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SiC composites manufactured by accumulative roll bonding (ARB) process
Author’s Accepted Manuscript Comparison of microstructure, toughness, mechanical properties and work hardening of titanium/TiO2 and titanium/SiC composites manufactured by accumulative roll bonding (ARB) process J. Moradgholi, A. Monshi, K. Farmanesh, M.R. Toroghinejad, M.R. Loghman-Estarki
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To appear in: Ceramics International Received date: 27 November 2016 Revised date: 11 March 2017 Accepted date: 11 March 2017 Cite this article as: J. Moradgholi, A. Monshi, K. Farmanesh, M.R. Toroghinejad and M.R. Loghman-Estarki, Comparison of microstructure, toughness, mechanical properties and work hardening of titanium/TiO2 and titanium/SiC composites manufactured by accumulative roll bonding (ARB) process, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.03.072 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Comparison of microstructure, toughness, mechanical properties and work hardening of titanium/TiO2 and titanium/SiC composites manufactured by accumulative roll bonding (ARB) process J.Moradgholia*, A.Monshia, K.Farmaneshb, M.R. Toroghinejada, M.R. Loghman-Estarkib* a
Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111,
Iran b
Department of Materials Engineering, Malek Ashtar University of Technology, P.O. Box
83145/115, Shahin Shahr, Isfahan, Iran [email protected] [email protected]
Abstract In this study, the effects of TiO2 ceramic nanoparticles and SiC microparticles on the microstructure, mechanical properties and toughness of titanium/TiO2 nanocomposite and titanium/SiC composite were investigated. To achieve this goal, TiO2 and SiC ceramic particles were incorporated as the reinforcement in titanium through the ARB (accumulative roll bonding) process. By adding SiC ceramic particles, the mechanical properties of the composite and the nanocomposite were enhanced, while their toughness was decreased, as compared to TiO2 nanoparticles. After applying 8 cycles of the ARB process, UTS in Ti/5 vol% SiC composite reached to about 1200 (MPa), as compared to that in Ti/0.5 wt% TiO2 nanocomposite, which was about 1100 (MPa). Furthermore, toughness in the Ti/5 vol.% SiC composite and the Ti/0.5 wt.%
TiO2 nanocomposite was 60 and 29 J/m3, respectively. Finally, SEM and TEM images showed SiC microparticles clustering in Ti/SiC composite samples and a suitable distribution of TiO2 nanoparticles in the Ti/TiO2 nanocomposite. By adding TiO2 nanoparticles, mechanical properties and work hardening coefficient were found to be increased, as compared to those of the monolithic samples. TiO2 nanoparticles, after being distributed in the titanium matrix through the ARB process, caused pin dislocations. As clearly shown in TEM images, dislocation tangles around TiO2 nanoparticles acted as the main mechanism improving the work hardening coefficient. Keywords: ceramic nanoparticles, nanocomposite, mechanical properties, toughness, TiO2 nanoparticles, SiC, work hardening, ARB Process.
INTRODUCTION Titanium and its alloys are well-known materials with widespread applications. In the recent decade, the focus of studies addressing these strategic alloys has been on their mechanical properties[1-14], as well as their corrosion [15] and creep [16-18] behaviors. Recently, severe plastic deformation (SPD) processes have been applied as powerful techniques for obtaining high strength metals by mechanical working and grain refinements, as these methods allow manufacturing bulk metals and nanostructured bulk specimens. The accumulative roll-bonding (ARB) process is a well-known SPD method applicable for manufacturing bulk industrial specimens. One of the major obstacles hindering the widespread use of bulk MMCs is their high cost. Thus, the ARB process can be used as a creative processing technique to overcome this problem and
manufacture high-strength Ti composites[1,13-14]. However, using ARB process to produce high strength monolithic and composite materials suffers a shortcoming; i.e., it strongly decreases elongation and toughness due to the effects of severe plastic deformation and secondary phase particles, especially in BCC and HCP metals. Work hardening mechanisms are well-known strengthening mechanisms in metals[19–28]. Base metal structures, alloying elements, workability, grain size, reinforcements, heat treatments and aging process are the major parameters in the work hardening of metals [20-22, 29–32]. The present study was conducted to investigate the effect of TiO2 nanoparticles and SiC microparticles on mechanical properties, toughness and the work hardening of titanium nanocomposite and to analyze the coefficient of work hardening in nanocomposite samples, as compared with the monolithic ones. Moreover, TEM images were applied to study the main mechanisms of work hardening in severe plastic deformation for samples with and without ceramic nanoparticles. Eventually, the effect of ceramic particles on the fracture surface of the nanocomposite was investigated.
MATERIALS AND METHODS Materials A commercial pure titanium sheet, 0.9 mm in thickness, with the specifications shown in Table 1 was used in present study. This sheet was cut into 15cm×4cm pieces, parallel to the sheet rolling direction. Also, TiO2 nanoparticles were used as the reinforcement. The TEM image of TiO2 nanoparticles showed that the TiO2 nanoparticles had a good distribution in size and a good similarity in shape, with the average size of 20 nm (fig.1a) . Furthermore, according to the SEM
image of SiC microparticles (Fig.1b), the irregular particles had an average size of ~12 µm. TiO2 nanoparticles and SiC microparticles were purchased from NanoAmor(USA) Company.
