發(fā)布時(shí)間：2018-05-07 14:29

  本文選題：穩(wěn)恒磁場 + 定向凝固　； 參考：《江蘇科技大學(xué)》2017年碩士論文

【摘要】：近十年,Co-Al-W基合金中發(fā)現(xiàn)的高溫穩(wěn)定的γ′相Co3(Al,W)成為國內(nèi)外學(xué)者的研究熱點(diǎn)。本文以Co-8.8Al-9.8W和Co-8.8Al-9.8W-2Ta兩種合金為研究對象,在穩(wěn)恒磁場下,進(jìn)行定向凝固實(shí)驗(yàn)。研究了定向凝固顯微組織的變化,考察了縱向和橫向穩(wěn)恒磁場對凝固過程中固-液界面組織和宏觀偏析的影響。在Co-8.8Al-9.8W合金定向凝固過程中,施加縱向穩(wěn)恒磁場導(dǎo)致固-液界面形貌和枝晶生長發(fā)生變化。在抽拉速率較低的情況下(R≤10um/s),施加小于等于1T的縱向磁場后,發(fā)現(xiàn)隨著磁場強(qiáng)度的增大,固-液界面平穩(wěn)生長,枝晶變得粗大,數(shù)目減少。當(dāng)磁場強(qiáng)度繼續(xù)增加時(shí),枝晶數(shù)目增多,枝晶細(xì)化。這是由于磁場強(qiáng)度較小時(shí),熱電磁流動(dòng)改變了界面前沿溶質(zhì)分布,二次枝晶發(fā)達(dá)。當(dāng)磁場強(qiáng)度繼續(xù)增加時(shí),磁阻尼占主導(dǎo)地位,抑制了熱電磁對流,二次枝晶被抑制,一次枝晶間距變小。而Co-8.8Al-9.8W-2Ta合金定向凝固過程中,在低抽拉速率的情況下,隨著磁場強(qiáng)度的增加枝晶開始變形,枝晶由穩(wěn)定生長的柱狀晶變得雜亂無章,產(chǎn)生的熱電磁對流使枝晶破碎斷裂。這是因?yàn)樵黾拥腡a元素是正偏析元素,加劇了合金凝固過程中的偏析行為,所以固-液界面不同于三元的Co-Al-W合金。當(dāng)抽拉速率較大(R≥50um/s)時(shí),磁場對兩種合金的固-液界面幾乎沒有影響,一次枝晶間距與低拉速下規(guī)律一致,隨著磁場強(qiáng)度的增加先變大后變小。對于縱向磁場下的偏析行為,兩種合金基本一致。磁場對兩種合金中Al元素的偏析存在一定的促進(jìn)作用,兩種合金在磁場下的偏析行為主要以Al元素為主,而且從實(shí)驗(yàn)結(jié)果可以看出隨著固相比例的提高而溶質(zhì)Al含量也在增加。而W元素的偏析,磁場對其影響較小。橫向穩(wěn)恒磁場對Co-8.8Al-9.8W合金定向凝固組織及界面形態(tài)有著不同的影響。在定向凝固Co-8.8Al-9.8W合金的過程中施加橫向穩(wěn)恒磁場,發(fā)現(xiàn)在低拉速的情況下(R=5um/s),固-液界面向試樣的右側(cè)凹陷并在右側(cè)產(chǎn)生溶質(zhì)Al的偏聚,而且在試樣的右側(cè)出現(xiàn)斑狀組織。導(dǎo)致此原因是由于合金固-液界面處試樣尺寸的宏觀熱電磁流動(dòng)(TEMCmac)和枝晶尺寸的微觀熱電磁流動(dòng)(TEMCmic)的耦合作用驅(qū)動(dòng)溶質(zhì)遷移所致。作者進(jìn)一步分析了橫向磁場對定向凝固過程中一次枝晶間距的影響,發(fā)現(xiàn)增大磁場強(qiáng)度可以使一次枝晶間距減小。這是因?yàn)樵谑┘哟艌龅倪^程中,TEMCmic隨著磁場強(qiáng)度的增大而不斷增大,使得枝晶間熔體流動(dòng)不斷加強(qiáng),一次枝晶間距不斷變小。當(dāng)拉速增加到50um/s時(shí),磁場對合金固-液界面幾乎沒有影響,一次枝晶間距隨著磁場強(qiáng)度的增加也沒有得到細(xì)化,這是因?yàn)槔佥^快橫向磁場導(dǎo)致的枝晶間熱電磁流動(dòng)TEMCmic作用時(shí)間變短,所以作用效果變低。
[Abstract]:In recent ten years, the stable 緯 '-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.
【學(xué)位授予單位】：江蘇科技大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：TG132.3

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本文編號：1857289