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
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 . 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
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  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 . 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
the plane of channels' intersection . 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:
In some cases using back pressure
helps to meet these 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.
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 . 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)
Literature Recommended for Further reading
- R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Progr. Mater. Sci. 45 (2000) 103-189.
- Y.T. Zhu and T.G. Langdon, JOM. Oct. (2004) 58-63.
- T.C. Lowe and R.Z. Valiev, JOM. Oct. (2004) 64-68.
- N. Hansen and D.J. Jensen, Phil. Trans. R. Soc. Lond. A357 (1999) 1447.
- V.V. Rybin, Large Plastic Deformations and Fracture of Metals. Moscow: Metallurgia Publ., 1986, 224 p (in Russian).
- R.Z. Valiev, A.V. Korznikov, and R.R. Mulyukov, Mater. Sci. Eng. A 186 (1993) 141.
- V.M. Segal et al., Russ. Metall. (Metally) 1 (1981) 99.
- R.Z. Valiev, N.A. Krasilnikov, and N.K. Tsenev, Mater. Sci. Eng., 137 (19991) 35.
- V.M. Segal. Mater. Saci. Eng. A271 (1995) 322.
- L.S. Toth et al. Acta Mater. 52 (2004) 1885.
- I.J. Beyerlein and C.N. Tome, Mater. Sci. Eng. A380 (2004) 171.
- G.J. Raab, R.Z. Valiev, T.C. Lowe and Y. T. Zhu. Mat. Sci. Eng. A 382 (2004) 30-34.
- R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, "Bulk Nanostructured Materials from Severe Plastic Deformation," Progr. Mater. Sci. 45 (2000) 103-189.
- 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.
- 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.
- Y.T. Zhu and T.G. Langdon, "Fundamentals of Nanostructured Materials by Severe Plastic Deformation" JOM, 58-63 (Oct. 2004).
- T.C. Lowe and R.Z. Valiev, "The Use of Severe Plastic Deformation Techniques in Grain Refinement " JOM, 64-68 (Oct. 2004).
- 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).
- R. Valiev. Nanostructuring of Metals by Severe Plastic Deformation for Advanced Properties// Nature Materials, 2004, Vol. 3, pp. 511-516.
- 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.
- 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.