Interdiffusion kinetics of the intermetallic coatings on AZ91D magnesium alloy formed in molten salts at lower temperatures Jingjing Le, Lei Liu, Fan Liu, Yida Deng, Cheng Zhong ⇑ , Wenbin Hu State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China article info Article history: Received 26 February 2014 Received in revised form 26 April 2014 Accepted 28 April 2014 Available online 9 May 2014 Keywords: Magnesium alloy Intermetallic coatings Molten salt Diffusion Interdiffusion coefficient Activation energy abstract A continuous Mg–Al diffusion coating can be formed on the AZ91D Mg alloy by the diffusion coating treatment in molten salts in the lower temperature range from 280 to 400 °C. The microstructure and composition of the diffusion coatings were investigated by scanning electron microscopy and energy dis- persive X-ray analysis. The results showed that the diffusion coating consists of continuous c-Mg 17 Al 12 phase and b-Mg 2 Al 3 phase. The b-Mg 2 Al 3 phase layer grows faster than the c-Mg 17 Al 12 phase layer. The interdiffusion coefficients for each phase were investigated by Heumann’s method. As the tempera- ture increases from 320 to 400 °C, the interdiffusion coefficient in c-Mg 17 Al 12 phase ( ~ D c ) increases from 2.2  10 –12 to 9.6  10 –11 cm 2 /s. When the temperature increases from 360 to 400 °C, the interdiffusion coefficient in b-Mg 2 Al 3 phase ( ~ D b ) increases from 3.5  10 –10 to 7.4  10 –10 cm 2 /s. The activation energies for the interdiffusion in c-Mg 17 Al 12 and b-Mg 2 Al 3 phases are 155.9 and 66.3 kJ/mol, respectively. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Mg and its alloys are promising light structural and functional materials being increasingly used in the automotive, aerospace, electronics and energy industries, owing to their unique character- istics such as high strength-to-weight ratio, high electrical conduc- tivity and thermal conductivity, and good recycling ability [1,2]. However, the further application of Mg alloys is limited by their poor corrosion and wear resistance [3]. Thus, various surface mod- ification technologies have been proposed to improve the surface properties of Mg alloys, such as anodizing [4], chemical conversion [5], electro/electroless plating [6], gas-phase deposition processes [7], diffusion coatings [8,9] and laser and ion beams [10,11]. Among these techniques, recently developed diffusion coating on Mg alloys is of great interest because of the following potential advantages [9]: (1) The adhesion strength of the coating is high since there is a metallurgical diffusion bond between the coating and the substrate. (2) The diffusion coating consists of intermetal- lic compounds, which may improve not only the corrosion resis- tance but also the wear resistance [12]. (3) The high thermal and electrical conductivity, as well as the electromagnetic shielding properties of the Mg alloys can be maintained by applying a metal- lic diffusion coating. Therefore, considerable research has been done trying to achieve diffusion coatings on Mg alloys [8,9,13]. For example, Shigematsu et al. [14] have obtained an Al-enriched diffusion coating by covering the Mg alloys with Al powders at 450 °C in 2000. It is found that the surface layer mainly consisted of c-phase Mg 17 Al 12 and its hardness was HV140-160, which was much higher than that of the AZ91D Mg alloy substrate (HV60). A pack cementation process with a powder mixture of Al and Zn has also been applied to obtain a diffusion coating on pure Mg at 480 °C [15], ZM5 Mg alloy at 470 °C [16], AZ91E alloy from 350 to 413 °C [13] and Mg alloys with various Al and Zn contents at 400 °C [8], respectively. Liu et al. [17,18] used a pack cementation process under a vacuum environment to form diffusion coatings on pure Mg from 400 to 445 °C. It was found that the microstructure of the diffusion coating was a hypoeutectic structure, which was similar to that reported by Shigematsu et al. [14] and Zhu et al. [19] Park et al. [20] carried out powder pack cementation process with the addition of halide salt activator (i.e., AlCl 3 ). Previous work from our group has also prepared Mg–Al and Mg–Zn intermetallic compounds on the surface of AZ91D Mg alloys at 427 °C by using Al powder as donor and ZnCl 2 as activator [21]. The ZnCl 2 reacts with the Al to form AlCl 3 which can react with the Mg, allowing the deposition of active Al atoms. Up to date, most of the work has used conventional powder pack cementation process which has to be carried out at high tempera- tures (near or even above the Mg–Al eutectic reaction temperature of 437 °C). However, such high temperatures may lead to the sur- face melting, cracking and distortion of the workpieces, which will limit its industrial application [13]. Therefore, the major challenge http://dx.doi.org/10.1016/j.jallcom.2014.04.209 0925-8388/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel./fax: +86 21 34202981. E-mail address: czhongcn@gmail.com (C. Zhong). Journal of Alloys and Compounds 610 (2014) 173–179 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom