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Preface to the viewpoint set: “Metals and alloys with a structural scale from the micrometre to the atomic dimensions”
Scripta Materialia 51 (2004) 751–753 www.actamat-journals.com
Preface to the viewpoint set: ‘‘Metals and alloys with a structural scale from the micrometre to the atomic dimensions’’
1. Introduction The microstructural scale of metals and alloys stretches typically from the micrometre level to the atomic scale, for instance, from the grain structure of a conventional polycrystalline metal to the amorphous structure of a rapidly solidified alloy. Research and development has at different times concentrated on materials groups representing different length scales and today, the emphasis is on nanocrystalline materials with a length scale below 100 nm. Such nanoscale materials have been the subject of a recent viewpoint set in Scripta Materialia 2003;4:625–680: Mechanical properties of fully dense nanocrystalline metals. The present viewpoint set also covers nanoscale materials but they are treated as part of a multiscale approach covering a structural scale from the micrometre/submicrometre level through the nanometre scale to atomic dimensions. Behind this approach is the wish to analyse in a holistic way the universality of fundamental principles and models over length scales covering both structures of conventional and of novel metallic materials. In this viewpoint set, a multiscale approach covering four length scales was the guide to papers on specific subjects. These papers cover many aspects of research and development in the general field of materials science and engineering such as synthesis/processing, structure/composition, properties, performance and theory.
(i)
(ii)
(iii)
2. Themes and contributions (iv) The themes of this viewpoint set encompass (i) processing (ii) characterization (iii) mechanical properties (iv) phase and structural stability (v) novel applications and (vi) dislocation structures and atomistic simulations.
Processing 1. Refinement and control of the metal structure elements by plastic deformation. A. Korbel and W. Bochniak. 2. Metallurgy of high strength Ni–Mn microsystems fabricated by electrodeposition. N.Y.C. Yang, T.J. Headley, J.J. Kelly and J. Hruby. 3. Formation of nanostructured steels by phase transformation. T. Yokota, C. Mateo and H.K.D.H. Bhadeshia. Characterization 4. Characterization of fine-scale microstructures by electron backscatter diffraction (EBSD). F.J. Humphreys. 5. Microstructure parameters from X-ray diffraction peak broadening. T. Unga´r. 6. Characterizing the dynamics of individual embedded dislocation structures. H.F. Poulsen, J. Bowen and C. Gundlach. 7. HRTEM analysis of nanostructured alloys processed by severe plastic deformation. C. Rentenberger, T. Waitz and H.P. Karnthaler. Mechanical properties 8. Ductilization of nanocrystalline materials for structural applications. J. Gil Sevillano and J. Aldazabal. 9. Hall–Petch relation and boundary strengthening. N. Hansen. 10. Fatique and microstructure of ultrafinegrained metals produced by severe plastic deformation. H. Mughrabi, H.W. Ho¨ppel and M. Kautz. Phase and structural stability 11. Two-phase equilibrium in small alloy particles. J. Weissmu¨ller, P. Bunzel and G. Wilde. 12. Stability of nanostructured metals and alloys. J.H. Driver.
1359-6462/$ - see front matter Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2004.06.012
752
(v)
(vi)
Preface / Scripta Materialia 51 (2004) 751–753
Novel applications 13. Performance and applications of nanostructured materials produced by severe plastic deformation. Y.T. Zhu, T.C. Lowe and T.G. Langdon. Dislocation structures and atomistic simulations 14. Physical parameters linking deformation microstructures over a wide range of length scales. A. Godfrey and D.A. Hughes. 15. Atomic-scale modelling of plastic deformation of nanocrystalline copper. J. Schiøtz.