Accumulative roll bonding (ARB) process The schematic illustration of the ARB process for manufacturing nanocomposite is shown in Fig.2, as already noted in the previous research1 of authors. In the present study, titanium strips were ARBed in two steps. At first, to produce a satisfactory bond between two layers of titanium strips in the cold roll bonding, it was essential to remove contaminations such as oil, grease, etc. on the joining surfaces of titanium sheets. Acetone bath, as a strong degreaser, was used for the initial preparation of titanium sheets surface. Then, they were scratch brushed with a stainless steel rotated wire cup brush 0.4 mm in wire diameter. Actually, these two steps of ARB in this study were similar, with the difference that adding TiO2 nanoparticles was only applied in the first step and then, the first step was employed only as an initial step of the ARBed process, and it was not repeated as done in the second step. The second step was repeated until the number of ARB cycles for producing samples was suitable.
Microstructure evaluation A field emission electron microscopy (TESCAN MIRA3) was employed for as the received titanium microstructure study. Before taking FESEM image, the sample coated with gold by Desk Sputter Coater (DST3 mode, Nanostructured Coating Company, made in Iran. Samples prepared by polishing with 600, 800, 1200 and 4000 meshes paper sheets polisher and Kroll’s reagent was used (2ml HF, 4ml HNO3 and 100ml H2O2) for etching in the microstructure study. Furthermore, the microstructure of the processed sheet was studied by transmission electron
microscope (TEM), using a PHILIPS CM30 operating at 200 kV and equipped with energy dispersive spectroscopy (EDS). The samples for TEM analysis were prepared using the twin-jet electropolishing method, with 10% hydrochloric acid in acetic acid at an operating voltage of 65 V.
Mechanical properties The tensile test samples were machined from the ARBed strips by wire electrical discharge machining, according to the ASTM E8M standard, and oriented along the rolling direction. The gauge width and length of the tensile test samples were 6 and 25mm, respectively. The tensile tests were conducted at room temperature on a Houndsfield H50KS testing machine. The total elongation of the specimens was determined as the difference between gauge lengths before and after testing. The Vickers hardness of the specimens was measured at 50 N of the load RD–TD plane. Hardness was measured randomly at five different points per one specimen, and the mean hardness value was calculated from three values, except for the maximum and minimum ones.
Strain hardening evaluation To study the strain hardening behavior of metals, various models are presented in terms of metal structure, production process, applied strain and material strength. In this research, three models were chosen and their consistency with the fabricated samples was examined. Significant efforts have been made and various experimental equations have been presented so far for explaining the work hardening of materials and polycrystalline alloys. Some of these
equations have been presented by Holomon, Ludwik, Swift, etc 33. In general, these correlations are summarized as follows: (1) (2) (3) The calculation methods for each of the above equations are as follows: 1. Holomon analysis In this method, it is assumed that material behavior follows Holomon’s equation (Eq.1). Both n and K values can be easily obtained by drawing lnσ-lnϵ curves.
2. The differential method of Crussard-Jaoul The flow stress curve analysis through the differential method of Crussard-Jaoul is applicable for many types of alloys. In this method, it is assumed that alloys behavior follows Ludwik’s equation (Eq.2). The following formula is derived by the differentiation of this equation: (4) ln(d / d ) ln(Kn ) (n 1) ln In the ln - ln(d / d )
curve, (n-1) is the slope of the curve and ln( Kn) is the intercept.
Therefore, the power of n and KC-J work hardening and, in turn, σ0 is calculated. 3. Modified Crussard-Jaoul analysis
Modified (C–J) analysis is presented by the Swift model, where the correlation between strain and stress is stated as follows:
0 c m (5) Here,
is the material constant, m is the reversed power of work hardening, and c is the
material constant. The following formula is extracted by the differentiation of this equation:
ln(d / d ) = (1 m)ln ln(cm) (6) Both m and c parameters are obtained from the slope of curve
with lnσ.
