View source for Balaban 10-cage

'''Severe plastic deformation''' ('''SPD''') is a generic term describing a group of [[metalworking]] techniques involving very large strains typically involving a complex stress state or high shear, resulting in a high [[Crystallographic defect|defect density]] and [[equiaxed]] "ultrafine" [[Crystallite|grain]] (UFG) size ([[diameter|d]] < 500 nm) or [[nanocrystalline]] (NC) structure (d < 100 nm).<ref>{{cite journal|last=Wei|first=Q|coauthors=Cheng, S; Ramesh, K.T; Ma, E|title=Effect of nanocrystalline and ultrafine grain sizes on the strain rate sensitivity and activation volume: fcc versus bcc metals|journal=Materials Science and Engineering: A|date=15 September 2004|volume=381|issue=1-2|pages=71–79|doi=10.1016/j.msea.2004.03.064}}</ref> 

==History==
The development of the principles underlying SPD techniques goes back to the pioneering work of [[Percy Williams Bridgman|P.W. Bridgman]] at [[Harvard University]] in the 1930s. This work concerned the effects on solids of combining large hydrostatic pressures with concurrent shear deformation and it led to the award of the [[Nobel Prize]] in Physics in 1946.<ref name=HPT /> Very successful early implementations of these principles, described in more detail below, are the processes of [[equal-channel angular pressing]] (ECAP) developed by V.M. Segal and co-workers in Minsk in the 1970s<ref name=ECAP /> and [[high-pressure torsion]], derived from Bridgman's work, but not widely developed until the 1980s at the Russian [[Russian Academy of Sciences|Institute of Metals Physics]] in modern-day [[Yekaterinburg]].<ref name=HPT /> 

Some definitions of SPD describe it as a process in which high strain is applied without any significant change in the dimensions of the workpiece, resulting in a large [[hydrostatic pressure]] component.<ref>{{cite journal|last=Valiev|first=Ruslan Z.|coauthors=Estrin, Yuri; Horita, Zenji; Langdon, Terence G.; Zechetbauer, Michael J.; Zhu, Yuntian T.|title=Producing bulk ultrafine-grained materials by severe plastic deformation|journal=JOM|date=April 2006|volume=58|issue=4|pages=33–39|doi=10.1007/s11837-006-0213-7}}</ref> However, the mechanisms that lead to grain refinement in SPD are the same as those originally developed for mechanical alloying, a powder process<ref name="CuAl-ECAP" /> that has been characterized as "severe plastic deformation" by authors as early as 1983.<ref>{{cite journal|last=Gilman|first=P.S.|coauthors=Benjamin, J.S.|title=Mechanical alloying|journal=Annual Review of Materials Science|year=1983|volume=13|pages=279-300|url=http://www.annualreviews.org/doi/pdf/10.1146/annurev.ms.13.080183.001431}}</ref> Additionally, some more recent processes such as asymmetric rolling, do result in a change in the dimensions of the workpiece, while still producing an ultrafine grain structure.<ref name="ASR-Al" /> The principles behind SPD have even been applied to surface treatments.<ref name="SMAT-Ti">{{cite journal|last=Zhu|first=K.Y.|coauthors=Vassel, A.; Brisset, F.; Lu, K.; Lu, J.|title=Nanostructure formation mechanism of α-titanium using SMAT|journal=Acta Materialia|date=16 August 2004|volume=52|issue=14|pages=4101–4110|doi=10.1016/j.actamat.2004.05.023}}</ref>

==Methods==
===Equal channel angular extrusion===
[[File:ECAP.png|thumb|During the ECAE process, the material is pressed through an angular die and experiences shear deformation, without changing its cross-sectional dimensions.]]
{{main|Equal channel angular extrusion}}
Equal channel angular extrusion (ECAE, sometimes called Equal channel angular pressing, ECAP) was developed in the 1970s. In this process, a metal billet is pressed through an angled (typically 90 degrees) channel. To achieve optimal results, the process may be repeated several times, changing the orientation of the billet with each pass. This produces a uniform shear throughout the bulk of the material.<ref name="ECAP">{{cite journal|last=Segal|first=V.M.|title=Equal channel angular extrusion: from macromechanics to structure formation|journal=Materials Science and Engineering: A|date=1 November 1999|volume=271|issue=1-2|pages=322–333|doi=10.1016/S0921-5093(99)00248-8}}</ref> 