3. Summary This first paper (1 by Korbel and Bochniak) summarizes many years of systematic work looking at microscopical evolution in terms of plastic strain path. The result is a new approach to metal forming where a significant reduction in strain hardening has been obtained by a change in strain path during processing. High strains can therefore be introduced even at low temperatures, which allows one to achieve nanostructures in commercially usable products. Paper 2 (by Yang and co-workers) offers an alternative approach to the fabrication of micrometer-size components by electrodeposition. A detailed chemical and structural characterization forms the basis for property, microstructural and microchemical correlations. By optimizing the chemical composition and the processing conditions microsystems have been obtained with a very high strength, which is ascribed to a very fine microstructure composed of submicron columnar grains with a high density of twins. The third paper (3 by Yokota and co-workers) considers a novel way of using phase transformation in steel (from austenite to martensite or bainite) to refine the microstructure, which then may be further refined by plastic deformation. The challenge is to obtain fine grain sizes, high strength and toughness in bulk samples at reasonable costs. The papers covering characterization techniques exemplify the range of experimental approaches aimed at looking at, how sub-micrometre microstructures develop. The first of this batch (4 by Humphreys) looks at how refined versions of the electron backscattered diffraction (EBSD) technique in a scanning electron microscope fitted with a field emission gun may be used to characterize grain structures with sizes in the sub-micrometre range. It is shown that significant improvements in the EBSD technique now allows structural characterization with a spatial resolution of about 100 nm. The next paper (6 by Unga´r) summarizes the use of X-ray diffraction peak profile analysis (XDPPA) to determine microstructural parameters such as crystallite size, size distribution and dislocation structure. A different and
novel use of X-rays is covered in paper 6 (by Poulsen and co-workers), where the use of hard X-rays from a synchrotron source forms the basis of a 3D X-ray diffraction (3DXRD) microscope. Here it is shown to be possible to look at the dynamics of the thermal and mechanical behaviour of embedded dislocation structures with a size limitation of 150 nm. In paper 8 (by Rentenberger and co-workers) some of the pitfalls of using high resolution transmission electron microscopy (HRTEM) for looking at nanostructures are covered in detail. In particular the density of grain boundaries in such structures is so high that often Moire´ effects are encountered, which may easily be misinterpreted. The third batch of papers all have as their theme some aspects of the mechanical properties of sub-micrometre scale materials. Thus paper 8 (by Gil Sevillano and Aldazabal) looks at possible schemes to give nanocrystalline materials sufficient ductility to be used in structural applications. By simple numerical simulations it is shown that an optimum in strength and ductility can be reached by a dispersion of a moderate volume fraction of soft regions (e.g. coarse grains) in a nanocrystalline matrix. The paper by Hansen (9) considers the applicability of Hall–Petch type relationships for the yield stress of undeformed polycrystalline metals and for the flow stress of deformed metals emphasizing that different mechanisms control the stress-grain size relationship in the two cases. An analysis of experimental data shows that a Hall–Petch relation can describe the yield stress of an undeformed polycrystalline metal over four length scales down to a grain size of about 20 nm in metals synthesized by electro-deposition or inert gas condensation. The last paper in this batch (10, by Mughrabi and co-workers) is concerned with the fatigue of ultrafine grained metals produced by the severe plastic deformation technique of equal channel angular pressing (ECAP). It is observed that in several cases the high cycle fatigue (HCF) strength is enhanced compared to coarse grained metals, while the low cycle fatigue (LCF) strength is reduced because of the lower ductility of the ultrafine grained metal. Possible schemes to enhance the low cycle fatigue resistance of these materials by annealing are discussed. The next two papers (11 by Weissu¨mller and coworkers and 12 by Driver) deal with various aspects concerning the thermodynamic equilibrium in small alloy particles (11) and the stability of nanostructures (12). In the first of these (11) it is emphasized that several of the rules that apply generally to the construction of phase diagrams for macroscopic alloy systems are violated at small particle sizes. By using an interfacial capillary argument it is shown that this changes the nature of the co-existence of two phases in particles of small size, so much so that eutectic points in conventional alloy phase diagrams degenerate into intervals of compositions, where the alloy melts discontinuously. In
Preface / Scripta Materialia 51 (2004) 751–753
paper 12 is discussed the inhibition of localized grain boundary motion in very fine grain sized metallic alloy samples using mechanisms such as Zener and solute drag. This problem of structural stability is important as a recovery heat treatment may be required in order to optimize the strength and ductility of metals processed by plastic deformation to large strains. The paper by Zhu and co-workers (13) is primarily concerned with the application of nanostructured materials processed by plastic deformation to large strains, in particular stressing their use in cases ranging from biomedical to aerospace industries. They consider the combination of properties achievable, performance and even address issues such as production costs. The final two papers give an overview of some theoretical issues (in 14 on dislocation structures by Godfrey and Hughes, while Schiøtz in 15 looks at some atomistic scale simulations of nano grain size metals). Godfrey and Hughes (14) apply scaling analysis in a study of the evolution of dislocation boundaries during plastic deformation and find that the strain dependency of structural parameters such as spacing between and misorientation across dislocation boundaries link the deformation behaviour over four length scales for different metals and processing routes. The existence of such links is supported by observations by HRTEM of glide dislocations in deformation microstructures of nanoscale dimensions. In a complementary paper using large scale molecular dynamics simulations, Schiøtz (15) considers the importance of dislocation nucleation, motion and pile-ups in nanocrystalline copper with grain sizes from 5 to 50 nm. These simulations show a shift in deformation mechanisms, when the grain size becomes smaller than 10–15 nm resulting in a maximum in the flow stress in this grain size range.
