The oxidation behaviour of an aligned Ni‐Al‐Cr 3 ‐C 2 eutectic alloy under isothermal and thermal cycling conditions

The isothermal and thermal cycling oxidation behaviour of a directionally solidified Ni‐Al‐Cr 3 C 2 eutectic alloy at temperatures from 800° to 1200 °C in flowing air have been investigated using several physical techniques. At all temperatures an initial, protective, external layer of α‐Al 2 O 3 develops on the alloy surface. However, this breaks down mechanically during thermal cycling, enabling a less protective Cr 2 O 3 ‐rich scale to form. The time of retention of the α‐Al 2 O 3 layer at temperature decreases with increasing temperature, failing after between 30 min and 2 h at 1100° and 1200 °C. However, if platinum metal is introduced into the hot zone at these temperatures, this period is increased to about 48 h. Following formation of the external Cr 2 O 3 scale, internal oxide penetration into the alloy can be considerable, involving preferential oxide penetration down the alloy/carbide fibre interfaces. Thermal cycling does not influence markedly the oxidation behaviour, although it does result in formation of a greater quantity of nickel‐rich oxide nodules on the scale surface following crack development in the Cr 2 O 3 ‐rich scale. This crack development is assisted by differential thermal contraction stresses.