Date of Award

2017

Document Type

Open Access Master's Thesis

Degree Name

Master of Science in Materials Science and Engineering (MS)

Administrative Home Department

Department of Materials Science and Engineering

Advisor 1

Jaroslaw Drelich

Committee Member 1

Jeremy Goldman

Committee Member 2

Daniel Seguin

Abstract

Stents made of biodegradable metallic materials are increasingly gaining interest within the biomaterials field because of their superior mechanical properties and biodegradation rates as compared to polymeric materials. Zinc and its alloys have been developed and investigated as possible candidates for biodegradable stent applications in the last five years. This study intended to formulate and characterize a new series of Zn-Ti alloys, with titanium additions of less than 1-3 wt%, with the primary objective to develop and select an alloy that meets benchmark values of mechanical properties for biodegradable stents. A series of Zn-Ti alloys was formulated through vacuum induction melting. The experimental approach was to analyze the effect of Ti alloying element addition on mechanical properties of zinc. The structure, mechanical properties and fractography of the as-cast alloys were investigated.

It was found that the grain size was reduced from above 600 µm to ~23 µm with the Ti content increasing from 0.01 wt% to 0.3 wt%. The amount of the intermetallic phase increased from 0.3 wt% to 2.5 wt% with Ti content. The results identify the formation of a eutectic phase of zinc with intermetallics at the primary grain boundaries. Zn16Ti was identified as the intermetallic phase formed in the as-cast Zn-Ti alloys. With increasing Ti content from 0.01 wt% to 1 wt%, the ultimate tensile strength and yield strength of the as cast Zn-Ti alloys increased from 101 and 64 MPa to 177 and 122 MPa, respectively. It is proposed that the strength of as-cast Zn-Ti alloys increases with the Ti content increasing from 0.01 wt% to 0.3 wt% due to grain refinement from a small percentage of titanium. The amount of the intermetallic phase increased with the Ti content increasing from 0.3 wt% to 2.5 wt%. It is proposed that the hardness and strength of the as-cast Zn-Ti alloys increased with the Ti content increasing from 0.3 to 2.5 wt% due to the increased formation of Zn-Ti intermetallic phases. The low elongation of the as-cast Zn-0.3 wt% Ti (3%), Zn-0.5 wt% Ti (4%), and Zn-1 wt% Ti alloys (2%) was also attributed to the increasing content of Zn-Ti intermetallic phases.

Based on the results of the structure and mechanical properties of as-cast Zn-Ti alloys, the most promising as-cast candidates were processed through hot extrusion. This phase study was focused on the structure-property relationships before and after hot extrusion. The as-extruded Zn-0.01 wt% Ti had the highest average ultimate tensile strength and yield strength of 269 and 177 MPa, respectively. It is proposed that a significant increase in the ultimate tensile strength and yield strength in Zn-0.01 wt% Ti alloy after hot extrusion is due to grain refinement and formation of precipitates. The as-extruded Zn-0.1 wt% Ti and Zn-0.3 wt% Ti alloys exhibited high ductility, with the elongation to failure of about 44% and 30%, respectively. It is proposed that the as-extruded Zn-0.1 wt% Ti alloy exhibited high ductility due to the grain refinement and grain shape adjustment after hot extrusion. The high elongation of the as-extruded Zn-0.1 wt% Ti and Zn-0.3 wt% Ti alloys is consistent with the microstructural observations of ductile fracture. The as-extruded Zn-0.1 wt% Ti alloy had the best combination of tensile mechanical properties (UTS=207 MPa, YS=163 MPa, and Elongation=44%), which nearly meet the mechanical requirements for stent application.

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