摘要
羽毛状晶是完全不同于常见柱状晶与树枝晶的一种异常晶粒。对于羽毛状晶生成条件及空间形貌的研究,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,而且对消除该晶体,获得细小等轴晶,提高材料力学性能,具有重要的应用价值。本文对Zn-Al系合金中出现的羽毛状晶展开研究,分析了铸造工艺参数及Si、P元素对ZA27合金微观组织的影响,研究了ZA27合金中羽毛状晶的形成条件、空间形貌。结果表明:
浇注温度、铸型激冷能力、试样尺寸对ZA27合金中初生a-A1的晶体形貌具有很大影响,其中浇注温度的影响最大。在较高的浇注温度(大于650℃)时,极易形成羽毛状晶。而且铸型激冷能力越大、尺寸锭料越小,越易形成羽毛状晶。试样的边缘处比中心处更容易形成羽毛状晶。这些不同的工艺参数,都反映了温度梯度在凝固中的作用:温度梯度越大,越容易形成羽毛状晶。
Si元素的加入减弱了羽毛状晶的形成,并随着Si含量的增大,其影响越来越大。初生Si作为先析出相悬浮在液相中,限制枝晶的生长。而经P变质后,因初生Si颗粒变小,对枝晶生长的阻碍作用减弱,羽毛状晶倾向加强。在同一浇注温度下,羽毛状晶形成的倾向由强到弱的顺序为:ZA27>ZA27-Si-P >ZA27-Si。
羽毛状晶在二维的金相上表现为长径比很大的柱状树枝晶,而在空间形貌上,羽毛状晶是一种非常发达的树枝晶,整体呈三维结构,枝晶朝多个方向生长;局部并排生长。羽毛状晶由主干和六个二次枝晶组成,其生长方向均为<110>方向。孪晶主干在(111)孪晶面内沿着<110>方向生长,一对二次枝晶位于孪晶面(111)上且对称生长,与主轴呈60°夹角。在孪晶面的两侧,一对二次枝晶臂与主轴呈60°夹角,关于孪晶面对称生长且与孪晶面夹角为70.5°。另外一对二次枝晶臂与主轴呈90°夹角,关于孪晶面对称生长且与孪晶面夹角为54.7°。羽毛状晶的形成是晶体学择优生长方向与外力综合作用的结果。另外,有时还会出现二维的羽毛状晶,它是羽毛状晶的一种特殊形态,由许多二维的层片有规律的排列而成。这是由于在生长过程中,其余的四个二次枝晶臂受到邻侧枝晶及空间的影响,逐渐演变为二维层片状。
Feathery dendrite is an unusual grain and it is completely different from the common columnar crystal and dendrite. It will not only greatly enrich the theories of metal solidification and crystal growth, but also has great significance for eliminating the feathery grains and obtaining fine grains. In this thesis, the formation conditions and morphologies of the feathery crystals in ZA27 alloy were studied by analyzing the effects of casting parameters and Si and P elements on the microstructures. The results are shown as followings:
The pouring temperature, chilled ability and casting ingot size have large effects on the morphologies of a-Al grains in the Zn-Al alloy. It was found that pouring temperature has greatest effect. The feathery dendrites were easy to form when the pouring temperature was at or above 650℃. The greater the chilled ability and the smaller the ingot size, the easier the feathery crystal formation. In addition, feathery dendrites in the edge of the cast specimen were easier to for than those in the center. Although the process parameters were different, all of the them reflected the role of temperature gradient during solidification:the greater the temperature gradient, the easier the feathery crystal formation.
The addition of Si element suppressed the formation of feathery crystals, and as the content of Si increased, the influence was enhanced. The primary Si plates suspended in the liquid and obstructed the growth of the dendrites. However, after being modified by P element, the primary Si particles became smaller and more dispersive, and the hindering effect on the dendrite growth was decreased. At the same pouring temperature, the formation tendancy of feathery dendrites became weak as the sequence of ZA27, ZA27-Si-P and ZA27-Si.
The feathery dendrite was in columnar dendrite morphologies with large ratio of length to diameter on two-dimension micrographs. But in three-dimension space, they were in three-dimensional developed dendrite morphologies with multiple-driection branches and in laminar structures in local zones. The feathery dendrite was composed of one primary trunk and six rows of secondary arms and all of them grew along<110> directions. The dendrite trunk grew along a<110> direction in the (111) twin plane and a pair of the secondary arms was in the (111) twin plane and symmetrically grew at 60°with respect to the trunk direction. At the both sides of the twinning plane, another pair of the secondary arms was at 60°with respect to the trunk direction and symmetrically grew at 70.5°with respect to the twin plane. The rest pair of the secondary arms was at 90°with respect to the trunk direction and symmetrically grew at 54.7°with respect to the twin plane.The formation of the feathery dendrites was attributed to the combined results of the preferred crystallographic growth direction and external forces. In addition,2-dimension feathery dendrites sometimes could be found and they should be a kind of special morphologies of feathery dendrites. They were composed of many parallel laminae.
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