Microstructural evolution during high-temperature exposure in a thixocast magnesium alloy

The microstructure of a thixoformed AZ91 (Mg-Al-Zn system) alloy was investigated by different techniques after various heat-treatment conditions. The material. Supplied in the as-thixocast state, consisted of large v. globules separated by a quasi-eutectic phase (alpha + beta). The alloy was exposed to high temperature (395, 415 and 435 degreesC) for different times to assess its response. Optical metallography showed that the alpha-Mg-based areas developed into individual grains, whereas beta-phase (Mg17Al12) particles were present only in the eutectic area of the base material. The eutectic was found on grain boundaries and alpha-Mg-based grains became enriched in aluminium and zinc due to simultaneous beta-phase dissolution and reduction in the amount of eutectic quantity. Static precipitation and/or dissolution were studied at the three treatment temperatures by TEM. A comprehensive study of precipitate evolution at temperatures very close to the solution is relevant because this thixocast magnesium alloy exhibits a good combination of high-temperature tensile strength and ductility in that temperature range. (C) 2002 Elsevier Science B.V. All rights reserved.