Microstructural evolution and mechanical strengthening mechanism of Mg-3Sn-1Mn-1La alloy after heat treatments

The Mg-3Sn-1Mn-1La alloy sheets were prepared by a continuous rheo-rolling process, and the effects of the solution and aging treatment on the microstructures and mechanical properties of the alloy were studied. The tensile strength and elongation at room temperature and 150 °C of the Mg-3Sn-1Mn-1La alloy sheets were decreased with increasing the solution time. The grain size was increased gradually. The plate-shaped MgSnLa compounds composed of La5Sn3, Mg2Sn and Mg17La2 phases and Mg2Sn phase gradually disappeared. At the same time, new irregular MgSnLa compounds were formed in grains. Aging treatment of the alloy was performed after solution treatment. The new spherical MgSnLa compounds composed of La5Sn3, Mg2Sn and Mg17La2 phase were formed, increased and distributed gradually homogeneously in the matrix with increasing the aging treatment time. Meanwhile, the Mg2Sn phases with spherical, short-rod and lath-shaped were formed in the matrix. After solution treatment at 550 °C for 24 h, aging at 250 °C for 48 h, the tensile strength at room temperature and at 150 °C of the Mg-3Sn-1Mn-1La alloy sheet was increased to 270 MPa and 233 MPa, with 19% and 20% increase, respectively. The elongation at room temperature and 150 °C was increased to 7.9% and 67.5%, with 6% and 4% increase, respectively. The formed spherical MgSnLa compounds and Mg2Sn phases which were coherent with the matrix and effectively pinned the dislocations and grain boundaries were responsible for the enhanced mechanical properties of the alloys.