磁场下新型钴基高温合金Co-Al-W定向凝固组织的研究

发布时间：2018-05-07 14:29

本文选题：稳恒磁场 + 定向凝固　；参考：《江苏科技大学》2017年硕士论文

【摘要】：近十年,Co-Al-W基合金中发现的高温稳定的γ′相Co3(Al,W)成为国内外学者的研究热点。本文以Co-8.8Al-9.8W和Co-8.8Al-9.8W-2Ta两种合金为研究对象,在稳恒磁场下,进行定向凝固实验。研究了定向凝固显微组织的变化,考察了纵向和横向稳恒磁场对凝固过程中固-液界面组织和宏观偏析的影响。在Co-8.8Al-9.8W合金定向凝固过程中,施加纵向稳恒磁场导致固-液界面形貌和枝晶生长发生变化。在抽拉速率较低的情况下(R≤10um/s),施加小于等于1T的纵向磁场后,发现随着磁场强度的增大,固-液界面平稳生长,枝晶变得粗大,数目减少。当磁场强度继续增加时,枝晶数目增多,枝晶细化。这是由于磁场强度较小时,热电磁流动改变了界面前沿溶质分布,二次枝晶发达。当磁场强度继续增加时,磁阻尼占主导地位,抑制了热电磁对流,二次枝晶被抑制,一次枝晶间距变小。而Co-8.8Al-9.8W-2Ta合金定向凝固过程中,在低抽拉速率的情况下,随着磁场强度的增加枝晶开始变形,枝晶由稳定生长的柱状晶变得杂乱无章,产生的热电磁对流使枝晶破碎断裂。这是因为增加的Ta元素是正偏析元素,加剧了合金凝固过程中的偏析行为,所以固-液界面不同于三元的Co-Al-W合金。当抽拉速率较大(R≥50um/s)时,磁场对两种合金的固-液界面几乎没有影响,一次枝晶间距与低拉速下规律一致,随着磁场强度的增加先变大后变小。对于纵向磁场下的偏析行为,两种合金基本一致。磁场对两种合金中Al元素的偏析存在一定的促进作用,两种合金在磁场下的偏析行为主要以Al元素为主,而且从实验结果可以看出随着固相比例的提高而溶质Al含量也在增加。而W元素的偏析,磁场对其影响较小。横向稳恒磁场对Co-8.8Al-9.8W合金定向凝固组织及界面形态有着不同的影响。在定向凝固Co-8.8Al-9.8W合金的过程中施加横向稳恒磁场,发现在低拉速的情况下(R=5um/s),固-液界面向试样的右侧凹陷并在右侧产生溶质Al的偏聚,而且在试样的右侧出现斑状组织。导致此原因是由于合金固-液界面处试样尺寸的宏观热电磁流动(TEMCmac)和枝晶尺寸的微观热电磁流动(TEMCmic)的耦合作用驱动溶质迁移所致。作者进一步分析了横向磁场对定向凝固过程中一次枝晶间距的影响,发现增大磁场强度可以使一次枝晶间距减小。这是因为在施加磁场的过程中,TEMCmic随着磁场强度的增大而不断增大,使得枝晶间熔体流动不断加强,一次枝晶间距不断变小。当拉速增加到50um/s时,磁场对合金固-液界面几乎没有影响,一次枝晶间距随着磁场强度的增加也没有得到细化,这是因为拉速较快横向磁场导致的枝晶间热电磁流动TEMCmic作用时间变短,所以作用效果变低。
[Abstract]:In recent ten years, the stable 纬 '-phase Co _ 3O _ 3 Al _ (W) found in Co-Al-W alloy has become a hot research topic at home and abroad. In this paper, the directional solidification experiments of Co-8.8Al-9.8W and Co-8.8Al-9.8W-2Ta alloys are carried out in a steady magnetic field. The changes of directional solidification microstructure were studied and the effects of longitudinal and transverse steady magnetic fields on the microstructure and macro segregation of solid-liquid interface during solidification were investigated. During directional solidification of Co-8.8Al-9.8W alloy, the morphology of solid-liquid interface and dendritic growth are changed by applying longitudinal steady magnetic field. When the pulling rate is lower than 10um / s, and the longitudinal magnetic field less than 1T is applied, it is found that with the increase of the magnetic field intensity, the solid-liquid interface grows steadily, the dendrite becomes coarse and the number decreases. When the magnetic field intensity continues to increase, the number of dendrite increases and the dendrite becomes fine. This is due to the fact that the distribution of solute at the front of the interface is changed by the thermo-electromagnetic flow when the magnetic field is small, and the secondary dendrite is developed. When the magnetic field intensity continues to increase, the magnetic damping dominates, and the thermal electromagnetic convection is restrained, the secondary dendrite is suppressed and the primary dendrite spacing is reduced. In the process of directional solidification of Co-8.8Al-9.8W-2Ta alloy, with the increase of magnetic field intensity, the dendrite begins to deform, and the dendrite changes from a stably growing columnar crystal to a chaotic one, and the thermal electromagnetic convection results in the breakup of the dendrite. This is because the added Ta element is a positive segregation element, which intensifies the segregation behavior during solidification of the alloy, so the solid-liquid interface is different from the ternary Co-Al-W alloy. When the pulling rate is larger than 50 um / s, the magnetic field has little effect on the solid-liquid interface of the two alloys. The primary dendrite spacing is consistent with the law at low drawing speed, and increases first and then decreases with the increase of magnetic field intensity. The segregation behavior of the two alloys under longitudinal magnetic field is basically the same. The magnetic field can promote the segregation of Al in the two alloys. The segregation behavior of the two alloys under the magnetic field is mainly Al, and the experimental results show that the content of solute Al increases with the increase of solid phase ratio. The magnetic field has little effect on the segregation of W element. The transverse steady magnetic field has different effects on the microstructure and interface morphology of Co-8.8Al-9.8W alloy. In the process of directional solidification of Co-8.8Al-9.8W alloy, the transverse steady magnetic field was applied, and it was found that at low drawing speed, the solid-liquid interface was depressed to the right side of the sample and the solute Al was segregated on the right side, and the porphyry structure appeared on the right side of the sample. This is due to the coupling effect of the macroscopical thermo-electromagnetic flow (TEMCmac) on the sample size at the solid-liquid interface of the alloy and the micro-thermo-electromagnetic flow (TEMCmic) at the dendritic size to drive the solute transport. The influence of transverse magnetic field on the primary dendrite spacing during directional solidification is further analyzed. It is found that increasing the magnetic field intensity can reduce the primary dendrite spacing. This is because the TEMCmic increases with the increase of magnetic field intensity, which makes the melt flow between dendrites become stronger and the primary dendrite spacing becomes smaller. When the drawing speed is increased to 50um/s, the magnetic field has little effect on the solid-liquid interface of the alloy, and the primary dendrite spacing is not refined with the increase of the magnetic field intensity. This is because the TEMCmic action time of the interdendritic thermoelectromagnetic flow is shorter due to the fast drawing speed and the transverse magnetic field, so the effect is low.
【学位授予单位】：江苏科技大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG132.3

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