The role of chromium in protective alumina scale formation during the oxidation of ternary TiAlCr alloys in air

Ternary TiAlCr alloys show promise for practical application as coating materials due to their excellent oxidation stability in air at elevated temperatures. The main feature of these alloys is the formation of a stable alumina layer at the surface during oxidation. Some alumina forming alloys were studied i.e. two ternary TiAlCr alloys, Ti0.25Al0.67Cr0.08 (τ-phase) and Ti0.28Al0.44Cr0.28 (Laves phase-based alloy), and compared with a binary Ti0.25lAl0.75 alloy and some ‘model’ single phase alloys. An intensive study of the surface characteristics using scanning and transmission electron microscopy, scanning Auger spectroscopy and secondary ion mass spectroscopy was performed. It was shown that to achieve an oxidation resistance greater than for the binary alloy, the components in a ternary TiAlCr alloy must collaborate synergistically during oxidation. Cr alloying of titanium aluminides leads to the formation of a fine-grained, protective alumina layer with little or no rutile formation. The multi-phase component Laves and τ phase alloys have superior oxidation resistance to single component alloys. Possible explanations of this phenomenon are discussed.