稀有金属(英文版) 2020,39(03),296-303

Deformation behavior and crack propagation on interface of Al/Cu laminated composites in uniaxial tensile test

Xiao-Bing Li Yuan Yang Yu-Song Xu Guo-Yin Zu

School of Metallurgical and Materials Engineering,Jiangsu University of Science and Technology (Zhangjiagang)

School of Materials Science and Engineering,Northeastern University

作者简介：*Xiao-Bing Li,e-mail:lxbing2009@126.com;

收稿日期：29 November 2016

基金：financially supported by the Natural Science Foundation of Higher Education Institutions in Jiangsu Province (No.16KJB430012);

Deformation behavior and crack propagation on interface of Al/Cu laminated composites in uniaxial tensile test

Xiao-Bing Li Yuan Yang Yu-Song Xu Guo-Yin Zu

School of Metallurgical and Materials Engineering,Jiangsu University of Science and Technology (Zhangjiagang)

School of Materials Science and Engineering,Northeastern University

Abstract：

The microstructural characterization and uniaxial tensile tests of Al/Cu laminated composites were taken to investigate the interface effect and fracture process of the composites.The electron microscopic graphs before and after tensile test were used to evaluate the fracture behavior.Experimental results show that the fracture surfaces of laminated composites mainly present brittle failure characteristics,accompanied with several dimples on the matrixes and a few tearing on the interface.Cracks generally initiate from the interfacial interlayer and variously propagate depending on the interfacial bonding.It is found that Cu/Al interface with enhanced bonding strength generally hinders the propagation of interlayer cracks,while the interface with weak bonding delaminates by the cracks propagation through the interfacial defects.The additional shear stress on the interface between Cu and Al layers due to their different tensile ductilities aggravates the interfacial propagation of cracks.The local plastic deformation of inpidual matrix layer then occurs after cracks coalesce and failure in the interface.Therefore,the strong bonding interface and matching properties between inpidual matrix layers are required to improve the fracture performance of Al/Cu laminated composites.

Keyword：

Laminated composite; Interface; Failure analysis; Bonding; Electron microscopy;

Received： 29 November 2016

1 Introduction

Metallic laminated composites have been attracting much attention from manufacture industry because of the property combination of components and the low material cost [ 1, 2] .The interfaces between inpidual components provide the important transition to the laminated structure and are often expected to be with a good bonding quality,such as with metallurgical bonding and without any structural defects [ 3] .Nevertheless,it is difficult to achieve the excellent bonding interface for the composites with large length-scale based on present preparation technologies,especially for the metal components with huge differences of mechanical properties and chemical activity.Particularly,Al/Cu laminated composites are a typical structure as mentioned above due to the remarkable reaction between Al and Cu inpidual layer.Several intermetallic compounds,such as CuAl₂,Cu₄Al₁₃,Cu₉Al₄ and CuAl,can form by the reaction on the Cu/Al interface [ 4, 5] .These compounds are usually brittle relative to the matrix metals and easily cause a destructive effect on the interface.For instance,the bonding strength of Cu/Al clad sheets produced by the liquid-forming technology clearly decreases when the thickness of interfacial compounds is in the scale of tens of micrometers [ 6] .Despite that,laminated composites from the bulk-forming technology need heat treatments to attain an appropriate interfacial interlayer to improve the bonding between inpidual layers [ 7] .Therefore,the interfacial composition and structure have always been the main concerns in the field of composites.

For laminated composites obtained by the bulk roll bonding,Jamaati has summarized the interfacial bonding strength between kinds of dissimilar metals and regarded the plastic deformation of metal components as the crucial factor on the metallic bonding at ambient temperature [ 8, 9] .The measurements have revealed that the interfacial bonding in the joint only with plastic deformation is weak.The bonding quality has been found to be relevant with the atomic diffusion and interaction on the interlaminar interface [ 10] .Al/Cu laminated composites can be prepared by the roll bonding at room temperature and evaluated temperature.During hot roll bonding process,oxidation and atomic reaction on the interface are difficult to control.Thus,the cold roll bonding and annealing technique have been used to obtain Al/Cu composites with improved interfaces.However,the demand of plastic deformation is extremely large in the cold roll bonding process and hinders the application in manufacturing composites [ 9] .

