The properties of Mg protected by Al- and Al/Zn-enriched layers containing intermetallic phases
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Faculty of Mechatronics and Mechanical Engineering, Kielce University of Technology, Al. Tysi˛aclecia P.P. 7, 25-314 Kielce, Poland (Received 17 July 2015; accepted 26 October 2015)
The alloyed layers were produced on Mg by heating the specimens in contact with pure Al powder or Al 1 Zn powder mixtures. The powder material acting as the source of diffusion elements was held under pressure during heating, and this led to the formation of thick, continuous layers in a short heating time (1 h). The layer formation process took place through partial melting at the substrate/ powder interface. The Al-enriched layer was characterized by a eutectic structure composed of an Mg17Al12 intermetallic phase and a solid solution of Al in Mg. The Al/Zn-enriched layers produced from Al 1 20% Zn and Al 1 40% Zn powder mixtures consisted of Mg17(Al,Zn)12 and Mg5Al2Zn2 and a solid solution of Al and Zn in Mg. The alloyed layers had higher hardness and better wear resistance than the Mg substrate. The results of the polarization measurements show that the Al- and the Al/Zn-enriched layers provide a certain level of protection to Mg against corrosion.
I. INTRODUCTION
Contributing Editor: Yanchun Zhou a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.351
atoms from the outside source to the substrate material. Alloyed layers can be produced from pure Al powder or an Al 1 Zn powder mixture. Shigematsu et al.20 obtained an alloyed surface layer on the AZ91D substrate by heating specimens at 450 °C in contact with Al powder using argon as the shielding gas. The heat treatment process was carried out at a temperature higher than the eutectic temperature (437 °C) of the Mg–Al system. Under such conditions, the layer formation process proceeded through partial melting at the AZ91-substrate/ Al-powder interface. The produced 780 lm thick layer with a hypoeutectic structure consisted of an Mg17Al12 intermetallic phase and a solid solution of Al in Mg. Zhu and Song21 carried out heat treatment of AZ91D magnesium alloy in contact with Al powder at 420 °C for 1.5 h in protective gas. Although the heating was performed at a temperature below 437 °C, dendrites of the Mg17Al12 phase were observed in the alloyed layer, which indicates that the layer formation occurred through partial melting at the reaction interface. The resultant layer was not defect-free; its thickness was not uniform (ranging from 10 to 200 lm) and some pinholes were observed. The researchers suggest that it was due to imperfect contact of Al powder with the AZ91D substrate in some local areas during heat treatment. They report that the coated specimens were more corrosion resistant and harder than the uncoated one. Liu and colleagues22,23 studied the effect of temperature on the microstructure and properties of the aluminized coatings on Mg. The specimens in contact with Al powder were heated in a vacuum furnace at 400 °C for 120 min, 420 °C for 90 min, and 445 °C for 90 min. The layer produced at 400 °C contained Mg2Al3 and Mg17Al
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