Surface alloying of magnesium and AZ91 by thermochemical treatment in a medium containing zinc chloride and potassium chloride

Abstract

Surface layers enriched with zinc were formed on pure magnesium and AZ91 alloy by heating the specimens in contact with paste containing zinc chloride and potassium chloride at 440 °C for 2 h, using no protective atmosphere. The study involved determining the microstructure, phase composition and microhardness of the resulting layers. During the layer formation process, a transient liquid phase occurred at the substrate/paste interface. The layers fabricated on both substrates were about 200 μm in thickness and they were metallurgically bonded to the substrate material. The same process conditions were used for both types of substrates. From the experimental data it is clear that the layer fabricated on magnesium differed in microstructure from that formed on AZ91. The alloyed layer on magnesium was characterised by dendrites of a solid solution of zinc in magnesium surrounded by a lamellar eutectoid structure composed of an MgZn intermetallic phase and a solid solution of zinc in magnesium. In the layer formed on AZ91, aluminium was detected in all the structural constituents. The layer was composed of a solid solution of zinc and aluminium in magnesium and Mg17(Al, Zn)12 and Mg5Al2Zn2 intermetallic phases. The alloyed layers had much higher hardness than the substrate materials.