Nowadays,the techniques of providing severe plastic deformation (SPD) have been increasingly used to improve the mechanical properties of metals and composites in the form of refining the grains into ultra-fine scale [ 11, 12] .Owing to the additional shear strain caused by the different speeds of two work rolls,asymmetrical rolling technique has been regarded as a potential method of severe deformation to manufacture the ultra-fine grained (UFG) metals [ 3, 13] .Owing to the additional cross shear region in the roll gap,it is useful for the reduction of roll pressure in the way of eliminating the friction effect.The SPD ability of asymmetrical rolling can enhance the interfacial deformation between matrix component layers and thus increase the interfacial bonding strength according to the solid-state bonding theory.Moreover,the additional cross shear region in the roll gap reduces effectively the roll pressure in the way of eliminating the friction effect.At present,it has been applied to clad metal layer on the matrix sheet with large width and has produced laminated composites with strong bonding between inpidual sheets [ 6, 14, 15] .Studies of deformation behavior by theoretical calculation and computational simulation show that interfacial microstructure and bonding of the dissimilar metals couple can be improved by asymmetrical cold roll bonding process [ 16] .

An appropriate resistance to cracks propagation in laminated composites is required to guarantee the structure and properties [ 2, 17] .The fracture analysis is always conducted to investigate the cracks behavior and failure process.Chen et al. [ 18] have found that large ductility of the layered nanostructured stainless steel is achieved by the interlaminar multiple cracking due to the interlaminar debonding of weak bonding interface and the crack deflection of strong bonding interface in tensile tests.Lee et al. [ 19] have found that interfacial delamination resulting from the intermetallics in roll-bonded stainless steel-AlMg clad sheet is retarded during three-point bending test and is accompanied with lateral crack propagation throughout the composites.Obviously,the crack growth and interaction with the interface determine the mechanical properties of laminated composites.Therefore,the in situ observation of cracks activity in laminated composites has been attracting much attention.The direct observation of crack propagation in copper-niobium multilayer has been conducted by Hattar et al. [ 20] through the in situ crosssectional transmission electron microscope (TEM) straining.The result indicates that failure occurs with no direct signs of interface debonding in nanolayered material.Owing to the strict experimental condition,initiation mechanism and propagation of cracks have been often investigated based on the fracture observation and numerical simulation.Rawer and Perry [ 21] have studied the crack initiation and propagation in laminated metal-intermetallic composite with the typical structure of alternating ductile with brittle layer,i.e.,Al-NiAl laminated composites.It is found the inhomogeneous performance between different layers influences the crack propagation through the inpidual layer and interfaces.Liu et al. [ 22] have studied the dynamic failure and damage evolution of the interface in laminated composites under impact loading according to the finite element analysis.

However,the effect of interface on the crack and fracture behavior in Al/Cu laminated composites is required to clarify.The failure behaviors of bonding interface and matrix layers need to be studied to optimize the properties of laminated structure [ 23, 24] .In this work,interfacial characterizations before and after tensile tests were taken to study the deformation and crack behavior of laminated composites.Cracks initiation and propagation mechanism were discussed based on the micrographs.

2 Experimental

The pure copper Cl 1000 (Cu:99.90 wt%,thickness of0.8 mm) and aluminum A1100 (Al:99.00 wt%,Si:0.40 wt%,Cu:0.05 wt%,thickness of 0.9 mm) sheets were used as raw materials.They were cut in dimensions of30 mm×150 mm and then were cold roll-bonded on the laboratory four-high mill with work roll diameter of92 mm.The metal surfaces were degreased and scratched to remove contaminations.The rotation speed of the lower work roll was fixed at 20 r·min^-1,which was 1.31 times faster than that of the upper work roll.The schematic of roll bonding process is shown in Fig.1a.The stacked layers were reduced to 0.75 mm in the roll gap with no lubrication.After that,as-rolled laminates were annealed at350℃for 30 min in the resistance furnace.

The metallographic observations were taken on the cross section of Al/Cu laminated composites by optical microscope (OM,OLYMPUS-PMG51) and scanning electron microscope (SEM,SSX-550).As illustrated in Fig.1b,theAl/Cu interface in laminated composites was defined as the interface near the upper roll,while the Cu/Al interface near the lower roll.The crystallographic features of the interface was obtained by TEM (Tecnai G2-20) equipped with energy diffraction spectroscopy (EDS) at 200 kV accelerating voltage.The cross-sectional films were thinned by precision ion-milling technique.

