Mechanical and microstructural investigation of Zn/Sn multilayered composites fabricated by accumulative roll bonding (ARB) process

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Mechanical and microstructural investigation of Zn/Sn multilayered composites fabricated by accumulative roll bonding (ARB) process

Amir Mashhadi

^a^,^b

, Amir Atrian

^a^,^b^,^*

, Laleh Ghalandari

^c

aDepartment of Mechanical Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Iran

bModern Manufacturing Technologies Research Center, Najafabad Branch, Islamic Azad University, Najafabad, Iran

cDepartment of Materials Science and Engineering, Shiraz Branch, Islamic Azad University, Shiraz, Iran

a r t i c l e i n f o

Article history:

Received 31 May 2017 Received in revised form 6 August 2017 Accepted 27 August 2017 Available online 30 August 2017 Keywords:

Accumulative roll bonding (ARB) Multilayer

Composite Tensile behavior Microstructure

a b s t r a c t

In the present research, Zn/Sn composites were produced for theﬁrst time by accumulative roll bonding (ARB) process up to seven cycles. The microstructure evolution was evaluated by scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer (EDS), X-ray diffraction (XRD) and the mechanical properties of Zn/Sn multilayered composites were measured by Vickers microhardness and tensile tests at different cycles of ARB process. It was observed that by increasing the ARB cycles, due to the differences in the mechanical behavior of the layers, Zn layers were necked atﬁrst, fractured and dispersed between Sn layers. After seven ARB cycles, a laminated composite with wavy interfaces and homogeneously distributed lenticular fragments of Zn was achieved. The XRD results showed that no new phase has been formed during the ARB process. Fracture surfaces of the samples after tensile tests were observed by SEM. Fracture surfaces revealed that fracture mode for the Zn layers was brittle at all cycles and fracture mode for the Sn layers was ductile at initial cycles and then changed to shear ductile fracture by increasing the ARB cycles.

1. Introduction

Severe plastic deformation (SPD) is the most famous type of top- down methods used in metals forming in order to achieve grain reﬁnement of metallic materials and provide desirable properties.

Various SPD processes such as equal channel angular pressing (ECAP) [1,2], high pressure torsion (HPT) [3], accumulative roll bonding (ARB) [4,5], cyclic extrusion compression (CEC) [6], etc have been proposed and were successfully applied to several materials. In these plastic deformation processes, an extremely high plastic strain is imposed to the work-piece at relatively low tem- peratures to exert changes in the microstructure and mechanical properties of the material.

Recently, among all the SPD techniques, the accumulative roll bonding (ARB) process has been developed as a SPD method to fabricate ultra-ﬁne grained (UFG) and nano structural metal matrix composites (MMCs) in the form of sheets without changing the specimen dimensions. The ARB process was developed by Saito

et al.[4]. This process consists of repetitive stages including: cut- ting, stacking and rolling of the sheets at room temperature. By repeating these procedures, very high strains have been imposed into the materials in several cycles which leads to the microstructural and mechanical properties evolution without any signiﬁcant changing of dimensions which is a unique feature of ARB process.

Severe plastic strains are introduced by large friction between rollers and the sheets besides. Up to now, various MMCs with similar or dissimilar metals have been fabricated by ARB process, due to their speciﬁc and superior properties. ARB has been used on commercial pure metals such as Al[7], Cu[8], etc. Recently, a great number of multilayered composites in the case of dissimilar layers have been successfully produced by ARB process. A lot of bimetal structures such as Al/Sn[9], Al/Zn[10], Cu/Ag[11], Cu/Ni[12]and Cu/Zn[13]have been fabricated by ARB process. Some authors have also investigated the behavior of tri-metallic systems processed by ARB such as Al/Ni/Cu[14], Cu/Zn/Al[15], Al/Cu/Sn[16]and Al/Ti/Mg [17]. Moreover, the ARB process is a suitable method to fabricate particulate MMCs with high uniform reinforced particles distribution such as Cu/Al₂O₃[18], Al/Al₂O₃[19]and Al/B₄C[20]. In addi- tion, a few attempts have been done to produce hybrid composites, a new kind of metal matrix composites with more than one type

*Corresponding author. Department of Mechanical Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Iran.

