Over the past decades, a variety of techniques, collectively known as Severe Plastic Deformation (SPD), have been recognized as a promising method for processing bulk nanostructured metals (BNM) and materials. These materials exhibit unique grain and subgrain configurations, typically within the scale of approximately 50–500 nm, surrounded by non-equilibrium grain boundaries. Employing SPD techniques enables the efficient production of such materials in large volumes, which is important for developing SPD-processed materials for a variety of applications.

Substantial research efforts have been in progress across numerous countries, resulting in significant breakthroughs in the SPD and BNM fields. This has led to the realization that the commercialization of materials processed through SPD techniques is now attainable [1]. The NanoSPD community has been expanding continuously, evidenced by a remarkable upswing in the volume of academic publications from researchers all over the world [2]. Moreover, the SPD research community has been building a strong network aiming to facilitate research collaboration, knowledge sharing, and collective contributions to educate young researchers and scientists. It can be represented by the holding of the NanoSPD conference series every three years, where the latest one, NanoSPD 8, took place on February 26–March 3, 2023, at the Indian Institute of Science (IISc) in Bangalore, India. Praveen Kumar, a Professor in the Department of Materials Engineering at the Indian Institute of Science, Bangalore, and the Co-Chair of the NanoSPD 8 conference, along with myself, Megumi Kawasaki, an Associate Professor at Oregon State University and a member of the NanoSPD international steering committee, as well as an editor of the Journal of Materials Science, have been eager to encapsulate the remarkable research advancements in the field of SPD research following the challenging times of COVID-19. Consequently, we find ourselves at an opportune time to publish a special issue on Processing Bulk Nanostructured Materials to summarize the current developments and achievements in the SPD community. Short bios of us, the special issue editors, are listed at the end of the preface.

This special issue comprises numerous aspects of SPD and BNM, such as the fundamental behaviors and enhanced mechanical properties of materials following nanostructuring through SPD and pre- and post-SPD heat treatments, functionalities of SPD-processed materials, and development as well as refinement of SPD techniques and the application of the processed materials. We are proud to announce a total of 28 invited reports, comprising two review papers and 26 original research reports, are compiled in this special issue. To illustrate the comprehensive coverage of important topics in Materials Science within this special issue, Table 1 provides a detailed list of seven key subjects along with specific information (keywords, SPD process, and materials) for the 28 invited articles and their respective lead authors.

Table 1 Seven research topics with the information of the 28 contributed articles for the special issue
Full size table

Presented below is a summary of the special issue outlined in Table 1. Topic 1 focuses on the advanced applications of the SPD-processed materials given by two review articles. In particular, Zohrevand et al. describe the application of several SPD techniques for the improvement of the properties of Zn-, Li-, Mg- and Al-based batteries, and Valiev et al. summarize the recent advancement in achieving a higher level of properties in the metallic biomaterials and the challenges and uncertainties to produce a new generation of medical implants. Topic 2 describes the fundamental but essential subject of the microstructure-mechanical property-SPD processing relationships by six articles. Special focuses are given to microstructure and texture evolution by processing, improvement in mechanical properties, and the changes in microstructure after heat treatments of various metallic materials, such as Al alloys, pure Ag, Ti alloy, and medium entropy alloys. Such changes in the SPD-processed materials have been attributed to the non-equilibrium grain boundaries with an excess number of defects, leading to unique features of SPD-processed materials. Subsequently, the three articles in Topic 3 examine the diffusion and kinetics of SPD-processed nanocrystalline materials, including Cu-Ag, Mg, and high-entropy alloys, during processing and post-processing heat treatments. Topic 4 relates the deformation mechanism of BNM, reported by Figueiredo in this issue, demonstrating theoretical relationships between strain rate sensitivity and ductility of nanostructured metals. Nine articles in Topic 5 demonstrate the utilization of SPD processing for fabricating functional materials having improved corrosion properties in Mg and high-entropy alloys, enhanced hydrogen absorption in a Mg alloy, electrical conductivity in Al–Mg-Zr and Cu-Cr-Hf alloys, magnetic properties in Ni-NiO composites and Fe–Ni-Mo alloy, high-temperature superconductivity in an Al-TiO system, and photocatalytic properties in Ti oxides. This topic, along with the review articles in Topic 1, opens the opportunities for SPD-processed materials for a variety of applications and industries. Topic 6, consisting of two articles, describes the further opportunity of SPD processing for the post-additive manufacturing treatment, by examining the microstructural changes and the density and porosity of additively manufactured metals of 316L stainless steel and Al–Fe alloys. Finally, five articles in Topic 7 describe the heart of the SPD research of understanding and invention of new SPD techniques and unique nanocrystalline structures in both experimental (Topic 6.1) and computational approaches (Topic 6.2). It is apparent from Table 1 that the well-developed SPD and BNM research is progressing to explore new opportunities in the application of processing techniques and fabricated materials across various industries. To illustrate the essence of this special issue, we present two distinct visualizations of bulk nanostructured materials in the Graphical Abstract. The images are sourced from the articles contributed by Cubero-Sesin et al. (depicted on the left) and Prakash (depicted on the right) within this special issue.

We hope that this special issue will attract numerous researchers, fostering the development and continued growth in this research field. We extend our sincere appreciation to all the authors who contributed to this special issue. Their dedication and enthusiasm for SPD and BNM research were instrumental in the successful completion of this special issue. Finally, we extend our special thanks to the Editor-in-Chief of the Journal of Materials Science, Professor Barry Carter, and the Deputy Editor-in-Chief, Prof. Grant Norton, for their kind assistance in ensuring the success of this special issue.