Fig.1 Schematic diagram of experimental:a asymmetrical cold roll bonding process,b Al/Cu laminated composite with Al/Cu interface and Cu/Al interface,c tensile specimen (RD rolling direction,ND normal direction,TD transverse direction)

Tensile tests and peeling tests of Al/Cu composites were conducted on a mechanical testing system CMT5000 at room temperature.Every test set was repeated with three duplications,and the mechanical properties were calculated in average of experimental results.The tensile specimens were made in the rolling direction following Fig.lc.The nominal strain rate was 0.1 s^-1.The mechanical performance and deformation behavior of laminated composites were then analyzed according to the measurements and characterizations.The peeling strength as the peeling force per width directly implied the interfacial bonding strength between different layers.

3 Results and discussion

3.1 Interfacial microstructure and bonding strength

The deformation of matrix metals during asymmetrical roll bonding process can be enhanced by the additional shear stress from different plastic flows of inpidual layers.It has been confirmed that the interfacial micros tructure between matrix layers is improved in several dissimilar metal couples [ 16] .Figure 2 shows the microstructure of Al/Cu laminated composites produced by asymmetrical cold roll bonding and annealing.Al layers tightly touch with Cu layer,accompanying with interlayer between those inpidual layers.It is well known that enhanced deformation induces extensive cracks on metal surfaces and promotes the metallic bonding between Al and Cu layers [ 25] .The tight interface makes it easy for the atomic migration from one layer to the other.In addition,vacancies induced by the enhanced deformation are supposed to promote the atomic diffusion.After annealing at 350℃,the diffusions of Al atom and Cu atom remarkably occur across the interface.When the interfacial diffusion is supersaturated,atomic reactions form and lead to the generation of interlayer,as indicated by arrows in Fig.2a.Owing to low temperature and short time in annealing process,the interfacial reaction is not significant [ 10] .However,it is found that the interlayer in Al/Cu interface is much thinner than that in Cu/Al interface.This asymmetry can be attributed to the different plastic deformations of inpidual layers during asymmetrical roll bonding process illustrated in Fig.la [ 26, 27] .

The high magnifications of bonding interfaces illustrated in Fig.2 show the microstructure of interfacial interlayer.It is seen from TEM image that the interlayer in Cu/Al interface and Al/Cu interface all contains two sublayers with different morphologies.The chemical composition in two typical points (A and B labeled in Fig.2b) was measured by EDS method.According to EDS results in Table 1and Cu-Al binary alloy phase diagram,the intermetallic compound in the sublayer near Cu matrix can be deduced as Cu₉Al₄ while that in the layer near Al matrix as CuAl₂.Furthermore,the selected-area electron diffraction (SAED)patterns of the interfacial layer,as inserted in Fig.2b,verify the intermetallic compounds on the basis of crystallographic feature.TEM images of bonding interfaces confirm the growth of interlayer by atomic diffusion.The growth track of interfacial compound can be seen from Fig.2c.Except that,a few microcracks indicated by the hollow arrow in Fig.2b exist between interfacial sublayers.They could be caused by the misfit volume of compounds or the original unbonded point between matrix layers.It is critical to reduce the interfacial defects to prevent the damage and failure of composites [ 28] .In this work,both the enhanced plastic deformation during asymmetrical roll bonding and modest atomic reaction on the interface during annealing provide an appropriate way to improve the interfacial microstructure.

Fig.2 Micrographs of cross section of Al/Cu laminated composites with bonding interfaces after annealing at 350℃for 30 min:a OM image of cross section,b TEM image of Cu/Al interface,and c TEM image of Al/Cu interface (insets in b being SAED pattern of sublayers)

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Table 1 EDS results of typical points in Fig.2b in Cu/Al interface

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Table 2 Bonding strength of different interfaces in Al/Cu laminated composite

The interfacial bonding strength is critical to the laminated composites and is significantly dependent on the interfacial microstructure.The bonding strength is generally improved by the atomic diffusion on the interface.Nevertheless,the bonding strength is undermined by the formation of intermetallic compounds in the bonding interface.In this work,the bonding strength of two interfaces was measured according to the peel tests and is listed in Table 2.The results reveal a higher bonding strength for Cu/Al interface than that of Al/Cu interface.It confirms the enhanced bonding of interlaminar interface through the improved microstructure.

