定向凝固高Nb含量TiAl合金微观组织演化与溶质偏析规律

MICROSTRUCTURE EVOLUTION AND SOLUTE SEGREGATION IN DIRECTIONALLY SOLIDIFIED TiAl ALLOYS WITH HIGH Nb CONTENT

在Bridγman定向凝固炉中对Ti-46Al-(8,9,10)Nb合金进行定向凝固实验,研究了生长速率和Nb含量对Ti Al-Nb合金的微观组织、相变路径以及溶质偏析的影响,获得了定向凝固Ti Al-Nb合金相变过程与微观组织选择图.结果表明,生长速率的增加使得固/液界面发生了平界面-胞状界面-枝晶界面转变,并促进完全b相凝固转变为具有L+β→α包晶反应凝固过程,最终组织由α2/γ层片组织转变为具有a2/γ层片和B2相的多相组织.溶质Nb的添加对b相具有稳定作用,促进了完全b相凝固过程的发生以及α2/γ层片和B2相多相组织的形成.生长速率和Nb含量对相变过程和微观组织的影响与溶质偏析(S型偏析,β型偏析)过程密切相关.其中S型偏析程度的提高有利于促进包晶反应的发生,从而导致最终组织发生严重的溶质偏析以及大量B2相在枝晶心部的集中分布.bβ型偏析中溶质Nb在残余β相中的富集是形成B2相的主要来源,Nb的富集程度直接决定了B2相的形态和尺寸.根据关于生长速率和Nb含量的Ti-46Al合金相变过程和微观组织选择图可知,选择较高的Nb含量和较低的冷却速率能获得完全β相凝固过程,可以得到组织分布均匀、溶质偏析程度较低的定向凝固组织.

TiAl-Nb alloys have been determined as the advanced direction for the development of the high temperature TiAl alloys, so being one of the advanced materials for turbines of aircraft engines and gas-burning power-generation plants. However, highly-Nb addition can lead to the complex solidification behavior of TiAI-Nb alloy and multi-phase microstructure, which is important for the mechanical properties during the alloy design. Bridgman type directional solidification experiments were conducted for Ti-46Al-（8, 9, 10）Nb alloy. The effect of the growth rate and Nb content on the microstructure, phase transition and microsegregation was investigated, and finally the selection diagram of the phase transition and the microstructure of the directional solidified TiAl-Nb al- loy were obtained. The results show that the planar-cellular-dendritic evolution of solid-liquid interface can be ob- served with the increase of the growth rate. Meanwhile the fully β phase solidification changes to the peritectic so- lidification with the increase of the growth rate, and correspondingly the final microstructure is composed of the α2/γ lamellar structure and a multiphase microstructure （B2 phase, α2/γ lamellar structure） respectively. The increase of the β-stabilizer Nb content can promote the fully β phase solidification and the formation of the multiphase micro- structure （B2 phase, α2/γ lamellar structure）. The contribution of the growth rate and the Nb content to the phase transition and the microstructure is connected with the solute segregation （S-segregation, β-segregation） closely. The increase of the S-segregation amplitude can easily promote the peritectic reaction, which always leads to the highly solute segregation and the concentrated distribution of plenty of B2 phase in the core of the dendrite, β-seg- regation is the mainly origin of the B2 phase formation, in which the Nb enrichment in the retained β phase directly determines the morphology and the dimension of the B2 phase. Finally according to the selection diagram of the so- lidification process and the microstructure of the Ti-46Al alloy with the growth rate and the Nb content, the high Nb content and the low growth rate during fully β solidification should be selected for the prefer microstructure with the homogeneous distribution and the low solute segregation.