An investigation on the effects of severe plastic deformation in elevated-temperature constrained groove pressing on the mechanical properties and mixed mode fracture parameters

نوع: Type: Thesis

مقطع: Segment: PHD

عنوان: Title: An investigation on the effects of severe plastic deformation in elevated-temperature constrained groove pressing on the mechanical properties and mixed mode fracture parameters

ارائه دهنده: Provider: Davood Zarini

اساتید راهنما: Supervisors: Dr. Faramarz Fereshteh-Saniee

اساتید مشاور: Advisory Professors:

اساتید ممتحن یا داور: Examining professors or referees: Dr. Rahman Seifi, Dr. Heshmatolah Haghighat, Dr. Ali Pourkamali

زمان و تاریخ ارائه: Time and date of presentation: 2026

مکان ارائه: Place of presentation: دانشکده مهندسی

چکیده: Abstract: With the increasing application of lightweight alloys in automotive, aerospace, and energy-related industries and considering the critical role of fracture behavior, toughness, and fatigue durability in structural design, the present research is conducted to advance fundamental understanding of mechanical performance optimization through microstructural engineering. In this dissertation, the effects of severe plastic deformation (SPD) imposed by the Constrained Groove Pressing process (CGP) on the microstructure, crystallographic texture, dislocation density, mechanical properties, fracture toughness, and crack growth behavior of two lightweight alloys, namely aluminium Al3105 and magnesium AZ80, are investigated. The primary objective of this study is to develop an in-depth understanding of how severe plastic deformation influences the fundamental properties of these alloys and to explain the differences in their deformation and fracture responses arising from their distinctly different crystal structures, i.e. face-centered cubic (FCC) for the Al alloy and hexagonal close-packed (HCP) for the Mg one. For the Al3105 alloy, the CGP process was performed using an available die, and its effects on microstructural evolution were examined by optical microscopy, X-ray diffraction, EDS analysis, and elemental mapping. The experimental results revealed that the application of high plastic strains activated dynamic recovery and induced significant changes in the secondary precipitates, including Al₆(Fe,Mn) and α-Al(Fe,Mn)Si phases. A considerable fraction of these precipitates dissolved into the aluminum matrix, while the remaining particles were fragmented and redistributed more uniformly. These microstructural changes, accompanied by grain growth and a pronounced weakening of the dominant cube and Goss textures, led to a reduction in the yield strength by up to 57 percent and a decrease in the ultimate tensile strength. In contrast, the elongation to failure increased up to 38 percent, the fracture toughness improved by up to 40 percent, and the plastic deformability and absorbed fracture energy increased markedly. Fractography analysis of the compact tension (CT) specimens indicated a transition from relatively brittle fracture to a ductile fracture mode characterized by large dimples and the formation of an extensive plastic zone around the crack tip. Overall, the FCC crystal structure of Al3105 facilitated dynamic recovery and grain growth, causing the CGP process to manifest primarily as an enhancement in ductility and fracture toughness. In the AZ80 magnesium alloy, the influence of the HCP crystal structure, the limited number of available slip systems, and the strong tendency toward dynamic recrystallization under severe plastic deformation were particularly pronounced. CGP processing at different temperatures demonstrated that increasing the processing temperature promotes dynamic recrystallization, and leads to the formation of a refined grain structure with a grain size reduction of up to 63 percent. In addition, β-Mg₁₇Al₁₂ precipitates were fragmented during CGP and redistributed more uniformly within the matrix, which played a significant role in enhancing the strength and hardness. Texture modification and crystallographic reorientation, especially the weakening of the basal texture, further contributed to improved mechanical properties and enhanced resistance to crack propagation. Uniaxial tensile tests combined with digital image correlation (DIC) strain-field measurements revealed increases in the yield strength by up to 37 percent and in the ultimate tensile strength, accompanied by a reduction in the elongation, which is attributed to the intrinsic characteristics of the HCP structure and dislocation-based strengthening mechanisms associated with a 56 percent increase in dislocation density. Fracture behavior was evaluated using compact tension tests under both the mode I and mixed-mode I+II loading conditions. The findings showed that CGP-processed AZ80 samples exhibited higher resistance to crack initiation and propagation compared with the annealed condition, with an improvement in fracture toughness of up to 70 percent. Fractographic observations additionally indicated that severe plastic deformation had led to the formation of more complex shear-dominated fracture regions and increased local resistance to crack opening. During the fatigue crack growth experiments, all the CGP-processed samples demonstrated a substantial improvement in the fatigue life by up to 14 times relative to the undeformed material. This enhancement was primarily attributed to a markedly increased resistance to crack initiation, as the contribution of the initiation stage increased from 18 percent in the annealed sample to over 90 percent in the CGP-processed ones. However, the crack growth rate behavior during the stable propagation regime was strongly dependent on the CGP processing circumstances. Under certain conditions, the crack growth rate decreased and the stability of the Paris law parameters improved, whereas in other cases an accelerated crack growth rate was observed. These findings have indicated that CGP can enhance the fatigue durability of AZ80 sheets through multiple interacting mechanisms. A comparative analysis of the two alloys specified that their responses to severe plastic deformation during CGP are strongly governed by their crystal structures and associated microstructural evolution mechanisms. In aluminum, the microstructure evolves predominantly toward dynamic recovery, resulting in reduced strength but enhanced toughness and ductility. In contrast, in magnesium, the combined effects of dynamic recrystallization, precipitate fragmentation, and increased dislocation density lead to significant improvements in the strength, hardness, and fracture resistance. Consequently, the selection of appropriate CGP processing parameters must be tailored to the intrinsic characteristics of each alloy system. The findings of this research provide a valuable foundation for the design of advanced forming processes and the optimization of structural performance in lightweight engineering applications.