TC17 钛合金在连续冷却过程中的组织析出特征

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 boundary α（α_GB）phase precipitated preferentially during the continuous cooling process，and its size was larger. Different α_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，while α_GB of interface instability nucleation was morphology of fishbone. α phase（α_W）close to the grain boundary and the intragranular α phase（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 and α_I phase was basket-weave structure. More fine α phases 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 of α phase，so that α phase had more precipitation sites in the grain，and the number of precipitated variants increased. This resulted in the cross distribution of α_W and α_I phases，which presented the basket-weave structure. The thickness of α_GB phase and α_W/α_I phases decreased significantly with the increase of cooling rate. The quantitative statistics showed that the thickness of α_GB phase was about 1.22 μm and the thickness of α_W/α_I phase was about 0.53 μm at the cooling rate of 0.1 ℃·s^-1. The size of α_GB phase was 0.69 μm，which as larger than that of α_W/α_I phase. When the cooling rate increased to 0.5 ℃·s^-1，the thickness of α_GB phase decreased to about 0.79 μm，the thickness of α_W/α_I phase decreased to about 0.32 μm，and the difference was 0.47 times. When the cooling rate increased to 1 ℃·s^-1，the thickness of α_GB phase and α_W/α_I phase approached 0.11 and 0.09 μm，respectively，with a difference of only 0.02 μm. It could be seen that the thickness difference of precipitated α_GB phase and α_W/α_I phase decreased with the increase of cooling rate. In addition，X-ray diffraction（XRD）results showed that only α phase 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 to α phase. However，a large amount of β matrix was retained and only a small amount of α phase 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 different α phase precipitation data. In the regions with less α phase precipitation，the contents of Cr and Mo elements were relatively low，while the contents of near α element Al were not significantly different. α phase precipitation in titanium alloy generally started at the grain boundary and then precipitated inside the grain. Wherever α phase precipitated，its crystal structure was affected by the orientation of the original β phase 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 and α_I phases in inverse pole figure（IPF）diagram of α phase were selected for analysis. The precipitation process of α_GB was controlled by crystal orientations of both sides β phase，and α_GB tended to maintain the Burgers orientation relationship（BOR）with one side β grain，but they did not strictly correspond. The other side β grain forced crystal structure of α_GB to rotate to adapt to both sides β phase. The precipitation of α_W was affected by α_GB and β，and it was characterized by colony characteristics. The precipitation of α_I was completely controlled by the matrix β phase，in this case，12 variants of α phase could be precipitated，presenting the characteristics of the basket-weave structure. In present work，the precipitation characteristics of α phase（α_GB，α_W，α_I）at different conditions during continuous cooling were studied，and the precipitation，evolution and mechanism of α phase were clarified. The research content of this work provided a basis for understanding the precipitation behavior of α phase of TC17 titanium alloy during continuous cooling. The relevant results could be used to select the appropriate process route and control the microstructure.