Results and discussion Mechanical properties By increasing the number of ARB cycles, within the first passes, the tensile strength process was increased while the elongation was declined. Fig.3a illustrates strain-stress curves for the titanium sample without adding titanium oxide nanoparticles at different ARB cycles. As shown, the tensile strength was noticeably raised with increasing the ARB cycles, probably by the reduction of grain size and the formation of nanocrystals with smaller dimensions. It is also notable that this increasing rate of tensile strength was reduced as ARB cycles were increased. It could be seen that the maximum increasing rate of strength was for the first pass of the process. Generally, through the ARB process, by increasing the ARB cycles, an increase in tensile strength was not obvious; rather, a declination in mechanical properties might be seen due to
microstructures such as dynamic recrystallization, which could occur within the final ARB cycles (Fig.3a). Moreover, the post-failure changes of elongation in stress-strain curves in this process indicated that an increase in the ARB cycles resulted in an accompanying drop in postfailure elongation. This declination at the primary cycle process occurred more quickly. In comparison, with an increase in the ARB cycles, this elongation rate was with a lower rate. It was noteworthy that the increasing number of ARB passes did not always result in a drop in the elongation rate; rather, in some cycles, the elongation value was increased, as compared to that of the previous cycle, probably because of dynamic recrystallization
34
. Fig.3a shows these
changes. Adding nanoparticles to samples during the ARB process did not considerably affect samples without titanium oxide nanoparticles in the early process. In contrast, an increase in the ARB passes resulted in tensile strength enhancement and a drop in the elongation percentage before failure in strain-stress samples [13-14, 35]. Figure 2b illustrates the changes in strain-stress curves for nanocomposite samples containing 0.5 wt.% titanium oxide nanoparticles. As shown, by increasing the ARB cycles, an increase in tensile strength was seen, as compared to the nanoparticle-free sample, while the reduction in elongation was visible when comparing Fig.3a and 3b. The toughness was decreased with applying the ARB process, especially for the composites. After the last cycle, the calculated toughness of the monolithic titanium, Ti–1.5 vol.% SiC, Ti–3 vol.% SiC and Ti–5 vol.% SiC samples reached to about 67, 64, 32, and 29 J/m3, respectively. In addition, for nanocomposite samples, the toughness reached was about 52, 48 and 60 J/m3 for Ti/0.1 wt.%, TiO2,Ti/0.3 wt.%TiO2 and Ti/0.5 wt.% TiO2, respectively. These results showed
that the nanoparticles improved the toughness, as compared with incorporating microparticles in the samples. The changes in particles distribution with ARB cycles were almost the same for all three powder contents. Powders distribution for Ti-5 vol.% SiC is shown in Fig.4 by SEM images. The highly non-uniform distribution of particles was observed in most areas of the second cycle specimen, such that the clustered areas with clumped particles and particle-free regions were visible on a macroscopic scale (of the order of 200 µm). Porosities in the matrix/ reinforcement interface, as well as fracturing and debonding of particles, were remarkable in this sample. Particle clustering arising from the initial dispersion process might negatively affect the mechanical properties. The constraints exerted by the elastic ceramic reinforcements on the metallic matrix could change the stress state around clusters, causing local triaxial stresses much higher than the external stress applied to the composite36,37. Consequently, clusters could act as void and/or crack nucleation sites at external stresses lower than the matrix yield strength, leading to the failure of MMC at unpredictable low-stress levels, as compared with those of the monolithic material. Furthermore, as shown in Fig.4, particles in the clustered regions were debonded and removed from their place with the grinding and polishing force. This behavior was probably due to the poor interfacial bonding and lack of sufficient surrounding metallic phase for the restraining particles. Since the poor interfacial bonding has been recognized as a factor leading to the reduced load transfer, it could result in the degradation of mechanical properties13.
Fig. 5 presents a TEM image of titanium nanocomposite with 0.5 wt.% of TiO2 nanoparticles after 8 cycles of ARB. The TiO2 nanoparticles (characterized by EDS analysis) had a good distribution in the titanium matrix without any clustering or voids. Strain hardening The strain hardening behavior for metals under severe strain could be considered as one of the important changes in mechanical parameters, an issue that has not been discussed much. Then, to investigate the effect of nanoparticles and the applied ARB process, as an important method in severe deformation and strain hardening behavior, and to examine its consistency with various models, the strain hardening behavior of fabricated samples (without nanoparticles and with the maximum size of nanoparticles (0.5%) after 8 ARB cycles) was investigated in the present research. In Fig.6, three curves based on three corresponding Hollomon (Fig.6.a), Crussard–Jaoul (Fig.6.b), and Crussard–Jaoul modified models (Fig.6.c) are presented. As shown in the Hollomon model, curves had two parts with different slopes for two samples. Based on Fig.6.a, which was drawn using the Hollomon model, for both samples with and without nanoparticles, two different slopes could be seen. For a sample without nanoparticles, the slope was more than that of the sample with 0.5wt.% nanoparticles. In the sample without nanoparticles, the slope was declined from 0.9 at the beginning of the path to 0.25. In comparison, the slope in the Hollomon model was n, or the strain hardening coefficient. Although this slope had significant amounts at low strains, the work hardening rate was dropped at slightly larger strains. The work hardening coefficient for nanocomposite samples containing 0.5wt.% nanoparticles at low strains was slightly lower than that of nanoparticles-free samples; on the other hand, by increasing the strain
