This work explores the compound effect of tension twins and compression twins on extruded Mg-5Sn-0.3Li, with an emphasis laid on the microstructure and mechanical properties. To induce both tension twins and compression twins, Mg-5Sn-0.3Li in the cast condition was chosen and subjected to 5% pre-compression. Both the pre-strained sample and unstrained sample were then exposed to extrusion. Our results reveal that pre-strained Mg-5Sn-0.3Li rods exhibit higher overall mechanical properties including yield strength, ultimate strength and elongation to failure in the extrusion condition. The relevant mechanisms were discussed.

•Morphology of Mg2Sn phase is distinct from irregular shape reported elsewhere.•With Ba addition, new BaMg2Sn2 phase forms distributed along the grain boundaries.•The increased Ba content refines the Mg2Sn phase and dendrite arm spacing of α-Mg.•The feather-shaped BaMg2Sn2 phase comprises nanocrystals rather than a monocrystal.This work lays great emphasis on the effect of Ba addition on microstructure evolution of magnesium alloy Mg–5Sn. It is found that the Mg2Sn phase in Mg–5Sn alloy adopts nearly spherical or plate-like morphology, the morphology distinct from the previously reported irregularshape. One thing to be noted is that most of those spherical particles have a distinct defect contrast in the interior. Our results reveal that with trace addition of Ba, a new BaMg2Sn2 phase forms distributed along the grain boundaries, and both the Mg2Sn phase and the secondary dendrite arm spacing of primary α-Mg phase are refined. Even more surprising is that the feather-shaped or needle-like BaMg2Sn2 phase is composed of numerous nanocrystals with a radius less than 50 nm rather than a monocrystal, in contrast to a large extent with Mg2Sn phase, a phase to be only a monocrystal. This unusual phenomenon seems to be greatly tied to the oriented attachment mediated growth mechanism of BaMg2Sn2 phase.

The effects of Sr on the microstructure and mechanical properties of Mg-5Zn-2Al alloy are studied. The results indicate that the as-cast ZA52 alloy mainly consists of α-Mg matrix, τ-Mg32(Al, Zn)49 and MgZn phases. Sr addition results in the precipitation of Al4Sr and suppression of MgZn phase. In high Sr-containing alloys, the Mg5Zn2Al2 phase can also be observed. Both the dendrite and grain size of the alloys can be refined by 1.0% Sr addition. However, the 0.2% Sr addition results in the grain coarsening in the alloy due to the decrease of contribution of Al and Zn solutes on the grain refinement. Both extrusion process and Sr addition give a response in refined microstructure. The yield strength and ultimate tensile strength of extruded alloys are increased, respectively, by about 167 MPa and 135 MPa in compared with the as-cast state. The elongations to failure enhanced by about 1 time, reaches 9.9–16.5%, with a strength of 322–338 MPa. Go along with Sr increase, yield strength increases and elongation decreases gradually in as-extruded alloy. The elongation of all the alloys decreases rapidly when Sr content exceeds 0.5% due to the formation of Mg5Zn2Al2 phase.

The microstructure and mechanical properties of AZ61 magnesium alloy with Ca, Sm addition were investigated. The results showed that the addition of 0.5 wt.% Ca reduced the quantity of Mg17Al12 phase, and formed a new Al4Ca phase which is reticular in AZ61 alloy. With the addition of Ca and Sm, the microstructure was further refined and new Al-Sm rich phases were formed in AZ61 alloy with 0.6 wt.%–1.5 wt.% Sm addition, the TEM analysis confirmed that they were Al2Sm and Al4Sm. Tensile tests showed that 1.0 wt.% Sm addition contributed to the formation of the Al2Sm and Al4Sm and the improvement in the ambient strength, i.e., an ultimate tensile strength of 327 MPa and an elongation of 10.1%. However, excessive Sm addition led to the coarsening of Al2Sm and Al4Sm phases, thus resulted in the decline of strength and plasticity.SEM images of as-cast alloys AZ61 (a) and AZ61-0.5Ca-xSm (x=0, 0.6, 1.0, 1.5) (b–e)

This work explores trace addition of Li on the magnesium alloy Mg–5Sn, with an emphasis laid on the microstructure, mechanical properties and fracture mechanism. The results reveal that with trace addition of Li, a new Li2MgSn phase forms distributed along the grain boundaries. When the concentration of Li increases, the secondary dendrite arm spacing of the primary α-Mg phase can be refined and the formation of Li2MgSn becomes more favored, and even totally substitute Mg2Sn phase finally. Trace addition of Li can significantly increase the yield stress and elongation to failure of Mg–5Sn alloy during both tensile and compressive tests, and manifests the highest ultimate strengthening effect during compression. The optimal comprehensive mechanical property is achieved in Mg–5Sn–0.3Li, with an ultimate compressive strength of 334 MPa. With the increment of Li, the fracture mechanism shifts from mixture of cleavage fracture and dimple fracture in Mg–5Sn alloy to the mechanism of cleavage fracture in Mg–5Sn–0.3Li and Mg–5Sn–1Li alloy.

The microstructure and mechanical properties of AZ91 magnesium with Ca and Sb additives were investigated. Adding 2 wt% Ca refines the microstructure, reduces the amount of the Mg17Al12 phase significantly, and forms a new Al4Ca phase that is reticular in AZ91 magnesium; the tensile strength and elongation properties of the AZ91–2Ca alloys decrease at room temperature (RT). However, with the addition of Ca and Sb, the microstructure is further refined, and a new Ca2Sb phase is formed in AZ91 magnesium with a 0.1%–1.0 wt% Sb addition. Furthermore, the microstructure and mechanical properties of the alloys with the 0.4 wt% Sb addition are found to be optimum.