I am not in particular field of interest but, since the question is intriguing I will share the thoughts. So please take my opinion merely as a hint.

Namely, cDRX is defined as oscillations between strain rate and strain. It happens when the energy required for crystallization is not enough. By observing the dimensions of the specimen with enough aquracy epsilon prime can be deduced. Fragmentation resembles with super-plastic deformation mechanism when new grain boundaries are formed. It is as a phenomenon attached to the high deformation rates that surpass grain changes made by diffusion. Although it is attached to the type of materials that can deform with high deformation rates, and are consisted of nanograins already. Mostly tensile deformation is observed. I would suggest dilatometric observation along with following microstructure observation at high heating rates for the already SPD obtained specimens. Relaxation of the induced defects can lead to an answer of their origin. I had particularly in mind the text book - Thomas J. Courtney - Mechanical behavior of materials.

Do you mean "Geometric dynamic recrystallization" by fragmentation mechanism? You can find useful information in "RECRYSTALLIZATION AND RELATED ANNEALING PHENOMENA" by F.J. HUMPHREYS and M. HATHERLY. See chapters: 13 and 14.

Nebojša Labus Firstly, Thanks for your response. I don't think it's a good and true definition for recrystallization in Metals and your definition can be related to powder metallurgy structures. As I know, Occurring dynamic recrystallization or fragmentation is not dependent to strain rate, especially in severe plastic deformation. Furthermore, relaxation can be an appropriate approach to obtain static recovery and recrystallization but I don't know how can be used to elucidate that dynamic recrystallization or fragmentation is the active mechanism in SPDed metal.

Ali Shabani NO! It's totally different. Geometric DRX is a process can be take place at high temperatures and generally in low strain rate and higher strains. and it's about some serrated boundaries that can be contact with each other and make new tangle scalloped boundaries that cause fine equiaxed microstructure. the texture will be differnet to cDRX.

Dear Ali,

As you know, we say CDRX because there is not distinct steps of nucleation and growth. During CDRX, whole of the microstructural evolution is continuous. So, it can be occurred by transformation of LAGBs to HAGBs, fragmentation, GDRX, etc. You can consider the GDRX and fragmentation mechanism as some types of CDRX. The term "fragmentation" some times has been used instead of CDRX in literature.

Please note that in most cases it is very hard to distinguish the different restoration mechanisms. For example, it is well developed that CDRX does not inherit the deformation texture. However, I found in my research that the existence of deformation textures in the material can not always be the sign of CDRX.

I think the story of dynamic restoration mechanisms have not been completed, yet.

Dr. Akbar Heidarzadeh thanks for your response. Since I have posed this question, I have found some hints. I don't think CDRX can comprise fragmentation, although both of them are analogous mechanism. but a difference between LAGBs in CDRX compare with fragmentation is mentioned in a paper. they suggest misorientation in CDRX is about to zero but in fargmetation it can be 1-2 degrees. I don't Know why! (unfortunately I don't access to the paper)

First, one can estimate the differences in the degree of misorientation between neighboring micro and sub micro volumes in a metallic materials using quantitative metallography techniques. If we nevertheless evaluate the mechanism of structural transformations in the event that conditions of superplastic flow (SPF) are reached, the effect should be divided in qualitatively different directions. One of them is the achievement of SPF conditions, which are caused by phase transformations. For carbon steels, this is plastic deformation at the α - γ transition temperatures. Another direction to solve the problem (for steel) is plastic flow at temperatures slightly lower 📷. At normal temperatures of deformation, the maximum level of plasticity of metals is due to the existence of a certain balance between the processes of nucleation of dislocations and their annihilation after the completion of the act of plastic deformation. The conditional separation of the formed interfaces between the micro volumes into more angular and little angular boundaries is due to their influence on the displacement of mobile dislocations during deformation. The fact is that the boundaries between grains of structurally free ferrite or interfacial ferrite-cementite pearlite colonies, being more angular, serve as the sites of origin of dislocations, and their annihilation after the act of flow. At low angular boundaries (sub-boundaries), the processes of nucleation and annihilation of dislocations are significantly hampered.

When SPF conditions are reached, a high diffusion mass transfer rate in a metallic material requires certain conditions to be met. The first is the maintenance of an isostructural state in the alloy during the process of high-temperature deformation (invariance of the shape and size of the grain of the metal matrix). This condition is achieved by maintaining a certain ratio between the velocity and temperature of deformation. A similar condition should be satisfied in the case of the implementation of SPF during phase transformations (α - γ transition for steel). Considering that the activation energy of the development of substructural changes in a deformed metal is somewhat lower than recrystallization, in the case of the formation of sub-boundaries, the formation of a recrystallization nucleus will inevitably occur. Then, the condition for the development of SPF - the maintenance of the isostructural state will not be fulfilled.

