https://material.karlov.mff.cuni.cz/people/janecek/studenti/FyzMetNano/Estrin_AM61_2013_782.pdf

Extreme grain refinement by severe plastic deformation: A wealth of challenging science

Deformation twinning in nanocrystalline materials

https://cyberleninka.org/article/n/149772.pdf

Mechanical properties, microstructure and thermal stability of a nanocrystalline CoCrFeMnNi high-entropy alloy after severe plastic deformation

After almost three decades of intensive fundamental research and development activities, intermetallic titanium aluminides based on the ordered γ-TiAl phase have found applications in automotive and aircraft engine industry. The advantages of this class of innovative high-temperature materials are their low density and their good strength and creep properties up to 750 °C as well as their good oxidation and burn resistance. Advanced TiAl alloys are complex multi-phase alloys which can be processed by ingot or powder metallurgy as well as precision casting methods. Each process leads to specific microstructures which can be altered and optimized by thermo-mechanical processing and/or subsequent heat treatments. The background of these heat treatments is at least twofold, i.e., concurrent increase of ductility at room temperature and creep strength at elevated temperature. This review gives a general survey of engineering γ-TiAl based alloys, but concentrates on β-solidifying γ-TiAl based alloys which show excellent hot-workability and balanced mechanical properties when subjected to adapted heat treatments. The content of this paper comprises alloy design strategies, progress in processing, evolution of microstructure, mechanical properties as well as application-oriented aspects, but also shows how sophisticated ex situ and in situ methods can be employed to establish phase diagrams and to investigate the evolution of the micro- and nanostructure during hot-working and subsequent heat treatments.

Design, Processing, Microstructure, Properties, and Applications of Advanced Intermetallic TiAl Alloys†

Forming alloys with impurity elements is a routine method for modifying the properties of metals. An alternative approach involves the incorporation of interfaces into the crystalline lattice to enhance the metal's properties without changing its chemical composition. The introduction of high-density interfaces in nanostructured materials results in greatly improved strength and hardness; however, interfaces at the nanoscale show low stability. In this Review, I discuss recent developments in the stabilization of nanostructured metals by modifying the architectures of their interfaces. The amount, structure and distribution of several types of interfaces, such as high- and low-angle grain boundaries and twin boundaries, are discussed. I survey several examples of materials with nanotwinned and nanolaminated structures, as well as with gradient nanostructures, describing the techniques used to produce such samples and tracing their exceptional performances back to the nanoscale architectures of their interfaces. The incorporation of structural defects, in particular of interfaces, into crystalline lattices results in enhanced material properties. In this Review, different types of boundaries and interfaces are discussed, including high- and low-angle grain boundaries, twin boundaries, nanotwinned and nanolaminated structures, and gradient nanostructures.

Stabilizing nanostructures in metals using grain and twin boundary architectures

In this review, we focus on the saturation microstructure that evolves during severe plastic deformation (SPD). These nanocrystalline or ultrafinegrained 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 lowtemperature 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.

Saturation of Fragmentation During Severe Plastic Deformation

The effect of the different processing parameters on the refinement during severe plastic deformation is evaluated. The attention is focused to very high strains, where saturation in the structural refinement is observed. The single phase metals and alloys show a relatively uniform behaviour, with increasing strain the size of structural elements decreases and reaches saturation between equivalent strains of 5 to 50. The resulting ultra fine or nanostructured granular microstructure contains mainly high angle grain boundaries. Alloying, the temperature, and the strain path are the most important parameters controlling the saturation in the structural refinement. The behavior of the dual and multiphase materials during SPD is more complex, it varies from simple homogenisation of the phase distribution, fragmentation of one phase to disintegration and supersaturation of the phases. Severe plastic deformation of these types of materials offers the potential for the production of new types of materials with a nanocrystalline or a nanocomposite structure for a broad range of industrial applications.

The Limits of Refinement by Severe Plastic Deformation

The improvements in the design of the HPT tools lead to a well defined torsion deformation and permits, therefore, a comparison with other SPD-techniques. The design of the tools, the advantages and disadvantages of HPT, as well as the limitation in the sample size are discussed.

Advantages and Limitations of HPT: A Review

Strain effects on the coarsening and softening of electrodeposited nanocrystalline Ni subjected to high pressure torsion

Nickel single crystals with different crystallographic orientations were deformed by high-pressure torsion. Special attention is devoted to examining the evolution of the micro-texture and microstructure. The initial crystal orientation was found to have a significant effect on the mechanical hardening and evolution of micro-texture at low and medium equivalent strains, whereas at very high strains no effect of the initial orientation was observed and the behaviour was very similar to a polycrystal. The evolution of micro-texture is in good qualitative agreement with the full constrained Taylor model. At very high equivalent strains the initial crystal orientation has no influence on micro-texture. At such strains, the hardening, the refinement of the structure and the texture reaches a saturation. The final micro-texture is explained by the change from one preferred crystallographic orientation to another.

/pdf/high-pressure-torsion-applied-to-nickel-single-crystals-3xbyc6wrsw.pdf

Martin Hafok

Papers

Saturation of Fragmentation During Severe Plastic Deformation

The Limits of Refinement by Severe Plastic Deformation

Advantages and Limitations of HPT: A Review

Strain effects on the coarsening and softening of electrodeposited nanocrystalline Ni subjected to high pressure torsion

High-pressure torsion applied to nickel single crystals