E-mail addresses:amiratrian@gmail.com,atrian@pmc.iaun.ac.ir(A. Atrian).

Contents lists available atScienceDirect

Journal of Alloys and Compounds

j o u r n a l h o m e p a g e : h t t p : / / w w w . e ls e v i e r . c o m / l o c a t e / j a l c o m

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reinforcing phase, such as Al/Al2O3/SiC[21], Al/Al2O3/B4C[22], Al/

B₄C/SiC[23].

Generally, during plastic deformation of dissimilar metals, plastic instabilities caused by differentﬂow properties and differences in mechanical properties of the layers, cause to necking, rupturing and fracture in the harder layer[13,24]. Eizadjou et al.

[25]investigated microstructures and mechanical properties of Al/

Cu composites produced by ARB process. They observed that by increasing number of ARB cycles, Cu layers started to neck and fractured. A multilayered composite with homogeneously distributed fragments of Cu layers in Al matrix was achieved after 5 ARB cycles. Sn is structurally a soft/weak metal with a low coefﬁcient of friction. This metal in laminated composites can acts as a lubricant and would be useful in corrosion and load-bearing applications, as self-lubricating parts[9,16]. Zn/Sn can be used in most applications such as electronic industries[26], automotive industries, radiation shielding, and is appropriate for use in coatings performances in architectural works. Lee et al.[26]investigated that Zn/Sn exhibit excellent oxidation resistance and good mechanical properties under severe thermal and humidity conditions. Also, Pstrus et al.

[27]reported that Zn/Sn alloys have good mechanical properties and can be used for soldering Cu and Al.

In the present study, ARB process have been performed up to seven cycles without any lubricant, at room temperature by using pure Zn and Sn sheets. It can be expected that, the difference between the Hexagonal Close-Packed (HCP) crystal structure of Zn and the Body-Centred Tetragonal (BCT) crystal structure of Sn and also, co-deformation of the components during ARB process would inﬂuence the mechanical properties and the microstructure evolution of Zn/Sn composites. In order to evaluate the fabricated Zn/

Sn multilayered composites, microstructure evolution and mechanical properties have been investigated at different ARB cycles using SEM, XRD, tensile test, and microhardness tests.

2. Materials and methods

2.1. Materials

The materials utilized in this study were commercially pure Zn (98.86 wt%) and Sn (>99.7 wt%) in the form of sheets with the speciﬁcations presented inTable 1. In order to compare the mechanical properties of the constituents, the strength coefﬁcient, strain-hardening exponent and tensile strength of the pure Zn and Sn metals are summarized inTable 2. The Zn and Sn strips with the dimensions of 50 mm150 mm and the thickness of 1 mm and 0.4 mm, respectively, were cut parallel to the original rolling direction from the cold rolled initial sheets. The Zn strips were annealed at 150 C for 30 min. The tensile test samples were wire cut from the ARBed strips, according to the ASTM E8M standard, ori- ented along the rolling direction (RD) plane.

2.2. Surfaces preparation

Zn and Sn strips were initially degreased by acetone and scratch brushed parallel to the rolling direction with a circular stainless steel brush having a 0.3 mm wire diameter. Preparation of the

surfaces was done prior to rolling process in order to increase adhesion and to avoid sliding the strips over each other. It is important that, the time between surface preparation and rolling process be less than 120 s to avoid oxide formation[28].

2.3. Accumulative roll bonding (ARB) process

Generally, the ARB process includes two main steps. Atﬁrst, in order to produce the primary composite or primary sandwich (zero cycle), two Zn sheets as the outer surfaces and one Sn sheet in the middle were stacked together and fastened via copper wires at the four corner points. After that, the stacked strips were roll bonded through a 57% reduction in thickness (ﬁrst step). In the second step, the length of produced sandwich was cut into two halves, surface prepared (degreased and wire-brushed), fastened and roll-bonded.