level, similar to the case of the nanoparticles-free sample, a drop in work hardening coefficient was seen. The work hardening coefficient for nanocomposite samples containing 0.5% nanoparticle was reduced from 0.6 to 0.167. However, it was notable that the change in the work hardening coefficient in nanoparticles-free samples occurred at high strains, as compared to those with the nanoparticles. Finally, it could be concluded that the nanoparticles-free samples, due to their higher work hardening coefficient, had a more suitable ductility in both phases, but those saturated at lower strains showed a reduction. This behavior could be linked to several parameters such as grain size, alloying elements, the material intrinsic properties and microstructure33. Fig.6. shows the TEM image of the titanium composite after 8 cycles of the ARB process (fig.7.a) and the titanium nanocomposite with 0.5 wt% TiO2 nanoparticles (fig.7.b). TiO2 nanoparticles had a good distribution, as shown in fig.7.b, and dislocation density around nanoparticles was increased, such that dislocation tangles could be observed around TiO2 nanoparticles. This phenomenon resulted in the work hardening of the titanium nanocomposite in severe strain, changing the work hardening slope in the second step of fig.6. It should be mentioned that EDS analysis of matrix and nanoparticles (marked by the red arrow in Fig. 7b) showed that the marked area was TiO2 nanoparticle (Fig.7d) and the matrix was composed of Ti element (Fig.7 c). However, a small amount of oxygen due to the surface oxidation of Ti was presented in the titanium matrix. Fig.6.b shows the Crussard-Jaoul model, with a slope equal to (n-1). As can be seen, in this model, the slope was high for nanoparticles-free samples at the beginning and started to reduce thereafter, reaching a constant value. In comparison, the diagram of the sample containing 0.5wt.% nanoparticles had a minor slope at the beginning and then started to decrease. This
behavior could also be observed in the modified Crussard-Jaoul model (Fig.6.c), probably due to the presence of nanoparticles as an isolated phase in the microstructure. C-J model and the modified C-J for samples containing different phases like steels with ferrite, pearlite and martensite had two different slopes, probably due to different deformation mechanisms in the presence and absence of nanoparticles33. Although, in the C-J model (Fig.6.b), both diagrams showed changes in the slope of the graph at the beginning, the change in the slope of the nanoparticles-free sample occurred more quickly. In the modified C-J diagram (Fig.6.c) , the slope of the graph was (1-m), where m was reverse work hardening coefficient (n). The remarkable point in this graph was the similarity between the C-J model and the modified C-J diagrams.
Conclusion In this work, mechanical properties, toughness and microstructure of titanium prepared by the ARB process with TiO2 nanoparticles and SiC microparticles were investigated for producing the titanium nanocomposite. Besides, the effect of TiO2 nanoparticles on the prepared nanocomposite was studied and compared with that of SiC microparticles. The main results of this work could be outlined as follows: Mechanical properties of Ti/SiC composite were higher than those of
the Ti/TiO2
nanocomposite. Furthermore, elongation of Ti/TiO2 nanocomposite was greater than that of the Ti/SiC composite. The uniaxial tensile strength (UTS) was enhanced by increasing the number of ARB processes in both TiO2 nanoparticles and SiC microparticles.
The elongation was decreased by increasing the ARB cycles in both Ti/TiO2 nanocomposite and Ti/SiC composite. Particle clustering, cracks and voids were observed only in the initial cycles of the ARB process of the Ti/SiC composite. The toughness of Ti/TiO2 nanocomposite samples was strongly improved, as compared with that of the Ti/SiC composite. Work hardening coefficient in CP Ti was different in various work hardening models; however, it seemed that the modified C-J model provided a better description of the effect of TiO2 nanoparticles on work hardening.
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Fig1: TEM image of TiO2 nanoparticles(a) and SEM image of SiC microparticles(b).
(2) Adding TiO2 nanoparticles
First step
(1) Surface preparation
(3) Stacking
(4) Rolling
Second step
(7) Stacking
(5) Cutting
(6) Surface preparation
Fig.2: The schematic illustration of the ARB process for manufacturing the nanocomposite
A
B
Fig.3: The stress-strain curve for a) titanium composites sample, b) titanium with 0.5 wt.% TiO2 nanoparticles, and c) titanium with 5 vol.% SiC microparticles13 at different cycles of the ARB process
2 cycles
4 cycles
6 cycles
8 cycles
Fig.4: Particles distribution for the composite produced by the ARB process with the final composition of Ti-5 vol.% SiC in the RD–TD plane. Right side images represent the rectangular regions of the left side images at a higher magnification13.
a
6000
TiK
b 5000
4000
3000
TiL OK
2000
1000
TiK
keV
0 0
5
10
Fig.5: (a) TEM image of titanium nanocomposite with 0.5 wt.% TiO2 nanoparticles after 8 cycles of ARB, (b) EDS point of yellow marked area in TEM image.
c
Fig. 6: Comparisons between nanoparticles-free samples strain hardening and samples with 0.5% nanoparticles based on a) the Hollomon model, b) the C-J model, and c) the modified C-J model.
Fig.7: TEM image of a) titanium composite, and b) titanium nanocomposite with 0.5 wt% TiO2 nanoparticles after 8 cycles of ARB and (c, d) EDS analysis of Ti (matrix, c) and TiO2 nanoparticles (d, marked with red arrow in TEM image) .
Table 1: Chemical composition of CP Ti (grade 2) asreceived sheet. Ti
Al
Mo
Sn
Zr
Mn
V
Fe
Nb
Cr
96.
<0.0
0.6
<0.
<0.1
<0.1
<0.1
<0.1
<0.
