【作者】 孙明杰；

【导师】 陈体军；

【作者基本信息】 兰州理工大学 ， 材料加工工程， 2012， 硕士

【摘要】 Zn-Al系合金具有良好的铸造性能、优良的耐磨、减摩性能，是替代价格昂贵的铜合金的理想材料。但研究中发现，在一定条件下，其组织出现较为发达的树枝状晶，即为羽毛状晶，会严重消弱力学性能。本文针对羽毛状晶形成的条件，控制一定的温度梯度，研究合金中各元素对羽毛状晶形成的影响，不仅会极大地丰富金属的凝固理论和晶体的生长理论，具有重要的理论价值，而且是消除该形貌晶体，获得细小等轴晶，提高材料力学性能的基础。本文对Zn-Al系合金中出现的羽毛状晶展开研究，分析了以ZA27合金为参考的各元素对Zn-Al系合金生成羽毛状晶的影响，得出如下结果：在Zn-Al二元合金中，对不同组分的各试样，在750℃下浇注，随着Al含量的降低，Zn-Al二元合金的组织从最初的等轴晶粒依次变为发达的等轴晶、柱状晶、羽毛状晶、柱状晶、等轴晶。即晶体的排列由杂乱变为整齐再到杂乱。从羽毛状晶的出现到羽毛状晶的消失，二次枝晶的生长受元素影响，分别按两个、四个、六个方向生长。在Zn-27Al的基础上，加入不同含量的Cu元素。随着Cu含量的增加，Zn-27Al合金的微观组织从粗大的等轴晶依次变为二次枝晶发达的树枝晶、羽毛状晶、二次枝晶发达的柱状晶。Cu相由弥散性分布逐渐变为富集于α-Al枝晶臂上。Cu元素使Zn-27Al合金的组织排列更具方向性，减少了枝晶偏离热流方向的趋向。合金中羽毛状晶的生长产生变化，枝晶主干受到抑制，二次枝晶、三次枝晶得到发展。在以R=30μm/s的凝固速率下，随着Al含量的降低，Zn-Al二元合金的定向凝固试样，一次枝晶间距逐渐减小，二次枝晶间距也逐渐变小；Zn-Al合金定向凝固试样的择优取向明显，随着Al含量的降低，合金组织的枝晶生长方向由Al含量为75%时的<100>晶向逐渐转变为Al含量为40%时的<110>晶向。对ZA27合金在不同浇注温度下的试样的EBSD研究，得出：600℃浇注的ZA27试样中，α-Al的二次枝晶生长方向以<100>为主。700℃浇注的ZA27试样中，α-Al的二次枝晶生长方向以<110>为主。更多还原

【Abstract】 Zn-Al alloys have good casting properties, excellent wear and friction properties, anideal alternative to expensive copper alloy materials. However, the very developed columnardendrites, especially the feathery crystals formed under certain conditions, can seriouslyweaken the mechanical properties, its organization. In this paper, the feathery crystalformation conditions such as temperature gradient, alloying elements have been studied,which will not only enrich metals’ solidification theory and crystal growth theory, but alsoprovide the foundation to eliminate the feathery crystals and improve mechanical propertiesby forming fine equiaxed grains.This thesis has studied the formation mechanisms of the feathery crystals. The effects ofalloying elements of ZA27alloy on this kind of crystals have discussed. The results indicate:For the Zn-Al binary alloys, the grains change from equiaxed grains in turn to columnardendrites, feathery grains, columnar grains and equiaxed dendrites with decreasing the Alcontent. Namely, the grains change from randomly distributed equiaxed dendrites todirectional lined columnar dendrites, and again to randomly distributed equiaxed dendrite.The growth direction of secondary dendritic arm is affected by the alloying elements, havingthree regimes, two, four and six directions.For the Zn-27Al alloy, its microstructure changes from developed equiaxed grains in turnof dendrites with developed secondary dendritic arms, feathery grains and dendrites withdeveloped secondary dendritic arms. The distribution of Cu-rich phase gradually change fromdispersive form to agglomeration on the crystal surfaces. This change leads the arms’ growthdirection to orientate to heat flux direction. When the growth of primary arms is restrainedand those of secondary and third arms are enhanced, the equiaxed grains form.Under the condition of directional solidification (a solidification rate of30μm/s), boththe primary arm interval and secondary arm interval decrease with decreasing Al content ofZn-Al binary alloy. The preferred growth orientation is very obvious. The growth direction ofprimary arms change from <100> to <110> when the Al content decreases to40%.For the ZA27alloy, the growth direction is basically the <100> at the pouringtemperature of600℃, but that is the <110> at700℃.更多还原