The Zr element is one of the important grain refiners for 7xxx series Al-Zn-Mg-Cu alloys, but the effect of Zr content more than 0.15 wt.% needs to be deeply investigated under the action of ultrasonic vibration. In this study, the effects of Zr contents (0.1 to 0.25 wt.%) on microstructure and mechanical properties of Al-Zn-Mg-Cu alloy were studied. The results showed that Zr element could refine grains, but when the Zr content was greater than 0.15 wt.%, the grain size was not uniform, the number of second phase particles increased, and the segregation of components became more serious. It was found that after ultrasonic treatment, the grain-size inhomogeneity was greatly improved, and the Zr content could be added up to 0.2 wt.%. When the Zr content is equal or lower than 0.2 wt.%, ultrasonic treatment can effectively improve the mechanical properties of materials by refining grains and weakening segregation. However, when the Zr content is up to 0.25 wt.%, the effect is getting worse.

中文翻译：

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Zr含量过多和超声处理对Al-Zn-Mg-Cu合金显微组织和力学性能的影响

Zr元素是7xxx系列Al-Zn-Mg-Cu合金的重要晶粒细化剂之一，但是在超声振动的作用下，需要深入研究Zr含量超过0.15wt。％的影响。在这项研究中，研究了Zr含量（0.1至0.25 wt。％）对Al-Zn-Mg-Cu合金的组织和力学性能的影响。结果表明，Zr元素可以细化晶粒，但是当Zr含量大于0.15 wt。％时，晶粒尺寸不均匀，第二相颗粒的数量增加，组分的偏析更加严重。发现在超声处理之后，晶粒尺寸不均匀性得到极大改善，并且Zr含量可以添加至0.2wt。％。当Zr含量等于或小于0.2重量％时，超声处理可以通过细化晶粒和弱化偏析来有效改善材料的机械性能。然而，当Zr含量高达0.25重量％时，效果变得更差。