===High pressure torsion===
[[File:HPT.png|thumb|200px|During the constrained HPT process, the material experiences shear deformation between a fixed and a rotating anvil, without losing its original dimensions.]]
High pressure torsion (HPT) can be traced back to the experiments that won [[Percy Bridgman]] the 1946 [[Nobel Prize in Physics]], though its use in metal processing is considerably more recent. In this method, a disk of the material to be strained is placed between 2 anvils. A large compressive stress (typically several [[gigapascal]]s) is applied, while one anvil is rotated to create a [[Torsion (mechanics)|torsion]] force. HPT can be performed unconstrained, in which the material is free to flow outward, fully constrained, or to some degree between in which outward flow is allowed, but limited.<ref name="HPT">{{cite journal|last=Zhilyaev|first=A|coauthors=Langdon, T|title=Using high-pressure torsion for metal processing: Fundamentals and applications|journal=Progress in Materials Science|date=1 August 2008|volume=53|issue=6|pages=893–979|doi=10.1016/j.pmatsci.2008.03.002}}</ref>

===Accumulative roll bonding===
{{main|Accumulative roll bonding}}
In accumulative roll bonding (ARB), 2 sheets of the same material are stacked, heated (to below the [[Recrystallization (metallurgy)|recrystallization]] temperature), and [[Rolling (metalworking)|rolled]], bonding the 2 sheets together. This sheet is cut in half, the 2 halves are stacked, and the process is repeated several times. Compared to other SPD processes, ARB has the benefit that it does not require specialized equipment or tooling, only a conventional rolling mill. However, the surfaces to be joined must be well-cleaned before rolling to ensure good bonding.<ref name=ARB>{{cite journal|last=Saito|first=Y.|coauthors=Utsunomiya, H.; Tsuji, N.; Sakai, T.|title=Novel ultra-high straining process for bulk materials—development of the accumulative roll-bonding (ARB) process|journal=Acta Materialia|year=1999|volume=47|issue=2|pages=579–583|doi=10.1016/S1359-6454(98)00365-6}}</ref>  

===Asymmetric rolling===
In asymmetric rolling (ASR), a rolling mill is modified such that one roll has a higher velocity than the other. This is typically done with either independent speed control or by using rolls of different size. This creates a region in which the frictional forces on the top and bottom of the sheet being rolled are opposite, creating shear stresses throughout the material in addition to the normal compressive stress from rolling. Unlike other SPD processes, ASR does not maintain the same net shape, but the effect on the microstructure of the material is similar.<ref name="ASR-Al">{{cite journal|last=Cui|first=Q.|coauthors=Ohori, K.|title=Grain refinement of high purity aluminium by asymmetric rolling|journal=Materials Science and Technology|date=October 2000|volume=16|issue=20|pages=1095-1101|doi=10.1179/026708300101507019}}</ref><ref>{{cite journal|last=Mousavi|first=S.A.A. Akbari|coauthors=Ebrahimi, S.M.; Madoliat, R.|title=Three dimensional numerical analyses of asymmetric rolling|journal=Journal of Materials Processing Technology|date=12 June 2007|volume=187-188|pages=725–729|doi=10.1016/j.jmatprotec.2006.11.045}}</ref> 

===Mechanical alloying===
{{main|Mechanical alloying}}
Mechanical alloying/milling (MA/MM) performed in a high-energy [[ball mill]] such as a shaker mill or planetary mill will also induce severe plastic deformation in metals. During milling, particles are fractured and [[cold weld]]ed together, resulting in large deformations. The end product is generally a powder that must then be consolidated in some way (often using other SPD processes), but some alloys have the ability to consolidate ''in-situ'' during milling. Mechanical alloying also allows powders of different metals to be alloyed together during processing.<ref>{{cite journal|last=Koch|first=C C|title=Materials Synthesis by Mechanical Alloying|journal=Annual Review of Materials Science|date=1 August 1989|volume=19|issue=1|pages=121–143|doi=10.1146/annurev.ms.19.080189.001005}}</ref><ref>{{cite journal|last=Zhang|first=X.|coauthors=Wang, H.; Kassem, M.; Narayan, J.; Koch, C.C.|title=Preparation of bulk ultrafine-grained and nanostructured Zn, Al and their alloys by in situ consolidation of powders during mechanical attrition|journal=Scripta Materialia|date=10 May 2002|volume=46|issue=9|pages=661–665|doi=10.1016/S1359-6462(02)00048-9}}</ref>