Fragmentation happens when two neighbouring parts of the crystal (or even two grains) deform differently due to different orientation (one being 'softer' than the other), and therefore they separate into different parts to accomodate the differences. This is driven by the need to maintain compatibility between neighbours.

CDRX happens by progressive rotation of subgrains to ultimately form a new grain. This is driven by microstructural restoration processes, aided by temperature. So atleast as far as their driving force and origin is concerned, the two appear to be opposite phenomena.

As for distinction, I can only hazard the guess that CDRX grains will have lower (intragranular) misorientation than fragmented grains, and possibly a progressive misorientation gradient from centre to radius (in CDRX).

You can find the detail mechanism in the following papers

https://www.scientific.net/MSF.941.1391

https://www.cambridge.org/core/journals/journal-of-materials-research/article/hot-deformation-behavior-of-the-highentropy-alloy-cocufemnni/F2B7CD0F82D2AE683609CAA56B49CEC7

Fragmentation mechanism is different from CDRX. CDRX involves in the formation of LAGBs into HAGBs (transformation of subgrains into grains). During CDRX, new grains forms as a result of the increase in sub boundary misorientation brought by continuous accumulation of the dislocations introduced by SPD. Misorientaion angle of the CDRX grains would be around 15-30 degree. one mechanism could be the migration or accumulation of dislocations in the LAGBs followed by grain boundary shearing. The transformation occurs when the subgrain boundaries contain precipitates, which pin them in place. As the subgrain boundaries absorb dislocations, the subgrains transform into grain by rotation, instead of growth. This process occurs generally at high temperatures which allows dislocations to glide and climb.

Fragmentation mechanism is also called as Geometric Dynamic Recrystallization (GDRX), in which a refined and nearly equiaxed grain structure is formed due to grain boundary which have become serrated during formation of subgrains in the course of SPD recombine as serrations pinch off or as the grains thin down. Here the deformation is accompanied by the serration of the grain boundaries due to surface tension effect where they are in contact with LAGBs of subgrains. Eventually the points of the serrations will come into contact. since the contacting points are defects of opposite sing they are able to annihilate and so reduce the total energy in the system. In effect, the grains will pinch in two new grains. These grains are having lower misorientation angle compare to CDRX (1-15 degree).

"...fragmentation is an accommodation process that matches micro and macrolevels in the development of plastic deformation to each other... "

"...this phenomenon is characterized by broad generality and manifested in that uniformly oriented grains of a polycrystal are divided in the process of developed plastic deformation independently of its initial structure and chemical and phase composition into a variety of microareas (fragments) with transversal sizes of nearly 0.2 μm and mutual misorientations that monotonically grow with an increase in the true strain ε to attain the values typical of high-angle intergrain boundaries at ε > 1 ..."[Rybin]

"... During the conventional discontinuous dynamic recrystallization (dDRX) that takes place at elevated temperatures, the new grains evolve by nucleation and growth in materials with low to medium stacking fault energies (SFE). On the other hand, new ultrafine grains can be produced in any material irrespective of the SFE by means of SPD at relatively low temperatures. These result from the gradual transformation of the dislocation sub-boundaries produced at low strains into ultrafine grains with high angle boundaries at large strains. This process, termed in situ or continuous dynamic recrystallization (cDRX) ..." [Sakai]

[Rybin] V. V. Rybin, V. N. Perevezentsev, and Yu. V. Svirina A Physical Model for the Initial Stages of the Fragmentation of Polycrystals in the Process of Developed Plastic Deformation // Physics of Metals and Metallography, 2017, Vol. 118, No. 12, pp. 1171–1175 DOI: 10.1134/S0031918X17120110.

[Sakai] Taku Sakai, Andrey Belyakov, Rustam Kaibyshev, Hiromi Miura, John J. Jonas Dynamic and post-dynamic recrystallization under hot, cold and severe plastic deformation conditions // Progress in Materials Science 60 (2014) 130–207 Article Dynamic and Post-Dynamic Recrystallization under Hot, Cold a...

CDRx is one of the processes of fragmentation taking place during SPD. When material is processed at heavy load, microstructure get fragmented. The fragmentation process involves CDRx, GDRx and DDRx depending on processing rate and temperature of deformation.