The roll bonding process was conducted by 50% reduction in the thickness (corresponding to a Von-Mises equivalent strain per cycle:ε¼0.8)[29]. This process was carried out at room temperature and repeated up to 7 cycles without any lubricant and annealing treatment between each cycle by 170 mm diameter rollers with a loading capacity of 20 ton. The rolling speed was set to 12 rpm.

2.4. Microstructural and mechanical properties studies

The microstructure of the ARB processed composites were evaluated at different ARB cycles on the Rolling Direction-Normal Direction (RD-ND) plane, using a scanning electron microscopy (SEM) equipped with an energy dispersive spectroscopy (EDS).

Also, SEM was used in order to observe the fracture surfaces of the samples after tensile tests. The X-ray diffraction (XRD) technique were carried out for phase identification on the surfaces of produced multilayered composites at different cycles in the range of 2q¼20e90by a Philips X’PERT diffractometer, operating at 40 kV and 30 mA with Cu Ka radiations and a step time of 4 s (l¼0.15406 nm). The crystallite (subgrain or dislocation cell) sizes of the ARB processed samples at different cycles were measured by analyzing the XRD patterns via the Materials Analysis Using Diffraction (MAUD) software[30e36]. The mechanical properties of Zn/Sn multilayers were evaluated by performing tensile tests and hardness measurements. The tensile tests were conducted at ambient temperature by a Houndsfield H50KS testing machine at an initial strain rate of 10⁴s¹. In order to attain accurate results, three tensile experiments were done on each sample and their average results were reported. The gauge length and width of the tensile test specimens were 25 mm and 6 mm, respectively. Total elongation of the specimens was measured from the difference between the gauge lengths before and after testing. The Vickers microhardness test was performed on Zn and Sn layers separately, at more thanfive random points on the RD-ND plane, using the Koopa apparatus under a load of 10 g for 15 s.

3. Results and discussion

3.1. Microstructural evolution and XRD analysis

Fig. 1shows the SEM micrographs of Zn/Sn composite strips on

Table 1

Speciﬁcations of the materials used in this study.

Materials Chemical composition (wt.%) Sheet dimensions (l, w, t)

(mmmmmm)

Condition

Zn (98.86) 0.493 Pb, 0.199 Al, 0.131 Si, 0.089 Mg, 0.0728 Fe, 0.0306 Ca, 0.0258 Sx, 0.0239 Ta, 0.0208 F, 0.0145 Cd, 0.0128 K, 0.0085 ln, 0.0081 Cl, 0.0067 Ge, 0.0043 Mn, 0.0028 Ti, 0.0017 Sc

150501 Annealed

Sn>(99.7) 0.04 Pb, 0.02 Cu, 0.02 Bi, 0.01 Fe, 0.01 Al,<0.01 Zn,<0.01 Sb 150500.4 e

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the RD-ND plane during different ARB cycles. It can be observed that as ARB proceeds, the thickness of the individual layers decreases gradually and the number of layers increases per unit thickness. Decreasing in thickness of the layers is due to enhancement of strain by increasing the number of ARB cycles. By repeating the ARB process up to 7 cycles, the shear deformation remarkably increased the equivalent strain and the whole thickness of the specimens were severely deformed. On the other hand, no lubricant condition led to high friction rolling condition and consequently, caused higher rolling pressure and higher deformation of layers.Table 3illustrates the amount of equivalent strain and the number of each Zn and Sn layers at different cycles of ARB process.

It is also noticed fromFig. 1that during the co-deformation of Zn/Sn multilayers, some plastic instabilities including necking and fracturing of layers occurred in the microstructure. AfterARB cycles, a composite with relatively homogeneous distributed Zn lenticular fragments in Sn (with narrow and wavy layers) was achieved.

To investigate the distribution of Zn and Sn layers, X-Ray maps of the samples at different ARB cycles are obtained, as depicted in Figs. 2 and 3. It should be noted that in all SEMﬁgures, bright areas are Sn, and other areas are related to Zn (seeFigs. 2-a and 3-a). At the early cycles, the interface between the layers is almost straight and the continuity of the layers retains up to the third cycle. In the zero cycle (primary sandwich), three layers (including 1 Sn layer and 2 Zn layers) were attached together with a 57% reduction in thickness. The initial bonding between the strips was occurred due to dry condition and adequate pressure applied to the sample by the rollers (seeFig. 2).

