Trans. Nonferrous Met. Soc. China 29(2019) 1353−1364 Individual and synergistic effect of gamma alumina (γ-Al 2 O 3 ) and strontium on microstructure and mechanical properties of Al−20Si alloy Mihira ACHARYA, Animesh MANDAL School of Minerals, Metallurgical and Materials Engineering, Indian Institute of Technology Bhubaneswar, Argul 752050, Odisha, India Received 14 September 2018; accepted 12 March 2019 Abstract: An optimized combination of gamma alumina (4 wt.%) and strontium (0.1 wt.%) was incorporated in cast Al−20Si alloy to obtain fine form of silicon. During casting process, the amount of γ-Al 2 O 3 was varied from 0.5−6 wt.% to refine primary Si and Sr was varied from 0.05−0.1 wt.% to modify eutectic Si. The results showed that the average size of primary Si is 24 μm for addition of 4 wt.% γ-Al 2 O 3 to the alloy whereas 0.1 wt.% Sr resulted in sphericity of eutectic Si to ~0.6 and average length of ~1.2 μm. The thermal analysis revealed that γ-Al 2 O 3 can act as potential heterogeneous nucleation sites. Moreover, simultaneous addition of γ-Al 2 O 3 and Sr does not poison γ-Al 2 O 3 particles and inhibit their nucleation efficiency as in the case of combined addition of phosphorous and strontium to Al−20Si alloy. Therefore, it was concluded that enhanced tensile strength, i.e., ultimate tensile strength (increase by 20%) and elongation (increase by 23%) in Al−20Si−4γ-Al 2 O 3 −0.1wt.%Sr alloy as compared to as-cast Al−20Si alloy can be attributed to refinement of primary Si, modification of eutectic Si and the presence of α(Al) in the alloy as evident from eutectic shift. Key words: Al−Si alloy; γ-Al 2 O 3 ; refinement; modification; primary Si 1 Introduction Hypereutectic Al−Si alloys are the preferred choice for aircraft and automobile industry due to their light weight and high specific strength [1]. These properties led to increased fuel economy and thus better vehicular emission standard. They have specific applications for automobile components subjected to continuous wear environment. Besides, hypereutectic Al−Si alloys exhibit excellent properties such as good corrosion resistance, high thermal conductivity [2] and low thermal coefficient of expansion [3]. These properties are regulated by the microstructure of the alloys. The typical microstructure of hypereutectic Al−Si alloy consists of coarse primary Si and relatively fine eutectic phases of eutectic Si and eutectic α(Al). The primary Si exists in various shapes such as star-like, polygonal, feathery types whereas eutectic Si appears in an acicular structure in the microstructure [4]. It is worth noting that the quantity and distribution of these phases in the microstructure dictate the properties of the alloy. The presence of primary Si with sharp edges leads to stress concentration with the application of load during service conditions [5]. The crack initiates near the edges and further propagates resulting in premature failure of the component. Moreover, the size and distribution of phases in hypereutectic Al−Si alloys influence the wear resistance of the alloy [6]. Hence, the refinement of primary Si, as well as eutectic Si, is crucial in achieving enhanced mechanical and tribological properties in such alloys. Several investigations have been carried out to alter the morphology of primary Si in hypereutectic Al−Si alloys by friction stir processing [7], rapid solidification processing, electromagnetic stirring [8] and semi-solid processing [9,10]. Powder metallurgy is also another popular route for incorporating ceramic reinforcements in the alloy and composites. However, the inability to produce intricate shapes limits its industrial applications [11]. The size of the product and complex shape are the primary reasons for not implementing it for mass production. As compared to powder metallurgy, better particle matrix bonding and near net shape can be achieved with liquid metallurgy route [12,13]. The liquid state processing is perceived as the better method for mass production as compared to powder metallurgy Corresponding author: Mihira ACHARYA; Tel : +91-943392133; E-mail: ma10@iitbbs.ac.in DOI: 10.1016/S1003-6326(19)65042-9