Growth Kinetics of TiAl 3 Diffusion Coating by Pack Cementation on Beta 21-S
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Growth Kinetics of TiAl3 Diffusion Coating by Pack Cementation on Beta 21-S N. Chaia1 • C. M. Cossu1 • C. J. Parrisch2 • J. D. Cotton3 • G. C. Coelho1 C. A. Nunes1
•
Submitted: 25 January 2020 / in revised form: 28 February 2020 ASM International 2020
Abstract The growth kinetics of protective diffusion aluminide coatings were investigated on Ti based alloy Beta21S. The coating was prepared by halide activated pack cementation using CrCl3 as transport agent and pure aluminum (high activity) as masteralloy. The coating was composed of only TiAl3 layer whose growth was controlled by solid state diffusion following a parabolic law. The morphology of the coating was quite similar with interdiffusion products present in a bulk semi-infinite diffusion couple as showed by previous literature studies. The transport phenomena are significantly affected by the alloying elements present in the alloys and dissolved in the coating layer. The activation energy estimated at 108 kJ mol-1 determined for temperatures in the range 660-760 C, indicates a bulk diffusion process with a & N. Chaia [email protected] C. M. Cossu [email protected] C. J. Parrisch [email protected] J. D. Cotton [email protected] G. C. Coelho [email protected] C. A. Nunes [email protected] 1
Escola de Engenharia de Lorena, Universidade de Sa˜o Paulo, Po´lo Urbo-Industrial Gleba AI-6, Lorena, SP 12602-810, Brazil
2
Boeing Research & Technology, Sa˜o Jose´ Dos Campos, SP, Brazil
3
Boeing Research & Technology, Seattle, WA
possible grain boundary contribution and/or kinetic limitations associated to the mobility of the gaseous species during the aluminization. Keywords pack cementation Beta-21S TiAl3 diffusion coating growth rate
1 Introduction Titanium alloys are widely used in the aerospace industry due to their low density and elevated mechanical strength at operating temperatures up to 850 C with the new class of c-TiAl based alloys.[1–5] Beta-21S alloy is a metastable titanium alloy characterized by good mechanical properties and elevated resistance to corrosion by hydraulic fluid known as Skydrol and hydrogen absorption.[6] The alloy has been used to substitute Ni based superalloys in the BOEING 777 airplane for plug-andnozzle assemblies which allowed for a considerable weight reduction of the exhaust structure.[7] In spite of the fact that titanium alloys exhibit attractive mechanical properties, they have inherently poor oxidation resistance, in particular at elevated temperatures and extended period of exposures under corrosive ambient.[8–13] Titanium and its alloys are however very sensitive to oxygen and other interstitials like C and N that are detrimental to their mechanical properties. In order to enhance the oxidation resistance, two ways can generally be considered: (i) modification of the alloys chemistry or (ii) deposition of protective coatings. This latter approach is potentially more attractive since it permits to avoid deterioration of the alloy’s mechanical properties and the difficulties to conc
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