Ultrafine-Grained Materials

Ultrafine-grained (UFG) materials are a new class of metals and alloys characterized by a microstructure with grain sizes less than 1 μm and nanostructural features that provide advanced multifunctional properties. This chapter considers the underlying principles for the formation of UFG materials using severe plastic deformation (SPD) and outlines the conditions for achieving the UFG structures with predominantly high angle grain boundaries, including low temperatures, high degrees of straining, high applied pressures, turbulent, non-monotonic nature of the material flow, achieving higher dislocation density and smaller grain size by using alloys with an ordered structure and materials with low stacking fault energy. The most popular techniques of SPD processing are analyzed, such as high-pressure torsion (HPT) and equal-channel angular pressing (ECAP). The schemes and mechanisms of grain refinement and formation of UFG structures are considered. At the same time, special attention is paid to the analysis of the evolution of microstructures and phase transformation during SPD processing following the results of computer simulation and numerous modern experimental methods of research. Grain boundaries, nanotwins, nanoscale particles, and segregations of alloying elements in nanostructured materials obtained by SPD techniques are described and used for developing a concept of nanostructural design for increasing material properties.

UFG metals and alloys attract the materials science community owing to their superior mechanical properties. This chapter considers achieving enhanced mechanical properties in the UFG materials processed by SPD techniques. Special emphasis is laid on the examples and origins of the phenomenon of superstrength as well as the description of the hardening mechanisms in the materials. The chapter views the manifestation of the SPD paradox caused by the formation of equiaxial UFG structure and control of grain boundary type and segregations. The SPD paradox is evident in the simultaneous growth of strength and ductility. Mechanisms for enhancing the ductility of UFG materials are considered. The chapter also focuses on the importance of grain refinement in increasing fatigue strength and endurance, creep resistance of materials, the manifestation of the superplasticity effect at lower temperatures, and higher strain rates and its use in superplastic forming of complex shape parts. Advanced mechanical properties of UFG materials provide excellent opportunities for manufacturing new promising products for practical applications.

The formation of ultrafine grains with nanostructural features in metallic materials by SPD processing can provide superior mechanical and, simultaneously, functional properties of metals and alloys, for example, high strength and electrical conductivity in Cu and Al alloys, enhanced fatigue endurance and high corrosion and erosion resistance in Ti alloys and many other so-called multifunctional properties. This chapter demonstrates that the level of the above properties is determined by small grain size, nonequilibrium grain boundaries with high density of grain boundary dislocations, nanoscale grain boundary segregations and precipitations, etc. Changes in the physical properties of UFG materials are associated with the comparability of grain sizes and characteristics, including the free path length of electrons and the size of magnetic domains.

In particular, nanocrystalline magnetic hard materials are characterized by high coercivity, which is due to the interaction of crystal structure defects with the walls of the magnetic domains during the movement of the latter. Nanocrystalline soft magnetic materials are characterized by high magnetic inductive capacity, low coercive force, and relatively high saturation magnetization. The enhancement of the above properties is the result of the domain size becoming smaller than the grain size.

UFG materials demonstrate enhanced biomedical properties, but the level of biocompatibility is strongly influenced by the substrate surface quality. Etching and formation of bioactive coatings contribute to the increase of biocompatibility.

The formation of UFG structures has a great positive effect on the manifestation of the shape memory effect and the superelasticity of materials. For example, the reactive force increases and the temperature range of the shape memory effect decreases.

Chapter 3 not only describes numerous examples of achieving unique multifunctional properties of nanostructured materials obtained by SPD techniques but also discusses their physical origin related to the influence of nanostructuring on the deformation and transport mechanisms that determine the properties of nanomaterials.

A continuous effort has been made in the research field of processing of ultrafine-grained materials (UFG) through the application of severe plastic deformation (SPD) over the last three decades. In particular, a new research focus was developed for the utilization of conventional SPD techniques with some modification in the procedures to synthesize advanced, hybrid nanocrystalline metals and materials demonstrating exceptional mechanical properties and functionalities. Hybrid nanocrystalline materials refer to composites consisting of two or more constituents at the nanometer level or the nano-to-micro level, leading to a gradient or heterogeneous microstructure formation. Accordingly, this chapter describes the recent developments in strategies for the formation of bulk nanostructured metal systems and UFG materials from powders and dissimilar bulk metals. The further sections discuss the uniquely tuned nanostructures, heterostructures, and multilayered laminates and their improved mechanical properties and additional functionalities of the hybrid nanocrystalline materials processed by different SPD techniques. Recent developments on architecturing of heterostructured nanostructures and their possible future applications are also discussed by considering SPD techniques-nanostructures synergy.

Ultrafine-grained (UFG) materials are innovative and quite promising for wide application. The important advantage associated with the use and commercialization of UFG materials includes first of all their advanced properties, also production efficiency is important due to the development of new SPD technique.

This chapter discusses the innovative potential and practical applications of UFG materials. Titanium implants made of nanoTi are promising for dentistry and orthopedics. UFG copper alloys can be used for making electrodes for welding. UFG composites based on copper matrix and WС are attractive for manufacturing current carrying cores, electric motors, contact wires. Nanocrystalline alloy Cu–10 at.% Ta is an exciting example illustrating the possibility of increasing thermal stability at elevated temperatures by creating clusters of the alloying element on the grain boundaries, preventing their migration. Another example of successful production of aluminum alloy 6101 wires with improved mechanical properties and high electrical conductivity by SPD processing has been demonstrated. The prospects of using UFG metallic alloys as promising materials for hydrogen storage and transportation have been analyzed. Manifestations of the shape memory effect are of great interest for the use of UFG alloys in medicine, as a material for joining pipes and other practical applications. Nanostructured magnetic materials are promising for the creation of new high-speed electric machines with high strength and less energy dissipation. The small grain size not only provides increased strength but also preserves the homogeneous microstructure and isotropic properties while miniaturizing products.