Deformation behavior and crack propagation on interface of Al/Cu laminated composites in uniaxial tensile test
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
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
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
An appropriate resistance to cracks propagation in laminated composites is required to guarantee the structure and properties
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
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
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
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)
Table 1 EDS results of typical points in Fig.2b in Cu/Al interface
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
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
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
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
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
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
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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