Directional Solidification of Mg-Al Alloys and Microsegregation Study of Mg Alloys AZ31 and AM50: Part I. Methodology

An appropriate method for the directional solidification (DS) of magnesium alloys AZ31 and AM50 was developed to enable the investigation of microstructures and microsegregation under controlled solidification conditions. These liquid alloys, containing both magnesium and aluminum, are highly reactive and attack both standard ceramic and metallic container materials. An integrated approach is developed that is comprised of the dedicated boron nitride (BN) protection of selected steel tube material and limitation of the melt temperature, with the retention of sufficiently high thermal gradients for DS. The well-known Scheil method for the thermodynamic calculation of segregation is extended to reflect the solute profiles of components in all precipitating phases. This Scheil-total profile is more realistic; its integral reflects the bulk composition of a multiphase alloy. It also predicts the primary crystallizing Al-Mn intermetallic phase experimentally detected in the microstructure. This prediction is also confirmed by the application of an advanced processing of quantitative electron probe microanalysis (EPMA) mapping data.