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 Seven

Cupronickel Alloys

Cu and Ni form a continuous series of solid solutions – as they are both FCC and have similar electronic structures and atomic sizes: Cu = 0.2556 nm; Ni = 0.2492 nm(These fit the Hume-Rothery rules of mixing of two elements for the formation of a single phase)

CupronickelWhen Cu-Ni alloy of composition “a” is cooled to the liquidus temperature

– solid of composition “b” is formed.

On further cooling to a’– as solid phase is precipitated- the composition of the solid should shift from “b” to “d” – while the liquid composition shifts from “a” to “c”.

Properties of Cu-Ni Alloys

Note – maximum electrical resistance, re, is ~50 Ni.

Properties of Cu-Ni Alloys

Three Cu-Ni alloys are of industrial importance

1) Cu - 66.7%Ni – known as Monel metal – (really a Ni-based alloy) – has the highest strength in the series – which can be further increased to 613 MPa by precipitation hardening with Ni3Al.

2) Cu - 30%Ni – has slightly lower strength than Monel – but it is preferred for marine applications – due to its high resistance to sea water corrosion.

3) Cu – 45%Ni – known as constantan – has the highest electrical resistivity – combined with a very low temperature dependence of resistivity – and is used for precision standard resistors* and rheostats.

* - for heating elements it is coated with SiO2 to reduce oxidation.

Mechanical Properties of Cu-Ni AlloysNote: The peaks in strength and hardness occur on the Ni side of 50% -

because the smaller sized Ni atoms in a copper matrix cause moreelastic misfit strain – than the larger Cu atom in a nickel matrix.

Both strength and ductility can be improved with more Ni.

Atomic size- RNi =0.162 nm, RCu=0.157 nm

Tin Bronzes (Cu-Sn alloys)

The FCC α-solid solution extends up to 15% Sn at 500 oC – but at lower temperatures the solubility of Sn drops to almost zero.

However - at normal cooling rates Cu3Sn (e-phase –circled on phase diagram) does not precipitate from a-phase alloys – due to the very low diffusion rate of Sn in Cu.

Tin Bronzes

Tin Bronzes

Aluminum Bronzes

Alloys of Cu-Al and Cu-Si are also referred to as bronzes – because they have similar colouration to the tin bronzes.

Single phase wrought alloys contain 5.0% Al – while heat treatable cast alloys contain 11% Al.

Aluminum Bronzes

Aluminium bronzes are most valued for their higher strength and corrosion resistance as compared to other bronze alloys.

These alloys are tarnish-resistant and show low rates of corrosion in atmospheric conditions, low oxidation rates at high temperatures, and low reactivity with sulfurouscompounds and other exhaust products of combustion.

They are also resistant to corrosion in sea water. Aluminium bronzes' resistance to corrosion rests in the aluminium component of the alloys, which reacts with atmospheric oxygen to form a thin, tough surface layer of alumina, which acts as a barrier to corrosion of the copper-rich alloy.

The addition of tin can improve corrosion resistance

Aluminum Bronzes

Another notable property of aluminium bronzes are their biostatic effects.

The copper component of the alloy prevents colonization by marine organisms including algae, lichens, barnacles, and mussels and therefore can be preferable to stainless steel or other non-cupric alloys in applications where such colonization would be unwanted.

Aluminium bronzes tend to have a golden color

Aluminum Bronzes

Aluminium bronzes are most commonly used in applications where their resistance to corrosion makes them preferable to other engineering materials.

These applications include plane bearings and landing gear components on aircraft, engine components (especially for seagoing ships), underwater fastenings in naval architecture, and ship propellers.

The attractive gold-toned coloration of aluminium bronzes has also led to their use in jewelry

Silicon Bronzes

The phase diagram for Cu-Si is similar in form to the Cu-Al system – but the a-solid only extends up to 4.65 wt% Si (9.95 at% Si).

Single phase wrought alloys in this system contain 1.5% Si and 3.0 % Si.