Some bright phases were observed in Zn layers. EDS analysis was conducted for characterization of these areas.Fig. 4shows the EDS results corresponding to the point A which indicates the presence of Pb on Zn surface (since Pb is insoluble in Zn). These results are consistent with the chemical composition of the as received Zn (Table 1) which contains the highest value of Pb as an impurity element.

As the number of cycles (i.e. the imposed strain) increased, some wavy irregularities occurred at the interfaces of the layers. Zn layers initiated to neck and fracture at various locations in the microstructure. In the present study, during the co-deformation of Zn/Sn multilayers, the differences between theﬂow properties and mechanical properties of Zn and Sn layers resulted in necking and fracturing in Zn layers (as harder layers)ﬁrst, as it was expected. Zn has lower formability because of the HCP structure and limited slip system. By increasing the number of cycles, Zn layers experienced sequential necking and became more susceptible to fracture in most regions. Meanwhile, Sn layers could endure additional plastic deformation and stretched more in compared with Zn layers, because of more active slip systems at room temperature (BCT structure) and higher ductility (Table 2). Fig. 5 illustrates the occurrence of necking in Zn layers as indicated by the arrows accompanied by the X-Ray maps after three ARB cycles.

Substantial rupturing along the necked regions produces the lenticular fragments in the microstructure [13], as illustrated in Fig. 6. This phenomenon is the most prominent feature at the last cycles. Zn layers are cut by the shear bands[17]which are formed as a result of activated slip systems in the softer phase (Sn), move through the Zn/Sn interfaces, concentrate shear strains and causing

instabilities in Zn layers. Also, stress and loading conditions may inﬂuence the initiation and development of shear bands [37].

Similar observations have been reported in other researches [10,13,17,28,38]. It is worth mentioning that, separation and fracture in Zn layers caused Sn layers to get in touch with each other in some areas.

As the ARB cycles increased, further fragmentation of Zn layers were observed. The fragmented Zn became smaller and distributed more uniformly through the thickness mainly due to increasing the number of shear bands. Eventually, after 7 cycles of ARB process, a laminated composite with wavy interfaces and relatively homogeneously distributed Zn fragments in Sn was achieved (Fig. 7).

Microstructural features observed in the present study from seven cycles of ARB process, were consistent with the observations of Ghalandari et al.[9]in the Al/Sn multilayered composite.

Fig. 8demonstrates the X-Ray diffraction (XRD) patterns of the specimens after different ARB cycles on the rolling direction- transverse direction (RDeTD) plane of the sheets. The XRD results indicated that only Zn and Sn peaks can be observed in the patterns and no intermetallic compounds can be detected. The XRD patterns for 0 (primary sandwich) and 1st ARB cycle, just showed the presence of Zn phase. After the third cycle, a little amount of Sn can be detected. At the last ARB cycles, intensity of Sn peaks slightly increased and more amount of Sn can be observed. At these cycles, the thickness of surface layer is lower than the x-ray penetration depth. So, a greater amount of Sn could be distinguished. Slight broadening and decreasing the heights of Zn and Sn peaks partic- ularly at low angles, can be related to grain reﬁnement. Moreover, another factor for the peak broadening may be the high residual stresses introduced by the rolling process. Increase in the intensity of Zn peaks can be attributed to texture formation.

The mean crystallite size from the XRD results (peak broadening) for different ARB cycles are presented inTable 4. The results show that the mean crystallite sizes in Zn and Sn have been reduced from 106 nm to 36 nm and 150 nme43 nm after eight ARB cycles, respectively. The mean crystallite size calculated by MAUD software has also been reported in the Al/Cu/Mn[30]composite.

3.2. Mechanical properties 3.2.1. Tensile tests

In order to study the effects of ARB cycles on the mechanical properties of Zn/Sn composites, tensile test was conducted.

