Texture evolution in high-pressure torsion processing

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Abstract

Among the severe plastic deformation processing (SPD) of materials, high-pressure torsion (HPT) is known to induce considerable amounts of plastic strain and crucial grain refinement up to the nanometer scale. There have been several reviews on the mechanical properties and/or microstructure correlations of SPD-processed materials published during the last two decades. By contrast, a review on the crystallographic texture evolution during HPT processing is not at present available. Consequently, this work was undertaken to review the texture evolution in Face-Centered Cubic (FCC), Body-Centered Cubic (BCC) and Hexagonal Close-Packed (HCP) crystal structure materials processed exclusively by HPT with a special emphasis on the influence of many intrinsic and extrinsic factors such as the imposed strain, the deformation temperature and the alloying elements.

Introduction

At the present time the application of severe plastic deformation (SPD) is known as the most effective ‘top-down’ procedure for achieving exceptional grain refinement in bulk solids through attaining ultrafine microstructures within the submicrometer (0.1–1 μm) or nanometer range (<100 nm) with relatively large fractions of high-angle grain boundaries (HAGBs) and also without changing significantly the overall dimensions of the sample. Usually this is possible through the application of very high strains via deformation under extensive hydrostatic pressure leading to an accumulation of a high dislocation density and/or extensive numbers of vacancies. A combination of an ultrafine grain size and a high density of lattice defects greatly enhances the mechanical and physical properties of the deformed materials such as strength, ductility, the occurrence of superplasticity and conductivity. Furthermore, SPD processing can impressively modify the nature of the solid-state phase transformation including the sequence and kinetics in various materials [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12].

Several SPD processing techniques have been proposed but it is noted that equal-channel angular pressing (ECAP) [13], accumulative roll-bonding (ARB) [14] and high-pressure torsion (HPT) [15] have received the most scientific and industrial attention. Fig. 1 shows a schematic illustration, reference system, and the appropriate relationship for the equivalent strain for each three of these SPD techniques where the relationships for the equivalent strains are given in [16] for ECAP, [17] for ARB and [18] for HPT.

In ECAP processing a billet with a round or square cross-section deforms by a simple shear strain when its passes through a special die (Fig. 1a). The cross-section of the billet remains unchanged which permits a repetition of the deformation and hence an accumulation of very large strains [13]. Different ECAP routes were proposed based on the rotation of the billet about the pressing axis between the various passes in different dies formed with different angles Φ and Ψ which lead to various microstructural and mechanical properties evolutions [19]. More details on the principles of ECAP processing are given in other reports [13], [20], [21].

ARB processing (Fig. 1b) consists of rolling a sheet usually to 50% reduction of thickness, cutting the sheet in two halves, stacking them together after degreasing and wire-brushing to achieve the original thickness of the sheet and then rolling again [14]. Consequently, several repetitions are possible to achieve extensive strains. A modified ARB processing procedure was proposed and labeled cross-ARB (CARB) in which the sheet is rotated about 90° around the normal direction after each ARB cycle [22]. This change in deformation path affects significantly the grain refinement and then the mechanical properties such as the tensile strength [22], [23], [24], [25], [26]. ARB processing is a useful method for manufacturing metal matrix composite materials and for multilayer composite materials such as Al/B4C [23], Al/Mg [27], and Cu/Nb [28] or with more than two different materials like Al/Ti/Nb [29] and Al/Cu/Mg [30].