<0.0
80
15
49
50
00
00
00
00
50
20
Cu
Ni
Pd
Eleme nts
Perce nt
(wt%)
<1. <0.00 0
30
0.1 95
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To appear in: Ceramics International Received date: 27 November 2016 Revised date: 11 March 2017 Accepted date: 11 March 2017 Cite this article as: J. Moradgholi, A. Monshi, K. Farmanesh, M.R. Toroghinejad and M.R. Loghman-Estarki, Comparison of microstructure, toughness, mechanical properties and work hardening of titanium/TiO2 and titanium/SiC composites manufactured by accumulative roll bonding (ARB) process, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.03.072 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Comparison of microstructure, toughness, mechanical properties and work hardening of titanium/TiO2 and titanium/SiC composites manufactured by accumulative roll bonding (ARB) process J.Moradgholia*, A.Monshia, K.Farmaneshb, M.R. Toroghinejada, M.R. Loghman-Estarkib* a
Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111,
Iran b
Department of Materials Engineering, Malek Ashtar University of Technology, P.O. Box
83145/115, Shahin Shahr, Isfahan, Iran [email protected] [email protected]
Abstract In this study, the effects of TiO2 ceramic nanoparticles and SiC microparticles on the microstructure, mechanical properties and toughness of titanium/TiO2 nanocomposite and titanium/SiC composite were investigated. To achieve this goal, TiO2 and SiC ceramic particles were incorporated as the reinforcement in titanium through the ARB (accumulative roll bonding) process. By adding SiC ceramic particles, the mechanical properties of the composite and the nanocomposite were enhanced, while their toughness was decreased, as compared to TiO2 nanoparticles. After applying 8 cycles of the ARB process, UTS in Ti/5 vol% SiC composite reached to about 1200 (MPa), as compared to that in Ti/0.5 wt% TiO2 nanocomposite, which was about 1100 (MPa). Furthermore, toughness in the Ti/5 vol.% SiC composite and the Ti/0.5 wt.%
TiO2 nanocomposite was 60 and 29 J/m3, respectively. Finally, SEM and TEM images showed SiC microparticles clustering in Ti/SiC composite samples and a suitable distribution of TiO2 nanoparticles in the Ti/TiO2 nanocomposite. By adding TiO2 nanoparticles, mechanical properties and work hardening coefficient were found to be increased, as compared to those of the monolithic samples. TiO2 nanoparticles, after being distributed in the titanium matrix through the ARB process, caused pin dislocations. As clearly shown in TEM images, dislocation tangles around TiO2 nanoparticles acted as the main mechanism improving the work hardening coefficient. Keywords: ceramic nanoparticles, nanocomposite, mechanical properties, toughness, TiO2 nanoparticles, SiC, work hardening, ARB Process.
INTRODUCTION Titanium and its alloys are well-known materials with widespread applications. In the recent decade, the focus of studies addressing these strategic alloys has been on their mechanical properties[1-14], as well as their corrosion [15] and creep [16-18] behaviors. Recently, severe plastic deformation (SPD) processes have been applied as powerful techniques for obtaining high strength metals by mechanical working and grain refinements, as these methods allow manufacturing bulk metals and nanostructured bulk specimens. The accumulative roll-bonding (ARB) process is a well-known SPD method applicable for manufacturing bulk industrial specimens. One of the major obstacles hindering the widespread use of bulk MMCs is their high cost. Thus, the ARB process can be used as a creative processing technique to overcome this problem and
manufacture high-strength Ti composites[1,13-14]. However, using ARB process to produce high strength monolithic and composite materials suffers a shortcoming; i.e., it strongly decreases elongation and toughness due to the effects of severe plastic deformation and secondary phase particles, especially in BCC and HCP metals. Work hardening mechanisms are well-known strengthening mechanisms in metals[19–28]. Base metal structures, alloying elements, workability, grain size, reinforcements, heat treatments and aging process are the major parameters in the work hardening of metals [20-22, 29–32]. The present study was conducted to investigate the effect of TiO2 nanoparticles and SiC microparticles on mechanical properties, toughness and the work hardening of titanium nanocomposite and to analyze the coefficient of work hardening in nanocomposite samples, as compared with the monolithic ones. Moreover, TEM images were applied to study the main mechanisms of work hardening in severe plastic deformation for samples with and without ceramic nanoparticles. Eventually, the effect of ceramic particles on the fracture surface of the nanocomposite was investigated.
MATERIALS AND METHODS Materials A commercial pure titanium sheet, 0.9 mm in thickness, with the specifications shown in Table 1 was used in present study. This sheet was cut into 15cm×4cm pieces, parallel to the sheet rolling direction. Also, TiO2 nanoparticles were used as the reinforcement. The TEM image of TiO2 nanoparticles showed that the TiO2 nanoparticles had a good distribution in size and a good similarity in shape, with the average size of 20 nm (fig.1a) . Furthermore, according to the SEM
image of SiC microparticles (Fig.1b), the irregular particles had an average size of ~12 µm. TiO2 nanoparticles and SiC microparticles were purchased from NanoAmor(USA) Company.
