Titanium, Stainless Steel and Cobalt-chromium-based Alloys: Classification and Diverse Applications

Titanium, stainless steel, and cobalt-chromium-based alloys represent the most widely used metallic biomaterials and high-performance structural materials owing to their outstanding corrosion resistance, mechanical properties, biocompatibility, and strength-to-weight ratio. This review provides a comprehensive classification of these alloys based on chemical composition and thermomechanical processing. Titanium alloys are categorized into α, near-α, α+β, metastable β, and stable β types; stainless steels into austenitic, ferritic, martensitic, duplex, and precipitation-hardened grades; while cobalt-chromium alloys are differentiated into cast (e.g., F75) and forged (e.g., F799) variants. The microstructure property relationships, influence of key alloying elements, and the effects of cold rolling and deformation processing on strength, fatigue resistance, and corrosion behavior are critically examined. Particular emphasis is placed on their diverse applications in the aerospace industry (airframe structures, jet engines, fasteners, and spacecraft components) and biomedical fields (orthopedic implants, cardiovascular devices, dental prosthetics, and trauma fixation devices). Advantages, limitations, biocompatibility issues (such as stress shielding and ion release), and recent advancements in surface modification techniques are discussed. Finally, future directions including the development of low-modulus β-titanium alloys, nickel-free stainless steels, improved Co-Cr alloys, and advanced additive manufacturing routes are outlined to address current challenges and meet the evolving demands of high-performance engineering and long-term biomedical applications.