The principle of HPT processing shown in Fig. 1c demonstrates that a disc sample undergoes a torsional shear straining under a high hydrostatic pressure (typically about 1–6 GPa). The equivalent strain is not homogeneously distributed across the disc and instead it varies with the distance from the center of the disc as shown by the equation in Fig. 1c [18]. More details on the fundamentals and applications of HPT processing and the historical development is given in other reports [31], [32], [33]. Several modified HPT processing methods were proposed over the last decade such as high-pressure tube twisting (HPTT) [34], tube high-pressure shearing (t-HPS) [35], [36], high-pressure-double-torsion (HPDT) [34], high-pressure sliding (HPS) [37], reversal high pressure torsion (RHPT) [38] and high pressure compressive shearing (HPCS) [39] in order to provide solutions to the limitations of HPT processing such as the inhomogeneity of the shear strain across the disc radius or the smallness of the sample [40]. Recently, it was demonstrated that HPT processing is an excellent tool for the consolidation of metal powders [41], [42], [43] and for fabricating hybrid systems in order to ultimately obtain metal matrix nanocomposites (MMNCs) such as Al–Mg [44], [45], [46], Al-Cu [47] and Zn-Mg [48].

At present, the ECAP and ARB processing techniques are widely investigated and they have attracted more industrial attention than HPT processing because they provide the capability of producing fairly large billets (for ECAP) and sheets (for ARB) so that they may be used in a range of structural applications [13], [49]. By contrast, HPT processing suffers from a significant handicap because of the smallness of the samples which appears to render this technique generally inappropriate for industrial implementation [33]. Nevertheless, it has been demonstrated that HPT processing can lead to more significant grain refinement than ARB and ECAP processing [49], [50], [51], [52], [53], [54] and in addition HPT produces a higher fraction of high-angle grains boundaries (HAGBs) [55], [56]. It is reasonable to conclude, therefore, that processing by HPT will attract more attention with the increasing current emphasis on the commercialization of bulk nanostructured materials [57].

The derived equation for the estimation of equivalent strain in HPT (Fig. 1) indicates that HPT processing is capable of reaching an unlimited amount of strain in a single operation due to the nature of the HPT process through the superimposition of hydrostatic pressure and torsion [15], [32]. By contrast, ECAP [18] and ARB [13] processing require several passes in order to achieve large strains. HPT processing allows materials with low crystal symmetry, such as magnesium-based alloys, to be severely deformed at room temperature (RT) without cracking due to the development of a hydrostatic stress [58], [59]. Moreover, HPT processing is considered a powerful process for solid-state consolidation from metallic powders and other constituents [60], [61], [62], [63], [64], [65], [66].

For many years, scientific research was focused on understanding the effect of grain refinement and dislocation density on the strain hardening of SPD-processed materials [2]. Recently, crystallographic texture was demonstrated to contribute significantly in the strain hardening, especially during the early stages of deformation [67], [68].

In practice, crystallographic texture (or preferred orientations) is defined as the quantitative orientation distribution of grains in a polycrystal aggregate. Often, the preferred orientation is mainly categorized into the “deformation texture” and the “recrystallization (or annealing) texture”. The development of the texture is observed during crystallization from a melt (casting) or more frequently during thermo-mechanical processing such as welding and heat treatment [69].

The texture mainly evolves during deformation processing from grain rotation via the activity of different slip systems and/or twinning which leads to the formation of intragranular substructures. These latter often exhibit orientation deviations from the initial parent grain orientation and the substructures will subsequently move towards characteristic preferred orientations dictated by the imposed deformation [70]. For example, materials develop a shear texture after ECAP and HPT processing because both processing methods are essentially shear-based processes [54], [68], [71], [72] while a typical rolling texture is usually observed for ARB-processed materials due to its plane-strain nature [27], [49], [73].

Consequently, texture development has a strong effect on different properties of the materials and this may be useful for achieving additional information in areas such as the deformation mechanisms [74], [75], the role of strain hardening [76], [77], [78], [79], [80], [81], phase transformations including precipitation kinetics [82] and recrystallization processes [83], [84], [85], [86], the significance of mechanical anisotropy [87], [88], [89] and other physical properties such as the magnetic, photovoltaic and corrosion properties [90], [91], [92], [93], [94], [95]. It is important to note that there is no evidence for any direct correlation between texture and the electrical conductivity evolution. However, there is an experimental study on a low carbon steel processed by ECAP followed by modeling which shows that the texture can induce grain coarsening during large strain deformation [96], [97], [98]. This was explained because if two neighboring grains develop similar orientations during deformation then the disorientation between them progressively diminishes so that high-angle grain boundaries transform to low-angle grain boundaries and this thereby produces a coalescence of neighboring grains into larger grains [96], [97].