===Surface treatments===
More recently, the principles behind SPD have been used to develop surface treatments that create a nanocrystalline layer on the surface of a material. In the surface mechanical attrition treatment (SMAT), an [[ultrasonic horn]] is connected to an ultrasonic (20 kHz) transducer), with small [[Ball (bearing)|balls]] on top of the horn. The workpiece is mounted a small distance above the horn. The high frequency results in a large number of collisions between the balls and the surface, creating a [[strain rate]] on the order of 10<sup>2</sup>–10<sup>3</sup> s<sup>-1</sup>. The NC surface layer developed can be on the order of 50 μm thick.<ref name=SMAT-Ti /> The process is similar to [[shot peening]], but the kinetic energy of the balls is much higher in SMAT.<ref>{{cite journal|last=Dai|first=K.|coauthors=Shaw, L.|title=Comparison between shot peening and surface nanocrystallization and hardening processes|journal=Materials Science and Engineering: A|date=15 August 2007|volume=463|issue=1-2|pages=46–53|doi=10.1016/j.msea.2006.07.159}}</ref>

==Applications==
Most research into SPD has focused on grain refinement, which has obvious applications in the development of high-strength materials as a result of the [[Hall-Petch]] relation. Conventionally processed industrial metals typically have a grain size from 10–100 μm. Reducing the grain size from 10 μm to 1 μm can increase the yield strength of metals by more than 100%. Techniques that use bulk materials such as ECAE can provide reliable and relatively inexpensive ways of producing ultrafine grain materials compared to rapid solidification techniques such as [[melt spinning]].<ref name=Segal-SPD>{{cite book|last=Segal|first=Vladimir M.|title=Fundamentals and engineering of severe plastic deformation|year=2010|publisher=Nova Science Publishers|location=Hauppauge, N.Y.|isbn=9781616681906|coauthors=Beyerlein, Irene J.; Tome, Carlos, N.; Chuvil'deev, Vladimir, N.; Kopylov, Vladimir I.}}</ref> 

However, other effects of SPD, such as [[Texture (crystalline)|texture]] modification also have potential industrial applications as properties such as the [[Lankford coefficient]] (important for [[deep drawing]] processes) and magnetic properties of [[electrical steel]] are highly dependent on texture.<ref name=Segal-SPD />

Processes such as ECAE and HPT have also been used to consolidate [[powder metallurgy|metal powders]] and composites without the need for the high temperatures used in conventional consolidation processes such as [[hot isostatic pressing]], allowing desirable characteristics such as nanorystalline grain sizes or [[amorphous metal|amorphous structures]] to be retained.<ref name=Segal-SPD /><ref>{{cite journal|last=Senkov|first=O.N.|coauthors=Senkova, S.V.; Scott, J.M.; Miracle, D.B.|title=Compaction of amorphous aluminum alloy powder by direct extrusion and equal channel angular extrusion|journal=Materials Science and Engineering: A|date=25 February 2005|volume=393|issue=1-2|pages=12–21|doi=10.1016/j.msea.2004.09.061}}</ref> 

Some known commercial application of SPD processes are in the production of [[Sputtering]] targets by [[Honeywell]]<ref name=Segal-SPD /> and UFG titanium for medical implants.<ref>{{cite patent | country = US | number = 6399215 | status = patent | title = Ultrafine-grained titanium for medical implants | gdate = 2002-06-04 | fdate = 2000-03-28 | inventor = Zhu, Y.T.; Lowe, T.C.; Valiev, R.Z.; Stolyarov, V.V.; Latysh, V.V.; Raab, G.J. | assign1 = The Regents Of The University Of California}}</ref>

