Saturation of Grain Refinement during Severe Plastic Deformation of Single Phase Materials: Reconsiderations, Current Status and Open Questions

Abstract

Grain refinement of materials by severe plastic deformation, investigation and understanding of their properties and phenomena has been a subject of intensive research over the last three decades. Along with the invention and development of these processes it has been recognized, that grain refinement is not indefinite but stagnates for single phase materials. Accordingly, the minimum grain sizes achievable are in the range between 50 and 500 nm. Motivated to find ways to overcome these limitations, effort has been made to understand the reasons behind. Various processes were suggested to cause saturation of grain fragmentation. While all of these assumptions and models were not based on direct observations, recently in-situ approaches allowed significant progress in understanding microstructural evolution and the principle restoration processes during severe straining. It is the aim of this work to recap important earlier findings, reconsider proposed models and to present our current understanding of the processes limiting grain refinement upon severe straining. Further it will be discussed, that similar processes generally occur during deformation of such fine scaled materials. They do not only govern saturation of grain refinement during severe deformation but more important also the mechanical response and performance of these materials. This motivates an in-depth understanding and therefore open questions as well as discrepancies between various experiments will be deliberately highlighted to stimulate further research and thus progress within this area.

Synthesis of Nanostructured TiFe Hydrogen Storage Material by Mechanical Alloying via High‐Pressure Torsion

“…The bright-field and dark-field images illustrate that the material has a distorted feature with a UFG microstructure similar to many other HPT-processed materials. [28][29][30][31] The ring pattern of the SAED analysis also confirms the presence of many nanograins with random orientations within the selected area. A comparison between the SAED analysis and diffraction pattern of the TiFe cubic phase with a lattice parameter of 0.298 nm, as shown in Figure 4b, confirms that the majority of grains in Figure 4a,c correspond to the TiFe phase.…”

“…Moreover, the high hardness level of 800 HV should be due to the formation of ultrafine grains (UFG) as well due to the mixing of Ti and Fe powders in the form of Ti-Fe composites (after N ¼ 2) and/or TiFe intermetallics (after N ¼ 10). The occurrence of an apparent steady state in Figure 2 after N ¼ 10 is not only due to a balance between the hardening phenomena (such as dislocation formation and grain fragmentation) and softening phenomena (such as recovery, recrystallization, and grain boundary migration) [28][29][30][31] but also due to a saturation in the phase transformation. [54,55] To assess the mechanical alloying, the distribution of Ti and Fe at the micrometer level was analyzed by scanning electron microscope and energy-dispersive X-ray spectroscopy (SEM-EDS) at different distances from the disc center for the sample processed by HPT for ten turns.…”

“…This average grain size is almost three times smaller than the grain size of HPT-processed Ti [62][63][64] and Fe [65][66][67] due to the formation of TiFe phase as well as due to the effect of Ti-Fe composite on hindering the recrystallization and grain boundary migration. [28][29][30][31] Automatic phase mapping using the ASTAR device in Figure 4e shows the presence of some amount of HCP phase even at 3-5 mm away from the disc center, suggesting that the mechanical alloying is not completed. To have a complete and uniform phase transformation, larger shear strains should be applied, as attempted earlier for other materials.…”

“…Motivated by earlier studies on the application of the HPT method to as-cast TiFe, [23][24][25] in this study and for the first time, we synthesize the TiFe hydrogen storage materials from the Ti and Fe powders by the HPT method and examine its activation (see the principles of HPT in the previous studies [28,29] ). It should be noted that HPT as a severe plastic deformation (SPD) method shows high potential to produce nanostructures, [30,31] control phase transformations, [32,33] and achieve solid-state reactions. [34,35] The method was used successfully by the group of authors to synthesize various kinds of nanostructured intermetallics in different systems, such as Al-Ni, [36] Al-Ti, [37] Al-Cu, [38] Al-Ti-Ni, [39] Fe-Ni, [40] Mg-Ti, [41,42] Mg-Zr, [43] Mg-Hf, [44] Mg-Ni-Pd, [45] and Mg-V-Cr.…”

Effect of Carbon in Severe Plastically Deformed Metals

“…This saturation is established by a dynamic equilibrium between the generation and annihilation of new boundaries, dislocations, and vacancies. [7,10,11] At low homologous SPD temperatures, the annihilation processes are mainly driven by the applied stresses and strains, supported by thermal activation. Therefore, the minimum grain size achievable at very high strains, when the dynamic equilibrium is reached, is determined by the thermally and mechanically driven grain boundary migration.…”