Precipitation of B2 and B19 phases in α2 lamella of Ti-44.5Al-4 Nb-1Mo-0.1B alloy

The α₂→B2 and α₂→B19 phase transformations were widely observed inside the same α₂ lamellae of the Ti-44.5Al-4 Nb-1Mo-0.1B alloy during the 1050 ℃ 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 α₂/γ lamellar interface morphologies of the B2 precipitate region sharply differ from those of the B19 precipitate region. B19 only precipitates at a relatively straight α₂/γ lamellar interface, while B2 only precipitates at a relatively curved α₂/γ lamellar interface. It was found that B2 and B19 precipitations inside the α₂ phase obey the orientation relationship with α₂: (100) B19[010] B19 // (0001) α₂ [112̅0] α₂, (011) B2 [1̅11] B2// (0001) α₂ [112̅0] α₂. The precipitation of B2/B19 after heat treatment at 1050 °C for 6 h is controlled by the element-diffusion mechanism. The difference between the B2 and B19 precipitation region interface distortion width and its α₂/γ interface morphologies are discussed by calculating lattice disregistry between α₂/B2 and α₂/B19.