Nb_(10)Ti_(61)Co_(29)包共晶合金定向凝固组织演化及其凝固路径模拟计算

Microstructure Evolution and Simulation of Solidification Path in Nb_(10)Ti_(61)Co_(29) Quasi-peritectic Alloy

选择包共晶点附近的Nb_(10)Ti_(61)Co_(29)合金为研究对象,利用Bridgman定向凝固技术对其开展了一系列定向凝固实验(v=1,3,5,15,30,70μm/s),然后利用XRD、SEM和EDS等分析了不同生长速率下的凝固组织,阐明定向凝固组织演化规律,最终得出相应的凝固机理。结果表明,不同生长速率下合金的凝固组织均包含初始过渡区、稳态生长区以及淬火区。随着生长速率的逐渐增大,初始过渡区上初始生长界面轮廓越来越清晰,并逐渐趋于平直状态,伴随上述变化,稳态生长区与初始过渡区关联性逐渐变小;其次,随着生长速率逐渐增大,合金淬火界面依次经历平界面向胞状晶再到树枝晶的转变,其中,淬火界面在生长速率为1μm/s时呈平直状态,在生长速率为3和5μm/s时,淬火界面大致呈胞状,当生长速率进一步增大时淬火界面呈现典型的枝晶生长;最后,利用CALPHAD方法计算得出了该合金在平衡凝固过程中会依次发生如下4个凝固反应:(1)L→α-Nb;(2)二元共晶反应L→α-Nb+TiCo;(3)三元包共晶反应L+TiCo→α-Nb+Ti_(2)Co;(4)二元共晶反应L→α-Nb+Ti_(2)Co。

The Nb_(10)Ti_(61)Co_(29)alloy near the quasi-peritectic point was selected as the research object in this paper,and a series of directional solidification experiments with different growth rates(ν=1,3,5,15,30,70μm/s)were carried out by Bridgman directional solidification technique.Then the solidification structure at each growth rate was analyzed by XRD,SEM and EDS,and the microstructure evolution law of these directionally solidified samples was clarified.The results show that the solidification structure of this alloy at different growth rates includes initial transition zone,steady-state growth zone and quenching zone.As the growth rate increases,the profile of the initial growth interface in the initial transition zone becomes more and more clear.With these changes,the relationship between the steady-state growth region and the initial transition region gradually becomes smaller.Moreover,the quenching interfaces undergoes the transformation from flat-bound to cell-oriented to dendrite in turn with the increase of the growth rate.Especially,the quenching interface is flat when the growth rate is 1μm/s,whereas the quenching interface is roughly cellular when the growth rates are 3 and/or 5μm/s.Lastly,the following four solidification reactions will occur successively in the process of equilibrium solidification,which was calculated by CALPHAD method,(a)L→α-Nb;(b)binary eutectic reaction L→α-Nb+TiCo;(c)ternary quasi-peritectic coated reaction L+TiCo→α-Nb+Ti_(2)Co and(d)binary eutectic reaction L→α-Nb+Ti_(2)Co.