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

Tension specimens of a γ-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 °C in the as-deposited condition and after a pre-oxidation treatment at 850 °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 °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 γ-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 °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.