Review
Grain boundaries in ultrafine grained materials processed by severe plastic deformation and related phenomena

https://doi.org/10.1016/j.msea.2012.01.080Get rights and content

Abstract

Grain boundaries in ultrafine grained (UFG) materials processed by severe plastic deformation (SPD) are often called “non-equilibrium” grain boundaries. Such boundaries are characterized by excess grain boundary energy, presence of long range elastic stresses and enhanced free volumes. These features and related phenomena (diffusion, segregation, etc.) have been the object of intense studies and the obtained results provide convincing evidence of the importance of a non-equilibrium state of high angle grain boundaries for UFG materials with unusual properties. The aims of the present paper are first to give a short overview of this research field and then to consider tangled, yet unclear issues and outline the ways of oncoming studies. A special emphasis is given on the specific structure of grain boundaries in ultrafine grained materials processed by SPD, on grain boundary segregation, on interfacial mixing linked to heterophase boundaries and on grain boundary diffusion. The connection between these unique features and the mechanical properties or the thermal stability of the ultrafine grained alloys is also discussed.

Highlights

► UFG materials produced by SPD do exhibit specific GBs defined earlier as “non-equilibrium GB”. ► Such boundaries differ from regular GBs on atomic structure and appearance of long-range stresses. ► The apparent thickness of “non-equilibrium” GB is typically in a range of 1 to 2 nm. ► GBs in SPD materials may promote segregations and mechanical mixing that affect the properties. ► “Non-equilibrium” GBs do significantly enhance the atomic mobility in SPD materials.

Introduction

With grain sizes in a submicron (100–1000 nm) or nanocrystalline (<100 nm) range ultrafine-grained (UFG) materials contain in their microstructure a very high density of grain boundaries (GBs), which can play a significant role in the development and exhibition of novel properties. For this reason, UFG materials can be typically considered as interface-controlled materials [1]. Unlikely to the nanocrystalline materials where grain boundary material can represent a significant, e.g. a percent or even larger fraction of the whole volume, the volume fraction of GBs in an UFG material is less than 1%. However, the structure, kinetic and thermodynamic properties of GBs could be modified so significantly that they start to dominate some important material properties.

Already in first works on nanocrystalline materials pioneered by Gleiter and colleagues it was suggested that grain boundaries can possess a number of peculiar features in terms of their atomic structure in contrast to grain boundaries in conventional polycrystalline materials [1], [2]. Further studies delivered plenty of indications towards this idea, evidencing simultaneously the fact that solely the grain size is not the deciding parameter. For example, specific grain boundaries were revealed in ultrafine-grained materials produced by severe plastic deformation (SPD) techniques [3]. In the recent decade the use of SPD techniques for grain refinement and nanostructuring of metals and alloys attracted intensive attention and received much development due to their possibility not only to enhance properties of different materials but also to produce mulifunctionality of the materials including commercial alloys and composites and presently, these developments witness the stage of transition from laboratory research to their practical application [4], [5], [6].

Depending on the regimes of SPD processing different types of grain boundaries can be formed in the UFG materials (high- and low-angle, special and random, equilibrium and so-called “non-equilibrium” grain boundaries) [3], [7], which paves the way to grain boundary engineering of UFG materials, i.e. to the control of their properties by means of varying the grain boundary structure. For example, recent studies demonstrated that transport properties of UFG materials (diffusion, segregation, etc.) are markedly affected by a so-called “non-equilibrium” grain boundary state [8], [9], [10]. At this place it is important to highlight that a broad spectrum of diffusivities of short-circuit paths is observed in UFG materials—contributions of high-angle grain boundaries with both “normal” and significantly enhanced diffusion rates can be differentiated in SPD-processed materials [11], [12]. In this context, the “normal” diffusion rates are those which reveal the relaxed general high-angle grain boundaries as they are present in well-annealed polycrystalline counterparts1 and the non-equilibrium interfaces are characterized by considerably higher diffusion coefficients. This hierarchy of interfaces in terms of their corresponding diffusivities is proposed [12] to explain the apparent contradictions between earlier publications that reported either conventional or unusual properties for grain boundaries in nanocrystalline or ultrafine grained materials.

