Microstructure Precipitation Characteristics of TC17 Titanium Alloy during Continuous Cooling

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（αGB）phase precipitated preferentially during the continuous cooling process，and its size was larger. Different αGB phases had different precipitation mechanisms，mainly manifested as sympathetic nucleation and interface instability nucleation，which resulted in the difference of morphology. αGB of sympathetic nucleation was flat，while αGB of interface instability nucleation was morphology of fishbone. α phase（αW）close to the grain boundary and the intragranular α phase（αI）precipitated later with smaller size. When the cooling rate was slower（0.1 and 0.5 ℃·s-1），αW phase was colony structure and αI 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 αW and αI phases，which presented the basket-weave structure. The thickness of αGB phase and αW/αI phases decreased significantly with the increase of cooling rate. The quantitative statistics showed that the thickness of αGB phase was about 1.22 μm and the thickness of αW/αI phase was about 0.53 μm at the cooling rate of 0.1 ℃·s-1. The size of αGB phase was 0.69 μm，which as larger than that of αW/αI phase. When the cooling rate increased to 0.5 ℃·s-1，the thickness of αGB phase decreased to about 0.79 μm，the thickness of αW/αI 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 αGB phase and αW/αI 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 αGB phase and αW/αI 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，αGB，αW and αI phases in inverse pole figure（IPF）diagram of α phase were selected for analysis. The precipitation process of αGB was controlled by crystal orientations of both sides β phase，and αGB 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 αGB to rotate to adapt to both sides β phase. The precipitation of αW was affected by αGB and β，and it was characterized by colony characteristics. The precipitation of αI 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（αGB，αW，αI）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. © 2024 Editorial Office of Chinese Journal of Rare Metals. All rights reserved.