Surface morphology of a die-cast Mg alloy

• Published in: Applied surface science 1999-12, Vol.153 (1), p.47-52
• Main Authors: Unigovski, Ya.B, Gutman, E.M
• Format: Article
• Language: English
• Subjects: Applied sciences; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Die casting; Exact sciences and technology; Magnesium alloy; Materials science; Metals. Metallurgy; Physics; Solid surfaces and solid-solid interfaces; Surface microstructure; Surface structure and topography; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Treatment of materials and its effects on microstructure and properties
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/S0169-4332(99)00337-2

We have studied the microstructure, phase composition and morphology of the intermetallic β-phase in the surface layer of the most widespread magnesium alloy AZ91D (Mg–9 wt.% Al–1 wt.% Zn) used in pressure die casting. Auger electron spectroscopy (AES), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM) and optical microscopy have been applied. The surface of the AZ91D Mg alloy consists of an oxide-metallic film with a thickness of about 0.15 micron containing Mg and Al oxides. Prolonged aging in air at 200°C promotes an increase in the MgO/Al 2O 3 ratio from 9±2 to 14±2. The aluminum concentration gradient in the surface layer is rather significant. The external layer of as-cast specimens contains up to 32 at.% Al, whereas α-Mg grains in the bulk contain, on the average, about 5 at.% Al. Aging leads to a substantial increase in the surface concentration of aluminum at the expense of the acceleration of its diffusion and the intensification of supersaturated solid solution decomposition. During creep tests at elevated temperatures the morphology of the β-phase is significantly affected by strain and, to a lesser extent, by the casting temperature.