The notions on non-equilibrium grain boundaries were first introduced in the scientific literature in the 1980s [13], [14] reasoning from investigations of interactions of lattice dislocations with grain boundaries. According to [14] the formation of a non-equilibrium grain boundary state is characterized by three main features, namely, excess grain boundary energy (at the specified crystallographic parameters of the boundary), the presence of long range elastic stresses (Fig. 1) and enhanced free volume. Discontinuous distortions of crystallographically ordered structures, that may come about by accommodation problems of differently oriented crystallites of finite sizes or by high densities of lattice dislocations and their interaction with grain boundaries can be considered as sources of elastic stress fields that modify the atomic structure of high angle grain boundaries so that their excess free energy becomes enhanced. Somewhat unfortunately, these “unusual” grain boundaries have been termed “non-equilibrium” grain boundaries although in a strict sense, each grain boundary is a non-equilibrium defect if segregation effects (see Section 3) are not to be considered. Since however the term has been accepted and utilized by the entire community who works on severe plastic deformation, we will also use it here.

A model for these non-equilibrium grain boundaries has been developed by Nazarov, Romanov and Valiev in a series of papers [15], [16] describing their formation. Lattice dislocations that are created during the plastic straining move towards high angle grain boundaries on their respective glide planes during continued straining and then, when reaching a high-angle grain boundary, transform into so-called “extrinsic grain boundary dislocations”, i.e. dislocations that do not contribute towards the misorientation of the two adjacent grains. As a net effect, high angle grain boundaries with high densities of such extrinsic grain boundary dislocations would also contain increased energy and free volume and considerable microstrain associated with the grain boundary region [15].

In recent years the non-equilibrium grain boundaries in UFG materials and related phenomena (diffusion, segregation, etc.) have been the object of intense studies performed by the authors of this paper and the obtained results provide convincing evidence of the importance of a non-equilibrium state of high angle grain boundaries for UFG materials with unusual properties. At the same time the complexity of such research becomes evident, involving the most contemporary techniques of structural analysis and, occasionally, different interpretation of the obtained results. All this specifies the aims of the present paper—first, to introduce the readers to this research field of recent studies of grain boundaries in bulk nanostructured materials where unique features about their structures and properties are outlined; second, to consider tangled, yet unclear issues and outline the ways of oncoming studies. The available models of the “non-equilibrium” GBs will be examined against the newest experimental data.

Section snippets

Structure of grain boundaries in ultrafine grained materials

The atomistic structure of random high-angle grain boundaries has been discussed since several decades by different models assuming quite different structural arrangements ranging from an amorphous structure to local structural units with high packing densities that are arranged non-periodically along the boundary plane, see, e.g. [17], [18], [19], to mention just a few examples. In recent years, atomistic simulations have considerably contributed to the understanding of grain boundary

SPD induced GB segregation

The grain size refinement mechanism during SPD is controlled by the generation of dislocations, the way they do dynamically reorganize to form low angle—and finally, for larger strains, high angle boundaries [3], [30]. On the other hand, it is also well known that impurities or solute elements may have strong interactions with dislocations. They usually lead to a stronger strain hardening due to a higher dislocation production rate during deformation [31], but alloying elements may also modify

Heterophase boundaries and multiphase alloys during SPD

It is known since a very long time that heterophase boundaries may promote the grain size reduction during deformation and thus the resulting strengthening. This is the typical case of drawn pearlitic steels, for which an interlamellar spacing of only 20 nm is commonly achieved in mass production leading to a yield stress of up to 3 GPa or more [74], [75], [76]. Following this approach, multiphase materials containing different phases with the capability of co-deformation are of particular

Diffusion along grain boundaries in ultrafine grained materials

As it was stated above (Section 2) the diffusion investigations are a highly sensitive probe for investigation of structural modifications on the atomic scale since the thermally activated diffusivity depends exponentially on the corresponding activation barriers which are determined by the interatomic potentials and the atomic environment. Thus, dedicated measurements of the atomic mobility at low temperatures, when diffusion within undisturbed regions of the crystal lattice is frozen, can be

Summary and outlook

The results presented in this paper provide a strong evidence that SPD-processing synthesizes material with a significant fraction of high angle grain boundaries that possess higher excess free energy density, enhanced atomic mobility along the boundary plane, significant residual strain fields located at the near-boundary region and strongly increased segregation at the boundary and in the near-boundary region. These observations agree with early experiments [134], [135] and models [15] of

Acknowledgements

The authors gratefully acknowledge funding by CNRS, DFG and RFBR of a French-German-Russian Tri-Lateral Seminar on “Atomic Transport Kinetics in Bulk Nanostructured Materials”, Rouen (France), May 2010, which nucleated the present overview paper. The authors also acknowledge individual funding on the above research topic by CNRS, DFG and RFBR.

