Microstructure and growth kinetics of silicide coatings for TiAl alloy

Abstract

In order to improve the oxidation resistance of TiAl alloy, silicide coatings were prepared by pack cementation method at 1273, 1323, and 1373K for 1-3 hours. Scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD) were employed to investigate the microstructures and phase constitutions of the coatings. The experimental results show that all silicon deposition coatings have multi-layer structure. The microstructure and composition of silicide coatings strongly depend on siliconizing temperatures. In order to investigate the rate controlling step of pack siliconizing on TiAl alloy, coating growth kinetics was analyzed by measuring the mass gains per unit area of silicided samples as a function of time and temperature. The results showed that the rate controlling step was gas-phase diffusion step and the growth rate constant (k) ranged from 1.53 mg²/(cm⁴·h²) to 2.3 mg²/(cm⁴·h²). Activation energy (Q) for the process was calculated as 109 kJ/mol, determined by Arrhenius' equation: k = k ₀ exp[–Q/(RT)].

Keywords

pack cementation coating / titanium aluminide / siliconizing kinetics / activation energy / rate controlling step

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