation density. The value of dislocation density first rapidly increased, then dwindled, and finally saturated by further ARB cycles. The presence of SiC microparticles and nanoparticles in the IF steel matrix increased the dislocation density during the ARB process. On the other hand, dislocation density of the nanocomposite was higher than that of the composite. After first cycle, a significant increase observed in the yield strength, from 84 MPa to 609, 682, and 689 MPa for pure steel, composite, and nanocomposite, respectively, which was almost 7.3, 8.1, and 8.2 times greater than that of the initial sample. There was no perfect saturation in yield strength of the pure IF steel with increasing the number of ARB cycles. Finally, the contribution of individual mechanisms such as the grain refinement, dislocation, second phase, and precipitation in strengthening of the IF steel was evaluated. The contribution of grain refinement and precipitation to the improvement in yield strength was maximum ( 67–72%) and minimum ( 3.1–3.7%), respectively. & 2015 Elsevier B.V. All rights reserved.
Keywords: Nanostructured materials Strengthening mechanisms Accumulative roll bonding
1. Introduction Strong materials are a classical goal for materials research and development. Today there is focus on nanostructured metals since they are found to have a very high strength as well as other excellent mechanical properties [1–5]. According to the well-known Hall Petch equation, the strength increases with a reduction in the grain size. Therefore, fabricating materials with a grain size in the nanorange (less than 100 nm) or ultrafine range (between 100 and 1000 nm) is an effective approach to increase the strength of materials [1–7]. Nanostructured metals can be processed by a number of different techniques and one promising method is to apply plastic deformation to very high strains [1–7]. The processing of metals through the application of severe plastic n
Corresponding author. E-mail addresses: [email protected], [email protected], [email protected] (R. Jamaati). http://dx.doi.org/10.1016/j.msea.2015.05.075 0921-5093/& 2015 Elsevier B.V. All rights reserved.
deformation (SPD) provides the potential for achieving exceptional grain refinement in bulk metal solids [1–7]. There are techniques for producing nanostructure and ultrafine grain (UFG) materials such as equal-channel angular pressing (ECAP), high-pressure torsion (HPT), and accumulative roll bonding (ARB) [1–7]. The ARB process is one of the SPD that was proposed by Saito et al. [8,9]. The ARB consists of multiple cycles of stacking, rolling and cutting in which large strains are imposed into the material without any change in the cross section [8,9]. During ARB, the sample is assumed to be deformed in a plain strain condition. Therefore, the equivalent strain εeq, can be calculated using the following equation [4]:
εeq =
h 2 2 1 n ln 0 = n ln h 1−r 3 3
(1)
where h0 is the initial thickness of the stacked sheets, h is the thickness after roll bonding, r is the reduction in thickness per cycle and n is the number of ARB cycles. The reduction per ARB
R. Jamaati et al. / Materials Science & Engineering A 639 (2015) 656–662
cycle is usually r ¼50%, which results in an equivalent strain of about 0.8 per cycle. The evolution of microstructures and the related mechanical properties during ARB process were studied for several metals such as commercial pure Al [4,8–12], Cu [13–15], Brass [16], Ti [17], Mg [18,19], and steel [20–23]. The ARB process is also applicable in the fabrication of multilayered composites [24–26] and metal matrix composites (MMCs) [27–33]. The density of dislocations is always difficult to determine experimentally. Traditionally there are some methods such as X-ray diffraction (XRD) [34–37], transmission electron microscopy (TEM) [38,39], electron backscatter diffraction (EBSD) [40,41], electron channeling contrast imaging [42], and hydrogen diffusivities [43]. Recently, the dislocation density has been estimated from hardness measurement [44,45]. Graca et al. [44] reported that it is possible to estimate the dislocation densities by the hardness indentation size using the Nix–Gao model [45]. In present study, the four-layer ARB process has been utilized at room temperature in presence of reinforcement particles to refine the grain size of interstitial free (IF) steel matrix. For a comparison, unreinforced pure IF steel was processed and characterized mechanically and microstructurally using the same methods. To quantify the microstructure, scanning transmission electron microscopy (STEM) is applied for fine-scale structures and scanning electron microscopy (SEM) is used for coarser-scale structures. Mechanical properties are determined by tensile testing at room temperature and are related to the structural parameters through the strength–structure relationship, which is discussed based on the operation of different strengthening mechanisms.