Figs. 9 and 10show the engineering stress-strain curves of the annealed materials and produced composites samples, respectively at room temperature. The curve shown inFigs. 9 and 10can be used to compare the tensile strength of ARBed samples and pure raw materials. The variation of the ultimate tensile stress (UTS) and the elongation for various samples at different ARB cycles are presented inFig. 11.

Regarding toFig. 11, it is obvious that the highest values of ultimate tensile strength of produced multilayered composites obtained in the zero cycle which was about 146 MPa, then reduced to 123 MPa in the second cycle and dropped continuously until the last cycle with UTS of about 55 MPa.

By comparing theﬁgures (Figs. 9 and 10), it can be observed that the maximum tensile strength of produced multilayer composites, Table 2

Mechanical properties of the constituents used in Zn/Sn multilayered composite.

Element Strain hardening exponent Coefﬁcient of strength

(MPa)

Tensile strength (MPa)

Zn 0.284 138.2 167

Sn 0.31 44.37 22

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Fig. 1.The SEM micrographs of ARB processed Zn/Sn composites after: (a) zero cycle (primary sandwich), (b) one cycle, (c) two cycles, (d) three cycles, (e) four cycles, (f)ﬁve cycles, (g) six cycles and (h) seven cycles.

Table 3

Variation in the number of layers and the amount of equivalent strain by increasing ARB cycles.

ARB Cycle (n) Number of Zn Layer (22ⁿ) Number of Sn Layer (2ⁿ) Total layer number (32ⁿ)

ε¼

2ffiffiffi p3

Inð2Þ

n¼0:8n

0 2 1 3 >0.8

1 4 2 6 1.6

2 8 4 12 2.4

3 16 8 24 3.2

4 32 16 48 4

5 64 32 96 4.8

6 128 64 192 5.6

7 256 128 384 6.4

Fig. 2.(a) SEM image of Zn/Sn composite after zero ARB cycle and the corresponding SEM X-Ray maps of (b) Sn and (c) Zn.

Fig. 3.(a) SEM image of Zn/Sn composite after one ARB cycle and the corresponding SEM X-Ray maps of (b) Sn and (c) Zn.

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was even less than the pure annealed Zn tensile strength. The values of tensile strength of pure Sn and Zn were 22 MPa and 167 MPa, respectively, as depicted inFig. 9. The maximum tensile strength of produced multilayered composites was about 6.6 times higher than the pure Sn and 0.8 of the annealed Zn.

In the case of MMCs produced by ARB process, it is believed that at primary cycle, bonding of the layers is not strong enough[39,40].

Consequently, during tensile test, micro cracks could be nucleated between the interfaces of the layers as a suitable region causing the failure of the composite. On the other hand, due to insufﬁcient strong bonding between the layers, porosities and micro cracks between the layers of the primary sandwich are formed. With the formation of microcracks and propagation of them between the layers, endurance of the composite components decreased and eventually led to a reduction in the composite tensile strength at this cycle. As shown inFig. 10, strength of the composite increased from ﬁrst cycle to the second cycle. The main reason for this strength increase is strain hardening by dislocations. Generally, the variations of the strength in the ARBed materials are followed by two major strengthening mechanisms[8,11,14,15,18,39,41e43]:

In the primary cycles of the ARB process, strain hardening or

Fig. 4.EDS results corresponding to the point A in Zn layer at the zero cycle.

Fig. 5.(a) SEM micrograph of Zn/Sn composite after three ARB cycles and the corresponding SEM X-Ray maps of (b) Zn and (c) Sn.

Fig. 6.SEM image of Zn/Sn composite after three cycles of ARB, showing fragmentation of Zn layers.

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dislocation strengthening plays the main role in enhancing the strength and by increasing the number of ARB cycles, higher strengths are related to grain reﬁnement (ultraﬁne grains plays the main role in strengthening).

