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In contrast, Hirsch et al.
A few attempts have been undertaken to suppress recovery, accumulate dislocations, and develop strain-induced UFG structures by rolling at cryogenic temperatures. However, Wang et al. ECAE of silver produced an ultrafine-grained microstructure with sharp high-angle boundaries and dislocation free-interiors.
The material strength increased after the first pass and then showed a plateau. After rolling of the same material with equivalent strains, the structure was formed by sub-grains and cells having diffused boundaries and a high density of dislocations, resulting in a continuous increase of the material strength. Several papers have also been published that compared various SPD techniques regarding structure refinement. In all cases, they confirm the same general trend: the more closely the deformation mode approximates to the simple shear, the more effective the SPD technique is for grain refinement.
Therefore, simple shear can be classified as the optimal mode for microstructure refinement and other structure modifications during SPD. For uniform processing by pure shear or simple shear, the stress-strain states shown in Figure 1 b,c for the small elements should be extended to the whole material with identical conditions at tool boundaries. As it was noted in Section 4.
During SPD with an exhausted hardening ability or even softening, the extensive free surfaces of the upset material present numerous options for localization along a multitude of systems of kinematical admissible shear planes. After localization begins, the deformation mode along the acting shear bands is simple shear rather than pure shear.
This effect of inversion of the deformation mode explains the inconsistency in some of the results when upsetting was used during SPD to simulate pure shear during rolling. Transition from pure shear to simple shear after flow localization during frictionless upsetting a and simple shear of thin layer b.
On the other hand, simple shear induced by internal flow localization presents significant practical interest for the processing of bulk materials. For large strains, such localization can be developed naturally or artificially. In the first case, the mechanics of plastic deformation shows strong inhomogeneity in distributions of strains and strain rates within plastic zones and along their boundaries.
The general characteristics of most forming operations are thin extended areas of large gradients of strains and strain rates [ 28 , 47 ]. For ideal-plastic solids, these areas are mathematically identified as surfaces of velocity discontinuities along slip lines, which are experimentally observed in most technological operations. Sufficiently uniform simple shear comprises the main part of the total shear strains, confirming the potential of using ordinary forming operations for SPD processing.
In the case of artificial localization, the processing mechanics is intentionally designed so that the plastic zone is transformed into a straight thin area of localized simple shear. This can be achieved for specific tool geometry, friction conditions, and kinematics. As a rule, simple shear processing is not accompanied by overall distortions, preserving the material shape and dimensions.
Simple shear techniques will be considered in the next sections. Some applications wear and corrosion resistance, fatigue require structure modification in a surface layer. Surface SPD achieved by local plastic contact during sliding, rolling, or impression was considered in [ 29 ]. Figure 11 shows slip line solutions for a steady flow during surface sliding a and rolling b with the formation of a plastic wave in front of the contact area with a tool.
Therefore, significant simple shear in the surface layer is accumulated after a few passes of surface sliding or rolling. Therefore, numerous impressions are necessary to accumulate large shears. The depth of the shear zone in all cases is comparable to the contact length d. Slip line solutions for surface SPD: a contact sliding; b contact rolling; c contact penetration. Another option produces surface shear by contact friction. A plastic state in the subsurface area can be developed under the maximum friction comprising high adhesion and mechanical components induced by interaction with tool asperities.
A related model [ 48 ] of a steady plastic flow around a unit micro-asperity of the tool surface is shown in Figure During sliding along a plurality of asperities, large shears are developed within the surface layer, where the thickness is comparable to the tool micro roughness R a. In some cases of thin materials, surface shear can penetrate through the whole thickness for a distance significantly greater than R a. In the following section, the main focus is on the basic techniques of SPD for the processing of bulk materials with an emphasis on the deformation mode and their practical applications.
Obviously, a large variety of other techniques can be realized by a combination of these techniques and with ordinary forming operations. As single-step operations usually cannot develop such large strains, multi-step processing with strain accumulation is necessary.
There are also limitations of strain per processing step in conjunction with the strain rate and temperature. Another challenge is the fabrication of bulk materials with large cross-section areas after SPD. In contrast, SPD techniques based on ordinary forming operations are associated with essential changes of the cross-section areas.
In such cases, cyclic loading or more complex deformation paths are used for periodical restoration of the original shape and dimensions. In terms of the processing mechanics, the effects of SPD also depend on tool geometry and contact friction. For most techniques, friction should be as low as possible. On the contrary, for friction shear techniques, maximum contact friction is necessary. In these cases, significant technical problems are a poor surface finish, intensive tool wear, and high energetic loses induced by friction.
Finally, engineering developments of processing tools and equipment have played a key role in transforming SPD techniques into industrial processes and technologies. Though SPD techniques are highly specialized metalworking processes, the general requirements remain the application of ordinary forming equipment; a reasonable cost; and productive, safe, and effective operations including material handling, lubrication, insertion, ejection, and inspection at each processing step.
Such processing does not allow tool lubrication and inspection after each pass, resulting in a high pressure, material galling of tools, a poor surface finish, and a short tool life.
In materials with work-hardening, the plastic flow around micro asperities does not lead to the lowest plastic work. Depending on the boundary conditions, flow localization along other kinematical admissible slip lines, such as line KG in Figure 12 , becomes more preferable. Thus, friction shear is transmitted into sub-surface layers, allowing the processing of thin materials.
Several processes of friction shear have been developed and applied as SPD techniques. The most important technique is high-pressure torsion HPT , which was introduced by Bridgman in [ 8 ]. Pippan and Hohenwarten [ 49 ] recently published a comprehensive overview of the instrumental aspects of HPT. The exceptional characteristic of this technique is practically unlimited one-step shear, which can be applied to strong and brittle materials under high hydrostatic pressure up to 8 GPa at low temperatures, producing the finest microstructures attainable by SPD.
However, HPT allows the processing of only small and thin discs. Additional disadvantages are the significant inhomogeneity of strains in the radial and axial directions due to twist straining and shear propagation through the material thickness. This can be explained by the simple model of Figure 13 a fully constrained HTP. Sample 1 of diameter D and thickness H is inserted into a die 2. Punch 3 first applies a large compressive pressure P, developing tight contact and large friction at the sample surfaces along with a torsion moment M.
Shear is initiated at the contact surface with the punch. After sufficient hardening in the subsurface layer, plastic straining is transferred to lower layers, forming parallel shear zones.
Geist et al. For SPD, the coefficient n may be positive, zero, or negative curves 1, 2, and 3 in Figure 13 b, respectively. All of the considered situations are possible depending on the material and the structure induced by SPD. Therefore, careful control of the processing characteristics is necessary to ensure structure uniformity. This conclusion is confirmed by numerous experimental and analytical results [ 49 , 50 , 51 , 52 ].
The industrial potential of HPT is still unclear. The process involves high demands in terms of pressure, torque, and energy, and it is hard to scale up. Additional drawbacks are a very slow processing speed, as well as short tool life and poor surface finish due to intensive dry friction. However, the situation may be changed for very specific applications. Hohenwarten [ 53 ] and Ivanisenko et al.
For the latter case, a simplified scheme is shown in Figure 13 c. A cylindrical sample 1 is placed into a split die having a rotating part 2 and a stationary part 3. Punches 4 and 5 squeeze the sample to develop large contact friction and then move it through the die with speed V. Simultaneously, rotating part 2 and punch 4 twists the material across a splitting plane A-A.
The concept was experimentally tested for sufficiently short samples and soft materials. The main concerns for practical applications are high contact friction; wasted billet length L 1 , which is necessary to start and complete processing; possible total length L; material ejection from the die; process stability; and complexity of the tool and equipment.