Evaluation of mechanical properties and structure of multilayered Al/Ni composites produced by accumulative roll bonding (ARB) process

Abstract

Multilayered Al/Ni composites were produced by accumulative roll bonding (ARB) process using Al 1060 and commercial Ni foils. In this process it was observed that nickel layers necked and fractured as accumulative roll bonding passes increased. After six ARB passes, a multilayered Al/Ni composite with homogeneously distributed fragmented nickel layers in aluminum matrix was produced. Structure and mechanical properties of these multilayer composites were evaluated at different passes of ARB process. During ARB, it was observed that as the strain increased with the number of passes, the strength, microhardness and elongation of produced composites increased as well. In addition, enhancement of the strength was shown to be higher than the tensile strength of Al/Al and Al/Cu multilayered composites produced by ARB process in the previous works by the same authors.

Introduction

Techniques of severe plastic deformation have been of continual interest in the production of novel metallic microstructures. Among these, accumulative roll bonding has been extensively used to produce nanocrystalline [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], as well as two phase nanocomposites [2], [5], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. Besides there are several reports of mixing and phase evolution of multicomponent systems during ARB processes [2], [7], [12], [14], [21]. In this technique, two strips of similar or dissimilar alloys are rolled together for several passes. In the simple case of two component system, the final structure consists of multilayers, which refines progressively with continuation of ARB process. The evolution of microstructures and related mechanical properties during ARB cycles at room temperature were studied for several metal strips such as commercial pure Al [1], [3], [4], Al based alloys [6], Cu–Ag alloys [7], Zr based alloys [8], IF steels [4], [8] and multilayer strips such as Al/Ni [11], [12], and Al/steel [13], Ti/Al/Nb [14], Ti/Zr/Ni [15], Ti/Ni [16], [17], Al/Pt [18], Al/Hf [18], Cu/Nb [19], Fe/Ag [20], and Al/Mg [21].

Interests in ARB are focused on mechanisms of grain refinement and the effect of strain on microstructural evolution. Concurrently, the average grain size of the two components is known to refine as the layered structure refines [12], [15], [20]. In the limit where the layers breakdown and the alloy reach a steady state structure, it is possible to form solid solution alloys, intermetallics and nanocomposite structures, depending upon the thermodynamics of the system and the characters of the mixing process [11], [14], [15], [16], [17]. Generally, metallic multilayer composites are produced by coating processes like ion sputtering and evaporation in order to make thin films [22], [23], [24] or by diffusion bonding of thin strips of different materials. Recently, production and development of bulk multilayer composites by means of deformation processes like ‘repeated press and rolling’ [20], [25] and ‘repeated folding and rolling’ [5], [26] have been practiced due to economical benefits as well as capability of mass productions [27]. It should be noted that most of the deformation processes require expensive tools and complex processes which have limited use at commercial and industrial scales. However, ARB process can be used as an innovative and appropriate way of multilayer composite production for the sake of its simplicity and cheaper primary commodity. With respect to other methods, the ARB process of elemental foil arrays allows for the retention of high purity during sample preparation [2], [21]. This is because, during deformation, the large interlayer interface areas are not exposed to the atmosphere. In addition, the sample temperature does not increase significantly above the ambient temperature during the process under low deformation rate (i.e., about 1 s⁻¹) [4], [5].

Severe plastic deformation by ARB for similar metals and alloys causes grain refinement by formation of IDB's (incidental dislocation boundaries) and GNB's (geometrically necessary boundaries). This leads to an ultrafine grained structure and equilibrium grain boundaries [1], [4], [7], [12]. Mostly, during co-deformation of dissimilar metal systems, plastic instabilities in one of the layers occur earlier than the other due to differences in mechanical properties. As strain increases, the harder layer experiences necking and premature fragmentation [2], [11], [12], [14], [15], [16], [17], [18], [19], [20], [21]. Multilayer foils have been of great interest owing to their new structural, chemical, magnetic, optical and electronic properties [14], [19], [22], [23], [24]. Especially, the Al–Ni system has been studied thoroughly as a model system for its attractive applications as coatings and microstructural features [11], [12], [23], [28], [29]. In the past studies, thin Al/Ni multilayers were mostly deposited by sputtering [23] or electron beam evaporation [28], while accumulative roll bonding process [12] and repeated folding and cold rolling of Al and Ni sheets and foils were also applied as an alternative process to produce the Al–Ni multilayer sheets and foils [5], [11], [29]. ARB provides a relatively inexpensive process, because starting materials are metal sheets or foils. These metal sheets and foils are available in a wide variety of thickness and purity, as compared with specially deposit layers. The ARB process also has a few problems; the most important defect of this method is edge cracks that form at the higher ARB cycles—these cracked parts should be trimmed.

In this study the Al/Ni multilayer composites were produced by ARB process. In addition to the microstructural studies mechanical properties of produced composites were investigated at different passes of ARB for the first time. Finally the properties of these composites were compared with Al/Cu multilayer composite [2] and Al nanostructure strip [3] produced previously by the same authors.