【作者】 蒋冰轮；

【导师】 李双明；

【作者基本信息】 西北工业大学 ， 材料加工工程， 2006， 硕士

【摘要】 本文研究了Ni-24.8wt%Nb过共晶合金恒速和跃迁变速定向凝固下的组织演变,重点探讨了通过跃迁加速扩大共晶共生区的可行性,同时观察和分析了跃迁加速过程中共晶层片间距的调整及其机制,最后通过压缩试验研究了跃迁加速后组织的改变对其力学性能的影响。 恒速定向凝固实验中,Ni-24.8wt%Nb合金在200K/cm的温度梯度下,凝固速度为1μm/s时合金以平界面耦合生长,得到的最终组织为规则的层片共晶,而理论计算表明凝固速度低于0.85μm/s时,合金可得到耦合生长的共晶组织,计算结果与实验结果较为一致。实验中凝固速度在5～100μm/s时,Ni₃Nb初生相领先于共晶相生长,并随着凝固速度的提高,初生相形态发生由长板条状向短条状转变,共晶相由规则层片转变为不规则层片,同时初生相的体积含量也相应减小。实验中当凝固速度达到10μm/s后,组织中出现Ni相共晶晕圈:而理论计算得到共晶晕圈相形成的临界速度为14μm/s,两者基本吻合。 通过跃迁加速定向凝固抑制了Ni-24.8wt%Nb过共晶合金中β-Ni₃Nb初生相的生长,使得在较高的凝固速度下仍然能获得耦合生长的共晶组织,这为提高高温自生复合材料的生产效率和扩大其共晶共生区提供了一个新的技术途径。 跃迁加速后共晶Ni相和Ni₃Nb相层片都发生了分叉,非稳态共晶组织的转变可分为三个阶段:规则共晶层片间距的减小:规则共晶向不规则共晶的转变;不规则共晶层片间距的调整,这三个阶段在转变过程并不是截然分开的。跃迁加速后的非稳态过渡区的长度随变速比的增加而缩短,加速后的非规则共晶稳态组织随变速比的增加而细化。 压缩试验结果显示:从定向凝固速率1μm/s跃迁加速到5μm/s以上的试样,无论是韧性,塑性还是抗压性能,与相同恒速凝固速度下获得的试样相比,都有明显的提高。含有β-Ni₃Nb初生相的试样,在压缩过程中出现脆性断裂。更多还原

【Abstract】 In this paper, directionally solidified microstructures of Ni-24.8%Nb hypereutectic were investigated at constant and changing growth rates. The solidification behaviors of Ni-24.8%Nb hypereutectic, including coupled growth zone and adjustment mechanisms of lamellar spacing of eutectic were researched by an abruptly changing growth rate in directional solidification. The influence of solidification process on the directionally solidified microstructures of Ni-24.8%Nb hypereutectic was presented. Finally, the mechanical property was studied through the compressive test.At the given temperature gradient of 200K/cm and the growth rate less than 1μm/s in directional solidification, coupled growth lamellar eutectic of Ni-24.8%Nb alloy was obtained, which was in agreement with the theoretical calculation result of 0.85μm/s. When the growth rate was larger than 5μm/s, primary Ni₃Nb phase grew ahead of the eutectic. With the increasing growth rate, the eutectic microstructure transformed from the regular lamellar to the irregular lamellar. When the growth rate was more than 10μm/s, α-Ni halos formed, which was closed to the calculated critical growth rate of 14μm/s for the formation of α-Ni halos.When the growth rate was increasing abruptly in directional solidification, the growth of β-Ni₃Nb primary phase was suppressed. The results showed that at higher growth rate, the coupled growth eutectic was obtained. It provided a new technical way for the production of in-situ composite effectively.Under the abruptly increase growth rate, eutectic lamellar would be branched. It took place both in Ni and Ni₃Nb phases. In the same time, the non-stability solidification microstructure was adjusted through three stages. Firstly, the regular lamellar spacing of eutectic reduced;secondly, the regular lamellar eutectic transformed to the irregular lamellar eutectic. Thirdly, the irregular lamellar spacing would be modified at various solidification conditions. With the higher acceleration growth rate, the length of non-stability transition zone reduced and the solidified microstructures were refined.Compressive tests showed that the compression strength and plasticity of samples, produced at the abruptly increase growth rate were improved than that of the samples manufactured at the constant growth rate in directional solidification. All the samples including primary Ni₃Nb phase appeared brittle fracture during the compression process. However;samples consisting of complete eutectic microstructure cannot break after compressive test, which showed the higher compression strength and toughness.更多还原