Effect of heat treatment and isothermal holding has been investigated on the microstructure and degree of globularity of Al–Mg2Si composites. Different contents of reinforcement, 15, 20, and 25% have been used in this study. Isothermal holding experiments were conducted at 585？°C for 140？min. Results showed that, upon heat treatment, grain size of dendrites was reduced while the degree of nodularity was increased. Results of nodularity were obtained using an image-analyzing software which gives the distribution of radius of curvature for different phases of particles. According to the results, in contrast to Al–15 and 25% Mg2Si, isothermal holding significantly influenced the microstructure of Al–20% Mg2Si composites. Two companion mechanisms have been proposed for the generation of globular grains. SEM investigations were also employed to confirm the optical observations. 1. Introduction Superior mechanical properties, as well as increased possibility to control the production process have been the main challenge with which manufacturing of engineering materials is faced. Semisolid processing is claimed to be one of the best approaches which can fulfill such properties. The aim of the semisolid processing is to achieve a fine globular structure. Thus, it is clear that controlling the microstructure of semisolid material is of great importance [1–3]. Possessing a fine globular (nondendritic) microstructure which can be obtained through semisolid-forming processes yields superior advantages over conventional casting and solid-forming methods among which one can mention high quality components capable of full-heat treatment to maximize properties, reduction of macrosegregation, solidification shrinkage, and forming temperature [1–4]. Different methods are classified in semisolid-processing techniques such as magnetohydrodynamic (MHD) stirring [7], spray forming [8], strain-induced melt-activated (SIMA)/recrystallization and partial melting (RAP) [9, 10], and liquidus/near-liquidus casting [11]. The key feature that permits the shaping of alloys in the semisolid state is the absence of dendrites [1, 4]. Hence, partial remelting and isothermal holding recently have been widely used specially for Al–Mg2Si composites which are also the subject of the current study. Partial remelting and isothermal holding provide a minimum total interfacial energy which is a key factor among composite materials [1, 12]. The Mg2Si-reinforced Al composites are great potential as automobile brake disc material because the Mg2Si intermetallic compound exhibits a high melting
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