电流对定向凝固Ti-48Al-2Cr-2Nb组织的影响

发布时间：2018-05-15 10:52

本文选题：TiAl合金 + 电流　；参考：《哈尔滨工业大学》2017年硕士论文

【摘要】：由于TiAl合金具有比强度高,高温性能优异等特点,所以在航空航天领域的使用十分广泛。然而,由于TiAl基合金的室温塑性差等缺点阻碍了其推广应用。电流凝固技术是当前采用比较多的用来改善材料组织,提升性能的新型凝固技术。本文重点研究了不同电流作用下定向凝固过程的TiAl基合金的组织变化规律。本文选Ti-48Al-2Cr-2Nb(at.%)合金作为研究对象,利用电磁冷坩埚定向凝固技术进行实验。在定向凝固过程中分别加载不同电流密度的直流与脉冲电流作用,分析不同电流对Ti-48Al-2Cr-2Nb合金定向凝固组织的影响及变化规律。首先,利用ANSYS对电流作用下TiAl基合金定向凝固过程的电磁场分布进行数值模拟。通过数值模拟发现,直流电流作用下磁感应强度及洛伦兹力由外及内逐渐降低,内部的值最低。而脉冲电流作用下,磁感应强度及洛伦兹力主要集中分布在表层趋肤层附近,对内部影响极小。从试样宏观组织观察到电流作用下使得试样两侧的柱状晶生长与抽拉方向夹角变小,定向效果更好,直流电流比脉冲电流作用更加明显。直流电流作用下当电流密度增大到74 m A/mm2时,稳定生长区的部分柱状晶转变为等轴晶。直流电流作用下随着电流密度的增加,柱状晶晶粒尺寸先变大后变小。脉冲电流作用下,柱状晶晶粒尺寸随着电流密度的增加先变小,后变大。直流电流作用下合金片层间距随着电流密度的增加,先减小后增大。脉冲电流作用下,片层间距随着电流的增大而减小。随电流密度增加,组织中与抽拉方向夹角在30~60°范围内的片层团增多,垂直夹角变少,夹角波动范围变小,有利于以β相为先析出相。电流作用下,组织中B2相的含量随电流密度的增加而减少,Al含量较高的γ单相组织也减少。电流作用使得溶质分布更加均匀,减少微观偏析。电流作用能使得定向凝固启动界面更加趋于平直化,而且随着电流密度的增加,铸态区的等轴晶逐渐增大,有向柱状晶转化的趋势,有利于柱状晶的生长。电流作用下在固液界面处既存在α枝晶也存在β枝晶,电流密度的增加,有利于减少固液界面界面前沿的成分过冷。对不同电流参数作用的定向试样做性能测试,结果表明:直流电流作用下合金的硬度随着电流密度的增加先变大后变小。脉冲电流作用下,随着电流密度的增加而变大。由于电流对固态相变过程影响较小,原始铸态区的硬度基本上无明显变化。直流电流作用下,随着电流密度的增加,抗压强度有增大后减小的趋势。脉冲电流作用下,抗压强度随着电流密度的增加而增大。随着电流密度的增加,合金室温压缩塑性有所提高。电流作用下合金的断裂韧性得到一定提高。直流电流作用下,随着电流密度的增加,断裂韧性先增大后减少,电流密度为49.7 m A/mm2时断裂韧性最大为22.91 MPa·m~(1/2)。脉冲电流作用下,随着电流密度的增加,合金断裂韧性不断增加。电流密度为74 m A/mm2,断裂韧性最大为20.09 MPa·m~(1/2),略低于直流电流作用下。
[Abstract]:Because of its high specific strength and excellent high temperature properties, TiAl alloys are widely used in the field of Aeronautics and Astronautics. However, due to the poor plasticity of TiAl based alloys at room temperature, their application is hindered. The current solidification technology is a new solidification technology used to improve the material organization and improve the performance. In this paper, the microstructure and change rules of TiAl based alloys with different current under directional solidification are studied. This paper selects Ti-48Al-2Cr-2Nb (at.%) alloy as the research object and uses the electromagnetic cold crucible directional solidification technology to carry out the experiment. In the directional solidification process, the direct current and pulse current of different electric current density are loaded respectively, and the analysis is different. The effect of current on the directional solidification structure of Ti-48Al-2Cr-2Nb alloy and its change law. First, the electromagnetic field distribution of the directional solidification of TiAl based alloy under the action of current is numerically simulated with ANSYS. Through numerical simulation, it is found that the magnetic induction intensity and Lorenz force are gradually reduced from outside and inside, and the internal value is the lowest. Under the action of pulse current, the magnetic induction intensity and Lorenz force are mainly concentrated near the surface layer of skin, which has little influence on the inner surface. From the macroscopic observation of the specimen, the angle of columnar crystal growth on both sides of the specimen is smaller, the orientation effect is better, the direct current is more obvious than the pulse current. When the current density increases to 74 m A/mm2, some columnar crystals in the stable growth region are transformed into equiaxed grains. With the increase of current density, the grain size of columnar crystals becomes larger and then smaller with the increase of current density. The grain size of columnar crystals decreases first and then becomes larger with the increase of current density. With the increase of current density, the spacing of lamellae decreases with the increase of current density. With the increase of current density, the lamellar mass with the angle of 30~60 degree increases with the increase of current density, the vertical angle becomes less and the range of the angle fluctuates smaller, which is beneficial to the first analysis of the beta phase. The content of B2 phase decreases with the increase of current density, and the high content of Al is also reduced. Current action makes the solute distribution more uniform and reduces the microsegregation. Current action can make the starting interface of directional solidification tend to be more straight, and as the current density increases, as cast region and so on The axial crystal gradually increases and has the tendency to convert to columnar crystal, which is beneficial to the growth of columnar crystal. Under the current action, there is not only an alpha dendrite but also a beta dendrite at the solid liquid interface, and the current density is increased, which is beneficial to reduce the component supercooling at the front of the solid liquid interface. The hardness of the alloy increases first and then decreases with the increase of current density. Under the action of pulse current, the hardness of the alloy becomes larger with the increase of current density. The hardness of the original cast state is basically no obvious change due to the small influence of the current on the solid state phase transformation process. The compressive strength of the alloy increases with the increase of current density. With the increase of current density, the compressive ductility of the alloy at room temperature increases. The fracture toughness of the alloy increases with current. The fracture toughness increases with the increase of current density, and the fracture toughness increases first and then decreases. When the current density is 49.7 m A/mm2, the maximum fracture toughness is 22.91 MPa. M~ (1/2). Under the action of pulse current, the fracture toughness of the alloy increases with the increase of current density. The current density is 74 m A/mm2, and the maximum fracture toughness is 20.09 MPa. M~ (1/2), slightly below the effect of DC current.

【学位授予单位】：哈尔滨工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG292;TG146.23

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