Severe plastic deformation (SPD) is effective in producing bulk ultrafine-grained and nanostructured materials with large densities of lattice defects. This field, also known as NanoSPD, experienced a significant progress within the past two decades. Beside classic SPD methods such as high-pressure torsion, equal-channel angular pressing, accumulative roll-bonding, twist extrusion, and multi-directional forging, various continuous techniques were introduced to produce upscaled samples. Moreover, numerous alloys, glasses, semiconductors, ceramics, polymers, and their composites were processed. The SPD methods were used to synthesize new materials or to stabilize metastable phases with advanced mechanical and functional properties. High strength combined with high ductility, low/room-temperature superplasticity, creep resistance, hydrogen storage, photocatalytic hydrogen production, photocatalytic CO₂ conversion, superconductivity, thermoelectric performance, radiation resistance, corrosion resistance, and biocompatibility are some highlighted properties of SPD-processed materials. This article reviews recent advances in the NanoSPD field and provides a brief history regarding its progress from the ancient times to modernity. Abbreviations: ARB: Accumulative Roll-Bonding; BCC: Body-Centered Cubic; DAC: Diamond Anvil Cell; EBSD: Electron Backscatter Diffraction; ECAP: Equal-Channel Angular Pressing (Extrusion); FCC: Face-Centered Cubic; FEM: Finite Element Method; FSP: Friction Stir Processing; HCP: Hexagonal Close-Packed; HPT: High-Pressure Torsion; HPTT: High-Pressure Tube Twisting; MDF: Multi-Directional (-Axial) Forging; NanoSPD: Nanomaterials by Severe Plastic Deformation; SDAC: Shear (Rotational) Diamond Anvil Cell; SEM: Scanning Electron Microscopy; SMAT: Surface Mechanical Attrition Treatment; SPD: Severe Plastic Deformation; TE: Twist Extrusion; TEM: Transmission Electron Microscopy; UFG: Ultrafine Grained GRAPHICAL ABSTRACT IMPACT STATEMENT This article comprehensively reviews recent advances on development of ultrafine-grained and nanostructured materials by severe plastic deformation and provides a brief history regarding the progress of this field.

In this review, we focus on the saturation microstructure that evolves during severe plastic deformation (SPD). These nanocrystalline or ultrafine-grained microstructures consist predominantly of high-angle boundaries, although low-angle boundaries are also present. Deformation temperature, alloying, and strain path are the dominant factors controlling the saturation grain size in single-phase materials. The saturation grain size decreases significantly with decreasing deformation temperature, although the dependency is stronger at medium homologous temperatures and less in the low-temperature regime. The saturation microstructure is sensitive to strain rate at medium temperatures and less so at low temperatures. The addition of alloying elements to pure metals also reduces the saturation grain size. The results indicate that grain boundary migration is the dominant process responsible for the limitation in refinement by SPD. Therefore, second-phase particles of the nanometer scale can stabilize even finer microstructures. This mechanism of stabilization of the microstructure is an effective tool for overcoming the limit in refinement of single-phase materials by SPD. The improved thermal stability of the obtained nanostructures is another benefit of the introduction of second-phase particles.

Fully dense bulk nanocomposites have been obtained by a novel two-step severe plastic deformation process in the immiscible Fe–Cu system. Elemental micrometer-sized Cu and Fe powders were first mixed in different compositions and subsequently high-pressure-torsion-consolidated and deformed in a two-step deformation process. Scanning electron microscopy, X-ray diffraction and atom probe investigations were performed to study the evolving far-from-equilibrium nanostructures which were observed at all compositions. For lower and higher Cu contents complete solid solutions of Cu in Fe and Fe in Cu, respectively, are obtained. In the near 50% regime a solid solution face-centred cubic and solid solution body-centred cubic nanograined composite has been formed. After an annealing treatment, these solid solutions decompose and form two-phase nanostructured Fe–Cu composites with a high hardness and an enhanced thermal stability. The grain size of the composites retained nanocrystalline up to high annealing temperatures.

High pressure torsion is a well known and widespread processing technique for severe plastic deformation. The aim of high pressure torsion and other comparable techniques is to obtain ultrafine-grained or even nanocrystalline materials with enhanced mechanical and physical properties compared with their coarse-grained counterparts. Generally this refinement process is strongly influenced by processing parameters such as temperature or accumulated strain, but can also simply be affected by the entire experimental setup. Therefore, the benefits and limitations of the process with regard to grain refinement, homogeneity and specimen size, underlined with experimental results using different tools, will be discussed.

The review focuses on the supersaturation process of immiscible elements induced by high‐pressure torsion deformation. A variety of investigated material systems offers the systematic analysis of the controlling factors for mechanical intermixing, which are mainly determined by the level of the positive heat of mixing and the deformation behavior. The homogeneity of the deformation process strongly influences the degree of supersaturation. Our results show that the fundamental mixing mechanism during deformation is not necessarily the same in all systems, but depends also on the strength differences of the phases and accordingly the deformation behavior.

In this article, the potential to fabricate composites with ultrafine grained structures, as well as composites with a nanostructure, by different severe plastic deformation methods, is reviewed. A broad spectrum of diverse composites produced by severe plastic deformation methods exist which include metal–metal composites, metal matrix composites and amorphous metal matrix composites. Furthermore, the influence of the strain path and initial structure on the final composite material is outlined.