Serge Naveos

Zirconium Modified Aluminide by a Vapour Pack Cementation Process for Thermal Barrier Applications: Formation Mechanisms and Properties

The main mechanism of degradation of thermal barrier systems is the spallation of the thermally grown oxide (TGO) formed at the ceramic/metallic sub-layer. Present trends are the use of a platinum modified nickel aluminide layer as metallic sub-layer. But the effect of few p.p.m. or few % at. of zirconium on the oxidation of hipped or cast nickel aluminide was also studied for higher temperature applications. It has been shown that the system ZrO2-Y2O3 (8% mass.) EBPVD / cast NiAl(Zr) had at 1200°C a life durability three times longer than the classical system ZrO2-Y2O3 (8% mass.) EBPVD /NiAlPt coating/nickel base superalloy. Similar encouraging results were obtained with the system ZrO2-Y2O3 (8% mass.) EBPVD / NiAl(Zr) coating deposited by physical vapour deposition/ nickel base superalloy. In this work, a zirconium modified nickel aluminide coating by vapour pack cementation process was developed. The structure of this modified coating is discussed. Cyclic oxidation test results and microscopic investigations are also presented.

Influence of surface modification and long-term exposure on mechanical creep properties of γ-TiAl G4

Abstract An investigation of the corrosion processes were performed for coated and uncoated γ-TiAl G4, an alloy designed to work in the temperature range 750 – 800°C, where oxidation and corrosion phenomena occur. An aluminising pack cementation treatment was used to improve the oxidation resistance of this γ-TiAl G4 alloy. Cyclic corrosion tests were performed at 800°C in air for up to 800 1-hour cycles with a Na2SO4/NaCl mixture. The influence of both aluminisation and the corrosion phenomena on the creep behaviour was investigated. The cyclic corrosion resistance of the coated γ- TiAl G4 was shown to be improved by aluminising. The pack cementation treatment had no detrimental effect on the creep behaviour. Moreover, neither is creep affected by the corrosion of coated specimens. As corroded uncoated specimen exhibited good creep behaviour, it can be concluded that this alloy is suitable, even without coating, for turbine applications in hot corrosion atmospheres at least up to 800°C.