Fig. 2 gives an example on the effect of different texture orientations on the forming anisotropy which is generally defined as the Lankford coefficient or R-value [88]. The R-value is defined as the ratio of the true strains in the width and thickness directions measured in tensile tests. Fig. 2 shows that Cube {0 0 1} 〈1 0 0〉 and Goss {1 1 0} 〈1 0 0〉 known as the recrystallization texture possessed low R-values compared to the rolling texture (the Copper {1 1 2} 〈1 1 1〉 and Brass {1 1 0} 〈1 1 2〉 components). The shear texture components (A{1 1 1} 〈1 1 0〉 and A* {1 1 1} 〈1 1 2〉 ) caused an increase in the R-value, while the C {0 0 1} 〈1 1 0〉 component has an insignificant influence on the R-value [88]. Such results demonstrate that rolling and shear textures can improve formability in deep drawing by comparison with the recrystallization texture. By contrast, sharpening the Goss {1 1 0} 〈1 0 0〉 and Cube {0 0 1} 〈1 0 0〉 components is beneficial for improving the magnetic and photovoltaic properties of materials, respectively [90], [92].

Several reviews on the resulting microstructures and the mechanical properties of materials and alloys processed by different SPD processing techniques are now available [15], [20], [21], [99], [100], [101], [102], [103], [104], [105], [106], [107], [108], [109], [110]. However, there are to date only very limited reviews dealing with the textural evolutions after SPD processing [70], [71] and the available information is especially limited for HPT processing. Hence, the main objective of the present review is to provide a comprehensive compilation of existing data on textural evolution of materials having FCC, BCC and HCP crystal structures when processed exclusively by HPT.

In the present review the texture evolution was first grouped based on the crystalline structure (FCC, BCC, and HCP). For each group, the texture review was divided into three sections: (1) effect of initial conditions of materials, (2) effect of HPT processing deformation conditions and (3) evolution of recrystallization texture of HPT-processed materials. In addition, this review attempts to provide a direct correlation between the texture evolution, grain refinement and different properties, especially the mechanical properties, based on the available published data for each crystalline structure. Finally, texture simulations during HPT processing are given for different materials and texture comparisons are presented with some HPT processing variants (HPDT, RHPT, HPTT and HPCS) and combinations with other SPD processing procedures.

Section snippets

Texture evolution after HPT processing for different materials

In principle, the texture may be determined either globally by X-ray or neutron diffraction or locally by electron backscatter diffraction (EBSD) measurements. Both of these procedures offer specific advantages and some limitations as a consequence of the fundamental characteristics of the radiation and the nature of the interaction with metals [111].

The ideal shear texture components present in FCC, BCC and HCP materials are described initially before providing reviews of the evolution of

Texture simulation during HPT processing for different materials

Over the last two or three decades, complex polycrystalline models for numerical prediction of the deformation processes and texture evolution have undergone a significant progression. Thus, models for texture prediction are now used as effective tools to connect the texture development to the deformation mechanisms and this assists in understanding the evolution of microstructure and mechanical properties. The models frequently used to predict the texture evolution during SPD, where the

Texture comparison with some HPT processing variants

Several modified HPT processing methods were proposed over the last decade mostly due to the apparent limitations in HPT processing such as the inhomogeneity of the shear strain across the disc radius or the generally relatively small size of the HPT samples. The schemes of these HPT variant processing methods and their investigations of texture evolution are depicted schematically in Fig. 42 a-c.

Summary and conclusions

The evolution of the deformation textures of FCC, BCC, and HCP materials after HPT processing and the resultant recrystallization texture during different heat treatments are reviewed in this report based on the published data available to date. The main findings may be summarized as follows:

  • For FCC materials, HPT processing leads to the formation of two texture fibers: an A-fiber containing A, A¯, A1 andA2 components and a B-fiber with A, A¯, B, B¯ and C components. The sharpness of the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The work of one of us was supported by the European Research Council under Grant Agreement No. 267464-SPDMETALS (TGL).