In the early stages of the ARB, a large number of structural de- fects such as dislocations and subgrains are formed as a result of cold deformation. The lamellae structure of Zn/Sn multilayer caused strengthening at this stage too. Because the interfaces between constituents could provide an effective barrier for dislocations mobility[44,45]. Another responsible factor for increase in Fig. 8.The XRD patterns of Zn/Sn composites subjected to (a) 0 cycle, (b) 1 cycle, (c) 3

cycles, (d) 5 cycles, (e) 6 cycles and (f) 7 cycles.

Table 4

The variation of the mean crystallite size in Zn and Sn by increasing the number of ARB cycles.

ARB Cycle (n) The mean crystallite size of Zn The mean crystallite size of Sn

3 106 nm 150 nm

5 44 nm 86 nm

6 40 nm 92 nm

7 36 nm 43 nm

Fig. 9.Engineering stressestrain curves for annealed elemental materials.

Fig. 10.Engineering stressestrain curves for Zn/Sn composites after different cycles of ARB process.

Fig. 7.(a) SEM image of Zn/Sn composite after seven ARB cycles and the corresponding SEM X-Ray maps of (b) Sn and (c) Zn.

0 1 2 3 4 5 6 7

0 40 80 120 160

0 20 40 60

Number of ARB cycles

Ultima te te nsile str ess(MPa ) Elongation(%)

Ultimate tensile stress(MPa) Elongation(%)

Fig. 11.Variations of tensile strength and elongation of Zn/Sn composites in different cycles of ARB process.

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the tensile strength of Zn/Sn composite was the bonding between Zn and Sn layers which became stronger and caused strength enhancement [10,46,47]. The stronger adhesion and improved unbonded regions at interfaces of Zn/Sn was the result of increasing ARB cycles and higher rolling pressure[39]. Strength reduction of Zn/Sn composites is related to reduction or annihilation of dislocations and strain hardening.

After three cycles, Zn layers could not support the load sufﬁ- ciently[48]and kept their continuity in the composite. The drop of strength after the second cycle can be attributed to substantial

necking of Zn layers which led to separation and fragmentation of Zn layers in most regions. Furthermore, the friction between the rollers and the surface of strips and the high imposed strain generate the internal heating in Sn[14]. It is noteworthy to mention that the large dislocation density can lower the recrystallization temperature of the metallic layers. Since the melting temperature of Sn is low, it is likely that the work softening phenomena such as dynamic recovery and recrystallization occur in Sn[28]. By pro- ceeding ARB process, the thickness of Sn layers reduced substan- tially and lost their resistance against the more deformations[9].

These results are in agreement with results reported by other re- searchers in the Cu/Zn[13], Cu/Ni[12]and Al/Sn[9]composites.

As stated earlier, the role of each layer (i.e. Zn and Sn) was individually investigated. So in summary, the strength reduction trend of Zn/Sn composites can be attributed to these factors:

1: Softening phenomena, which acts as a dominant mechanism on the strength enhancement and grain reﬁnement (Recrystal- lization in the structure of Sn and reduction in density of dislocations).

2: The embrittlement of Zn and continuous fragmentations of its layers (which was as a hard phase and as a reinforcement agent) The strength enhancement in the ARB processed composites is a typical behavior, but in this research, above factors overcame on the strength enhancement and acted as the dominant cases. A reduction in the tensile strength, has also been reported for the Al/Cu [28], Al/Sn[9], Cu/Zn[13], Cu/Ni[12], Mg/Al[49], Al/Fe[48], Al/Cu/

Al2O3[39]and Al/Cu/Sn[16]composites.

Variation of elongation with ARB cycles (Fig. 11) demonstrates that the elongation of multilayers decreased gradually by Fig. 12.Microhardness variation for individual Zinc and Tin layers of Zn/Sn ARB pro-

cessed composites.

Fig. 13.Tensile fracture surfaces of Zn/Sn multilayered composites at: (a) primary sandwich, (b) third cycle, (c)ﬁfth cycle and (d) seventh cycle.

