Microstructures and dislocation configurations in nanostructured Cu processed by repetitive corrugation and straightening

Abstract

The microstructures and dislocation configurations in nanostructured Cu processed by a new technique, repetitive corrugation and straightening (RCS), were studied using transmission electron microcopy (TEM) and high resolution TEM. Most dislocations belong to 60° type and tend to pile up along the {111} slip planes. Microstructural features including low-angle grain boundaries (GBs), high-angle GBs, and equilibrium and non-equilibrium GBs and subgrain boundaries were observed. Dislocation structures at an intermediate deformation strain were studied to investigate the microstructural evolutions, which revealed some unique microstructural features such as isolated dislocation cell (IDC), dislocation tangle zones (DTZs), and uncondensed dislocation walls (UDWs).

Introduction

Many methods have been used to synthesize materials with ultrafine grain sizes (10–1000 nm), including inert gas condensation [1], high-energy ball milling [2], sliding wear [3], etc. These techniques are attractive for producing powders with grain sizes below 100 nm, but cannot be used to make bulk samples. To consolidate the nanometer-sized powders into bulk materials, high pressure and moderate temperature are usually needed. Grains might grow during consolidation, making the bulk materials partially or completely lose the nanocharacteristics. It is usually impossible to completely eliminate porosity, even in materials consolidated under very high pressure and temperature. In addition, nanopowders are very susceptible to oxidation and absorb large quantities of impurities such as O₂, H₂ and N₂, making it difficult to obtain clean bulk materials. The porosity as well as impurities significantly affect the mechanical properties of the bulk materials, often making them brittle [4], [5], [6], [7], [8]. These problems prevent us from studying the intrinsic properties of bulk nanomaterials. As a consequence of these difficulties, much attention has been paid to alternative procedures of introducing ultrafine grains in materials by severe plastic deformation (SPD) [9], [10], [11], [12].

One of the SPD variants, equal-channel angular pressing (ECAP), has been used to refine bulk, coarse-grained metals and alloys to grain sizes ranging from <0.1 to 1 μm [9], [10], [11], [12]. However, ECAP is difficult to scale up to process volumes of materials much larger than the 20×20×100 mm³ samples that are typically produced today. Furthermore, current implementations of ECAP are discontinuous, requiring labor intensive handling of the work-piece between process steps. These difficulties in fabricating bulk, nanostructured materials have been substantial road-blocks to the structural applications of nanostructured materials. Other SPD techniques that have been reported in the literature include multipass-coin-forge (MCF) [13] and multi-axis deformation [14]. Both of them have certain advantages over the ECAP process. However, they also employ batch processing, which is not efficient for large-scale production.

Recently, we have developed a new technique, repetitive corrugation and straightening (RCS), that can not only create bulk nanostructured materials free of contamination and porosity, but can also be easily adapted to large-scale industrial production [15]. In the RCS process, a work-piece is repetitively bent and straightened without significantly changing the cross-section geometry of the work-piece, during which large plastic strains are imparted into the materials, which leads to the refinement of the microstructure.

Hansen and coworkers [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27] have systematically studied the evolution of microstructures and defined microstructural features in rolled face-centered cubic (fcc) metals with medium to high stacking fault energies, such as Al and Cu. However, the deformation mode in the RCS process is different from that of rolling and is expected to result in different microstructural evolution and consequently different microstructures.

A controversial microstructural feature in nanostructured materials processed by SPD techniques is non-equilibrium grain boundary (non-equilibrium GB) [28], [29]. Valiev et al. [28] defined it as GB that contains extrinsic dislocations that are not needed to accommodate the misorientation across the GB. The extrinsic dislocations are usually lattice dislocations trapped at the GB. They cause lattice distortion near the GB and increase the GB energy [30]. Although the non-equilibrium GB has been mentioned by many researchers [12], [30], [31], [32], [33], [34], it has not been directly proved experimentally and has been controversial.

The objectives of this work are: (1) to study the microstructural features and dislocation configurations in nanostructured Cu processed by RCS; (2) to investigate the microstructural evolutions and grain-refinement mechanisms through TEM observations of dislocation structures at an intermediate deformation strain; and (3) to clarify the existence of non-equilibrium GBs produced by SPD.