Fig.3 Stress-strain curve of Al/Cu laminated composites under uniaxial tensile test

3.2 Fracture analysis of laminated composites

The mechanical properties of Al/Cu composites were obtained from the engineering stress-strain profile shown in Fig.3 according to tensile tests.The ultimate tensile strength (UTS) and elongation after fracture are215.97 MPa and 7.3%,respectively.While the composites produced by cold roll bonding process with symmetrical roll speed possess the ultimate strength of 208.04 MPa,it is found that Al/Cu composites in this work with asymmetrical roll bonding approach can be strengthened by the improvement of bonding interfaces.Nevertheless,the increase in UTS is not remarkable in the specimens produced by asymmetrical roll bonding process.It is suggested that the improved interfacial bonding only hinders the crack propagation along the interlaminar interface,but not completely resists the cracks motion in the matrix layer.The results also indicate that the mechanical properties of laminated composites can be enhanced by the interface,but be crucially determined by the mechanical performance of matrix materials.

It is notable that two turning points exist at the beginning stage of plastic deformation in stress-strain curve.However,the tensile curves of pure copper and aluminum present smooth feature in the stage of uniform plastic deformation.According to the microstructure of laminated composite,the turning points can be ascribed to the deformation of interfacial interlayer.The Cu/Al interface and Al/Cu interface in laminated composites possess different microstructures and bonding strengths,thus differently influencing the mechanical performance of composites.At Point 1 in Fig.3,the weak interface with low bonding strength early fractures and causes the first yield of composites.With the strain increasing,the strong interface with high bonding strength fractures and results in the second transition in the stress-strain curve.The deformation behavior of different bonding interfaces was analyzed by the fractography in the following section.

SEM image in Fig.4 indicates that Cu and Al matrixes in the fracture mainly contain the smooth surface in terms of brittle fracture mode,which consists with the low elongation ratio of composites.After heat treatment in short time,the significant work hardening formed in SPD process incompletely releases and results in the low plasticity of laminated composites.Among Al and Cu layers,the interfacial interlayers with supersaturated solid solution and intermetallics firstly fracture during tensile tests [ 2] .Therefore,the available area bearing tension stress reduces in composites.It causes the reduction of resistance to uniaxial tensile load and then brings the plastic instability to composites.Afterward,cracks rapidly form and grow until the serious fragmentation of bonding interface.Owing to the strain hardening of Al and Cu matrix,the layers get notable strain strengthening in unstable deformation process and suppress the premature necking.Finally,a few of ductile dimples indicated by the white arrows in Fig.4a form on the fracture of matrixes.Considering the availability of layered structure,the laminated composites are deprived of the efficiency when the bonding interface ruptures and grows into the delamination [ 29] .Therefore,the subsequent deformation of inpidual matrixes hardly contributes to the desired plasticity of Al/Cu composites.

When a fragmentation occurs on the interface,the nearby matrix layer will independently deform under the tensile load.The concentrated deformation leads to the local necking of inpidual layers.At this moment,the different deformation degrees of Cu and Al matrix layer introduce an additional shear stress to the associated interface [ 2, 21] .It is found that the interface with high bonding strength can resist the additional stress,while the weak interface delaminates,as shown by the hollow arrow in Fig.4a,due to the additional stress.The Cu/Al interface is magnified in Fig.4b to evaluate the damage.It indicates that a layered structure with tight bonding interface retains between Cu and Al layers,accompanied with the interlayer and shear tracks on the matrixes shown by white arrows.The smooth surfaces of matrixes adjacent to the interface reveal that brittle fracture initiates from the interfacial interlayer and then induces the necking and rupture of metal matrixes.

In this work,the fractures of laminated composites are still observed on longitudinal section to evaluate the deformation behavior of bonding interface.SEM image shown in Fig.5a corresponds to the fracture in Fig.4a.It is found that the interfaces in Al/Cu laminated composite demonstrate different fracture patterns.Al/Cu interface presents the evident delamination between Al and Cu matrixes,while Cu/Al interface only contains some voids indicated by the arrows in Fig.5a with the regular layered structure.The formation of delamination on weak bonding interface demonstrates the serious failure of laminated composites.The fractures reveal that the bonding strength of the interlaminar interface is critical to the crack resistance of laminated composites under external loading [ 30] .

The magnification of Cu/Al interface shown in Fig.5b indicates that some cracks form in the interfacial interlayer at local points and then propagate in the tensile direction and finally cause partial separations of the interlayer.When the interface possesses a high bonding strength,the interlayer can resist the delamination and keep its original structure.However,some cracks form in the interlayer and extend along the defective locations,as illustrated by Fig.5b.According to the interfacial characterizations in Fig.2,it is known that cracks grow along the boundary of different intermetallics in the interlayer.Nevertheless,the damage of interlayer in Cu/Al interface is restricted due to the small amount of intermetallics.The results reveal the disadvantage of the excessive intermetallics in bonding interface.