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increasing ARB cycles. Accordingly, elongation was decreased signiﬁcantly from the zero cycle up to the third cycle, while for the next cycles it was decreased slightly and reached about a plateau value. Variation of elongation during ARB process can be attributed to strain hardening, layers bonding at the interfaces and distribution of Zn fragments between Sn layers, in Zn/Sn composites[45].

At the preliminary cycles, the rapid increase of strain hardening (accumulation of dislocations and internal stresses) played a sig- niﬁcant role in the ductility. Due to decreasing mobility of dislocations, elongation decreases with strain hardening[12,14]. With increasing the number of ARB cycles, the bonding strength increasing affects the ductility in an opposite direction and causes the elongation decreased. Similar trends were also reported for the 6061 aluminum alloy[37]and Al/Cu composite[28]produced by the ARB process.

3.2.2. Microhardness test

Vickers microhardness variations of Zn and Sn layers at different cycles of ARB process are plotted inFig. 12.

As can be seen inFig. 12, the microhardness measurement is carried out individually for each layer. The microhardness measurement for Zn layers was performed up to the last ARB cycle, but the microhardness of Sn layers was measured just for two cycles. In the next cycles, the thicknesses of Sn layers became too small to measure the hardness and the mean diameter of the indenter was larger than the thickness of Sn layers. Similar restrictions on hardness measurement of layers in multilayer composites in other investigations are also reported [9,15]. Comparison of hardness variation indicates that the hardness of Zn layers is much higher than Sn layers. It is noteworthy that the differences in microhardness values can be attributed to the different strain hardening rate of Zn and Sn layers during plastic deformation[39]. As illustrated in Fig. 12, the hardness of Zn layers increased after primary sandwich preparation and then dropped in the third cycle. Afterwards, the hardness reached to its maximum value (42 Vickers) and dropped again in following cycles. Increasing in the microhardness of Zn layers after primary sandwich preparation (at relatively low strains) can be attributed to the strain hardening (as a result of high dislocations density), dislocations interaction and subgrains formation[25,37,38]. Microhardness enhancement after the third ARB cycle could be related to grain reﬁnement[10,17,50]. It is reported that strain hardening occurred in the initial cycles of ARB process, has more effect on increasing microhardness than the grain reﬁnement[29]. A gradual drop happened from the fourth cycle to the last cycle in the hardness of Zn layers may be due to activation of the softening mechanisms[17]or the dislocations and residual stresses annihilation (dynamic recovery) [16,51]. Fig. 12 also revealed that, the ARB process led to increase the hardness of Sn layers (from 6.9 Vickers to 7.5 Vickers). The main reason for increasing the hardness of Sn layers is strain hardening as a result of high dislocations density[38].

3.3. Fractography

The fractured surfaces of the samples subjected to tensile test were examined using SEM. Fig. 13 Shows the tensile fractured surfaces of the ARB-processed composites after different cycles.

Regarding toFig. 13(a), it is clear that Sn and Zn layers have different types of fracture. The fractured surfaces reveal equiaxed dimples and shear zones in Sn layers which indicate that the fracture mode is ductile in Sn layers. Also, fractured surfaces in Zn layers are bright and smooth and show cleavage characteristics accompanied by a lot of cracks which indicate that the fracture mode is brittle [15]. These failure modes were observed in other in-

vestigations of materials under ARB process. In the same cases, ^{Fig. 14.}SEM micrographs of Zn/Sn composite afterﬁve ARB cycles with (a,b) low and (c) high magniﬁcations.

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ductile fracture of Sn layers in the Al/Sn[9]and Al/Cu/Sn[16]and brittle fracture of Zn layers in the Al/Zn [10] and Cu/Zn/Al [15]

