In this study, multi-pass friction stir processing (FSP) was used to produce magnesium (Mg) matrix composites reinforced with CoCrFeNiMoTi high entropy alloy (HEA) particles. The effect of the HEA reinforcements on microstructure evaluation, mechanical properties (hardness and tensile) and wear performance was investigated. The composites were produced with similar pass number (three passes) and tool rotation speed (1250 rpm) and different tool transverse speeds (25 and 50 mm/min) and compared with the Mg matrix without reinforcing particles. XRD results indicated no significant reaction at the interface of the HEA and the matrix during FSP. Microstructure results showed the distribution improvement of the HEA reinforcements and elimination of lamellar structure (inhomogeneous structure) with the decrease of the tool transverse speed from 50 to 25 mm/min. Microstructures of all specimens illustrated four microstructural regions including stirring zone, thermo-mechanical affected zone, heat-affected zone and base metal. The average grain size of the stir zone reduced significantly from 12 µm for the Mg-50 to 2.5 µm and 1.6 µm for the Mg/HEAs-50 and Mg/HEAs-25 composites. The hardness value was increased from 34 HV for the base alloy to 113 HV for the FSPed composite at 25 mm/min (232 % improvement). The maximum yield and tensile strengths were obtained as 86.4 MPa and 114 MPa for the FSPed composite at 25 mm/min which presented enhancement of 92 % and 43 %, respectively as compared with the FSPed Mg alloy. The wear resistance results showed the reduction of the friction coefficient and wear rate with the addition of the HEA reinforcements with a considerable decrease in wear debris, groove depth, micro-cracks and delamination. Comparative plots confirmed that the hardening and strengthening effects of the HEA reinforcements were significantly higher than other reinforcements in Mg matrix composites.

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