Advantages and limitations of HPT: a review Reinhard ...

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Materials Science Forum Vols. 584-586 (2008) pp 16-21 online at http://www.scientific.net © (2008) Trans Tech Publications, Switzerland Online available since 2008/Jun/17

Advantages and limitations of HPT: a review Reinhard Pippan1,2, a, Stephan Scheriau1,2, Anton Hohenwarter1,2, Martin Hafok1,2 1

Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstraße 12, A8700 Leoben, Austria 2 Christian Doppler Laboratory for Local Analysis of Deformation and Fracture, Jahnstraße 12, A8700 Leoben, Austria a [email protected] Keywords: High pressure torsion, severe plastic deformation, processing

Abstract. The improvements in the design of the HPT tools lead to a well defined torsion deformation and permits, therefore, a comparison with other SPD-techniques. The design of the tools, the advantages and disadvantages of HPT, as well as the limitation in the sample size are discussed. Introduction For a long time torsion has been frequently used to determine the stress strain behaviour at large strains. In free torsion, the geometrical changes are usually very small. The fracture strain is significantly larger than in tensile experiments due to the vanishing macroscopic hydrostatic tension component of the stress. By applying an additional hydrostatic compression stress, the fracture strain can be further enhanced and can be increased to infinity at very high hydrostatic compression stresses. High pressure torsion, HPT, is a realisation of such a torsion experiment [1-3]. One can distinguish between constrained and unconstrained version. In the latter case the sample is pressed between two plane anvils, during the deformation the thickness of the sample decreases continuously. In the constrained version this decrease in thickness is significantly reduced and is therefore mainly used in the severe plastic deformation (SPD) community. The improvements in the design of the tool leads to a relatively well defined torsion deformation, therefore the interest in the HPT deformation has remarkably increased. Despite the relatively large number of published papers in the last 5 years related to the structural evolution during HPT, several discrepancies remains concerning the applied strain and the limits of the sample dimensions, etc. In order to clarify few of these points a short summary of our obtained experience is presented in this paper. At first the deviation of real HPT from ideal torsion, then the advantages and disadvantages, limitation in sample size and the up-scaleability will be discussed. Idealized torsion and real HPT In pure torsion the shear strain γ 2π n γ = r t , where r, n, and t are the distance from the axis of rotation, the number or revolutions and the thickness of the sample, respectively. In order to compare the shear strain with strains achieved by other methods, for example, rolling, it is helpful to define an equivalent strain. The most common definition of equivalent strain, εv, is based on comparable plastic work spent to a volume element. Assuming von Mises yield laws leads then to

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(a)

(b)

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(c)

Fig. 1. Schematic illustration of High Pressure Torsion, HPT, equipment: (a) unconstrained HPT, (b) idealized HPT and (c) practical HPT.

εv =

2π ⋅ n ⋅ r

3⋅t . In HPT a disc-shaped specimen is deformed between two anvils, which rotate with respect to one another. In order to avoid a pronounced decrease of the sample thickness and to generate a nearly constant pressure during the deformation in the sample the constrained version of HPT is now usually used (see Fig. 1). An idealized constrained HPT process is depicted in Fig. 1b, where the sample should be deformed by pure torsion under hydrostatic pressure without changing the sample geometry. In order to realize such ideal torsion, no friction at the outside of the cylinder sample should occur. Since such frictionless constrain of radial deformation is impossible, the height of the sample is limited in order to ensure homogeneous deformation in the axial direction (i.e, over the height of the sample). The idealized version is difficult to realize and has several disadvantages. It requires very accurate tools, the handling is more difficult and the maximum applicable pressure is significantly lower than in the set-up, which is now most frequently used. The new most used practical setup [3] is depicted in Fig. 1c, which permits a deformation very similar to the idealized HPT. Both anvils are provided with a cylindrical cavity of about the same dimension. The diameter of the cylindrical cavity and the initial diameter of the HPT sample are identical. The sum of both depends on the cavities and should be somewhat smaller than the initial height of the HPT sample. Thus during loading, a small amount of material flows in the ring shaped region between the anvils. The friction in this single region confines the free flow of the material out of the HPT tool; this leads to a back pressure and therefore induces a well defined hydrostatic pressure within the processing zone of the tool. In addition, the material in this ring region prevents touching of the anvils and prevents failure of the anvils during the torsional deformation. The bottom and the top face of the cylindrical cavity of the tool are sandblasted in order to clean the surface and to provide the necessary micro-roughness, which delivers sufficient friction for a continuous torsion deformation. Furthermore, the surface of the sample should also be cleaned. As mentioned above, during the axial loading of the anvils a certain amount of material flows into the ring shaped region, i.e. at the beginning a certain compression deformation of about 5-10% is applied to the sample. During subsequent torsional deformation, an additional small amount of material flows out, which again reduces the height of the sample somewhat and it induces an additional compression deformation of a few percent. The amount of this compression

