Thermal behavior of copper processed by ECAP with and without back pressure

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Abstract

Samples of electrolytic tough pitch (ETP) pure copper were subjected to 12 passes of Equal-Channel Angular Pressing (ECAP) at room temperature with and without back pressure. Subsequent annealing was performed to evaluate the influence of back pressure during ECAP on the thermal behavior of ultrafine-grained copper. The microstructural and hardness changes caused by annealing were characterized by orientation imaging microscopy (OIM) and microhardness measurements. The application of back pressure resulted in an earlier drop in hardness upon annealing, which is believed to be associated with a lower critical temperature for the initiation of recrystallization and a rapid coarsening of microstructure. Regardless of whether back pressure was applied or not, structure coarsening during short-time annealing of ECAP-processed copper was governed by discontinuous static recrystallization. This is seen as a result of microstructure heterogeneity. Analysis of recrystallization kinetics was carried out based on observations of the microstructure after annealing in terms of the Avrami equation. The magnitude of the apparent activation energies for recrystallization in the absence of back pressure and in the case of back pressure of 100 MPa was estimated to be ~99 kJ/mol and ~91 kJ/mol, respectively. The reasons for reduced activation energy in the case of processing with back pressure are discussed.

Introduction

Equal Channel Angular Pressing (ECAP) as one of severe plastic deformation (SPD) methods has enjoyed great popularity over the last two decades. This technique has the capability to produce bulk ultafine-grained (UFG) materials with dimensions sufficient for potential practical application. For example, high-purity copper processed by ECAP was employed to manufacture premium quality sputtering targets [1], [2]. However, UFG materials manufactured by SPD methods are metastable in nature, and some of them readily recrystallize even at ambient temperature [3], [4]. As a result of deformation to giant plastic strains involved in SPD processing, a large energy is stored in the material, owing to a high concentration of crystal lattice defects, such as vacancies [5], dislocations [6], and non-equilibrium grain boundaries [7]. From the viewpoint of industrial application, the investigation of the thermal stability of the severely deformed materials, i.e. their resistance to the microstructural restoration processes (recovery and recrystallization), is a necessary step in evaluating their suitability.

A considerable amount of work has concentrated on the thermal stability of ECAP processed materials, in which the influential factors can mainly be put in two categories: (i) extrinsic, processing-dependent parameters (the magnitude of plastic strain [8], [9], [10] as well as the strain path, including the processing route [11], [12], [13] and the post-ECAP deformation [14], [15] and subsequent treatment during annealing [16]), and (ii) intrinsic, i.e. material-dependent parameters, such as the purity [17] and the stacking fault energy (SFE) [18]. These studies show that both the extrinsic and the intrinsic variables play an important role in thermal stability of UFG materials [19].

The application of back pressure (BP) during ECAP processing has attracted wide attention due to the profound advantages it provides. Thus, it has been reported that back pressure assists in achieving greater grain refinement, improvement in mechanical properties and production of high density bulk materials, as well as preventing fracture initiation [20], [21], [22], [23]. An important factor is an increase in the fraction of high-angle grain boundaries and a more uniform grain structure the material attains if back pressure is imposed [20], [23]. Despite the importance of the ability of these structures to resist restoration, studies of thermal stability of ECAP-induced structures produced under back-pressure are rather scarce. In our preliminary investigation [24], the effect of back pressure on thermal stability was studied for the case of ECAP-processed copper. Thermal stability was found to decrease marginally when back pressure was imposed during ECAP. The objective of the present work is to provide a detailed evaluation of post-deformation thermal annealing of copper processed by ECAP with and without back pressure. The evolution of the microstructure with annealing time and the ensuing mechanical properties were investigated in a systematic way as a function of the annealing temperature. Analysis of the recrystallization kinetics in terms of the recrystallization mechanisms was also conducted. The outcomes of the experiments and their interpretation are presented below.

Section snippets

Experimental

Electrolytic Tough Pitch (ETP) copper billets of 99.9 wt% purity were studied. Prior to deformation by ECAP, rectangular samples of dimensions 20×20×110 mm3 were annealed at 600 °С for 2 h to obtain a homogenous coarse-grained structure. The resulting microstructure is presented in Fig. 1. The average grain size was about 28 µm and the corresponding microhardness was 45.5 kgf/mm2. ECAP with and without a back pressure of 100 MPa were conducted at room temperature up to 12 passes using an angular die

Deformation microstructure

The characterization of as-deformed microstructures was carried out by means of TEM. As shown in Fig. 3, irrespective of the level of back pressure, largely homogenous granular structures occurred in both cases; they were characterized by fuzzy and diffuse boundaries, representative of high-energy non-equilibrium configurations. The microstructure is consistent with those reported earlier [28], [29]. A comparison of the SAD patterns for both conditions presented as insets in Fig. 3(a) and (b)

Discussion

Irrespective of whether back pressure is or is not employed in ECAP processing, the variation of microhardness and microstructure of deformed samples with the annealing temperature exhibits similar trends. The observed steep drop in microhardness within a narrow temperature interval, and the corresponding evolution of a heterogeneous microstructure towards a duplex structure tend to meet the definition of discontinuous recrystallization, which involves the formation of nuclei of recrystallized

Conclusions

In this work, the effect of back pressure on thermal behavior of UFG copper processed by Equal-Channel Angular Pressing was studied. The following conclusions can be drawn based on the foregoing discussion:

  • (1)

    The thermal behavior of UFG copper processed by ECAP in the annealing temperature range of 80–200 °С can be characterized by subdividing the temperature interval in three regions, based on the microhardness variation upon short time annealing: a relatively small decline at low annealing

Acknowledgments

The authors acknowledge the Monash Center for Electron Microscopy (MCEM) for providing access to experimental facilities. This work was supported by MOST of China (Grant no. 2012CB932203) and a Key project of Chinese Ministry of Education (No. 311035). One of the authors (Ya Li Wang) is grateful to the China Scholarship Council (CSC) for the financial support received (Grant no. 2011684020). The authors are thankful to Dr. Hoi Pang Ng for useful comments.

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