Author ORCID Identifier

https://orcid.org/0000-0002-7959-1426

Date of Award

28-6-2025

Document Type

Thesis

School

School of Mechanical Engineering

Programme

Ph.D.-Doctoral of Philosophy

First Advisor

Dr.S.Raghuraman

Keywords

Laser Shock Peening, Hot Extrusion, Rare Earth Element, Biodegradable, Orthopedic Implant

Abstract

Magnesium (Mg) and its alloys are of much interest as promising third-generation bio-materials for bone implant applications; however, challenges remain for the alloy's mechanical integrity and bio-corrosion behaviour. Therefore, appropriate alloying elements and surface-modification techniques are required to overcome these limitations. Zinc (Zn) and Scandium (Sc) elements were found to have good biocompatibility and biodegradability; hence, they were selected as suitable alloying elements for magnesium.

This research work aims to enhance the mechanical integrity and bio-corrosion resistance behaviour of a novel Mg-1wt% Zn-0.5wt% Sc cast alloy by laser shock peening surface treatment with multiple passes (LSP-2 Pass and LSP-3 Pass). Based on the characterization studies, LSP induced beneficial compressive residual stress around the surface and sub-surface region of Mg-1Zn-0.5Sc alloy due to severe plastic deformation, which led to grain refinement through the twinning mechanism of the Mg alloy.

Also, the dispersion of second-phase particles (β-ScZn) was observed while analysing the XRD profiles. Strain hardening and grain refinement have been attributed to the evolution of structure and texture, which enhanced the strength, ductility, and corrosion resistance of novel Mg-1Zn-0.5Sc alloy compared to the as-cast condition. From in vitro studies, a low rate of corrosion in simulated body fluid, uniform hydroxyapatite layer formation on the surface and less cytotoxic behaviour was observed for the LSP-3 pass Mg-1Zn-0.5Sc alloy, which is suitable for bone implant applications.

However, some limitations remain, and challenges need to be addressed. In the LSP-3 pass, Mg material had only a marginal strength and a corrosion rate of cortical bone properties for orthopaedic implant applications. To address these challenges, Mg-1Zn-0.5Sc alloy was developed through thermomechanical processing at various temperatures.

During extrusion at 250°C and 350°C, the microstructure and texture evolution of the hot extruded at 350°C alloy exhibited complete dynamic recrystallization, and texture evolved to strong basal plane orientation, influencing an increased strength of 152 MPa, reduced ductility of 10%, and improved corrosion resistance. Moreover, this hot extruded at 350°C processed Mg alloy underwent multipass laser shock peening surface treatment to synergistically enhance its strength and ductility properties via structural and textural evolution.

Hence, laser shock peening on this extruded alloy surface leads to further recrystallizations owing to the effect of developed compressive residual stress near the peened surface, resulting in large grain formation. Also, favourable texture evolution of basal and non-basal planes was observed due to this LSP-induced plastic strain, with significant improvement in strength and ductility to 230MPa and 16%, respectively.

Biocompatibility analysis shows that both extruded and LSP-treated alloys have uniform bio-friendly hydroxyapatite layer formation and exhibit good cell viability after 72h of incubation. It also shows a reduced corrosion rate compared to the as-cast alloy, which can also harmonize with tissue healing and implant degradation. The extruded Mg alloy was developed through the LSP technique and can be a candidate material for orthopaedic implant application.

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