In the present investigation, Al–X wt-% Mg2Si (X = 0, 5, 10, 15 and 20) in situ composites are successfully synthesised by one-step gravity casting technique. Commercially pure Al, Mg and Si are used as raw materials. Microstructural evaluation and correlation of micro- and bulk hardness properties have been studied on developing composites. The composites consist of mainly three phases: matrix (α-Al), reinforcing (primary Mg2Si) and binary eutectic (Al–Mg2Si) phase. Primary Mg2Si particles are formed by pseudo-eutectic transformation during solidification and surrounded by matrix and binary eutectic phase. It is found that Mg2Si concentration has a significant impact on morphology and volume per cent of the above-mentioned phases. Primary Mg2Si particles’ size and volume per cent increase with increasing wt-% of Mg2Si. Volume per cent of individual phases and Mg2Si concentration have great impact on hardness properties of composites. Bulk hardness increases with increasing wt-% of Mg2Si concentration, but micro-hardness of primary Mg2Si particle decreases slightly. Mg2Si concentration also has significant impact on micro-hardness of individual phases
