Characterizing the microstructure and mechanical behavior of a two-phase NiCoCrAlY bond coat for thermal barrier systems

The performance and reliability of thermal barrier systems are greatly affected by the behavior of their bond coats. Microsample tensile experiments conducted on commercial two-phase NiCoCrAlY bond coats have unveiled impressive room temperature strength and tensile ductility; in the as-processed state yield strengths exceeding 1200 MPa and elongations to failure of up to 0.75% plastic strain have been measured. Thermally cycled specimens show a lower yield strength of 660 MPa and up to 1.5% plastic strain. Elevated temperature microsample experiments revealed less impressive properties. Transmission electron microscopy points to the importance of nano-scale γ′ precipitates as a contributor to room temperature strength. Thermal cycling has been related to a general coarsening of the microstructure and a modest reduction in room temperature strength. Thermal cycling also results in the transformation of β-grains from an ordered B2 structure to L10 martensite, but the significance of this transformation remains to be elucidated.