strathSEEA

The remaining samples used in the investigation were nickel based, nickel-chromium alloys. Nickel and chromium are often found together as chromium is very soluble in nickel, even at room temperature as can be seen in the phase diagram. The addition of chromium also acts to greatly enhance the oxidisation resistance, especially at high temperatures, as it does when added to an iron based alloy. The presence of nickel also makes the alloy much more wear resistant than iron-chromium alloys. Finally the addition of chromium increases the electrical resistivity of the alloy, making it an ideal material to be used as electrical resistance wires in applications such as heating elements.

 

As can be seen from the phase diagram, for an alloy with a nickel-chromium alloy with a nickel weight % of greater than approximately 70% below roughly 1400°C an FCC structure is present.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

This means that the alloy is in an austenitic (γ) phase and so can be used as a suitable comparison to stainless steels alloys in the austenitic phase.

 

By referring to the Alloying Elements Effects section, it can be seen that the addition of nickel results in the promotion of an austenite phase, this meaning that as the nickel content is increased, the temperature required for the polymorphic transformation in steel to austenite is decreased.

Also, the addition of nickel gives increased corrosion resistance and improved toughness and ductility. Therefore for alloys where nickel is the dominant element, these effects are magnified.

 

Nickel based alloys are traditionally used in environments of more extreme heat and pressure, where an increased corrosion effect is present. A chromium based stainless steel is not suitable given such conditions. This is due to the nickel strengthening the oxide layer. Furthermore the increased content of nickel can change the physical structure of the stainless steel enough to alter its magnetic properties to non-magnetic.