Microstructural evolution, microhardness and thermal stability of HPT-processed Cu

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Abstract

Coarse-grained copper was subject to high-pressure torsion (HPT) and thermal treatment to study the effects of increasing amounts of deformation and subsequent annealing on the evolution of microstructure and microhardness. Cellular subgrains with low-angle grain boundaries were first formed at low strain. Some of the low-angle subgrain boundaries transformed to high-angle grain boundaries at higher strains, refining the average grain size from 200 μm to 150 nm. X-ray diffraction patterns showed the formation of crystallographic texture. Microhardness increased monotonically with increasing torsional strain. High internal stress and nonequilibrium grain boundaries were observed in unannealed samples. Annealing as-deformed samples at temperatures as low as 50°C decreased the microhardness, indicating a very low thermal stability of the deformation induced microstructures. Differential scanning calorimetry (DSC) revealed an exothermal peak between 180 and 280°C, caused by recrystallization. Annealing twins were also formed during recrystallization.

Introduction

Ultrafine-grained (UFG) materials with grain sizes in the range 10–1000 nm (nanocrystalline and submicrocrystalline) contain a large volume fraction of grain boundaries and, therefore, exhibit a wide spectrum of unique properties and property combinations [1], [2], [3], [4], [5]. In the pioneering work by Gleiter [1], [2], [4], materials with grain sizes less than 100 nm were fabricated using an inert gas condensation (IGC) technique. Studies of these materials revealed very unique microstructures and superior properties. Driven by these promising early results, worldwide efforts have been aimed at the synthesis and characterization of an ever-growing variety of UFG materials using an expanding variety of techniques. Among the synthesis technologies are ball milling [6], spark erosion [7], crystallization of amorphous precursors [8], pulsed electrolytic deposition [9], and severe plastic deformation (SPD) [10], [11], [12], [13].

The processing of UFG materials by severe plastic deformation (SPD) has been receiving increasing attention [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. The major advantage of SPD techniques is their ability to produce bulk UFG materials free of porosity. This is important for the study of the physical and mechanical properties of UFG materials and for their commercial applications. Two of the most common variants of SPD processing are equal channel angular pressing (ECAP) [10], [11], [12], [15], [16], [17], [18], which has also been called equal channel angular extrusion (ECAE) [13], [19], [20], [23], [24], and high-pressure torsion (HPT), which has been referred to as torsional straining [11] and severe plastic torsional straining (SPTS) [21], [22], [25] in the literature. Recently, the scientific community has renamed the latter HPT to better reflect the physical process. A substantial body of work on processing by ECAP [10], [11], [12], [15], [16], [17], [18], [19], [20], [23], [24], [26], [27], [28], [29], [30] has created a basic understanding of how mechanical properties and microstructures evolve in UFG materials. However, little has been reported on the evolution of microstructure and properties of UFG materials processed via HPT [31].

There has also been little systematic study of the thermal stability of UFG materials processed by SPD. This topic is particularly important for SPD-processed materials since, in addition to small grain size, they also have a high dislocation density and high internal stress. These characteristics tend to make their microstructures thermally unstable. Differential scanning calorimetry (DSC) is a powerful tool for studying the thermal stability of UFG materials because it can measure energy release during thermal annealing. The energy release is directly related to microstructural changes such as dislocation density decrease and grain growth. Islamgaliev et al. [32], [33] studied the thermal stability of SPD-processed copper and nickel using DSC, transmission electron microscopy (TEM), and other techniques. However, the annealing conditions for the samples they used for TEM did not have any correlation with the DSC curves and, therefore, no connection could be made between the energy release measured by DSC and the observations of microstructure.

In this investigation, we have studied the evolution of the microstructures and microhardness of UFG copper with increasing torsional strains. The thermal stability of UFG copper was studied using DSC, TEM, and microhardness measurement, with a direct correlation of the results obtained by these three techniques. Copper was chosen because it has been processed into UFG state by various techniques including ECAP [11], [33], [34] and, as a result, a wealth of reference experimental data on UFG Cu is available [11], [33], [35], [36], [37], [38], [39].

Section snippets

Experimental procedures

Samples were prepared from 98.51 wt.% Cu and 1.49 wt.% Si rod with an initial average grain size of 200 μm. The Si impurity remains in solution with Cu at this composition [40]. Coarse-grained samples were HPT-processed into UFG copper (see Refs. [25], [41] for more details on the process). A disk with a thickness of 1 mm and a diameter of 10 mm was cut from a coarse-grained copper rod. As illustrated in Fig. 1, the disk was placed between two vertically opposed steel surfaces, and deformed by

Microstructural evolution in copper during HPT

Fig. 3 shows the XRD patterns of the as-received copper sample and samples processed by HPT to four levels of deformation, corresponding to 1/2, 1, 3, and 5 turns of the torsional press. The maximum shear strain that could be induced by these deformations can be computed as:γmax=2πrntwhere t is the sample thickness, r is the radius and n is the rotation in turns. As discussed earlier, due to the lack of side constraint, the sample flowed in the radius direction during the HPT process, resulting

Summary and conclusions

HPT is more effective in refining grain size than ECAP. The initial application of high pressure decreased the fraction of {111} planes parallel to the sample surface. Subsequent HPT strain increased the {111} crystallographic texture with {111} planes parallel to the sample surface. The {111} texture became saturated after the first HPT turn. Low-angle subgrain boundaries were formed first, some of which subsequently evolved into high-angle grain boundaries with further HPT. The grain

Acknowledgements

This work was performed under the auspices of the US Department of Energy (Contract W-7405-ENG-36).

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