multilayered composites are reported. Ductile fracture occurs by nucleation and coalescence of microvoids ahead of the main crack and very limited dislocations activity. It should be noted that, this type of fracture in most materials, have a grayﬁbrous appearance with equiaxed or hemispheroidal dimples[37]. In the present study, fracture surfaces of the composites exhibited a combination of ductile and brittle fractures while the brittle fracture is the dominant fracture mode. The brittle behavior of fractured surfaces of specimens could be related to the embrittlement of Zn layers with HCP structure and also their higher amounts in the composites as discussed earlier. By increasing the load during tensile tests, the coalescence of the cracks resulted in fracture of Sn layers. By pro- ceeding ARB process, fracture mechanism of Sn layers in composites is changed and the size and depth of the dimples in Sn layers is reduced (Fig. 13(b) and (c)). According to these observations, dimples are not as deep as dimples in the initial cycles and equiaxed dimples changed to shallow and small elongated shear dimples due to shear stresses. So, the fracture mechanism is changed to shear ductile fracture. According to previous investigations in various composites, the decrease in the dimple size is due to grain reﬁne- ment, work hardening and fragmentation of hard phase[52]. The shear dimples have also been observed in other researches such as Al/Cu[25], Cu/Ni[12], Al/Sn[9], Al/Zn[10]and Al/Ni/Cu[14].

Fig. 14shows SEM micrographs of Zn/Sn composite afterfive ARB cycles. The ductile and brittle failures of the composite surfaces are visible in both low and high magnifications. As this figure suggests, it is comparatively difficult to identify the individual layers due to increasing the number of layers and decreasing the thickness of layers. Some distances and separations occurred between the layers are probably due to existence of tensile stresses during tensile test and delamination of the layers.Fig. 15shows SEM micrograph and the corresponding X-Ray maps of the fractured surfaces of Zn/Sn multilayered composites afterfive cycles.

Fig. 16shows SEM micrographs of Zn/Sn composite after six ARB cycles with a widespread smooth and cleavage surfaces. As mentioned before, by increasing ARB cycles, the plastic deformation and elongation values in ARBed samples decreased. It should be noted that, debonding in interfaces has an important role in decreasing the ductility of the specimens[37]. On the other hand, decreasing the ductility is associated to formation of microcracks and embrittlement of Zn layers with the HCP structure which cannot tolerate more strains (mainly due to strain hardening which causes to decrease the mobility of dislocations. Alvandi et al.[52]

have also observed similar trend in 7075 aluminum alloy which fracture surfaces of the composites gone to more brittleness as the ARB cycles proceeded.

4. Conclusions

In the present study, microstructure evolution and mechanical

properties of Zn/Sn multilayer composites were evaluated during different ARB cycles and following conclusions were obtained:

1. Up to the second ARB cycle, the interfaces between the layers were almost straight. After the third cycles, Zn layers initiated to neck and were fractured in the subsequent cycles. After seven ARB cycles, a laminated composite with narrow and wavy layers of Sn and distributed lenticular fragments of Zn was achieved.

2. The XRD analysis showed that no intermetallic phase has been formed and only the major peaks of pure elements are visible in the patterns.

3. The accumulative roll bonding process led to decreasing the mean crystallite sizes of both Zn and Sn.

4. The maximum value of the tensile strength of produced composites was in the zero cycle with about 146 MPa. It was about 6.6 times higher than the pure Sn and 0.8 of the annealed Zn. By increasing the ARB cycles (except in theﬁrst cycle with the amount of 123 MPa), the tensile strength decreased continuously up to the last cycle due to the softening mechanism and embrittlement of Zn layers.

5. By increasing the number of ARB cycles, ductility of the composites was signiﬁcantly decreased due to strain hardening and reached to a plateau in the last cycles.

6. Vickers microhardness of Zn layers was increased in theﬁrst and the fourth cycles and decreased in other cycles. It was also found that, microhardness of Sn layers was increased from 6.9 to 7.5 Vickers for theﬁrst cycle.

7. The SEM micrographs of fractured surfaces exhibited a combination of ductile and brittle fractures. Rupture mode of Sn layers Fig. 15.(a) The SEM micrographs of fracture surfaces of Zn/Sn composite afterﬁve ARB cycles and the corresponding X-Ray maps for (b) Sn and (c) Zn.

Fig. 16.Fractured surface of Zn/Sn composite after six ARB cycles.

(10)

was ductile in initial cycles and changed to shear ductile fracture in the next cycles. Fracture mode of Zn layers was brittle at all cycles.

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