657
Fig. 1. SEM micrograph of the SiC nanoparticles.
2. Experimental procedure The materials used in this study were fully annealed sheets of interstitial free steel (specifications are given in Table 1) and SiC microparticles (50 mm) and nanoparticles (50 nm) (Fig. 1). Four sheets of 150 mm 50 mm 0.7 mm were degreased via acetone and scratch brushed with a stainless steel wire brush 0.25 mm in diameter. For fabrication of steel-based composite (containing microparticles) and nanocomposite (containing nanoparticles), after the surface preparation, SiC micro/nano-particles were uniformly dispersed between the four sheets. To achieve a uniform dispersion of SiC particles between IF steel sheets, an acetonebased suspension was prepared. After surface preparation, the SiC particles in acetone were sprayed between the four sheets with an atomizer. Then, SiC particles were deposited and the acetone evaporated in air, so that the brushed surfaces of sheets were uniformly covered with SiC particles. The sheets were then stacked over each other and fastened at both ends by steel wires. Attention was also paid to proper alignment of the four sheets surfaces prior to rolling. The roll bonding process was carried out with no lubrication and with an amount of thickness reduction equal to 75% corresponding to a von Mises equivalent strain evM of 1.6 per cycle (first step). Then, the roll bonded sheets were cut into four pieces. Then, to achieve a uniform distribution of SiC particles in the IF steel matrix, the above procedure was repeated again up to fourth cycle without adding particles (second step). The schematic illustration of the ARB process for fabrication of steel-based composite and nanocomposite samples is shown in Fig. 2. Table 1 Chemical composition of IF steel (wt%). C
N
Si
Mn
Cu
Ni
Ti
Fe
0.002
0.004
0.01
0.14
0.01
0.018
0.055
Bal.
Fig. 2. Schematic illustration of ARB process.
The samples for scanning electron microscopy (SEM) observations were cut from the sheets and this was mounted in bakelite. Then, the samples were polished using 80–4000 grit water-proof SiC paper. Finally, the polishing was finished on a cloth using alumina paste of 3 mm. Scanning electron microscopy PHILIPS XL30 was used. The microstructural observations were performed using scanning transmission electron microscopy (STEM). Thin foils were prepared with electropolishing conducted at 30 °C using 60 V in a 5% perchloric acid/95% methanol solution. Multiple disc samples with 3 mm diameter were separated from thin foils. Then, the discs were prepared using a low angle Ion Milling System from Fischione Model 1010 with 5 kV operating voltage, 5 mA current, 2.5 h time duration, and angle of 10° conducted at 40 °C. A Hitachi S-4800 field emission scanning electron microscope was used. Vickers microhardness was measured according to the ASTM: E384-11e1 standard. The surfaces used for indentation testing were ground with SiC papers and polished with a sequence of alumina particles suspensions. Vickers indentation tests were performed using loads in the range 0.01– 2 N. The tensile test samples were machined from the ARB-processed sheets, according to the ASTM: E8M tensile sample,
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oriented along the rolling direction. The gauge width and length of the tensile test samples were 6 and 25 mm, respectively. The tensile tests were conducted at room temperature on a Hounsfield H50KS testing machine at an initial strain rate of 1.67 10 4 s 1.
