【摘要】 为改善Ni₃Al合金的室温塑性,研究了不同电流强度的直流电场对冷坩埚定向凝固Ni₃Al合金微观组织的影响。结果表明,在直流电场作用下,随着电流强度的增加,Ni₃Al合金的定向凝固组织的一次枝晶间距变小,凝固界面变得平直。未施加直流电流时,凝固组织由L12型结构的Ni₃Al基体和B2型结构的Ni Al析出相两相组成。当定向凝固过程中施加直流电流时,凝固组织中析出相由B2型NiAl相转变为呈薄层状、晶面对称的孪晶马氏体NiAl相。更多还原

【Abstract】 Due to their excellent high-temperature strength, and good oxidation resistance, Ni₃Al based alloys have long attracted considerable interest as a class of high-temperature structural material.These properties, combined with their unique high thermal conductivity, make them ideal for special applications, such as blades in gas turbines and jet engines. However, polycrystalline Ni₃Al alloys show almost no ductility and extremely low fracture resistance at ambient temperatures. Ni₃Al alloys with the high ductility at room-temperature can be adjusted by the microstructure through directional solidification（DS） and matching. It has been shown that the electric field can refine the solidification structure, reduce the dendrite spacing, promote the diffusion and change the solute redistribution in the solidification process. In order to improve the room temperature ductility of Ni₃Al alloy, the effect of current intensity on microstructure of DS Ni-25 Al alloy is investigated. In this work, the effects of constant current intensity and Ni Al phase on the microstructure are researched. The results show that in the DC electric field, as the result of the aggregation of current along dendrite tip and the Joule heat at the tip of dendrite, the primary dendritic spacing（l） decreases with the increasing of current intensity. And the solid-liquid interface tends to be straight resulting from the Joule heat and Peltier effect caused by the segregation of current and the difference in conductivity between solid and liquid interface. When no direct current is applied the DS sam-ples contain the L12 structure of Ni₃Al matrix and B2 structure of NiAl precipitate phase. The microstructure is a duplex structure which consist of gray Ni₃Al matrix and black Ni Al precipitates. Ni Al precipitates with regular shape and has obvious orientation along with the growth direction. When the DC current is applied, Ni Al precipitates is irregular and dispersion and has no obvious directionality, due to Joule heat effect generated by the current effect, the undercooling increased and the precipitated Ni Al phase transformed into thin martensite Ni Al phase with twin crystal symmetry from the Ni Al-B2 type structure.更多还原