Study On Abnormal Phenomenon Of Expansion Curve During Phase Transformation Of Typical Titanium Alloy

With the development of high speed and complexity in the aerospace industry,the properties of titanium alloy are required higher.The heat treatment of titanium alloy can make it match the performance of strength,plasticity and toughness,and the rule of solid phase transformation of the alloy is the theoretical basis for the heat treatment process.Therefore,it has certain theoretical value to study the volume effect and the distribution of alloying elements in the process of phase transformation of titanium alloy.In this paper,TC17,BT25Y and Ti650 titanium alloy is taken as the main research object.The three titanium alloys are studied by dilatometric method combined with OM,SEM,TEM and XRD to study the changes in the expansion curve and the distribution of the alloying elements in the process of continuous heating and cooling.At the same time,the activation energy of?+???phase transition occurred in TC17 titanium alloy during continuous heating and cooling was calculated by Kissinger equation,and the continuous cooling transformation kinetics diagram of TC17 titanium alloy was established.The main conclusions of the study are as follows:(1)The expansion curves of TC17,BT25Y and Ti650 titanium alloys during continuous heating and cooling process are consistent.The change is obvious in the continuous heating process:the expansion trend of the expansion curve of the alloy increases slowly with the temperature,then shows the shrinkage trend,and then returns to the linearity with the rise of temperature.The shrinkage trend of the expansion curve is with the TC17,BT25Y and Ti650 increased in turn.There is no obvious change in the continuous cooling process,but the derivative curve of the expansion curve indicates the local shrinkage of titanium alloy in this process.(2)The change of expansion curve of titanium alloy during continuous heating and cooling is the result of the difference of phase structure and thermodynamics and diffusion kinetics,which results in the aggregation of alloying elements in different phases of titanium alloy.In addition,the alloy elements Al,Mo,Cr,Sn and Zr will be formed during the phase transition of titanium alloy in?and?phases.(3)During the kinetic study of TC17 titanium alloy,it was found that the activation energy of?+???was 302.7 kJ·mol~-11 during continuous heating,and the activation energy of?_m??+?was 218.4kJ·mol~-11 during continuous cooling.(4)The continuous cooling transformation kinetics diagram(CCT)of TC17titanium alloy was obtained.The results show that the critical cooling rate of TC17titanium alloy during the cooling process is 3.5?/s.