3. Results and discussion One of the major microstructural features of influence in the strengthening of metals is grain size. The microstructure of the initial sample is shown in Fig. 3. This figure indicates that the initial grain size before deformation is about 25 μm. Fig. 4 demonstrates STEM microstructure of the pure steel, composite, and nanocomposite fabricated by ARB process after the final cycle in RD–TD plane. Average grain size of the pure steel, composite, and nanocomposite is 95, 73, 55 nm, respectively and the microstructures consist of equiaxed grains. Therefore, the grain size decreases into the nanostructured domain during ARB process, reaching grain sizes of below 100 nm. It should be noted that the final grain size of the ARB-processed samples decreases with the addition of silicon carbide microparticles and nanoparticles compared to the unreinforced IF steel. This matter can be attributed to the following two main reasons. Firstly, the particulates may induce recrystallization of the IF steel matrix through particle stimulation of nucleation (PSN) mechanism at the interfaces of particles and the IF steel matrix. Secondly, the added reinforcement particles could restrict the grain growth of new recrystallized grains formed during the ARB process. Another major microstructural features of effect in the strengthening of metals is dislocation density. In order to quantitative investigation of dislocation density variations during ARB process, a method based on the application of an ISE model [45] is used. This method developed by Graca et al. [44] which can determines the dislocation density of a metal from microhardness measurements. They deduced a simple expression to relate hardness with indentation depth [44]:
⎛ H ⎞2 ⎛ 1⎞ ⎜ ⎟ = 1 + h*⎜ ⎟ ⎝h⎠ ⎝ H0 ⎠
(2)
where H0 is the hardness in the limitation of infinite depth (bulk hardness) and h* is a characteristic length and H is the hardness value corresponding to indentation depth h. The correlation between the dislocation density stored in the sample and the h* is expressed as follows [45]:
Fig. 4. STEM micrographs from RD–TD plane after final cycle for: (a) pure steel, (b) composite, and (c) nanocomposite.
ρ=
Fig. 3. Microstructure of the initial sample.
3 1 tan2θ 2 f 3 bh*
(3)
where θ is the angle between the surface of the sheet and the indenter, b is the burgers vector of the dislocation and f is a correction factor for the size of the plastic zone. It should be noted that the indentation depth, h, was calculated by [46,47]
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Fig. 5. The hn values for ARB-processed samples calculated by Eq. (2).
h=
d 2 2 tan
θ 2
(4)
where d is the indentation diameter. In this research θ ¼22°, f¼ 1.9, and b¼ 0.248 nm [48,49]. Introducing these values in Eq. (3) a dislocation density is obtained. Fig. 5 shows the value of h* in the ARB-processed samples through 0, 1, 2, 3, and 4 cycles which are determined by fitting the Eq. (2) to experimental data. The dislocation density of samples versus number of cycles calculated from Eq. (3) is presented in Fig. 6. It is clear that with increasing the number of ARB cycles, the dislocation density of samples increases. The dislocation density increased from 2.02 109 cm 2 (for the initial sample) to 6.53 109 cm 2 for pure steel, 6.91 109 cm 2 for composite, and 7.01 109 cm 2 for nanocomposite, respectively, registering 223%, 242%, and 247% increase. It can be concluded that the first cycle of ARB process has significant effect on the dislocation density. The dislocation density increased continually until the dislocation density of the pure steel, composite and nanocomposite after fourth cycle was about 4.7 times (9.47 109 cm 2), 5.3 times (10.73 109 cm 2),
Fig. 6. The dislocation densities for ARB-processed samples calculated by Eq. (3) based on hn values of Fig. 5.
659
and 6.1 times (12.29 109 cm 2) higher than that of the initial sample, respectively. Regarding Fig. 6, the value of dislocation density first rapidly increased, then dwindled, and finally saturated by further ARB cycles. The saturation of dislocation density occurs because the material reached to the steady-state density of dislocation. The steady-state density of dislocations is attributed to dynamic balance between dislocation generation during ARB process and annihilation by occurrence of dynamic restoration phenomena. It can be concluded that, the ARB process leads to a remarkable increase in the dislocation density of the material. In addition, a further enhancement in dislocation density can be achieved by adding the second phase particles in the material. In fact, the presence of SiC microparticles and nanoparticles in the IF steel matrix increases the dislocation density during the ARB process because these particles act as obstacles for dislocation movement. On the other hand, dislocation density of the nanocomposite is higher than that of the composite. The enhanced dislocation density with decreasing particle size is a result of the larger amount of particle/ matrix interfacial area, which results in a higher amount of processing-induced dislocation during the ARB process. During the ARB process, the nanoparticles increase dislocation density in the interfaces much more than microparticles due to strain incompatibility between matrix and reinforcement, and difference in thermal expansion between IF steel and SiC particles after the ARB process and during cooling. In addition, the presence of the SiC nanoparticles in the IF steel matrix during the ARB process can also activate other strengthening mechanisms such as the Orowan strengthening mechanism in the matrix (Fig. 7). The variations of yield strength for pure steel, composite and nanocomposite versus the number of cycles are shown in Fig. 8. After first cycle, a significant increase can be observed in the yield strength, from 84 MPa to 609, 682, and 689 MPa for pure steel, composite, and nanocomposite, respectively, which is almost 7.3, 8.1, and 8.2 times greater than that of the initial sample. The significant increase in yield strength at the first cycle can be attributed to strain hardening and formation of subgrains. After final cycle, the yield strength value of pure steel, composite, and nanocomposite increased to 909, 1061, and 1189 MPa, respectively. It can be said that that yield strength variations in the ARB-processed materials are governed by two main strengthening mechanisms: strain hardening and grain refinement. In the early stages of ARB process, strain hardening plays a main role in increasing the yield strength, while the formation of subgrains or dislocation cells also contributes to strength. But at higher cycles, higher yield strength is achieved by grain refinement as the ARB
Fig. 7. The Orowan loops in the IF steel microstructure.
