Precipitation of B2 and B19 phases in alpha 2 lamella of Ti-44.5Al-4 Nb-1Mo-0.1B alloy

The alpha 2 -> B2 and alpha 2 -> B19 phase transformations were widely observed inside the same alpha 2 lamellae of the Ti-44.5Al-4 Nb-1Mo-0.1B alloy during the 1050 degrees C short term heat treatment without external stress. The differences between the B2 and B19 precipitates were characterized by transmission electron microscopy (TEM). In general, the alpha 2/gamma lamellar interface morphologies of the B2 precipitate region sharply differ from those of the B19 precipitate region. B19 only precipitates at a relatively straight alpha 2/gamma lamellar interface, while B2 only precipitates at a relatively curved alpha 2/gamma lamellar interface. It was found that B2 and B19 precipitations inside the alpha 2 phase obey the orientation relationship with alpha 2: (100) B19[010] B19 // (0001) alpha 2 [1120] alpha 2, (011) B2 [111] B2// (0001) alpha 2 [1120] alpha 2. The precipitation of B2/B19 after heat treatment at 1050 degrees C for 6 h is controlled by the element-diffusion mechanism. The difference between the B2 and B19 pre-cipitation region interface distortion width and its alpha 2/gamma interface morphologies are discussed by calculating lattice disregistry between alpha 2/B2 and alpha 2/B19. (c) 2023 Published by Elsevier B.V.