Accumulative roll bonding (ARB) process The schematic illustration of the ARB process for manufacturing nanocomposite is shown in Fig.2, as already noted in the previous research1 of authors. In the present study, titanium strips were ARBed in two steps. At first, to produce a satisfactory bond between two layers of titanium strips in the cold roll bonding, it was essential to remove contaminations such as oil, grease, etc. on the joining surfaces of titanium sheets. Acetone bath, as a strong degreaser, was used for the initial preparation of titanium sheets surface. Then, they were scratch brushed with a stainless steel rotated wire cup brush 0.4 mm in wire diameter. Actually, these two steps of ARB in this study were similar, with the difference that adding TiO2 nanoparticles was only applied in the first step and then, the first step was employed only as an initial step of the ARBed process, and it was not repeated as done in the second step. The second step was repeated until the number of ARB cycles for producing samples was suitable.
Microstructure evaluation A field emission electron microscopy (TESCAN MIRA3) was employed for as the received titanium microstructure study. Before taking FESEM image, the sample coated with gold by Desk Sputter Coater (DST3 mode, Nanostructured Coating Company, made in Iran. Samples prepared by polishing with 600, 800, 1200 and 4000 meshes paper sheets polisher and Kroll’s reagent was used (2ml HF, 4ml HNO3 and 100ml H2O2) for etching in the microstructure study. Furthermore, the microstructure of the processed sheet was studied by transmission electron
microscope (TEM), using a PHILIPS CM30 operating at 200 kV and equipped with energy dispersive spectroscopy (EDS). The samples for TEM analysis were prepared using the twin-jet electropolishing method, with 10% hydrochloric acid in acetic acid at an operating voltage of 65 V.
Mechanical properties The tensile test samples were machined from the ARBed strips by wire electrical discharge machining, according to the ASTM E8M standard, and oriented along the rolling direction. The gauge width and length of the tensile test samples were 6 and 25mm, respectively. The tensile tests were conducted at room temperature on a Houndsfield H50KS testing machine. The total elongation of the specimens was determined as the difference between gauge lengths before and after testing. The Vickers hardness of the specimens was measured at 50 N of the load RD–TD plane. Hardness was measured randomly at five different points per one specimen, and the mean hardness value was calculated from three values, except for the maximum and minimum ones.
Strain hardening evaluation To study the strain hardening behavior of metals, various models are presented in terms of metal structure, production process, applied strain and material strength. In this research, three models were chosen and their consistency with the fabricated samples was examined. Significant efforts have been made and various experimental equations have been presented so far for explaining the work hardening of materials and polycrystalline alloys. Some of these
equations have been presented by Holomon, Ludwik, Swift, etc 33. In general, these correlations are summarized as follows: (1) (2) (3) The calculation methods for each of the above equations are as follows: 1. Holomon analysis In this method, it is assumed that material behavior follows Holomon’s equation (Eq.1). Both n and K values can be easily obtained by drawing lnσ-lnϵ curves.
2. The differential method of Crussard-Jaoul The flow stress curve analysis through the differential method of Crussard-Jaoul is applicable for many types of alloys. In this method, it is assumed that alloys behavior follows Ludwik’s equation (Eq.2). The following formula is derived by the differentiation of this equation: (4) ln(d / d ) ln(Kn ) (n 1) ln In the ln - ln(d / d )
curve, (n-1) is the slope of the curve and ln( Kn) is the intercept.
Therefore, the power of n and KC-J work hardening and, in turn, σ0 is calculated. 3. Modified Crussard-Jaoul analysis
Modified (C–J) analysis is presented by the Swift model, where the correlation between strain and stress is stated as follows:
0 c m (5) Here,
is the material constant, m is the reversed power of work hardening, and c is the
material constant. The following formula is extracted by the differentiation of this equation:
ln(d / d ) = (1 m)ln ln(cm) (6) Both m and c parameters are obtained from the slope of curve
with lnσ.