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Fig. 2: Schematic illustration of the HPT equipment used. It permits deformation between -196 and 500°C, measurement of the applied torque and ability to change the direction of rotation [4-5]. deformation depends on the difference in volume between the sample and the anvil cavity, the applied load and the strength of the material. However, it should be noted that this compression deformation is usually very small in relation to the applied torsion strain, except of the centre of the sample. Due to the inhomogeneous elastic deformation of the tool, the height of the sample in the centre is somewhat larger than at the edge, even if the cavity formed by the anvils has a perfectly cylindrical shape. This effect increases with increasing pressure. An example, which demonstrates this effect of deviation from the cylindrical shape, is presented in Fig. 3 for a rail steel deformed at a nominal pressure of 7 GPa. The homogeneity of the deformation In an ideal HPT experiment the strain should linearly increase with the distance from the axis of rotation. It is therefore surprising that a uniform deformed microstructure in HPT samples with no significant radial dependence of the hardness has been reported several times (see for example [7]). However, in recent years carefully performed HPT experiments showed clearly that the linear increase of the shear strain is well fulfilled, see, e.g. [3]. Furthermore one

Fig. 3. Shape of a originally cylindrical rail steel sample loaded in a HPT tool at a nominal pressure of 7 GPa, diameter 8mm.

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Fig. 4: (a) illustrates a micro grid on a HPT sample split at a radius of 2 mm from the axis of rotation and polished. The Ni sample was deformed to γ about 170. (b) shows the grid after a further shear deformation of γ=1.4, which demonstrates the very homogeneous deformation in the axial direction even on the micro-scale for the saturation microstructure.

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Fig. 5: Illustration of the macroscopic homogeneous deformation of a large Cu HPT sample. A Ni wire was filled at a radius of about 9 mm in a HPT sample (thickness 7 mm, diameter 30 mm). Two revolutions were applied to this sample. A removing of the outer region of the HPT sample indicates the very homogeneous torsion deformation, even for this relative thick HPT sample

should take into account that most single phase materials develop a saturation microstructure and a saturation in hardening at strains larger than 15. In typical HPT samples (diameter 8 mm, thickness 0.7 mm) one overcomes this saturation strain after 3 revolutions for radii larger than about 1 mm. Hence, it is not surprising that most parts of a typical HPT sample exhibit a uniform microstructure. If one looks to the central region of a HPT sample and further takes into account that some deviation from the ideal HPT causes an additional deformation of the HPT sample. Few of them are listed in the following: • In addition to the shear deformation of the sample, a compression deformation during the compression loading and during the initial phase of the following torsional deformation takes place as already mentioned. The sum of these compression deformations is typically between 5 and 20%, which is usually very small in relation to the applied strain in torsion. • A parallel shift of the axis of the upper and the lower anvils can introduce an additional shear deformation also in the centre of the sample [3]. • Furthermore a misalignment of the axes of the anvils can make the deformation during the compression as well as during the rotation even more complex. • Deviation form the axial symmetrical shape of the cavities introduces similar effects as discussed above. In ideal torsion in axial direction (in the height direction of the HPT sample) the applied strain is constant. In real HPT at the edge of the sample a deviation due to the friction occurs. The size of the zone, where one should expect a deviation from the ideal torsion deformation depends on different parameters. A hardening and a high strain rate sensitivity should reduce the size of this regime. However, in most cases at the edge the HPT sample is deformed to the microstructural saturation, only the strain rate sensitivity stabilizes the deformation in the height direction. The friction at the edge of the cavity, the pressure and strength of the material have also an important effect. This edge problem limits the height of the HPT sample. For thickness, t, to diameter, d, smaller than 1/8, we usually observed a very homogenous deformation up to a radius of r < d/2 - t/2 even in the

