Strengthening mechanisms of semi-coherent boundaries between Al8Mn4Y and the Mg matrix in magnesium alloys

Cracking at phase or grain boundaries has been demonstrated as the dominant failure mode in Mg alloys, emphasizing the necessity for boundary enhancement to improve both the strength and ductility of the alloy. However, producing strongly bonded twin boundaries in Mg alloys is challenging. Therefore, a novel method to enhance the bonding between intermetallics and the matrix was proposed in this work by forming semi-coherent boundaries. Utilizing transmission electron microscopy (TEM), a previously unexplored intermetallic compound (IMC), Al8Mn4Y, has been identified and systematically investigated. The orientation relationship between lattices of Al8Mn4Y and Mg matrix revealed that the angle between {0 1‾ 0}Al8Mn4Y and {000 1‾}α-Mg planes is only 3.9°. Furthermore, typical lattice mismatches were observed in every four Al8Mn4Y lattices along the <010>Al8Mn4Y direction at the bonding interface, demonstrating their semi-coherent relationship. Subsequently, the mechanical properties of Al8Mn4Y were measured using nanoindentation, and the corresponding impact of Y-containing intermetallics on the mechanical properties of the alloy was also evaluated through tensile tests and fracture analyses. The results revealed that the refinement of conventional intermetallics originally present in the alloy, in conjunction with the newly generated Y-containing particles and the resulting semi-coherent boundaries following the addition of Y, effectively alleviated the stress concentrations and reduced cracking at phase and grain boundaries, contributing to the simultaneous enhancement of both strength and ductility in Mg alloys. Ultimately, the effects of Y on the elastic and plastic deformation of Mg alloys were quantified using four individual contributions, and the mechanisms responsible for the concurrent augmentation of strength and ductility were elucidated, offering valuable insights into the design strategies for future Mg alloys.