Results and discussion Mechanical properties By increasing the number of ARB cycles, within the first passes, the tensile strength process was increased while the elongation was declined. Fig.3a illustrates strain-stress curves for the titanium sample without adding titanium oxide nanoparticles at different ARB cycles. As shown, the tensile strength was noticeably raised with increasing the ARB cycles, probably by the reduction of grain size and the formation of nanocrystals with smaller dimensions. It is also notable that this increasing rate of tensile strength was reduced as ARB cycles were increased. It could be seen that the maximum increasing rate of strength was for the first pass of the process. Generally, through the ARB process, by increasing the ARB cycles, an increase in tensile strength was not obvious; rather, a declination in mechanical properties might be seen due to
microstructures such as dynamic recrystallization, which could occur within the final ARB cycles (Fig.3a). Moreover, the post-failure changes of elongation in stress-strain curves in this process indicated that an increase in the ARB cycles resulted in an accompanying drop in postfailure elongation. This declination at the primary cycle process occurred more quickly. In comparison, with an increase in the ARB cycles, this elongation rate was with a lower rate. It was noteworthy that the increasing number of ARB passes did not always result in a drop in the elongation rate; rather, in some cycles, the elongation value was increased, as compared to that of the previous cycle, probably because of dynamic recrystallization
34
. Fig.3a shows these
changes. Adding nanoparticles to samples during the ARB process did not considerably affect samples without titanium oxide nanoparticles in the early process. In contrast, an increase in the ARB passes resulted in tensile strength enhancement and a drop in the elongation percentage before failure in strain-stress samples [13-14, 35]. Figure 2b illustrates the changes in strain-stress curves for nanocomposite samples containing 0.5 wt.% titanium oxide nanoparticles. As shown, by increasing the ARB cycles, an increase in tensile strength was seen, as compared to the nanoparticle-free sample, while the reduction in elongation was visible when comparing Fig.3a and 3b. The toughness was decreased with applying the ARB process, especially for the composites. After the last cycle, the calculated toughness of the monolithic titanium, Ti–1.5 vol.% SiC, Ti–3 vol.% SiC and Ti–5 vol.% SiC samples reached to about 67, 64, 32, and 29 J/m3, respectively. In addition, for nanocomposite samples, the toughness reached was about 52, 48 and 60 J/m3 for Ti/0.1 wt.%, TiO2,Ti/0.3 wt.%TiO2 and Ti/0.5 wt.% TiO2, respectively. These results showed
that the nanoparticles improved the toughness, as compared with incorporating microparticles in the samples. The changes in particles distribution with ARB cycles were almost the same for all three powder contents. Powders distribution for Ti-5 vol.% SiC is shown in Fig.4 by SEM images. The highly non-uniform distribution of particles was observed in most areas of the second cycle specimen, such that the clustered areas with clumped particles and particle-free regions were visible on a macroscopic scale (of the order of 200 µm). Porosities in the matrix/ reinforcement interface, as well as fracturing and debonding of particles, were remarkable in this sample. Particle clustering arising from the initial dispersion process might negatively affect the mechanical properties. The constraints exerted by the elastic ceramic reinforcements on the metallic matrix could change the stress state around clusters, causing local triaxial stresses much higher than the external stress applied to the composite36,37. Consequently, clusters could act as void and/or crack nucleation sites at external stresses lower than the matrix yield strength, leading to the failure of MMC at unpredictable low-stress levels, as compared with those of the monolithic material. Furthermore, as shown in Fig.4, particles in the clustered regions were debonded and removed from their place with the grinding and polishing force. This behavior was probably due to the poor interfacial bonding and lack of sufficient surrounding metallic phase for the restraining particles. Since the poor interfacial bonding has been recognized as a factor leading to the reduced load transfer, it could result in the degradation of mechanical properties13.
Fig. 5 presents a TEM image of titanium nanocomposite with 0.5 wt.% of TiO2 nanoparticles after 8 cycles of ARB. The TiO2 nanoparticles (characterized by EDS analysis) had a good distribution in the titanium matrix without any clustering or voids. Strain hardening The strain hardening behavior for metals under severe strain could be considered as one of the important changes in mechanical parameters, an issue that has not been discussed much. Then, to investigate the effect of nanoparticles and the applied ARB process, as an important method in severe deformation and strain hardening behavior, and to examine its consistency with various models, the strain hardening behavior of fabricated samples (without nanoparticles and with the maximum size of nanoparticles (0.5%) after 8 ARB cycles) was investigated in the present research. In Fig.6, three curves based on three corresponding Hollomon (Fig.6.a), Crussard–Jaoul (Fig.6.b), and Crussard–Jaoul modified models (Fig.6.c) are presented. As shown in the Hollomon model, curves had two parts with different slopes for two samples. Based on Fig.6.a, which was drawn using the Hollomon model, for both samples with and without nanoparticles, two different slopes could be seen. For a sample without nanoparticles, the slope was more than that of the sample with 0.5wt.% nanoparticles. In the sample without nanoparticles, the slope was declined from 0.9 at the beginning of the path to 0.25. In comparison, the slope in the Hollomon model was n, or the strain hardening coefficient. Although this slope had significant amounts at low strains, the work hardening rate was dropped at slightly larger strains. The work hardening coefficient for nanocomposite samples containing 0.5wt.% nanoparticles at low strains was slightly lower than that of nanoparticles-free samples; on the other hand, by increasing the strain