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estimated on the basis of a simple rule-of-mixtures [50–52]:
σ = σm(1 − f ) + σrf
Fig. 8. Variation of yield strength in ARB-processed samples for different cycles.
cycles increase. This result is consistent with the discussion of Figs. 4 and 6. It can be also found that with the addition of SiC microparticles and nanoparticles, the yield strength of the composite and nanocomposite are enhanced compared with the pure steel. The grains of IF steel matrix in the composite and nanocomposite samples were finer than that of pure steel sample after ARB process as shown in Fig. 4. Therefore, according to Hall–Petch equation, the yield strength of composite and nanocomposite increases. In addition, the grain size of matrix in the nanocomposite is smaller than that of composite with the same ARB process resulting in higher yield strength. There is one important point that should be considered. In the present work, unlike the dislocation density, there is no perfect saturation in yield strength of the pure IF steel with increasing the number of ARB cycles. This is attributed to pinning effect of oxide particles which forms after every ARB cycle. According to Fig. 2, after each cycle, a large number of interfaces including oxide film are introduced. These films break and fragment during the ARB process and transform to oxide particles. The oxide particles act as obstacles for grain boundary and dislocation motion and therefore lead to dislocations accumulation and increasing the yield strength even at high number of ARB cycles. It should be noted that the SiC microparticles and nanoparticles also have an outstanding effect on the tensile behavior of samples. The onset of plastic deformation in the composite and nanocomposite represents a deformation mechanism of dislocation activity, such as the dislocation accumulation, the dislocation multi-plication and the interactions of dislocations in the primary slip system. Thus, the SiC microparticles and nanoparticles would hinder the dislocations slip during tensile process. There are two different strengthening mechanisms that are typically associated with composite and nanocomposite. First, direct strengthening resulting from load transfer from the IF steel matrix to the reinforcing particles and indirect strengthening resulting from the influence of reinforcement particles on the IF steel matrix microstructure or deformation mode, such as dislocation strengthening induced by the deformation mismatch between the reinforcement and the matrix. The steel/SiC interface plays an important role in determining the mechanical response of the composite and nanocomposite, and its effect can be examined in the context of a load transfer mechanism. Under these conditions, the contribution of the microstructure to the mechanical properties can be
(5)
where sm is the yield strength of the metal matrix, sr is the yield strength of the reinforcement phase, and f is the volume fraction of the reinforcement phase. In the present work s¼ 1061 MPa (for composite) and 1189 MPa (for nanocomposite), f ¼0.02, and sr ¼3440 MPa [53,54]. Therefore, the yield strength of the IF steel matrix (sm) is calculated as 1012.5 MPa (for composite) and 1143.1 MPa (for nanocomposite). It should be noted that the contribution of reinforcement microparticles and nanoparticles to the improvement in yield strength of the IF steel matrix through its direct strengthening effect is equal to 68.8 MPa. On the other hand the IF steel matrix can be strengthened by three different mechanisms including grain refinement strengthening, dislocation strengthening, and precipitation strengthening. It is clear that sm must be a direct consequence of the aforementioned strengthening effects. With this in mind, an attempt has been made to separate the influence of certain parameters on strengthening of composites. The contribution of various strengthening mechanisms to the improvement in sm is discussed as follows. sm can be estimated by the following equation:
