Formation mechanism of lamellar alpha in titanium through accurate simulation

Abstract

The lamellar α, transformed β matrix and spheroidization products of them combine to form all typical microstructures in titanium alloys. Accurately predicting the formation of lamellar α and revealing the precipitation mechanism therefore become vital for tailoring the microstructure and thus optimizing the properties of titanium alloys. A mixed-mode model combined with cellular automaton is proposed to simulate the process of nucleation and growth of lamellar α phase in isothermal treatment of Ti alloys. In this model, the driving force that is dependent on temperature, the solute concentration in two phases and interface is derived; the soft impingement effect, considering the microscopic morphology evolution of lamellar α phase in two dimensions, is well described by not only approximating the diffusion field function in front of the interface as a polynomial but also introducing the area and interface length of lamellar α into the mass conservation law. A comparision with the interface-controlled and diffusion-controlled models shows that the mixed-mode model gives obviously more accurate description of β-to-α phase transformation kinetics. Furthermore, it is also found that the interfacial migration coefficient, and the microscopic morphology of lamellar α phase to be precipitated have great effects on the mixed mode character (in especial the change in controlled mode) and the kinetics of entire phase transformation process.