Clarifying the adverse effect of secondary phase on the tribological property of Mg alloy

• Mg-Al precipitate can inhibit the formation of mechanical twins in the worn subsurface. • The wear hardening capacity of AZ-Mg alloys decrease with increasing precipitate content. • Loads can amplify the inhibition of precipitate on the formation of mechanical twins. • Tribolayer hardness increases at 3 N but decreases at 15 N with increasing precipitate content. • The negative effect of precipitate on wear resistance of Mg alloys is its inhibition on twins. Introduction of hard particles is considered an effective approach to improve alloy wear resistances. However, the wear resistances of Mg alloys could be deteriorated by increasing the hard particle content in many researches. To reveal the underlying negative effect of precipitate on the wear resistance, the wear behaviors of three AZ-Mg alloys (precipitate contents: AZ31: 2.1 %, AZ61: 3.8 %, AZ91: 5.0 %) at the axial loads of 3 and 15 N were investigated. The results indicated that although wear volume of the AZ-Mg alloys decreased with the increasing Mg 17 Al 12 content at 3 N (0.30→0.24→0.20 µm 3 ) and 15 N (1.04→0.88→0.85 µm 3 ), the relative wear resistances of AZ61 and AZ91 to AZ31 decreased with increasing load (AZ61: 1.25→1.17, AZ91: 1.50→1.22) and the reduction was proportional to the precipitates content (AZ61:7 %, AZ91:28 %). That is because the wear volume of AZ-Mg was mainly attributed to micro-cutting, which was negatively correlated with the precipitate content and tribolayer hardness. However, the wear hardening ability of AZ-Mg alloys was weakened by precipitate for its inhibition on the formation of mechanical twins that the precursors for the tribolayer. Moreover, the inhibition of the precipitate on tribolayer could be amplified by the load, resulting in an increase in tribolayer hardness at 3 N (AZ31: 0.94, AZ61: 1.03, AZ91: 1.10 GPa) but a decrease at 15 N (AZ31: 1.77, AZ61: 1.73, AZ91: 1.62 GPa). Therefore, the formation of twin was inhibited by precipitates, which is detrimental to the wear resistance of Mg alloys. That means the wear resistance could be enhanced by promoting twin formation, which provides a new concept for the design of wear-resistant Mg alloys. Schematic diagram of the adverse effect of secondary phase on the tribological property of Mg alloy