==Grain refinement mechanism==
The presence of a high hydrostatic pressure, in combination with large shear strains, is essential for producing high densities of crystal lattice defects, particularly [[dislocations]], which can result in a significant [[Grain boundary strengthening|refining of the grains]]. Grain refinment in SPD processes occurs by a multi-step process:
#[[Dislocation]]s, which are initially distributed throughout the grains, rearrange and group together into dislocation "cells" to reduce the total strain energy.
#As deformation continues and more dislocations are generated, misorientation develops between the cells, forming "subgrains"
#The process repeats within the subgrains until the size becomes sufficiently small such that the subgrains can rotate
#Additional deformation causes the subgrains to rotate into high-angle grain boundaries, typically with an [[Equiaxed crystals|equiaxed]] shape.<ref name=Mishra2007>{{cite journal|last=Mishra|first=A|coauthors=Kad, B; Gregori, F; Meyers, M|title=Microstructural evolution in copper subjected to severe plastic deformation: Experiments and analysis|journal=Acta Materialia|date=1 January 2007|volume=55|issue=1|pages=13–28|doi=10.1016/j.actamat.2006.07.008}}</ref> 

The mechanism by which the subgrains rotate is less understood. Wu ''et al.'' describe a process in which dislocation motion becomes restricted due to the small subgrain size and grain rotation becomes more energetically favorable.<ref name=Wu2002>{{cite journal|last=Wu|first=X|coauthors=Tao, N; Hong, Y; Xu, B; Lu, J; Lu, K|title=Microstructure and evolution of mechanically-induced ultrafine grain in surface layer of AL-alloy subjected to USSP|journal=Acta Materialia|year=2002|volume=50|issue=8|pages=2075–2084|doi=10.1016/S1359-6454(02)00051-4}}</ref> Mishra ''et al.'' propose a slightly different explanation, in which the rotation is aided by [[Atomic diffusion|diffusion]] along the grain boundaries (which is much faster than through the bulk).<ref name=Mishra2007 />

F.A. Mohamad has proposed a model for the minimum grain size achievable using [[mechanical alloying|mechanical milling]]. The model is based on the concept that the grain size is dependent on the rates at which dislocations are generated and annihilated. The full model is given by

<math>
\frac{d_{min}}{b}=A_3 \left ( e^{-\tfrac{\beta Q}{4RT}} \right ) {\left ( \frac{D_{p0} G b^2 }{\nu_0 k T} \right )}^{0.25} {\left ( \frac{ \gamma }{ G b } \right )}^{0.5} {\left ( \frac{ G }{ H } \right )}^{1.25}
</math>

*On the left side of the equation: d<sub>min</sub> is the minimum grain size and b is the [[Burgers vector]].
*A<sub>3</sub> is a constant.
*β=Q<sub>p</sub>−Q<sub>m</sub>/Q (Q<sub>p</sub> is the activation energy for pipe diffusion along dislocations, Q<sub>m</sub> is the activation energy for vacancy migration, and Q is the activation energy for self-diffusion), βQ represents the activation energy for recovery, R is the gas constant, and T is the processing temperature.
*D<sub>p0</sub> is the temperature-independent component of the pipe [[diffusion coefficient]], G is the [[shear modulus]], ν<sub>0</sub> is the dislocation velocity, k is the [[Boltzmann constant]], γ is the [[stacking fault energy]], and H is the [[hardness]].<ref name=Mohamed2003>{{cite journal|last=Mohamed|first=Farghalli A.|title=A dislocation model for the minimum grain size obtainable by milling|journal=Acta Materialia|year=2003|volume=51|issue=14|pages=4107–4119|doi=10.1016/S1359-6454(03)00230-1}}</ref> 

While the model was developed specifically for mechanical milling, it has also been successfully applied to other SPD processes. Frequently only a portion of the model is used (typically the term involving the stacking fault energy) as the other terms are often unknown and difficult to measure. This is still useful as it implies that all other things remaining equal, reducing the stacking fault energy, a property that is a function of the alloying elements, will allow for better grain refinement.<ref name="HPT" /><ref name="CuAl-ECAP">{{cite journal|last=Qu|first=S.|coauthors=An, X.H.; Yang, H.J.; Huang, C.X.; Yang, G.; Zang, Q.S.; Wang, Z.G.; Wu, S.D.; Zhang, Z.F.|title=Microstructural evolution and mechanical properties of Cu–Al alloys subjected to equal channel angular pressing|journal=Acta Materialia|year=2009|volume=57|issue=5|pages=1586–1601|doi=10.1016/j.actamat.2008.12.002}}</ref>

==References==
{{reflist}}

[[Category:Deformation]]
[[Category:Metal forming]]
[[Category:Materials science]]