Co-based Alloys and Their Applications

A metal containing two or more elements.

The ability of a material to perform within an appropriate host response in a specific application.

Breakdown of a material by chemical reactions with the environment.

A substance that harms living cells.

Metals used for medical implants (e.g., Fe, Cr, Co, Ni, Ti, Ta, Mo, W).

Alloys often have better strength than pure metals.

Materials released during metal corrosion.

Materials used for implants and medical devices.

Carbon is a key element in steel, influencing strength and ductility. Higher carbon content leads to higher strength but lower ductility. Structural steels typically have carbon content between 0.15% and 0.30%.

Nickel improves steel's corrosion resistance and low-temperature toughness. Its use in structural steels varies, with some grades containing 0.30% to 1.50% nickel.

Vanadium, similar to manganese, molybdenum, and columbium, refines the steel's microstructure, improving fracture toughness.

Structural steel generally contains between 0.15% and 0.30% carbon.

ASTM A514 steel contains 0.30% to 1.50% nickel.

ASTM A588 steel contains 0.25% to 1.25% nickel.

Vanadium contents in ASTM A572 and A588 range from 0.02% to 0.15%.

A514 steel contains 0.03% to 0.08% vanadium.

Molybdenum increases the strength of steel, especially at higher temperatures and improves corrosion resistance.

Molybdenum is often combined with manganese or vanadium in steel, especially for A588 and A514 grades.

Chromium enhances the corrosion resistance of steel, often used with nickel and copper.

Steel with significant amounts of chromium, typically 18-8 (18% nickel, 8% chromium) for improved corrosion resistance.

A type of stainless steel containing molybdenum for enhanced corrosion resistance, especially in salty environments.

A variation of 18-8Mo with reduced carbon content (0.03% max) for improved corrosion resistance in chloride solutions.

A low-carbon version of 316 with even better corrosion resistance against chloride and is known for its biocompatibility.

Stainless steels have a high chromium content, which forms a protective oxide layer that prevents rusting.

A type of metal alloy commonly used in medical implants, often composed of cobalt, chromium, molybdenum, and other elements.

A type of Co-Based alloy that is melted and poured into a mold to create its final shape. It is commonly used in dentistry and for artificial joints.

A type of Co-Based alloy that is shaped by forging (heating and hammering) and is known for its high strength. Used in stems of prostheses for heavily loaded joints.

Castable Co-based alloys are formed by pouring molten metal into a mold, while wrought alloys are shaped by forging, heating, and hammering.

ASTM designation for castable CoCrMo alloy, commonly used for medical implants.

ASTM designation for wrought CoCrWNi alloy, used in medical implants.

ASTM designation for wrought CoNiCrMo alloy, used in medical implants.

ASTM designation for wrought CoNiCrMoWFe alloy, used in medical implants.

A widely used titanium alloy in implants, primarily composed of titanium (Ti), aluminum (Al) (5.5-6.5%) and vanadium (V) (3.5-4.5%).

The high aluminum content in titanium alloys significantly contributes to their strength and oxidation resistance, ensuring they hold up well against the body's environment.

Titanium alloys possess comparable strength to 316 stainless steel or cobalt-based alloys, making them suitable for load-bearing applications in implants.

Titanium alloys exhibit poor shear strength, making them less ideal for bone screws and plates. They also tend to gall or seize when in contact with themselves or other metals.

Titanium resists corrosion by forming a solid oxide layer (TiO2). This oxide layer acts as a protective barrier, preventing further corrosion.

Under in vivo conditions, titanium forms a stable oxide layer (TiO2) that adheres to the metal surface, protecting it from corrosion.

The phenomenon responsible for inhibiting corrosion in a given environment. This is often achieved by forming an oxide film on the metal surface, slowing down the corrosion process.

The mechanism by which a protective oxide layer forms on a metal surface, separating the metal from the environment and reducing the rate of corrosion reaction.

MP35N is a cobalt-based alloy (CoNiCrMo) known for its high corrosion resistance to seawater (containing chloride ions) under stress. It's composed of approximately 35% cobalt and nickel each.

In contrast to cast alloys, wrought cobalt-chromium alloys require a lower carbon content (0.05% versus 0.25% or higher). This is because a lower carbon content reduces the formation of carbides, which are responsible for strengthening in cast alloys.

To enhance fabricability, chromium levels are generally reduced and nickel is added in wrought cobalt-chromium alloys. This change improves the alloy's ability to be shaped and worked without compromising corrosion resistance.

The use of titanium for implant fabrication dates back to the late 1930s, when it was observed to be tolerated in cat femurs similar to stainless steel and Vitallium® (CoCrMo alloy).

Titanium is favoured due to its lightness (4.5 g/cm3), good mechanical properties, and biocompatibility. Compared to 316 stainless steel (7.9 g/cm3), cast CoCrMo (8.3 g/cm3), and wrought CoNiCrMo (9.2 g/cm3), titanium offers a lighter alternative without compromising strength.

There are four grades of unalloyed titanium used for implants, distinguished by their impurity levels, specifically oxygen, iron, and nitrogen. Oxygen plays a crucial role in affecting the titanium's ductility and strength.

Unalloyed titanium for implants typically contains 98.9–99.5% titanium, with up to 0.50% iron, 0.40% oxygen, 0.08% carbon, 0.05% nitrogen, and 0.015% hydrogen.

Oxygen content plays a crucial role in unalloyed titanium used for implants. It significantly affects the titanium's ductility and strength. Controlling oxygen levels ensures optimal mechanical properties for implant applications.