MSE 258:Physical Metallurgy of Non-Ferrous Metals

Dr. Emmanuel Kwesi Arthur

Department of Materials Engineering,

Kwame Nkrumah University of Science and Technology, Kumasi

2019

Lecture Six

Copper Alloys

There are several ways to increase the strength of Cu alloys

They can be cold-worked or solid-solution strengthened

Cold-working has a substantial effect, much like it did for Al Alloys.

The process of age hardening is an important method of strengthening alloys of materials and it illustrates the power of knowledge of the phase diagrams and microstructures of materials.

Strengthening Mechanism

Copper and copper alloys

4

Hume – Rothery Rules

The Hume-Rothery rules give the tendency of two substances to form a substitutional solid solution.

The formation of substitutional solid solutions is encouraged by the following characteristics of materials:

• 1) the atomic sizes of the elements must not differ by more than about 15%

• 2) the crystal structure of the atoms must be the same

• 3) there should be no appreciable difference in the electronegativities of the elements so that compounds will not form

• 4) the two elements should have the same valence.

Atomic size- RNi =0.162 nm, RCu=0.157 nmElectronegativity- Ni=1.91, Cu=1.9Valence- Ni=2, Cu=2

Atomic size- RZn =0.153 nm, RCu=0.157 nmElectronegativity- Zn=1.65, Cu=1.9Valence- Zn=2, Cu=2

Commercially important alloys of copper and zinc

Have composition up to 40% Zn

Addition of zinc to copper increases in strength because zinc enters into solid solution in the copper.

Unusual feature: ductility also increases with dissolved zinc content, reaches a maximum ductility value at a zinc content of 30%

Zinc is cheaper than copper, so brasses are more economical than pure copper.

Brasses are widely used in a variety of applications

where electrical conductivity is important, pure Cu is used.

Brasses - general

1. Brasses - alloys of copper and zinc Alpha brasses - alloys containing up to 40% zinc

Red brasses - 5 to 20% zinc Yellow alpha brasses - 20 to 40% zincEg. Cartridge brass (70Cu 30Zn) is single phase Zinc (0.5 to 3%) is always added to copper to increase machinability

Alpha plus beta brasses - 54 to 62% copper. Eg. Muntz brass (60Cu 40Zn) is two phase

2. Bronzes - up to 10% of alloying element Tin bronzes Silicon bronzes Aluminium bronzes Beryllium bronzes

Most important commercial copper alloys

Stronger and less corrosive than Cu-Zn bronzes.

Up to 16% Sn is added to alloys that are used for high strength bearings.

3. Cupronickels - alloys of copper and nickel 4. Nickel silvers - alloys of copper, nickel, and zinc.

Copper Alloys

Cupro Nickel 90/10 Pipe Fittings

Copper - zinc phase diagram

Copper-zinc phase diagram

Commercial Brasses

The FCC a-solid solution extends up to 40% Zn – at 400oC but – due to low diffusion – very little of the β-phase precipitates on cooling to RT.

Properties of Wrought a-phase Cu-Zn Alloys

The colour of the a-brasses depends on the Zn content: Guilding metal: 5%Zn – has a golden colour – used for cheap jewelry

Commercial bronze: 10%Zn – has a bronze colour – similar to Sn bronze.

Red Brass: 15%Zn – has a bright red tint like copper

Cartridge brass: 30%Zn – has a bright yellow colour

The UTS increases only gradually across the a-phase – because of the similarity in the atomic sizes of Cu and Zn.

In contrast to most other alloy solid solutions – the ductility of the a-phase increases with Zn concentration – so that 30%Zn – the strongestalloy – is also the most ductile!

Properties of Wrought a-phase Cu-Zn Alloys

The a-phase has excellent deep drawing properties – and is used for making gun cartridges from flat rolled strip – but it is also very susceptible to stress corrosion cracking in alkaline environments.

The (a+b) 40%Zn alloy is stronger than the a-phase alloys – but has reduced ductility equivalent to the 10%Zn a-brass.

It is commonly alloyed with small amounts of Sn, Pb and Mn to increase its corrosion resistance in saline solutions.

Work Harrdening of Cu-Zn Brasses

Note: The 30% Zn alloy has a greater strength and elongation compared to the 15% Zn alloy.

It’s rare to increase strength and ductility at the same time.

Properties of Wrought (a+b)-phase Cu-Zn Alloys

Muntz Metal – the pure 40% Zn alloy – has greater strength – but lower ductility – than the 30% Zn a-phase alloy

The addition of 2% Pb to Muntz metal does not affect the annealed strength – but increases its machinability – this alloy is used for manufacturing brass screws and fittings in high speed automatic machines.

Naval brass – with 1% Sn – has increased strength compared to Muntz metal in both the annealed and cold worked condition – and improved corrosion resistance – it is used for forgings and marine machinery.

Properties of Wrought (a+b)-phase Cu-Zn Alloys

Mn bronze (is misnamed as it’s really a brass) – with 1%Fe + 0.03% Mn –has even greater strength in both the annealed and cold worked condition.

Brasses for casting are (a+b) Muntz metal alloys with additional alloying elements – to control strength, shrinkage, castability and machinability

For comparison – their properties are listed with the cast bronzes –which are also two-phase Cu-based alloys.