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Preface to the viewpoint set on grain boundary engineering
Scripta Materialia 54 (2006) 961–962 www.actamat-journals.com
Editorial
Preface to the viewpoint set on grain boundary engineering
The manipulation of microstructures to obtain desired properties is one of the fundamental goals of the field of materials science and engineering. While control of the microstructural length scale, as for instance, grain or domain size, has been in common use for enhancements in mechanical properties, the idea of grain boundary engineering, where the objective is to control the population of grain boundary types, was first proposed only in the mid-1980s by Professor Tadao Watanabe at Tohoku University in Japan. This idea has been vigorously pursued by the Watanabe group in an academic setting, and also by a group led by Dr. Gino Palumbo at a nuclear utility company in Canada that advanced the industrial applications by employing novel thermo-mechanical processing to achieve microstructures with enhanced properties. Early work in this field demonstrated that significant improvements in corrosion and creep resistance, as an example, were empirically correlated with the fraction of so-called ‘‘special’’ boundaries in the microstructure, based upon Bollmann’s coincidence site lattice model for the crystallography of a grain boundary. These studies used a variety of processing approaches, largely based on trialand-error, to manipulate the special boundary fraction. Today, the study of grain boundary engineering processing has advanced from these early works, and this viewpoint set begins with a series of papers highlighting recent progress in this regard. These papers include advances in the area of traditional bulk structural metals processed through novel deformation/annealing routes, or using the application of fields. There are also papers reflecting significant new domains of grain boundary engineering processing, relevant in growing technological areas such as thin films, interconnects, and MEMS devices.
The characterization of grain boundary engineered materials has also evolved rapidly since the inception of the field, at which time the most detailed quantitative studies of boundary character required laborious transmission electron microscopy (TEM) observations. The statistical characterization of grain boundary character has since benefited enormously from the advent of scanning electron microscopy (SEM)-based automated electron backscatter diffraction (EBSD) techniques, so much so that the grain boundary character distribution is now routinely reported as another measure of the microstructure. This development has enabled the field to turn its attention to more sophisticated methods of characterizing the grain boundary character and connectivity and is the focus of the next group of papers in this viewpoint set. The articles assess the spectrum of boundary character in light of the additional degrees of freedom associated with the boundary plane inclination. Other papers in the set treat the obvious shortcoming of the special boundary fraction as a microstructural state variable, being as it is a scalar measure that does not adequately address path dependency of physical properties such as intergranular damage propagation. The viewpoint papers on boundary connectivity, boundary junctions, and correlations among grain boundary types highlight these important issues, and emphasize the importance of studying topology of the grain boundary network in three dimensions. In the end, the field of grain boundary engineering is focused upon improving material properties, and the potentially revolutionary improvements promised by this type of ‘‘microstructure design’’ have, from the beginning, driven progress in the field. The final group of papers in the viewpoint set reflect an increasing sophistication in the way measured properties (such as creep, grain growth,
1359-6462/$ - see front matter Ó 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.11.059
962
Editorial / Scripta Materialia 54 (2006) 961–962
and impurity segregation) are controlled by the grain boundary engineered microstructure, and provide further impetus for progress in the areas of processing and structure characterization. With significant recent advances in processing methodologies, grain boundary character and network characterization, and improved interpretation of property measurements, grain boundary engineering is a field on the verge of a major expansion. The present viewpoint set provides a benchmark, giving a perspective of where the field has come from, and the broad directions faced by ‘‘grain boundary engineers’’ going forward.
Mukul Kumar Lawrence Livermore National Laboratory, Livermore, CA 94550, U.S.A. E-mail address: [email protected] Christopher A. Schuh MIT, Department of Materials Science and Engineering, Cambridge, MA 02139, U.S.A. Available online 27 December 2005
Editorial
Preface to the viewpoint set on grain boundary engineering
The manipulation of microstructures to obtain desired properties is one of the fundamental goals of the field of materials science and engineering. While control of the microstructural length scale, as for instance, grain or domain size, has been in common use for enhancements in mechanical properties, the idea of grain boundary engineering, where the objective is to control the population of grain boundary types, was first proposed only in the mid-1980s by Professor Tadao Watanabe at Tohoku University in Japan. This idea has been vigorously pursued by the Watanabe group in an academic setting, and also by a group led by Dr. Gino Palumbo at a nuclear utility company in Canada that advanced the industrial applications by employing novel thermo-mechanical processing to achieve microstructures with enhanced properties. Early work in this field demonstrated that significant improvements in corrosion and creep resistance, as an example, were empirically correlated with the fraction of so-called ‘‘special’’ boundaries in the microstructure, based upon Bollmann’s coincidence site lattice model for the crystallography of a grain boundary. These studies used a variety of processing approaches, largely based on trialand-error, to manipulate the special boundary fraction. Today, the study of grain boundary engineering processing has advanced from these early works, and this viewpoint set begins with a series of papers highlighting recent progress in this regard. These papers include advances in the area of traditional bulk structural metals processed through novel deformation/annealing routes, or using the application of fields. There are also papers reflecting significant new domains of grain boundary engineering processing, relevant in growing technological areas such as thin films, interconnects, and MEMS devices.
The characterization of grain boundary engineered materials has also evolved rapidly since the inception of the field, at which time the most detailed quantitative studies of boundary character required laborious transmission electron microscopy (TEM) observations. The statistical characterization of grain boundary character has since benefited enormously from the advent of scanning electron microscopy (SEM)-based automated electron backscatter diffraction (EBSD) techniques, so much so that the grain boundary character distribution is now routinely reported as another measure of the microstructure. This development has enabled the field to turn its attention to more sophisticated methods of characterizing the grain boundary character and connectivity and is the focus of the next group of papers in this viewpoint set. The articles assess the spectrum of boundary character in light of the additional degrees of freedom associated with the boundary plane inclination. Other papers in the set treat the obvious shortcoming of the special boundary fraction as a microstructural state variable, being as it is a scalar measure that does not adequately address path dependency of physical properties such as intergranular damage propagation. The viewpoint papers on boundary connectivity, boundary junctions, and correlations among grain boundary types highlight these important issues, and emphasize the importance of studying topology of the grain boundary network in three dimensions. In the end, the field of grain boundary engineering is focused upon improving material properties, and the potentially revolutionary improvements promised by this type of ‘‘microstructure design’’ have, from the beginning, driven progress in the field. The final group of papers in the viewpoint set reflect an increasing sophistication in the way measured properties (such as creep, grain growth,
1359-6462/$ - see front matter Ó 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.11.059
962
Editorial / Scripta Materialia 54 (2006) 961–962
and impurity segregation) are controlled by the grain boundary engineered microstructure, and provide further impetus for progress in the areas of processing and structure characterization. With significant recent advances in processing methodologies, grain boundary character and network characterization, and improved interpretation of property measurements, grain boundary engineering is a field on the verge of a major expansion. The present viewpoint set provides a benchmark, giving a perspective of where the field has come from, and the broad directions faced by ‘‘grain boundary engineers’’ going forward.
Mukul Kumar Lawrence Livermore National Laboratory, Livermore, CA 94550, U.S.A. E-mail address: [email protected] Christopher A. Schuh MIT, Department of Materials Science and Engineering, Cambridge, MA 02139, U.S.A. Available online 27 December 2005