level, similar to the case of the nanoparticles-free sample, a drop in work hardening coefficient was seen. The work hardening coefficient for nanocomposite samples containing 0.5% nanoparticle was reduced from 0.6 to 0.167. However, it was notable that the change in the work hardening coefficient in nanoparticles-free samples occurred at high strains, as compared to those with the nanoparticles. Finally, it could be concluded that the nanoparticles-free samples, due to their higher work hardening coefficient, had a more suitable ductility in both phases, but those saturated at lower strains showed a reduction. This behavior could be linked to several parameters such as grain size, alloying elements, the material intrinsic properties and microstructure33. Fig.6. shows the TEM image of the titanium composite after 8 cycles of the ARB process (fig.7.a) and the titanium nanocomposite with 0.5 wt% TiO2 nanoparticles (fig.7.b). TiO2 nanoparticles had a good distribution, as shown in fig.7.b, and dislocation density around nanoparticles was increased, such that dislocation tangles could be observed around TiO2 nanoparticles. This phenomenon resulted in the work hardening of the titanium nanocomposite in severe strain, changing the work hardening slope in the second step of fig.6. It should be mentioned that EDS analysis of matrix and nanoparticles (marked by the red arrow in Fig. 7b) showed that the marked area was TiO2 nanoparticle (Fig.7d) and the matrix was composed of Ti element (Fig.7 c). However, a small amount of oxygen due to the surface oxidation of Ti was presented in the titanium matrix. Fig.6.b shows the Crussard-Jaoul model, with a slope equal to (n-1). As can be seen, in this model, the slope was high for nanoparticles-free samples at the beginning and started to reduce thereafter, reaching a constant value. In comparison, the diagram of the sample containing 0.5wt.% nanoparticles had a minor slope at the beginning and then started to decrease. This
behavior could also be observed in the modified Crussard-Jaoul model (Fig.6.c), probably due to the presence of nanoparticles as an isolated phase in the microstructure. C-J model and the modified C-J for samples containing different phases like steels with ferrite, pearlite and martensite had two different slopes, probably due to different deformation mechanisms in the presence and absence of nanoparticles33. Although, in the C-J model (Fig.6.b), both diagrams showed changes in the slope of the graph at the beginning, the change in the slope of the nanoparticles-free sample occurred more quickly. In the modified C-J diagram (Fig.6.c) , the slope of the graph was (1-m), where m was reverse work hardening coefficient (n). The remarkable point in this graph was the similarity between the C-J model and the modified C-J diagrams.
Conclusion In this work, mechanical properties, toughness and microstructure of titanium prepared by the ARB process with TiO2 nanoparticles and SiC microparticles were investigated for producing the titanium nanocomposite. Besides, the effect of TiO2 nanoparticles on the prepared nanocomposite was studied and compared with that of SiC microparticles. The main results of this work could be outlined as follows: Mechanical properties of Ti/SiC composite were higher than those of
the Ti/TiO2
nanocomposite. Furthermore, elongation of Ti/TiO2 nanocomposite was greater than that of the Ti/SiC composite. The uniaxial tensile strength (UTS) was enhanced by increasing the number of ARB processes in both TiO2 nanoparticles and SiC microparticles.
The elongation was decreased by increasing the ARB cycles in both Ti/TiO2 nanocomposite and Ti/SiC composite. Particle clustering, cracks and voids were observed only in the initial cycles of the ARB process of the Ti/SiC composite. The toughness of Ti/TiO2 nanocomposite samples was strongly improved, as compared with that of the Ti/SiC composite. Work hardening coefficient in CP Ti was different in various work hardening models; however, it seemed that the modified C-J model provided a better description of the effect of TiO2 nanoparticles on work hardening.
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Fig1: TEM image of TiO2 nanoparticles(a) and SEM image of SiC microparticles(b).
(2) Adding TiO2 nanoparticles
First step
(1) Surface preparation
(3) Stacking
(4) Rolling
Second step
(7) Stacking
(5) Cutting
(6) Surface preparation
Fig.2: The schematic illustration of the ARB process for manufacturing the nanocomposite
A
B
Fig.3: The stress-strain curve for a) titanium composites sample, b) titanium with 0.5 wt.% TiO2 nanoparticles, and c) titanium with 5 vol.% SiC microparticles13 at different cycles of the ARB process
2 cycles
4 cycles
6 cycles
8 cycles
Fig.4: Particles distribution for the composite produced by the ARB process with the final composition of Ti-5 vol.% SiC in the RD–TD plane. Right side images represent the rectangular regions of the left side images at a higher magnification13.
a
6000
TiK
b 5000
4000
3000
TiL OK
2000
1000
TiK
keV
0 0
5
10
Fig.5: (a) TEM image of titanium nanocomposite with 0.5 wt.% TiO2 nanoparticles after 8 cycles of ARB, (b) EDS point of yellow marked area in TEM image.
c
Fig. 6: Comparisons between nanoparticles-free samples strain hardening and samples with 0.5% nanoparticles based on a) the Hollomon model, b) the C-J model, and c) the modified C-J model.
Fig.7: TEM image of a) titanium composite, and b) titanium nanocomposite with 0.5 wt% TiO2 nanoparticles after 8 cycles of ARB and (c, d) EDS analysis of Ti (matrix, c) and TiO2 nanoparticles (d, marked with red arrow in TEM image) .
Table 1: Chemical composition of CP Ti (grade 2) asreceived sheet. Ti
Al
Mo
Sn
Zr
Mn
V
Fe
Nb
Cr
96.
<0.0
0.6
<0.
<0.1
<0.1
<0.1
<0.1
<0.
<0.0
80
15
49
50
00
00
00
00
50
20
Cu
Ni
Pd
Eleme nts
Perce nt
(wt%)
<1. <0.00 0
30
0.1 95