Nanostructuring by Severe Plastic Deformation
Severe plastic deformation (SPD) means an imposition of extremely large plastic strain on a sample that does not results in a significant change of the sample's geometry [1-3]. Microstructure refinement by SPD is based on a general trend for the formation and evolution of dislocation substructures consisting of cells and subgrains during large plastic strains common for all metals and alloys [4,5]. Substructure development during traditional metal forming processes such as extrusion and cold rolling has been studied for a long time. These straining methods, however, yield anisotropic, elongated substructures with predomination of low angle misorientations and cause significant changes in the geometry of samples at relatively low values of the strain. To obtain ultrafine-grained microstructures with mainly high-angle grain boundaries extremely large plastic deformations are required. Such deformations can be imposed by special deformation techniques which are now commonly referred to as severe plastic deformation techniques. The SPD techniques are capable of producing true polycrystalline structures with high angle misorientations and grain sizes of the order of 100 nm and even less. Nanostructuring through SPD based on the grain refinement without desintegration of material is a "top-down" approach to the processing of nanostructured materials.

There are two fundamental ways of SPD, now generally accepted: high-pressure torsion (HPT) and equal channel angular pressing/extrusion (ECAP/ECAE).

HPT technique is based on the use of Bridgeman anvil-type device. The possibility of the formation of a nanocrystalline structure by this method was first demonstrated in [6]. A disk-shaped sample is put between two anvils and subjected to a high pressure of (2-5) GPa (Figure 1). Rotation of one of the anvils forces the sample to deform by torsion. Up to five rotations of the anvil are usually enough to form a homogeneous microstructure with the grain size typically about 100 nm, in some metals and alloys with high melting temperature as small as 50 nm. This method enables the preparation of disc-shaped samples with diameter up to 20 mm and thickness about 0.2 mm, which are good for fundamental studies of the structure-property relationships for nanomaterials.

Figure 1. Schematic rendering of high pressure torsion

The ECAP technique was invented as a method for the shape-maintaining plastic deformation of bulk materials in [7] and for the first time has been shown to be applicable for the formation of an ultra-fine grained structure in metals and alloys in [8]. The method utilizes a die containing two channels of equal cross sections intersecting at an angle 2Φ, which normally varies in the range 90°-135° (Figure 2). In the vicinity of the plane of intersection of these channels material undergoes severe plastic deformation, which is mainly of simple shear character. On passing the two channels a sample maintains its shape except for small portions at the ends (Figure 3). There have been done many analyses of the mechanics of deformation during ECAP to understand the exact character of plastic strain during ECAP, its dependence on the location of material elements with respect to the channel walls and to evaluate the accumulated strain [9-11]. The simplest approximation is the model of simple shear according to which materials is subject to simple shear strain of ε=2/√3cot(Φ/2) singularly at the plane of channels' intersection [9]. For 2Φ=90° the highest strain per pass equal to ε=1.15 is achieved. Dies with such an angle are used for pure metals and easily deformable alloys. For hard-to-deform materials strain is imposed at elevated temperatures and/or with the channels' intersection angle 2Φ>90°.

To accumulate very large strains sample can be forced to pass through the die several times. Strain path can be easily changed by turning the sample around its longitudinal axis between subsequent passes. Four standard routes have been established referred to as A, BA, BC, and C [1,2,9]. A sample is rotated around its axis to an angle of 0°, 90°, and 180° for the routes A, B, and C, respectively. When using route BA, consecutive 90° rotations have opposite senses, while in route BC the sample is rotated in the same direction.

Figure 2. Schematic view of equal channel angular pressing

Parameters of the die and deformation route can be chosen for any material to meet the following main requirements:
  • Formation of an UFG structure with mainly high-angle grain boundaries,
  • The absence of macroscopic damages and cracks in the samples,
  • Microstructural homogeneity in the most volume of the samples, and
  • Formation of equiaxed grains.
In some cases using back pressure helps to meet these requirements.

The original ECAP is a discontinuous process and as such has a low production efficiency and high cost. It is considered as a basic way to understand the principles of SPD fabrication of nanomaterials that then can be used in further developments aimed at scaling up of the process and production of low-cost nanomaterials in large quantities.

