Cobalt Chromium Alloys

Question 1

What is the (usual) composition of Cobalt Chromium Alloys?

Answer 1

90%: chromium(35-65%), cobalt(23-30%), nickel(0-20%)
Small quantities of: manganese, silicon, boron, beryllium, iron, copper, aluminium, molybdenum, carbon

Question 2

What does cobalt provide to the alloy?

Answer 2

It increases
1. Fracture strength
2. Hardeness
3. Modulus of elasticity

Question 3

What does chromium provide to the cobalt-chromium alloy?

Answer 3

• increases tarnish resistance & stainless properties: form inert/passive chromium oxide layer on surface
• solid solution hardener
• > 30%, difficult to cast

Question 4

What does nickel provide to the cobalt chromium alloy?

Answer 4

(As nickel replaces cobalt)
- decreases strength, hardness, modulus or elasticity and fusion temp
- increase in ductility

• prof say cheap jewellery has a lot of nickel, ez to bend
• can cause allergies

Question 5

What are some oxide scavengers used when making cobalt chromium alloys?

Answer 5

Manganese and silicon.

• oxide scavengers prevent oxidation
• solid solution hardeners
• tend to cause brittleness in a cobalt base alloy

Question 6

Why do we add carbon into cobalt chromium alloys?

Answer 6

Carbon is
- critical cuz small variations have a pronounced effect on strength, hardness, ductility of alloy
- forms carbides w any of the metallic constituents (Co, Cr or Mo)
- carbide precipitation is very impt in strengthening the alloys, but in excess, can produce severe brittleness