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saturation regime. We have determined the axial distribution several times on the microscale and on the macroscale for different strains even up to the saturation regime [6]. It was surprising that even in the saturation regime in single phase materials the shear was very homogeneously distributed down to the microscale as depicted in Fig. 4. An example, how we prove the axial homogeneity in the deformation is depicted in Fig. 5. This example clearly demonstrates that even for somewhat larger t/d ratios in most parts of the HPT sample (even in very large ones) the shear strain is constant over the height of the sample. Advantages and disadvantages of the HPT process Compared to other severe plastic deformation processes the HPT technique offers a large number of advantages, listed in the following: • Most SPD processes offer only a stepwise application of strain. HPT permits a defined continuous variation of strain. • The most important advantage of HPT is the very simple way in which extremely high shear strain can be achieved. γ > 1000 is in many materials no problem. • Relatively brittle or high strength materials can be severely deformed, which is often impossible by other SPD processes. • Strain rate at a specific radius can be precisely controlled. • Heating or cooling of the anvils permits a well defined deformation temperature. • The total torque vs. angle of rotation can be measured comfortably. This permits an estimation of the development of the flow stress [4]. • A change of the direction of rotation permits also a severe plastic deformation, which is typical for many other SPD processes [5]. Despite these advantages of HPT, especially for fundamental studies of the potential of SPDmicrostructures, there are some disadvantages, especially for large scale industrial application. In industrial application one usually needs homogeneous material properties. Standard HPT has an inherent radius dependence of the strain. Only if the sample is deformed in the saturation regime, a uniform microstructure can be achieved except the central region. The limitation in the size of the HPT sample is a further disadvantage, the limits for the upscaling are discussed in the following Upscaling and limits of size of HPT samples In principle, there exists no limit for an increase in the diameter of the HPT-sample, only the ratio of thickness, t, to diameter, d, should be smaller than a certain value to obtain in most parts of the sample on homogeneous deformation in the axial direction. As mentioned for t/d < 1/10 only in a very small region at the edge of the sample deviation from a constant strain in axial direction was observed in many different types of materials. Even in the case of t/d ~ 1/5 a homogeneous distribution in axial direction in a relatively large part of the HPT sample was observed (see Fig. 5). To avoid sliding between the anvils and the top and bottom face of the sample, the necessary pressure is usually very high. As a rule of thumb, the pressure should be larger than the hardness of the material, or in other words, at least 3 times larger than the yield stress of the material in order to get a perfect fitting between the rough anvils and the sample. The maximum applicable pressure is determined by the hardness of the tool material. Therefore, the limit for the usually used tool steels is about 8 GPa. The limit in the size is therefore given by maximum force of the used pressure and the processed material. Furthermore, one has to take into account that the necessary torque increases with d3. In our standard HPT equipment with a force of 400 KN, “low strength” (for example, Al and Al alloys, pure Cu, Fe, Ni), we process samples up to a diameter of 15 mm, in high strength materials, rail steel at very high strains or W-alloys, we go down to d of 6 mm (with WC-Co tools). We have recently built a new HPT with 4000 KN, which allows us to increase d and t by a factor of 3. In our small HPT device the effect of heating could be neglected. However, in the new larger HPT the strain rate has to be limited to avoid a too large increase in temperature, and to avoid strain localization due to plasticity induced heating.

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Finally, it should be noted that besides the simple upscaling, a new severe torsion process, torsion extrusion, has been proposed for a mass production [9] where there exist no limit in length cylindrical sample.

Fig. 6: The development of anvil shape and dimension in the last few years in our institute. Acknowledgements The work was supported by the Austrian “Fonds zur Förderung der wissenschaftlichen Forschung”, Project P17096-N02 and S10402-N16. References [1] J. Bridgmen, in: Processing of metals under high pressure conditions, M. Techizdat, ed., Moscow, p.230, (1936). [2] R.Z. Valiev, R.K. Ilsamgaliev, I.V. Alexandrov: Prog Mater Sci 45 (2000), 101. [3] A. Vorhauer, R. Pippan: Scripta Mater 51 (2004), 921-925. [4] A. Vorhauer, R. Pippan: Metallurgical and Materials Transactions 38A (2008), 417-429. [5] F. Wetscher, R. Pippan: Phil. Mag 86 (2006), 5867-5883. [6] R. Pippan, F. Wetscher, M. Hafok, A. Vorhauer, I. Sabirov: Advanced Eng Mater 8 (2006), 1046-1056 [7] A.P. Zhilyaev, G.V. Nurislamova, B.-K. Kim, M.D. Bar, J.S: Szpunar, T.G. Langdon: Acta Mater 51 (2003), 753. [8] M. Hafok, R. Pippan: Scripta Materialia 56 (2007), 757-760. [9] S. Mizunuma: Materials Science Forum 503-504 (2006) 185-190.