One of numerous ECAP-based continuous SPD methods is the ECAP-Conform process developed at IPAM [12]. The ECAP-Conform set up is schematically illustrated in Figure 4. A rotating shaft in the center contains a groove, into which the work-piece is fed. The work-piece is driven forward by frictional forces on the three contact interfaces with the groove, which makes the work-piece rotate with the shaft. The work-piece is constrained to the groove by a stationary constraint die. The stationary constraint die also stops the work-piece and forces it to turn an angle by shear as in a regular ECAP process. The angle is about 90°, which is the most commonly used channel intersection angle in ECAP. This set up effectively makes ECAP continuous. Other ECAP parameters (die angle, strain rate, etc.) can also be used. Our preliminary results have shown that the ECAP-Conform process can effectively refine grains of coarse-grained Al and improve its mechanical properties in a way similar to the conventional ECAP.

Figure 4. Schematic view of ECAP-Conform set-up

Severe plastic deformation has a significant effect on the microstructure and mechanical properties of obtained UFG materials). The following structural parameters can be varied: grain size and shape, internal stresses, density of dislocations in grains and grain boundaries, grain boundary misorientations, crystallographic texture, microstructural homogeneity, changes of phase composition etc. This allows for the variation of materials properties in a wide interval and for tailoring materials with desired mechanical and functional properties. Examples of this see at (Reference on appropriate pages)


References

  1. R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Progr. Mater. Sci. 45 (2000) 103-189.
  2. Y.T. Zhu and T.G. Langdon, JOM. Oct. (2004) 58-63.
  3. T.C. Lowe and R.Z. Valiev, JOM. Oct. (2004) 64-68.
  4. N. Hansen and D.J. Jensen, Phil. Trans. R. Soc. Lond. A357 (1999) 1447.
  5. V.V. Rybin, Large Plastic Deformations and Fracture of Metals. Moscow: Metallurgia Publ., 1986, 224 p (in Russian).
  6. R.Z. Valiev, A.V. Korznikov, and R.R. Mulyukov, Mater. Sci. Eng. A 186 (1993) 141.
  7. V.M. Segal et al., Russ. Metall. (Metally) 1 (1981) 99.
  8. R.Z. Valiev, N.A. Krasilnikov, and N.K. Tsenev, Mater. Sci. Eng., 137 (19991) 35.
  9. V.M. Segal. Mater. Saci. Eng. A271 (1995) 322.
  10. L.S. Toth et al. Acta Mater. 52 (2004) 1885.
  11. I.J. Beyerlein and C.N. Tome, Mater. Sci. Eng. A380 (2004) 171.
  12. G.J. Raab, R.Z. Valiev, T.C. Lowe and Y. T. Zhu. Mat. Sci. Eng. A 382 (2004) 30-34.
Literature Recommended for Further reading

  1. R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, "Bulk Nanostructured Materials from Severe Plastic Deformation," Progr. Mater. Sci. 45 (2000) 103-189.
  2. M. Furukawa, Z. Horita, M. Nemoto, T.G. Langdon, "Review: Processing of Metals by Equal-Channel Angular Pressing," J. Mater. Sci. 36 (2001) 2835-2843.
  3. Y.T.Zhu and D.P. Butt, "Nanomaterials by Severe Plastic Deformation," Encyclopedia of Nanotechnology, American Scientific Publishers, Stevenson Ranch, CA, volume 6, 2004, pp. 843-856.
  4. Y.T. Zhu and T.G. Langdon, "Fundamentals of Nanostructured Materials by Severe Plastic Deformation" JOM, 58-63 (Oct. 2004).
  5. T.C. Lowe and R.Z. Valiev, "The Use of Severe Plastic Deformation Techniques in Grain Refinement " JOM, 64-68 (Oct. 2004).
  6. S.L. Semiatin, A.A. Salem and M.J. Saran, "Models for Severe Plastic Deformation by Equal-Channel Angular Extrusion, " JOM, 58-63 (Oct. 2004).
  7. R. Valiev. Nanostructuring of Metals by Severe Plastic Deformation for Advanced Properties// Nature Materials, 2004, Vol. 3, pp. 511-516.
  8. G.J. Raab, R.Z. Valiev, T.C. Lowe and Y.T. Zhu. Continuous processing of ultrafine grained Al by ECAP - Conform// Mat. Sci. Eng., 2004, Vol. A 382, pp.30-34.
  9. R.Z. Valiev. Development of equal-channel angular pressing to produce ultrafine-grained metals and alloys// Rus. Metall. (Metally), 2004, Vol. 2004, No. 1, pp. 10-15.