References (139)

  • H. Gleiter

    Prog. Mater. Sci.

    (1989)
  • H. Gleiter

    Acta Mater.

    (2000)
  • R.Z. Valiev et al.

    Prog. Mater. Sci.

    (2000)
  • Yu. R. Kolobov et al.

    Scripta Mater.

    (2001)
  • Y. Amouyal et al.

    Acta Mater.

    (2007)
  • A.A. Nazarov et al.

    Acta Metall. Mater.

    (1993)
  • A.A. Nazarov et al.

    Scripta Metall. Mater.

    (1990)
  • H. Gleiter

    Mater. Sci. Eng.

    (1982)
  • D. Wolf

    Curr. Opin. Solid State Mater. Sci.

    (2001)
  • H. Van Swygenhoven et al.

    Mater. Today

    (2006)
  • Z.H. Jin et al.

    Acta Mater.

    (2008)
  • Y. Mishin et al.

    Acta Mater.

    (2010)
  • G.P. Dinda et al.

    Scripta Mater.

    (2005)
  • S.V. Divinski et al.

    Acta Mater.

    (2011)
  • M.J. Hÿtch et al.

    Ultramicroscopy

    (1998)
  • H. Rösner et al.

    Acta Mater.

    (2010)
  • Y. Ito et al.

    Mater. Sci. Eng. A

    (2009)
  • T. Morishige et al.

    Scripta Mater.

    (2011)
  • Y.H. Zhao et al.

    Mater. Sci. Eng. A

    (2008)
  • A.P. Zhilyaev et al.

    Mater. Sci. Eng. A

    (2005)
  • Z. Horita et al.

    Mater. Sci. Eng. A

    (2005)
  • H.W. Zhang et al.

    Acta Mater.

    (2010)
  • X.-Y. Liu et al.

    Acta Mater.

    (1998)
  • P. Lejcek et al.

    Acta Mater.

    (1997)
  • D. Blavette et al.

    Acta Mater.

    (1996)
  • T. Surholt et al.

    Acta Mater.

    (1997)
  • S.V. Divinski et al.

    Acta Mater.

    (2010)
  • X.F. Zhang

    Mater. Sci. Eng. A

    (2010)
  • J. Schäfer et al.

    Acta Mater.

    (2011)
  • S. Cheng et al.

    Acta Mater.

    (2007)
  • J.K. Kim et al.

    Scripta Mater.

    (2005)
  • Y.H. Zhao et al.

    Acta Mater.

    (2004)
  • K. Ohashi et al.

    Mater. Sci. Eng. A

    (2006)
  • G. Sha et al.

    Acta Mater.

    (2009)
  • R.Z. Valiev et al.

    Scripta Mater.

    (2010)
  • S.-H. Song et al.

    Mater. Sci. Eng. A

    (2008)
  • J.D. Embury et al.

    Acta Metall.

    (1966)
  • J. Languillaume et al.

    Acta Mater.

    (1997)
  • Z. Horita et al.

    Sci. Technol. Adv. Mater.

    (2006)
  • M. Murayama et al.

    Acta Mater.

    (2001)
  • K. Matsubara et al.

    Acta Mater.

    (2003)
  • H. Hasegawa et al.

    Mater. Sci. Eng. A

    (1999)
  • S. Lee et al.

    Acta Mater.

    (2002)
  • M. Furukawa et al.

    Mater. Sci. Eng. A

    (2002)
  • X. Sauvage et al.

    Scripta Mater.

    (2008)
  • I. Sabirov et al.

    Scripta Mater.

    (2005)
  • G. Wilde et al.

    Scripta Mater.

    (1999)
  • X. Sauvage et al.

    Scripta Mater.

    (2007)
  • V.V. Stolyarov et al.

    Mater. Sci. Eng. A

    (2000)
  • T. Tokunaga et al.

    Mater. Sci. Eng. A

    (2008)
  • Cited by (449)

    View all citing articles on Scopus

    At the invitation of the MSE A editors, the winner of the 2011 Materials Science and Engineering: A prize, Professor Rusian Z. Valiev has contributed this paper co-authored by X. Sauvage, G. Wilde, S.V. Divinski and Z. Horita, which considers the nature of grain boundaries in alloys subjected to severe plastic deformation. MSE A editors and staff all want to again congratulate Prof. Valiev and thank him for submission of this manuscript.

    View full text