Morteza Shaker
Congratulations to Dr. Morteza Ardakani for defending his PhD dissertation and graduating in Spring 2022.!
Morteza dissertation is entitled Engineering Mechanically-Stable Zinc-based Alloys for Medical Implants, and the following is the abstract:
Zinc-based alloys have been introduced as candidate absorbable materials for biomedical implants due to their biocompatibility and application-appropriate corrosion rate. However, poor mechanical strength and ductility, as well as susceptibility to creep and fatigue, restrict their usage in load-bearing stenting applications. Many studies have been conducted that attempt to overcome these limitations by using a combination of alloying and microstructural refinement; these have resulted in some successes in increasing their strength. Nevertheless, room temperature aging, strain softening, and strain rate sensitivity within these alloys remain unresolved. These properties can negatively affect the elongation to failure, as well as the creep and fatigue resistance of the zinc-based alloys.
This Ph.D. research identifies the mechanisms of the mechanical instabilities that are inherent to these alloys, including natural (room temperature) aging, strain softening during deformation, and a sensitivity to deformation rate (i.e., strain rate sensitivity). Natural aging of a zinc-magnesium binary alloy was studied and a successful strategy to reduce the effect was identified by introduction of a third alloying element to the zinc solid solution. This method was used for the first time in zinc-based alloys to eliminate natural aging. Strain softening and strain rate sensitivity of cold-rolled zinc-based alloys were reduced through heat treatments that act to modify the grain size and the fraction of the secondary phase. Specifically, short-time annealing alters the microstructure in a way such that the predominant deformation mechanism shifts from dynamic recovery and recrystallization to homogenized dislocation slipping. This is due to the reduction of the deformation gradient resulting from the reintroduction of the alloying elements and grain growth after annealing. Finally, cross rolling and hot rolling were hypothesized and studied as strategies that would serve to promote the development of a lower-energy substructure and to reduce the driving force for following dynamic recrystallization.
The presented fundamental study introduces a new approach for stabilizing the microstructure and mechanical properties of zinc-based alloys by using a specific group of alloying elements followed by performing post-deformation annealing. The results provide promising progress in the use of these alloys in load-bearing applications as biodegradable metallic materials.
