TC17钛合金在连续冷却过程中的组织析出特征

Microstructure Precipitation Characteristics of TC17 Titanium Alloy during Continuous Cooling

研究了连续冷却过程中TC17钛合金在不同冷速条件下的相析出和组织演变行为。结果表明,连续冷却过程中,晶界α相(α_(GB))优先析出,尺寸较大,靠近晶界的α相(α_(W))和晶内α相(αI)随后析出,尺寸较小。随着冷却速度增加,析出的α相变细、变少,而且晶界和晶内α相的厚度差异逐渐降低。α_(GB)的析出过程受两侧β相晶体取向影响,α_(GB)倾向于一侧β晶粒保持Burgers取向关系,但并不严格对应,另一侧的β晶粒促使α_(GB)相的晶体结构发生一定的旋转以尽可能地适应两侧的β相。α_(W)的析出受α_(GB)和β相的共同影响,一般呈集束特征,αI的析出由β基体相决定,可析出12种变体,呈网篮结构特征。

In this work,a thermal dilatometer was used to accurately control the cooling rate after heat treatment.The behavior of phase precipitation and microstructure evolution of TC17 titanium alloy were studied under different cooling rates during the continuous cooling process.The results showed that the grain boundaryα(α_(GB))phase precipitated preferentially during the continuous cooling process,and its size was larger.Differentα_(GB)phases had different precipitation mechanisms,mainly manifested as sympathetic nucleation and interface instability nucleation,which resulted in the difference of morphology.α_(GB)of sympathetic nucleation was flat,whileα_(GB)of interface instability nucleation was morphology of fishbone.αphase(α_(W))close to the grain boundary and the intragranularαphase(α_I)precipitated later with smaller size.When the cooling rate was slower(0.1 and 0.5℃·s^(-1)),α_(W) phase was colony structure andα_I phase was basket-weave structure.More fineαphases were precipitated when the cooling rate increased to 1℃·s^(-1).Moreover,as the cooling rate increased,the increasing velocity gradient increased the driving force to promote the precipitation ofαphase,so thatαphase had more precipitation sites in the grain,and the number of precipitated variants increased.This resulted in the cross distribution ofα_(W) andα_I phases,which presented the basket-weave structure.The thickness ofα_(GB)phase andα_(W)/α_I phases decreased significantly with the increase of cooling rate.The quantitative statistics showed that the thickness ofα_(GB)phase was about 1.22μm and the thickness ofα_(W)/α_I phase was about 0.53μm at the cooling rate of 0.1℃·s^(-1).The size ofα_(GB)phase was 0.69μm,which as larger than that ofα_(W)/α_I phase.When the cooling rate increased to 0.5℃·s^(-1),the thickness ofα_(GB)phase decreased to about 0.79μm,the thickness ofα_(W)/α_I phase decreased to about 0.32μm,and the difference was 0.47 times.When the cooling rate increased to 1℃·s^(-1),the thickness ofα_(GB)phase andα_(W)/α_I phase approached 0.11 and 0.09μm,respectively,with a difference of only 0.02μm.It could be seen that the thickness difference of precipitatedα_(GB)phase andα_(W)/α_I phase decreased with the increase of cooling rate.In addition,X-ray diffraction(XRD)results showed that onlyαphase was precipitated when the cooling rate was 0.1℃·s^(-1).When the cooling rate increased to 0.5 and 1℃·s^(-1),α′phase was precipitated in addition toαphase.However,a large amount ofβmatrix was retained and only a small amount ofαphase was precipitated when the cooling rate was 5℃·s^(-1).The results of energy dispersive spectroscopy(EDS)showed that there were differences in the microcompositions in the regions with differentαphase precipitation data.In the regions with lessαphase precipitation,the contents of Cr and Mo elements were relatively low,while the contents of nearαelement Al were not significantly different.αphase precipitation in titanium alloy generally started at the grain boundary and then precipitated inside the grain.Whereverαphase precipitated,its crystal structure was affected by the orientation of the originalβphase crystal structure.The samples with cooling rate of 0.1℃·s^(-1)were selected for electron back-scattered diffraction(EBSD)analysis.According to the structure characteristics of the morphology and crystal orientation,α_(GB),α_(W) andα_I phases in inverse pole figure(IPF)diagram ofαphase were selected for analysis.The precipitation process ofα_(GB)was controlled by crystal orientations of both sidesβphase,andα_(GB)tended to maintain the Burgers orientation relationship(BOR)with one sideβgrain,but they did not strictly correspond.The other sideβgrain forced crystal structure ofα_(GB)to rotate to adapt to both sidesβphase.The precipitation ofα_(W) was affected byα_(GB)andβ,and it was characterized by colony characteristics.The precipitation ofα_I was completely controlled by the matrixβphase,in this case,12 variants ofαphase could be precipitated,presenting the characteristics of the basket-weave structure.In present work,the precipitation characteristics ofαphase(α_(GB),α_(W),α_I)at different conditions during continuous cooling were studied,and the precipitation,evolution and mechanism ofαphase were clarified.The research content of this work provided a basis for understanding the precipitation behavior ofαphase of TC17 titanium alloy during continuous cooling.The relevant results could be used to select the appropriate process route and control the microstructure.