【作者】 吴强；

【导师】 司乃潮；

【作者基本信息】 江苏大学 ， 材料加工工程， 2007， 硕士

【摘要】 目前国内外对于定向凝固组织的影响因素的研究一般集中在定性分析,关于定向凝固组织与可控凝固参数关系的定量研究极少。在前人研究的基础上,本文分析了定向凝固中的主要影响因素,研究定向凝固Al-4.5%Cu合金组织的演变规律;着重研究了抽拉速率与枝晶间距的定量关系,建立了界面局域平衡条件下定向凝固枝晶间距与抽拉速率的理论模型;并对不同抽拉速率的定向凝固Al-4.5%Cu合金的力学性能及断裂机理进行了比较研究。本文用下拉式定向凝固设备,当抽拉速率为20～220μm/s时,制备了一系列定向凝固Al-4.5%Cu合金试样。首先研究了微观组织演变,特别是一次枝晶间距变化规律。结果表明:定向凝固组织具有明显的方向性,晶粒沿固液界面法线方向生长,偏离轴向生长的晶粒被淘汰。在特定温度梯度下,抽拉速率由小到大变化时,微观组织由粗大低速胞状晶、胞枝晶混合组织、树枝状晶依次变化;抽拉速率增大使组织细化,小于100μm/s时,枝晶间距减小幅度较大,大于100μm/s时,减小幅度较为平缓。本文着重研究了抽拉速率对定向凝固组织性能的影响,根据Hunt模型和Bower的理论,建立界面局域平衡条件下预测定向凝固次枝晶间距的理论模型。当抽拉速率为100～220μm/s时,一次枝晶间距的理论模型计算值与实测值相吻合,误差小于6.5%。本模型可为预选定向凝固工艺参数及组织分析提供参考。本文通过SEM原位拉伸实验及抗拉强度、延伸率、硬度等力学性能检测,进一步研究了定向凝固Al-4.5%Cu合金的断裂机理和抽拉速率对其基本力学性能的影响。结果表明,抽拉速率为U=200μm/s的试样B与抽拉速率为U=60μm/s的试样A相比,抗拉强度提高16.7%,同时延伸率提高85.9%,硬度提高67.5%;随着抽拉速率的增加,合金的断裂机理由准解理断裂机理转变为延性断裂机理,说明抽拉速率对定向凝固Al-4.5%Cu合金的断裂机理和基本力学性能影响显著。更多还原

【Abstract】 There is plenty of investigation on the effect factors of unidirectional solidification in home and abroad. However, they are basically qualitative analysis. At present there isn’t investigation on quantitative analysis between microstructure and controllable parameters. On the groundwork of former researches, we have investigated the development of microstructures, especially dentritic spacing; put emphasis on the studies of quantitative relationship between primary arm spacing of Al-4.5%Cu Alloy and withdrawal rate during unidirectional solidification, which make a theoretical model founded under the condition of local equilibrium of interface and furtherly researched the fracture mechanism and mechanical properties of experimental alloy.A series of Al-4.5%Cu Alloy samples are prepared by the use of drawing crucible unidirectional solidification device, when the withdrawal rate is in the range of 20～220μm/s. The development of microstructures, especially dentntic spacing, is investigated. The results show that the microstructure of unidirectional solidification has obvious directivty; the crystal grows along the normal direction; the grains that depart axial direction fall into disuse.The microstructure changes from cellular crystal to cellular-dentritic crystal and then dentritic crystal. The increase of withdrawal rate leads to microstructure refinement. The primary arm spacing decreases sharply with a withdrawal rate below 100μm/s, and decreases more gently when beyond 100μm/s.We have paid great attention on the investigation of relationship between primary arm spacing of Al-4.5%Cu Alloy and withdrawal rate during unidirectional solidification. Based on the theories of Hunt and Bower, a theoretical model is founded to pridict primary dentritic arm spacing, under the condition of local equilibrium of interface. The experimental data are well inosculating to calculation result by the withdrawal rate of 100～220μm/s, and the error is less than 6.5%. It is hoped that the function can be useful for microstructure analysis and to be helpful to obtain a good match of parameters during unidirectional solidification.We have also furtherly studied the fracture mechanism and mechanical properties of experimental alloy, through SEM in-situ study and tests of tensile strength, extensibility and hardness. The results show that compared with Specimen A (withdrawal rate is 60μm/s), tensile strength of Specimen B (withdrawal rate is 200μm/s) increase 16.7% and simultaneously extensibility improve 85.9% and hardness 67.5%. With the increase of withdrawal rate, fracture mechanism of Al-4.5%Cu changes from quasi-cleavage fracture to ductile fracture, which indicates that there is great effect of withdrawal rate on the fracture mechanism and mechanical properties of unidirectional solidification Al-4.5%Cu.更多还原