Secondly, it has been indicated tha

Secondly, it has been indicated that broken W-phase has essentially strengthening effect on as-extruded Mg-Zn-Er alloy [20]. Fine W-phase particles exert positive effects on strengthening by effectively retarding the dislocation movement during the tensile test which contributes to the enhanced strength. In this work, a small amount of Er addition lead to formation of fine W-phase that can act as second phase strengthening. However, too much Er addition results in more W-phase which gradually becomes the dominating secondary phase in the LZ105-xEr alloy and is detrimental to the strength of LZ105-xEr alloy. Because too much Er in LAZ105-xEr alloys is likely to cause aggregation of the Wphase which will lead to stress concentration and ultimately fracture initiation at the interfaces between W-phase and matrix during tensile deformation. Thus it is possible that not all second phase precipitates formed contribute to the entire strengthening effects of LZ105-xEr alloys. Not only volume fraction, but also the size, distribution and morphology of precipitates play an important role on mechanical behavior. When the size and volume fraction of W-phase increase to a certain extent, W-phase will be obviously coarsened at the grain boundaries and it is harmful to the strength. Normally, the embedment of the coarse second phase particle into the ductile metal matrix can simultaneously induce high stress concentrations at the interface of particle/matrix that result in low EL. While the EL of LZ105-3.5Er alloy with coarse W-phase is still higher than that of LZ105 alloy because the negative effect of large W-phase particles on the EL is compensated by the coordinating deformation of finer β-Li phase with more Er addition. Thirdly, with Er addition, the Mg-Zn-Er phase is formed that also means the solid solution strengthening effect caused by Zn element is weaker because more Zn is consumed on forming second phase. Thus, there exists a balance between the fine Mg-Zn-Er phase strengthening and solid solution strengthening of Zn element. A suitable amount of Er content can let these two reinforcement ways comprehensively work to the maximum for LZ105 alloy.Thirdly, with Er addition, the Mg-Zn-Er phase is formed that also means the solid solution strengthening effect caused by Zn element is weaker because more Zn is consumed on forming second phase. Thus, there exists a balance between the fine Mg-Zn-Er phase strengthening and solid solution strengthening of Zn element. A suitable amount of Er content can let these two reinforcement ways comprehensively work to the maximum for LZ105 alloy.