4. Concluding remarks Overall this collection of papers demonstrate the novelty of research and development covering metals and alloys with structural scales from the micrometre/submicrometre level to the atomic dimension. The top end of this scale covers many traditional metallic materials, where improvement in properties especially strength and toughness is obtained through a structural refinement by plastic deformation or phase transformation. An outstanding example is steel produced industrially
753
with a submicrometre scale microstructure for high strength applications. At the lower end of the length scale metals and alloys synthesized by a variety of methods offer potential for materials with outstanding properties for niche applications. However, research covering the whole range of length scales show unique opportunities to extend classical material science and engineering into the nanoscale regime. This extension is at present taking place rapidly, when it comes to development of advanced processing techniques and rapid and precise characterization methods. However, there is a significant back-log when it comes to the theoretical development, which has not fully explored the significant advances in the processing and characterization. A back-log also exists in exploration of the effect of alloying elements on the structure and properties of ultrafine structured metals. However, the present collection of papers demonstrate the wide ranging interest in exploring these issues based on critical experiments and further technical advancements especially in 2D and 3D characterization techniques. Also coming through the presentations given here is the sense of excitement about this field, which not only bridges length scales in terms of structural scales from micrometres to atomic dimensions but also in terms of research approaches so that this field links people interested in fundamental mechanisms with those working close to industrial applications.
Acknowledgments The organizer thanks B. Ralph and G. Winther for many discussions on the themes of this viewpoint set and E. Nielsen for assistance with the manuscripts. The organizer gratefully acknowledges the Danish National Research Foundation for supporting the Center for Fundamental Research; Metal Structures in Four Dimensions within which the organization of this viewpoint set was performed. Niels Hansen Center for Fundamental Research Metal Structures in Four Dimensions Materials Research Department Riso National Laboratory DK-4000 Roskilde, Denmark Tel.: +45 4677 5769; fax: +45 4677 5758 E-mail address: [email protected]
Preface to the viewpoint set: ‘‘Metals and alloys with a structural scale from the micrometre to the atomic dimensions’’
1. Introduction The microstructural scale of metals and alloys stretches typically from the micrometre level to the atomic scale, for instance, from the grain structure of a conventional polycrystalline metal to the amorphous structure of a rapidly solidified alloy. Research and development has at different times concentrated on materials groups representing different length scales and today, the emphasis is on nanocrystalline materials with a length scale below 100 nm. Such nanoscale materials have been the subject of a recent viewpoint set in Scripta Materialia 2003;4:625–680: Mechanical properties of fully dense nanocrystalline metals. The present viewpoint set also covers nanoscale materials but they are treated as part of a multiscale approach covering a structural scale from the micrometre/submicrometre level through the nanometre scale to atomic dimensions. Behind this approach is the wish to analyse in a holistic way the universality of fundamental principles and models over length scales covering both structures of conventional and of novel metallic materials. In this viewpoint set, a multiscale approach covering four length scales was the guide to papers on specific subjects. These papers cover many aspects of research and development in the general field of materials science and engineering such as synthesis/processing, structure/composition, properties, performance and theory.
(i)
(ii)
(iii)
2. Themes and contributions (iv) The themes of this viewpoint set encompass (i) processing (ii) characterization (iii) mechanical properties (iv) phase and structural stability (v) novel applications and (vi) dislocation structures and atomistic simulations.
