The fabrication of high strength Zr/Nb nanocomposites using high-pressure torsion

Abstract

Nanocomposites of Zr/Nb with exceptionally high hardness were fabricated successfully through the high-pressure torsion (HPT) processing of prepacked Nb/Zr/Nb sandwich samples at ambient temperature. The initial layers of Nb and Zr became fragmented during HPT processing with the formation of many fine-scale intermixed Zr/Nb layers. The intermixing of these Zr/Nb layers increased both with increasing HPT revolutions from 10 to 100 and with increasing radial positions on the disks. The Vickers microhardness, Hv, increased with increasing revolutions and with radial position reaching a maximum of ~700 Hv at the edge of the 100 turns sample. Exceptional grain refinement to the range of ~20–40 nm and the occurrence of twinning were associated with the HPT-processed Zr/Nb composites after 100 turns. These results suggest a potential route for fabricating high strength bulk Zr/Nb nanocomposites.

Introduction

In order to enhance the mechanical properties of metals, various elements may be mixed in a controllable way to form composite structures [[1], [2], [3]] and the mixture can then be tuned so that the required properties of the composite are enhanced and may even surpass the properties of the comprising elements [2,4]. In recent years, bimetallic composites have attracted considerable attention as they exhibit enhanced strength, hardness and plastic properties even under extreme conditions as well as resistance to shock and radiation damage compared to the constituent materials in bulk form. Examples of these composites include Al/Cu, Cu/Nb, V/Ag, etc. [[5], [6], [7], [8], [9], [10], [11], [12]].

To date, most of the research interest in metallic composites has focused on using combinations of crystal structures of bcc/fcc, fcc/fcc and bcc/bcc materials [[13], [14], [15], [16], [17], [18], [19]] but recently there has been an interest in developing cubic/hcp combinations [[20], [21], [22], [23], [24], [25]]. Considering the Zr/Nb system, it is well-known that zirconium and zircalloys are widely employed in the nuclear industry due to their small capture cross-section for thermal neutrons, their relatively good high temperature strength and their resistance to corrosion [26]. Therefore, it is reasonable to anticipate that Zr/Nb bimetallic nanoscale composites may be promising candidate materials for use in the nuclear industry due to the development of intrinsic highly efficient interfaces which will protect these materials against radiation damage [27,28].

Metallic composites of this type are generally obtained by using deposition methods such as magnetron sputtering [29] and already there is a report of the preparation of Zr/Nb nano-multilayers using this procedure [26]. Nevertheless, the relatively low deposition rate and the overall complexity of the equipment that is needed for this synthesis technique make this type of processing difficult for the fabrication of large volumes of material. Mechanical alloying (MA) is also a potential approach for preparing these composites [5] but a simpler and more direct procedure is through the use of a severe plastic deformation (SPD) technique such as high-pressure torsion (HPT) where this procedure is now routinely used to produce bulk nanostructured metallic materials [30]. Processing by HPT has been employed recently to produce nanostructured intermetallics and metal matrix composites and the results have shown that the procedure provides the potential for achieving true nanostructures [[31], [32], [33], [34], [35]]. Non-equilibrium phases, such as supersaturated solid solutions [36], may be formed using this procedure and the resultant mechanical properties are enhanced due both to the significant grain refinement and to the intensive introduction of point and line defects during processing [37].

An earlier report described the fabrication of a Zr/Nb composite using two semi-circular sections of a disk which were placed together to form a single whole disk within the HPT facility [5]. Processing by HPT produced an ultrafine-grained structure with grain sizes of less than 100 nm and a supersaturated solid solution with a hardness of up to 500 Hv at the disk edge after 100 turns of HPT [5]. In practice, most reports on the fabrication of bulk Zr/Nb composites have used the technique of accumulative roll bonding (ARB) [[38], [39], [40], [41]] and very limited information is at present available on the effect of heavy shear strains on the fabrication of bulk Zr/Nb composites when using the HPT technique. Furthermore, the early study using HPT produced a Zr/Nb composite by taking two semi-circular disks [5] and this is similar to early reports for other materials using two semi-circular disks [42] or four quarter disks [43], respectively. More recent experiments have been conducted differently by stacking whole disks in a sandwich-like configuration [31] where this procedure provides a better opportunity for evaluating the significance of mixing during the HPT process.

The only report on the HPT processing of a Zr/Nb composite available to date has limitations due to the necessity of cutting semi-circular Zr and Nb disks [5]. Accordingly, the present research was initiated to develop a novel and compact approach for making Zr/Nb composites by stacking Nb/Zr/Nb disks in a sandwich-like structure for the HPT processing. Therefore, a comprehensive investigation was conducted to examine the production, microstructure and the microhardness of a Zr/Nb composite after processing at room temperature. The results demonstrate the potential for using HPT for the fabrication of nanocomposites having exceptionally high levels of hardness.