Ti6Al4V-0.55Fe合金连续升温过程的相变研究

Phase Transformation of Ti6Al4V-0.55Fe Alloy During Continuous Heating Process

通过连续升温热膨胀法(DIL)研究了Ti6Al4V-0.55Fe合金在连续升温过程中的α相回溶(α+β→β)的热膨胀行为和显微组织演化。采用了1、5、10、15 K/min的4种升温速率对Ti6Al4V-0.55Fe合金进行热膨胀实验,结果发现:不同升温速率的α相回溶曲线都展现出典型的“S”型曲线,表明α相回溶是一种由形核长大控制的过程。通过Kissinger-Akahira-Sunose(KAS)方法和Kolmogorov-Johnson-Mehl-Avrami(KJMA)模型分别得到α相回溶转变平均相变激活能E和随着α相回溶体积的增大所对应的Avrami指数n,可分为3个阶段,即相变初期(0<f<0.01)、相变中期(0.01<f<0.95)和相变终期(0.95<f<1),表明Ti6Al4V-0.55Fe合金的α相回溶过程机制在不同时期是不同的。同时采用背散射电子像(BSE)进行Ti6Al4V-0.55Fe合金组织演化验证分析,发现α相回溶温度区间与升温速率为15 K/min的热膨胀曲线确定的温度区间885~1043℃相同,增加了热膨胀实验结果的可靠性。最后,结合相转变曲线,给出了合金连续升温过程α+β→β相转变的连续升温相转变图。将Ti6Al4V-0.55Fe与Ti6Al4V合金比较,发现两合金的热动力学差别主要为Ti6Al4V-0.55Fe合金α相回溶平均相变激活能比Ti6Al4V低,表明Ti6Al4V-0.55Fe合金比Ti6Al4V合金的α相回溶更容易。

The dilatometry behavior and microstructure of theα+β→βphase transformation in the Ti6Al4V-0.55Fe alloy were investigated during continuous heating process.Four heating rates(1,5,10,15 K/min)were applied for the dilatometry behavior research.The results show that for theα+β→βphase transformation curve,the S-shaped pattern under the different heating rates indicates that theα+β→βphase transformation is a nucleation-growth-controlled process.The mean phase transition activation energy E was obtained by the Kissinger-Akahira-Sunose(KAS)method which equals to 200 kJ/mol.By calculating the corresponding Avrami index n with the increase of theβphase volume under the Kolmogorov-Johnson-Mehl-Avrami(KJMA)model,we can divide the Avrami index n into three stages which include the initial phase transformation stage(0<f<0.01),the middle phase transformation stage(0.01<f<0.95)and the phase transformation end stage(0.95<f<1).It indicates thatα+β→βtransformation mechanism in the Ti6Al4V-0.55Fe alloy varies at different stages.At the meantime,the back scattered electron(BSE)micrograph was applied for the microstructure analysis.The microstructural aspects perfectly match the previous dilatometry research,which validate our assumptions.Finally,a continuous heating transformation(CHT)diagram ofα+β→βphase transition in the Ti6Al4V-0.55Fe alloy was obtained.It can be found that the Ti6Al4V-0.55Fe alloy has a lowerα+β→βphase transition activation energy compared with Ti6Al4V alloy,which indicates that theαphase re-dissolution is easier in the Ti6Al4V-0.55Fe alloy.