Effect of intermetallic coatings on the tensile properties of a γ-TiAl based TNM alloy

Tension specimens of a gamma-TiAl based TNM-B1 alloy (nominal composition in at%: Ti-43.5Al-4Nb-1Mo-0.1B) were coated with oxidation protective intermetallic Ti-Al-Cr layers containing yttrium and zirconium. The coated specimens were tensile tested in air at room temperature (RT) and 850 degrees C in the as-deposited condition and after a pre-oxidation treatment at 850 degrees C for 300 h in air. For comparison, uncoated specimens with and without pre oxidation were also tested. The elongation after fracture of the TNM-B1 alloy was 0.23% at RT; it increased to 8.3% at 850 degrees C. The as-deposited coatings spalled off during tensile testing at RT. No coating spallation was observed at elevated temperature and after pre-oxidation due to improved adhesion by interdiffusion bonding. Pre-oxidation deteriorated the RT tensile properties of the gamma-TiAl based alloy. The RT ductility determined for pre-oxidized coated specimens was even more reduced probably caused by the brittleness of the coatings initiating premature cracks. When tensile tested at 850 degrees C, the coated specimens exhibited considerably enhanced elongation after fracture in both the as-deposited and pre-oxidized conditions in comparison to the bare substrate alloy. The increase in ductility was attributed to the high environmental protection of the substrate by the coatings.