Effect of Cooling Rate on Growth Mechanism of Primary Α and Precipitation of Secondary Α of a Near Α Titanium

This paper focused on the investigation of the effect of cooling rate on the growth mechanism of the primary 𝛼 and the precipitation mechanism of the secondary 𝛼. A low cooling rate could provide sufficient time for the primary α phase to grow. Rim-𝛼, formed by the migration from the α p /β interface to the β matrix, was observed during the cooling process, which gave proof of the epitaxial growth of the primary 𝛼. One key finding is that once the 𝛼 GB (grain boundary 𝛼) nucleated on the primary 𝛼 could grow forward and do not keep the Burgers orientation relationship (BOR) with the adjacent 𝛽 grains. HADDF and Fourier-filtered HRTEM images elucidated that two adjacent primary α p particles share a common plane but not the same atomic arrangement direction in crystal space. Three precipitation types of the secondary α s were discussed through EBSD and spatial crystal structure analysis. Type-I is featured as the primary α p keeping the BOR with the pre-existing β grain, the secondary α s has prone to share a common {11 0} plane with the primary α p grain and their c axis are at an angle of 60 º. As for Type-II, the secondary α s still share the common {11 0} plane with the primary α p and their c axis are at an angle of 80 º but do not obey the BOR with the pre-existing β grain. Type-III is characterized by the secondary α s sharing the common {10 0} plane with the primary α p and does not maintain the BOR with the pre-existing β grain.