Quantitative characterization of Mg17Al12 phase and grain size in HPDC AZ91 magnesium alloy

Quantitative characterization of size and spatial arrangement of beta phase particles (beta-PP) and grain size by the developed image processing algorithm was studied in this paper. This is expected to be useful to understand the processing-property-microstructure correlation of high-pressure die cast (HPDC) AZ91 alloy. The important parameters of image processing like montage creation, separation of different phases and grains and the characterization procedure are explained. The microstructural quantities such as average size, average nearest neighbour distance (NND), area fraction, grain size and clustering tendency (CT) of beta-PP obtained as a function of distance from surface to centre of the casting. The beta phase particles are more in number and finer in the surface region compared to the centre of the casting, the average particles size in centre region is one and half times of surface region. There is no considerable change in the area fraction of beta phase in different regions throughout the casting and this is a contradictory conclusion from the previous understanding. A minor clustering nature of beta-PP is observed in the surface region and the random nature is increasing from the surface to centre of the casting. In addition the grains are very fine in the surface region compared to the centre of the castings. (C) 2007 Elsevier B.V. All rights reserved.