Microstructure and mechanical properties of pure Cu processed by high-pressure torsion
Introduction
Considerable interest has developed over the last decade in processing materials through the application of severe plastic deformation (SPD) in order to achieve grain sizes at the submicrometer or nanometer level [1], [2], [3]. Although several different SPD processing techniques are available, the most promising procedure appears to be the high-pressure torsion (HPT) [4], [5]. Experiments have shown that HPT is especially effective in producing extremely small grain sizes when compared with other SPD processing techniques [6], [7], [8].
Bridgman who proposed the idea of HPT suggested the use of a cylindrical hollow specimen to produce a homogeneous microstructure [9]. This idea was further developed by Erbel [10] and Saunders and Nutting [11], and very recently by Harai et al. who carried out HPT using ring samples with a simple modification of the technology currently in use for disc samples [12].
The strain in HPT is introduced in proportion to the distance from the disc center so that the strain at the disc center is theoretically zero and accordingly the microstructural development is inhomogeneous across the disc diameter [13], [14], [15], [16], [17], [18]. However, the use of ring samples overcomes such a microstructural inhomogeniety and homogeneous microstructure is developed throughout the sample. The advantage of using the ring sample is that not only a homogeneous microstructure is developed but also the diameter of the ring can be increased by the area corresponding to the central hollow area [12].
Using both disc and ring samples, Harai et al. [12] showed that the hardness variation is represented as a unique function of the equivalent strain. High purity Al with 99.99% was used for this demonstration. The hardness initially increased to a maximum at an equivalent strain of ∼2 and decreased with a further increase in the equivalent strain. The hardness leveled off at an equivalent strain of ∼6 and this was followed by the onset of a steady-state where the hardness remained unchanged with straining. Along with this change in hardness, the microstructural evolution was examined in terms of dislocations and grain boundaries including misorientation angles using transmission electron microscopy and electron back scatter diffraction analysis [19], [20].
In the present investigation, HPT using both disc and ring samples is applied to high purity Cu (99.99%) and the hardness variation with respect to equivalent strain is examined if it is expressed with a single unique function of the equivalent strain. The microstructural evolution is also examined with respect to the equivalent strain and the mechanism for grain refinement is discussed in comparison with pure Al.
Section snippets
Experimental materials and procedures
High purity copper (99.99%) was received in the form of a cold-rolled plate having dimensions of 10 mm × 100 mm × 200 mm. The plate was cut to cylinders with 10 mm diameter and 10 mm height and to inner-hollow cylinders with inner and outer diameters of 24 and 30 mm using a wire-cutting electric discharge machine. Both cylinders were annealed for 1 h at 873 K to give an initial grain size of ∼150 μm. They were further sliced to discs and rings with thicknesses of 0.8 mm.
HPT was conducted using the facilities
Results
Fig. 4 shows the hardness variation with the distance from the centers of disc and ring samples after revolutions from 1/8 to 10 under a pressure of 2 GPa. The microhardness increases with an increasing number of revolutions and an increasing distance from the center for the disc samples. The saturation of the hardness level appears in the disc samples after one or more revolutions and in particular the hardness values after 10 revolutions lie close to this saturated level. The hardness is
Discussion
This study has shown that the hardness variation is expressed by a unique function of the equivalent strain so that all data points fall well on a single curve. This is also valid for the tensile strength and elongation to failure including uniform elongation. The present results thus indicate clearly that there is no difference between the disc samples and the ring samples for introducing strain despite the fact the higher strain rate is achieved in the ring sample because the diameter is
Summary and conclusions
- (1)
All values of Vickers microhardness fall on a unique singe line when plotted as a function of equivalent strain for both disc and ring samples of pure Cu processed by HPT.
- (2)
The hardness increases with an increase in the equivalent strain at an early stage of straining but levels off and enters into a steady-state where the hardness remains unchanged with further straining.
- (3)
Tensile properties such as the ultimate tensile strength, uniform elongation and elongation to failure are also expressed by
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan, in Priority Areas “Giant Straining Process for Advanced Materials Containing Ultra-High Density Lattice Defects”. One of the authors (KE) would like to thank Islamic Development Bank for his scholarship.
References (22)
- et al.
Prog. Mater. Sci.
(2000) - et al.
Prog. Mater. Sci.
(2006) - et al.
Acta Mater.
(2005) - et al.
Mater. Sci. Eng. A
(2007) - et al.
Scripta Mater.
(2008) - et al.
Scripta Mater.
(2001) - et al.
Acta Mater.
(2003) - et al.
Mater. Sci. Eng. A
(2004) - et al.
Scripta Mater.
(2004) - et al.
Mater. Sci. Eng. A
(2005)
Acta Mater.
Cited by (205)
Phase decomposition behavior and its impact on mechanical properties in bulk nanostructured Cu-20 at.%Fe supersaturated solid solution
2024, Journal of Materials Science and TechnologyRecent progress in gradient-structured metals and alloys
2023, Progress in Materials ScienceNano‑carbon-mediated microstructure evolution and superior performance in Ti-based nanocomposites
2023, Materials CharacterizationThe effect of initial grain size on the strength property of copper with gradient microstructure
2023, Journal of Materials Research and TechnologyFormation of nanostructures in α-uranium processed by high pressure torsion
2022, Materials Science and Engineering: A