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In this work, the influence of element Y on the mechanical properties of the as-cast Mg-5.5Zn-xY-0.8Zr alloys (with element Y content of 0, 1.08, 1.97 and 3.08 wt.%) has been investigated and compared. The results showed that the alloy with Y content of 1.08 wt.% had the superior tensile strength, but its ductility was the lowest. When Y content reached 1.97 or 3.08 wt.%, the tensile strength of the alloys decreased obviously, but the ductility had a little improvement. X-ray phase analysis showed that when Y content was 1.08 wt.%, the alloy mainly contained I-phase and alpha-Mg matrix, whereas when Y content was 1.97 or 3.08 wt.%, besides I-phase and alpha-Mg matrix, W-phase would be formed. And the diffraction peak of W-phase intensified with the increase of element Y content. Based on fracture observations, the influence of I-phase and W-phase on the mechanical properties of the alloys has been explained, respectively. It revealed that I-phase was closely bonded with the Mg matrix and could effectively retard the basal slip, and then strengthened the alloy greatly. However, W-phase was easily cracked during the tensile process, which degraded the mechanical properties. (c) 2006 Elsevier B.V. All rights reserved.