σm = σ0 + Δσg + Δσdis + Δσp
(6)
where s0 is the yield strength of undeformed pure single-crystal IF steel (s0 ¼ 30 MPa [48]), and Δsg, Δsdis, and Δsp are the increases in the yield strength caused by grain refinement, dislocation, and precipitation, respectively. The contribution of the grain refinement Δsg is first evaluated. Δsg can be calculated by the following equation [48,49]: 1
Δσg = kd− 2
(7)
where k is the Hall–Petch slope (k¼ 200 MPa μm0.5 for IF steel [49]) and d is the average grain size (95 nm for pure steel, 73 nm for composite, and 55 nm for nanocomposite). The contribution of grain refinement to the improvement in yield strength of the samples is then calculated as 649 MPa for pure steel, 707.1 MPa for composite, and 852.8 MPa for nanocomposite. Next, the contribution of dislocation strengthening is analyzed. Δsdis can be calculated by the following equation [48,49]: 1
Δσdis = MαGbρ 2
(8)
where M ¼3.2 [49] is the Taylor factor, α ¼0.33 [48] is a constant, G ¼77 GPa [49] is shear modulus, b ¼0.248 nm [48] is Burgers vector, and ρ is the dislocation density (9.47 109 cm 2 for pure steel, 10.73 109 cm 2 for composite, and 12.29 109 cm 2 for nanocomposite). The contribution of dislocation strengthening to the improvement in yield strength of the IF steel matrix is then calculated as 196 MPa for pure steel, 208.9 MPa for composite, and 223.6 MPa for nanocomposite. It should be noted that the Δsdis of samples depends on thermal expansion mismatch, Orowan mechanism, and other mechanisms:
Δσdis = ΔσCTE + ΔσOr + Δσother
(9)
Thermal mismatch between silicon carbide microparticles and nanoparticles and the IF steel matrix can result in internal stresses. These internal stresses are the generator of dislocations leading to increase of dislocation density in the IF steel matrix. Arsenault and Shi [55] developed the model describing the density of dislocations formed due to thermal mismatch between reinforcement particles and the matrix. Dislocation density generated due to thermal mismatch (ρCTE) is given by [55,56]:
R. Jamaati et al. / Materials Science & Engineering A 639 (2015) 656–662
ρCTE =
12ΔT ΔαVp bdp
(10)
where b is Burgers vector (b¼0.248), dp is diameter of SiC microparticles (50 mm) and nanoparticles (50 nm), Vp is the volume fraction of reinforcement particles (Vp ¼0.02), ΔT is the temperature difference (ΔT¼80 K, considering the temperature rising during ARB process was 80 K), and Δα is the difference in coefficient of thermal expansion between the IF steel matrix and the SiC particles (Δα ¼ αSteel αSiC ¼11 10 6 K 1 4.5 10 6 K 1 ¼6.5 10 6 K 1). The ρCTE is then calculated as 1.02 106 cm 2 for composite and 1.02 109 cm 2 for nanocomposite. According to Eq. (8), the contribution of thermal expansion mismatch to the improvement in dislocation strengthening of the IF steel matrix is then determined as 2.1 MPa for composite and 64.4 MPa for nanocomposite. The interaction of dislocations with the non-sharable nanoprecipitates increases the dislocation density of steel-based nanocomposite according to the Orowan mechanism. Owing to the presence of the dispersed nanocarbides and nanonitrides in the IF steel matrix, dislocation loops form as dislocation lines and bypass the particles (Fig. 7). In fact, since Orowan loops are present around the nanoprecipitates, there would be contributions from the Orowan strengthening effect. ΔsOr can be calculated as [56,57]
ΔσOr =
661
dpr 0.13Gb ln λ 2b
(11)
where G ¼77 GPa, b¼ 0.248 nm, dpr is diameter of nanoprecipitates (dpr ¼20 nm), and is the inter-particle spacing that is expressed as [56] 1 ⎤ ⎡ ⎥ ⎢⎛ 1 ⎞ 3 ⎟⎟ − 1⎥ λ ≈ dpr ⎢⎜⎜ 2Vpr ⎠ ⎝ ⎥⎦ ⎢⎣
(12)
