Tribo-induced surface deformation mechanisms govern friction and wear in ultra-light HCP and duplex Mg-Li alloys

Adding lithium into Mg can activate more deformation modes in both hexagonal close packed (hcp) and body centered cubic (bcc) Mg-Li alloys. In this work, friction and wear tests were performed on coarse-grained Mg-xLi-3Al-3Zn (x = 4 and 10 wt %) alloys and detailed surface deformation mechanisms were uncovered. Compared with the duplex Mg-10Li alloy, the hcp Mg-4Li shows a 40% lower friction coefficient and a significantly smaller wear rate by nearly one order of magnitude. The low wear rate stems from formation of hard and stable nanograined surface layer through rapid grain refinement of the hcp grains upon sliding. While for the dual -phase alloy, dynamic recrystallization (DRX) dominates the deformation of bcc grains, unavoidably causing its cracking and delamination within the ultra-fine grained DRX layer. Our results are suggestive in designing wear resistant ultra-light Mg-Li alloys through in-depth understanding of tribo-induced surface deformation mechanisms.