Fig.4 SEM images of fracture of Al/Cu laminated composites.a Fracture morphology and b magnification of Cu/Al interface in selected zone in a

Fig.5 SEM images of fractured Al/Cu laminated composites observing on longitudinal section:a fracture on longitudinal section and b magnification of Cu/Al interface indicated by selected zone in a

In addition,the interfacial microstructure away from the fracture can be used to reveal the crack propagation and failure behavior.As shown in Fig.6,high magnifications of the interfaces are taken on ion-milled specimens by the field emission SEM (FESEM).The delamination clearly exists on the Al/Cu interface.Apparently,some cracks laterally form in the brittle interlayer under tensile strain and grow along the weak bonding interface by the additional shear stress in the interface of Cu and Al layer.If the interfacial bonding is strengthened,crack propagation can be hindered and thus discontinuous cracks will form in the interlaminar interface.However,it is noteworthy in Fig.6b that a gap exists between matrix layers and deviates from the regular path of the interface.According to the bonding theory of solid pressure welding,the gap can be ascribed to the cleavage of the interface without underlying extrusion and interlocking during roll bonding process [ 9] .

3.3 Failure behavior of laminated composites

The experimental results demonstrate that the interfacial bonding quality determines the crack propagation on the interlaminar interface and the failure behavior of laminated composites.As shown in Fig.7,a simple model is proposed to understand the interfacial deformation behavior of laminated composites.At the early stage of tensile loading,laminated composite keeps its layered structure and soundly resists to the load F₁.With the tensile load increasing,the total strain of composites exceeds the ultimate strain of the relatively brittle interlayer on the interface and leads to the formation of cracks between Al and Cu matrix layers [ 21] .Afterward,the matrix layers are inpidually extended at the crack zones as indicated in the stage of F₃.For the different bonding interfaces,two different failure modes can be concluded as follows.One is for composites with high bonding strength acting as a strong resistance to the crack propagation,and no delamination forms on the interface as shown by the stage of F₄in Fig.7.Nevertheless,the other condition is for composites with weak bonding interface.At the stage of moderate strain,the additional shear stress from the misfit ductility between Al and Cu layers leads to the crack propagation along the interface.Finally,the laminated composite fractures with serious interfacial delamination.

Fig.6 FESEM images of Al/Cu laminated composites observing on longitudinal section with a distance of 2 mm from fracture:a Al/Cu interface and b Cu/Al interface

Fig.7 Schematic diagram of deformation behavior of Al/Cu lami nated composites during tensile test with tensile load increasing

Based on the characterizations and analysis,it can be achieved that cracks mostly initiate from the interfacial interlayer and variously propagate dependent on the interfacial resistance derived from bonding strength [ 31, 32] .In tensile loading of the composites with strong bonding interfaces,interfacial microcracks are extended but not yet grow into the coalescence.In this case,the interfaces keep the transition effect between matrix layers and guarantee the availability of laminated composites.Nevertheless,the weak bonding interface is destroyed due to the rapid growth and propagation of cracks on the interface.This action then induces the consequent delamination of composites.

4 Conclusion

The microstructure and bonding strength of interlaminar interface in Al/Cu laminated composites are improved due to the enhanced deformation in asymmetrical cold roll bonding and the modest atomic diffusion in annealing process.During uniaxial tensile test of laminated composites,the cracks generally initiate from the interfacial interlayer with intermetallic compounds,such as CuAl₂,Cu₉Al₄,and then primarily propagate along the interface with structural defects and weak bonding strength.The crack motions finally lead to the delamination between different matrix layers.The Cu/Al interface with high bonding strength can resist the crack propagation and maintain the layered structure of laminated composites.The additional shear stress between Cu and Al layers with different ductilities aggravates the propagation of interfacial cracks.Owing to the deformation and first rupture of interfacial interlayer,the fracture of laminated composites mainly presents a brittle failure mode,accompanied with several ductile characteristics such as dimples and tearing on the matrix layers.

Acknowledgements This work was financially supported by the Natural Science Foundation of Higher Education Institutions in Jiangsu Province (No.16KJB430012).

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