Titanium-aluminium-chromium protect TiAI from oxidation

Titanium-aluminium-chromium protect TiAI from oxidation

Keywords Alloys, Coatings, NASA, Turbines

A coating alloy composed of titanium, aluminium and chromium has reportedly been developed at NASA Lewis Research Center, Cleveland, Ohio, to protect gamma titanium aluminide alloys for application in turbine engines. At half the density of superalloys, these titanium aluminides are promising materials for turbine engines. However,protective coatings are needed because of inadequate resistance to oxidation at high temperatures.

Other coatings have not provided satisfactory protection because of poor mechanical properties, mismatch of thermal expansion coefficients, or chemical incompatibility. However, the new alloy is said to exhibit excellent compatibility and improved mechanical properties, as well as oxidation resistance. The alloy composition (in at. per cent), is Ti-51.25Al12.25Cr. This composition was selected because its microstructure consists primarily of the gamma phase, with a minor volume of the oxidation resistant Laves phase, Ti(Cr, Al)₂. Mechanical properties and compatibility with gamma titanium aluminide are optimised as the alloy is based on the gamma phase, and the Laves phase is kept to a minimum as it is extremely brittle. To characterise the material, it was applied to a substrate of the titanium aluminide alloy Ti-48Al-2Cr-2Nb by low pressure plasma spray. Subsequent oxidisation tests at 800 and 1,000°C (1,470 and 1,830°F) in air indicated that the coating did protect the titanium aluminide.

Further information from Dr James L. Smialek or Dr William J. Brindley, NASA Lewis Center, 21000 Brookpark Road, Cleveland, Ohio 44135, USA. Tel: +1 216/433 4000.