Formation Mechanism of Aluminide Diffusion Coatings on Ti and Ti-6Al-4V Alloy at the Early Stages of Deposition by Pack Cementation

Hailiang Du; Ning Tan; Li Fan; Jiajie Zhuang; Zhichao Qiu; Yanhua Lei

doi:10.3390/ma12193097

Formation Mechanism of Aluminide Diffusion Coatings on Ti and Ti-6Al-4V Alloy at the Early Stages of Deposition by Pack Cementation

Materials (Basel). 2019 Sep 23;12(19):3097. doi: 10.3390/ma12193097.

Authors

Hailiang Du^{1

2}, Ning Tan³, Li Fan⁴, Jiajie Zhuang⁵, Zhichao Qiu⁶, Yanhua Lei⁷

Affiliations

¹ College of Mechanical and Electronic Engineering, Shanghai Jian Qiao University, Shanghai 201306, China. hailiang_du@hotmail.com.
² Institute of Marine Materials Science and Engineering, College of Ocean Science and Engineering, Shanghai Maritime University, Shanghai 201306, China. hailiang_du@hotmail.com.
³ Institute of Marine Materials Science and Engineering, College of Ocean Science and Engineering, Shanghai Maritime University, Shanghai 201306, China. tanning986@163.com.
⁴ College of Mechanical and Electronic Engineering, Shanghai Jian Qiao University, Shanghai 201306, China. 18126@gench.edu.cn.
⁵ College of Mechanical and Electronic Engineering, Shanghai Jian Qiao University, Shanghai 201306, China. nhezzjj17@163.com.
⁶ Institute of Marine Materials Science and Engineering, College of Ocean Science and Engineering, Shanghai Maritime University, Shanghai 201306, China. qzc4321@163.com.
⁷ Institute of Marine Materials Science and Engineering, College of Ocean Science and Engineering, Shanghai Maritime University, Shanghai 201306, China. yhlei@shmtu.edu.cn.

Abstract

The diffusion coatings were deposited on commercially pure Ti and Ti-6Al-4V alloy at up to 1000 °C for up to 10 h using the pack cementation method. The pack powders consisted of 4 wt% Al (Al reservoir) and 4 wt% NH₄Cl (activator) which were balanced with Al₂O₃ (inert filler). The growth kinetics of coatings were gravimetrically measured by a high precision balance. The aluminised specimens were characterised by means of scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). At the early stages of deposition, a TiO₂ (rutile) scale, other than aluminide coating, was developed on both materials at <900 °C. As the experimental temperature arose above 900 °C, the rutile layer became unstable and reduced to the low oxidation state of Ti oxides. When the temperature increased to 1000 °C, the TiO₂ scale dissociated almost completely and the aluminide coating began to develop. After a triple-layered coating was generated, the coating growth was governed by the outward migration of Ti species from the substrates and obeyed the parabolic law. The coating formed consisted of an outer layer of Al₃Ti, a mid-layer of Al₂Ti and an inner layer of AlTi. The outer layer of Al₃Ti dominated the thickness of the aluminide coating.

Keywords: Ti; Ti-6Al-4V; aluminide; diffusion coating; mechanism; pack cementation.

Abstract

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