where dpr ¼ 20 nm and Vpr is the volume fraction of nanoprecipitates (Vpr ¼10 4). The λ is then calculated as 342 nm. According to Eq. (11), the contribution of Orowan mechanism to the improvement in dislocation strengthening of the IF steel matrix is then determined as 26.8 MPa for pure steel, composite and nanocomposite. Based on 196, 208.9, and 223.6 MPa of the total contributions of dislocation strengthening for pure steel, composite and nanocomposite, respectively, and according to Eq. (9), the contribution of other mechanisms to the improvement in yield strength of samples is then calculated as 169.2 MPa (for pure steel), 180 MPa (for composite), and 132.4 MPa (for nanocomposite). Finally, the contribution of precipitation strengthening of nanocarbides and nanonitrides is determined. Based on 909 MPa (for pure steel), 1012.5 MPa (for composite), and 1143.1 MPa (for nanocomposite) of the total yield strength of the IF steel matrix and according to Eq. (6), the contribution of precipitation to the improvement is then calculated as 34, 33.4, and 37.3 MPa for pure steel, composite, and nanocomposite, respectively. Fig. 9 shows the contribution of various strengthening mechanisms to the improvement in yield strength of ARB-processed samples. As seen, the contributions of different strengthening methods are different. The contribution of grain refinement and precipitation to the improvement in yield strength is maximum ( 67–72%) and minimum ( 3.1–3.7%), respectively. It can be concluded that the predominant method for the improvement of IF steel strength is the grain refinement.
4. Conclusions In the present work, the strengthening mechanisms in nanostructured IF steel deformed to high strain by four-layer
Fig. 9. Contribution of strengthening mechanisms to the improvement in yield strength of ARB-processed samples.
accumulative roll bonding (ARB) process at room temperature in absence and presence of SiC particles were investigated. The conclusions drawn from the results can be summarized as follows: (1) The average grain size of the pure steel, composite, and nanocomposite was 95, 73, and 55 nm, respectively and the microstructures consisted of equiaxed grains. (2) With increasing the number of ARB cycles, the dislocation density of samples increased. (3) After first cycle, the dislocation density increased from 2.02 109 cm 2 (for the initial sample) to 6.53 109 cm 2 for pure steel, 6.91 109 cm 2 for composite, and 7.01 109 cm 2 for nanocomposite, respectively, registering 223%, 242%, and 247% increase. The first cycle of ARB process had significant effect on the dislocation density. (4) The dislocation density increased continually until the dislocation density of the pure steel, composite and nanocomposite after fourth cycle was about 4.7 times (9.47 109 cm 2), 5.3 times (10.73 109 cm 2), and 6.1 times (12.29 109 cm 2) higher than that of the initial sample, respectively. (5) The value of dislocation density first rapidly increased, then dwindled, and finally saturated by further ARB cycles. (6) The presence of SiC microparticles and nanoparticles in the IF steel matrix increased the dislocation density during the ARB process because these particles acted as obstacles for dislocation movement. On the other hand, dislocation density of the nanocomposite was higher than that of the composite. (7) After first cycle, a remarkable increase observed in the yield strength, from 84 MPa to 609, 682, and 689 MPa for pure steel, composite, and nanocomposite, respectively, which was almost 7.3, 8.1, and 8.2 times greater than that of the initial sample. After final cycle, the yield strength value of pure steel, composite, and nanocomposite increased to 909, 1061, and 1189 MPa, respectively. (8) There was no perfect saturation in yield strength of the pure IF steel with increasing the number of ARB cycles. (9) The contribution of grain refinement and precipitation to the improvement in yield strength was maximum (67–72%) and minimum ( 3.1–3.7%), respectively. The predominant method for the improvement of IF steel strength was the grain refinement.
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