Processing 1. Refinement and control of the metal structure elements by plastic deformation. A. Korbel and W. Bochniak. 2. Metallurgy of high strength Ni–Mn microsystems fabricated by electrodeposition. N.Y.C. Yang, T.J. Headley, J.J. Kelly and J. Hruby. 3. Formation of nanostructured steels by phase transformation. T. Yokota, C. Mateo and H.K.D.H. Bhadeshia. Characterization 4. Characterization of fine-scale microstructures by electron backscatter diffraction (EBSD). F.J. Humphreys. 5. Microstructure parameters from X-ray diffraction peak broadening. T. Unga´r. 6. Characterizing the dynamics of individual embedded dislocation structures. H.F. Poulsen, J. Bowen and C. Gundlach. 7. HRTEM analysis of nanostructured alloys processed by severe plastic deformation. C. Rentenberger, T. Waitz and H.P. Karnthaler. Mechanical properties 8. Ductilization of nanocrystalline materials for structural applications. J. Gil Sevillano and J. Aldazabal. 9. Hall–Petch relation and boundary strengthening. N. Hansen. 10. Fatique and microstructure of ultrafinegrained metals produced by severe plastic deformation. H. Mughrabi, H.W. Ho¨ppel and M. Kautz. Phase and structural stability 11. Two-phase equilibrium in small alloy particles. J. Weissmu¨ller, P. Bunzel and G. Wilde. 12. Stability of nanostructured metals and alloys. J.H. Driver.
1359-6462/$ - see front matter Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2004.06.012
752
(v)
(vi)
Preface / Scripta Materialia 51 (2004) 751–753
Novel applications 13. Performance and applications of nanostructured materials produced by severe plastic deformation. Y.T. Zhu, T.C. Lowe and T.G. Langdon. Dislocation structures and atomistic simulations 14. Physical parameters linking deformation microstructures over a wide range of length scales. A. Godfrey and D.A. Hughes. 15. Atomic-scale modelling of plastic deformation of nanocrystalline copper. J. Schiøtz.
3. Summary This first paper (1 by Korbel and Bochniak) summarizes many years of systematic work looking at microscopical evolution in terms of plastic strain path. The result is a new approach to metal forming where a significant reduction in strain hardening has been obtained by a change in strain path during processing. High strains can therefore be introduced even at low temperatures, which allows one to achieve nanostructures in commercially usable products. Paper 2 (by Yang and co-workers) offers an alternative approach to the fabrication of micrometer-size components by electrodeposition. A detailed chemical and structural characterization forms the basis for property, microstructural and microchemical correlations. By optimizing the chemical composition and the processing conditions microsystems have been obtained with a very high strength, which is ascribed to a very fine microstructure composed of submicron columnar grains with a high density of twins. The third paper (3 by Yokota and co-workers) considers a novel way of using phase transformation in steel (from austenite to martensite or bainite) to refine the microstructure, which then may be further refined by plastic deformation. The challenge is to obtain fine grain sizes, high strength and toughness in bulk samples at reasonable costs. The papers covering characterization techniques exemplify the range of experimental approaches aimed at looking at, how sub-micrometre microstructures develop. The first of this batch (4 by Humphreys) looks at how refined versions of the electron backscattered diffraction (EBSD) technique in a scanning electron microscope fitted with a field emission gun may be used to characterize grain structures with sizes in the sub-micrometre range. It is shown that significant improvements in the EBSD technique now allows structural characterization with a spatial resolution of about 100 nm. The next paper (6 by Unga´r) summarizes the use of X-ray diffraction peak profile analysis (XDPPA) to determine microstructural parameters such as crystallite size, size distribution and dislocation structure. A different and
novel use of X-rays is covered in paper 6 (by Poulsen and co-workers), where the use of hard X-rays from a synchrotron source forms the basis of a 3D X-ray diffraction (3DXRD) microscope. Here it is shown to be possible to look at the dynamics of the thermal and mechanical behaviour of embedded dislocation structures with a size limitation of 150 nm. In paper 8 (by Rentenberger and co-workers) some of the pitfalls of using high resolution transmission electron microscopy (HRTEM) for looking at nanostructures are covered in detail. In particular the density of grain boundaries in such structures is so high that often Moire´ effects are encountered, which may easily be misinterpreted. The third batch of papers all have as their theme some aspects of the mechanical properties of sub-micrometre scale materials. Thus paper 8 (by Gil Sevillano and Aldazabal) looks at possible schemes to give nanocrystalline materials sufficient ductility to be used in structural applications. By simple numerical simulations it is shown that an optimum in strength and ductility can be reached by a dispersion of a moderate volume fraction of soft regions (e.g. coarse grains) in a nanocrystalline matrix. The paper by Hansen (9) considers the applicability of Hall–Petch type relationships for the yield stress of undeformed polycrystalline metals and for the flow stress of deformed metals emphasizing that different mechanisms control the stress-grain size relationship in the two cases. An analysis of experimental data shows that a Hall–Petch relation can describe the yield stress of an undeformed polycrystalline metal over four length scales down to a grain size of about 20 nm in metals synthesized by electro-deposition or inert gas condensation. The