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Developing Dislocation Density Based Model for Severe Plastic Deformation Considering Strengthening Mechanism

Parvin, Hooman | 2024

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 57095 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Kazeminezhad, Mohsen
  7. Abstract:
  8. In recent years, severe plastic deformation has received significant attention as a technique for improving the mechanical properties of materials. In this regard, modeling the deformation behavior of materials during severe plastic deformation is of importance. As previous investigations have shown, dislocation density is one of the most applicable parameters for the mentioned purpose. On the other hand, strengthening mechanisms and the presence of strengthening agents in the material are important metallurgical phenomena that can play a significant role during severe plastic deformation. In this regard, the present research investigates the development of a dislocation density-based model for severe plastic deformation, considering the strengthening mechanism. To do this, the effects of dislocation density and the other strengthening agents such as solute atoms, second-phase precipitate particles, insoluble second-phase particles, etc. are considered. At first, the model and its details are presented. Then, the presented model is applied to severe plastic deformation, and the evolution of microstructure and strength is investigated. To verify the obtained modeling results, they are compared with various experimental measurements, and good agreement is obtained. About 40 materials with different conditions are investigated, and the obtained results are discussed in detail. The model is applied to Al-Mg, Cu-Mg, Cu-Al, Al-Cu-Mg, Cu-Ag, and Cu-Cr-Zr alloys containing solute atoms, Cu-Cr-Zr, Cu-Mg-Ca, Al-Zn-Mg-Zr, Cu-Ag-Zr and Al-Zn-Mg alloys in the presence of second-phase precipitate particles, Cu and Al in the pure state, materials containing insoluble second-phase particles composed of SiCp and Al2O3p, such as metal-matrix composites fabricated by accumulative roll bonding process, including Al-SiCp, Al-Al2O3p and monolithic Al. Moreover, the model considers two roles for the strengthening agents. The first role is the direct effect of these agents on the strength of materials (explicit effect). The second one is the effect on dislocation evolutions (implicit effect), which affects strength indirectly. These effects are evaluated quantitatively. The model also shows that although the strengthening agents can alter the strength, it is their combination with severe plastic deformation that can produce a more considerable, ~5 times, improvement in the strength. In another part of the research, the secondary phenomena occurring during severe plastic deformation are studied. The results show that even after considering the role of the strengthening agents, the model still needs further development due to the occurrence of secondary phenomena. In this regard, the model has been further developed by considering the mentioned phenomena. In this part, various items, such as the fragmentation of the particles in the course of severe plastic deformation, the second-phase particle distribution and its variation during the process, the deformation-induced re-dissolution of the precipitates, etc., are investigated. The model indicates that the strengthening agents and the deformation behavior have a mutual effect on each other, and takes into account the effects of the process on the agents. Furthermore, this mutual effect is studied for both the implicit and explicit roles of the strengthening agents. The results also show that severe plastic deformation itself can affect the strengthening agents considerably. In another section of the research, metal-matrix composites fabricated by accumulative roll bonding are studied. The products are investigated under conditions with and without particle addition. The model evaluations show that for metal-matrix composites, an additional effect should be incorporated into the model since the particles are different from soluble second-phase particles and are embedded into the matrix during the cycles of the composite fabrication. In this regard, various functions are examined. A logistic function is proposed to describe the above-mentioned effect. Moreover, the deformation behavior of the product is studied in two aspects: monolithic-like behavior and composite-like behavior. The model shows that there is a critical strain at which monolithic-like behavior initiates to decline. The process also shows a minimum strain at which composite-like behavior dominates. Moreover, the results are compared to the previous model suggested for the metal-matrix composites. The present model takes into account the presence of particles, includes the particle parameters, considers the effect of cycling, and gives a better agreement with experimental data. In another section, it is shown that the model is designed in such a way that it can also be reduced to a model for the pure state, i.e., a situation without any strengthening agents. With this ability, the model can also be applicable to pure materials. Moreover, it can compare the pure state with the condition in which the strengthening agents are present
  9. Keywords:
  10. Strength Mechanism ; Dislocation Density ; Severe Plastic Deformation ; Metal Matrix Composite (MMC) ; Second Phase Particle ; Solute Atom ; Dislocation Density-Based Modeling

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