Hiba Azzeddine is Professor in the Faculty of Technology at University of M’sila, Algeria, where she joined the faculty as an Assistant Professor in 2012.

Hiba obtained her Diploma of High Studies (D.E.S) degree from University of M’sila in Algeria in 2005 and then received a Magister degree in 2008 and Ph.D. degree in 2012 in Materials Science at the University of Sciences and Technology Houari Boumediene, Algeria.

She has collaborated actively with many researchers around the world and has

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    Hiba Azzeddine is Professor in the Faculty of Technology at University of M’sila, Algeria, where she joined the faculty as an Assistant Professor in 2012.

    Hiba obtained her Diploma of High Studies (D.E.S) degree from University of M’sila in Algeria in 2005 and then received a Magister degree in 2008 and Ph.D. degree in 2012 in Materials Science at the University of Sciences and Technology Houari Boumediene, Algeria.

    She has collaborated actively with many researchers around the world and has published more than 52 peer-reviewed papers. Values of h-index: 13 (Scopus), 14 (Google Scholar). Her research interests include non-ferrous materials, severe plastic deformation, Recrystallization, Texture and microstructure evolution.

    https://orcid.org/0000-0001-6680-5045

    https://scholar.google.fr/citations?user=ceOK5bwAAAAJ&hl=fr

    Djamel Bradai is Professor at the Faculty of Physics, University of Sciences and Technology Houari Boumediene (USTHB) Algiers, Algeria. Djamel defended his State Doctorat (Doctorat d’Etat) in 1999. He obtained a Diploma of High Studies (D.E.S) in June 1985 and graduated a Magister degree in 1990 in Physics at the USTHB. He is the leader of the scientific staff “Texture, Microstructure and Phase Transformations” at the Laboratory “Materials Physics”, USTHB. He has supervised more than 12 PhD theses and established a broad collaboration with many teams in France, Spain, Germany and the UK. His main interests are the characterization of the texture and microstructure in non-ferrous alloys after conventional and severe plastic deformation. He has published more than 81 publications. Values of h-index: 19 (Scopus).

    Thierry Baudin is research director at the french National Center of Scientific Research (CNRS) at the University of Paris-Saclay.

    He is engineer in mechanical science (Institut National des Sciences Appliquées, INSA, Lyon) and he has obtained his Doctoral thesis in material science at the Ecole des Mines de Paris (Centre de Mise en Forme des matériaux, Sophia-Antipolis) in 1988.

    He has supervised (or co-supervised) about 40 Ph.D thesis and published more than 200 refereed publications, 13 book chapters and 200 conference proceedings.

    His main research interests are linked to the characterization of microstructure and texture of metallic materials (SEM, EBSD, X-ray and neutron diffraction) in the objective to study (hyper) deformation, recovery, recrystallization and grain growth mechanisms (from experimental and numerical approaches) in correlation with the material properties.

    https://orcid.org/0000-0002-6765-360X

    Terence G. Langdon graduated from the University of Bristol and then obtained his PhD at Imperial College, University of London, UK.  After short appointments at the University of California in Berkeley, USA, the University of Cambridge, UK, and the University of British Columbia, Canada, he joined the University of Southern California (USC) in Los Angeles, USA, and remained there for 40 years.  He is now Professor of Materials Science at the University of Southampton, UK, and Professor of Engineering Emeritus at USC.  He is a Fellow of the Royal Academy of Engineering and he was the recipient of the Acta Materialia Gold Medal in 2012.  He has published more than 1200 papers in peer-reviewed journals and on Google Scholar he has over 88,000 citations with an h-index of 141.

    https://orcid.org/0000-0003-3541-9250

    https://scholar.google.fr/citations?user=CntRsTYAAAAJ&hl=fr

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