last paper in this batch (10, by Mughrabi and co-workers) is concerned with the fatigue of ultrafine grained metals produced by the severe plastic deformation technique of equal channel angular pressing (ECAP). It is observed that in several cases the high cycle fatigue (HCF) strength is enhanced compared to coarse grained metals, while the low cycle fatigue (LCF) strength is reduced because of the lower ductility of the ultrafine grained metal. Possible schemes to enhance the low cycle fatigue resistance of these materials by annealing are discussed. The next two papers (11 by Weissu¨mller and coworkers and 12 by Driver) deal with various aspects concerning the thermodynamic equilibrium in small alloy particles (11) and the stability of nanostructures (12). In the first of these (11) it is emphasized that several of the rules that apply generally to the construction of phase diagrams for macroscopic alloy systems are violated at small particle sizes. By using an interfacial capillary argument it is shown that this changes the nature of the co-existence of two phases in particles of small size, so much so that eutectic points in conventional alloy phase diagrams degenerate into intervals of compositions, where the alloy melts discontinuously. In
Preface / Scripta Materialia 51 (2004) 751–753
paper 12 is discussed the inhibition of localized grain boundary motion in very fine grain sized metallic alloy samples using mechanisms such as Zener and solute drag. This problem of structural stability is important as a recovery heat treatment may be required in order to optimize the strength and ductility of metals processed by plastic deformation to large strains. The paper by Zhu and co-workers (13) is primarily concerned with the application of nanostructured materials processed by plastic deformation to large strains, in particular stressing their use in cases ranging from biomedical to aerospace industries. They consider the combination of properties achievable, performance and even address issues such as production costs. The final two papers give an overview of some theoretical issues (in 14 on dislocation structures by Godfrey and Hughes, while Schiøtz in 15 looks at some atomistic scale simulations of nano grain size metals). Godfrey and Hughes (14) apply scaling analysis in a study of the evolution of dislocation boundaries during plastic deformation and find that the strain dependency of structural parameters such as spacing between and misorientation across dislocation boundaries link the deformation behaviour over four length scales for different metals and processing routes. The existence of such links is supported by observations by HRTEM of glide dislocations in deformation microstructures of nanoscale dimensions. In a complementary paper using large scale molecular dynamics simulations, Schiøtz (15) considers the importance of dislocation nucleation, motion and pile-ups in nanocrystalline copper with grain sizes from 5 to 50 nm. These simulations show a shift in deformation mechanisms, when the grain size becomes smaller than 10–15 nm resulting in a maximum in the flow stress in this grain size range.
4. Concluding remarks Overall this collection of papers demonstrate the novelty of research and development covering metals and alloys with structural scales from the micrometre/submicrometre level to the atomic dimension. The top end of this scale covers many traditional metallic materials, where improvement in properties especially strength and toughness is obtained through a structural refinement by plastic deformation or phase transformation. An outstanding example is steel produced industrially
753
with a submicrometre scale microstructure for high strength applications. At the lower end of the length scale metals and alloys synthesized by a variety of methods offer potential for materials with outstanding properties for niche applications. However, research covering the whole range of length scales show unique opportunities to extend classical material science and engineering into the nanoscale regime. This extension is at present taking place rapidly, when it comes to development of advanced processing techniques and rapid and precise characterization methods. However, there is a significant back-log when it comes to the theoretical development, which has not fully explored the significant advances in the processing and characterization. A back-log also exists in exploration of the effect of alloying elements on the structure and properties of ultrafine structured metals. However, the present collection of papers demonstrate the wide ranging interest in exploring these issues based on critical experiments and further technical advancements especially in 2D and 3D characterization techniques. Also coming through the presentations given here is the sense of excitement about this field, which not only bridges length scales in terms of structural scales from micrometres to atomic dimensions but also in terms of research approaches so that this field links people interested in fundamental mechanisms with those working close to industrial applications.
Acknowledgments The organizer thanks B. Ralph and G. Winther for many discussions on the themes of this viewpoint set and E. Nielsen for assistance with the manuscripts. The organizer gratefully acknowledges the Danish National Research Foundation for supporting the Center for Fundamental Research; Metal Structures in Four Dimensions within which the organization of this viewpoint set was performed. Niels Hansen Center for Fundamental Research Metal Structures in Four Dimensions Materials Research Department Riso National Laboratory DK-4000 Roskilde, Denmark Tel.: +45 4677 5769; fax: +45 4677 5758 E-mail address: [email protected]