Quasi-in-situ investigation on complete lamellar fragmentation of β-solidified TiAl alloy during uniaxial isothermal compression

Abstract

The coarse as-cast lamellar microstructure in TiAl alloys is difficult to be broken completely by thermomechanical processing. Some remnant lamellar colonies in the deformed microstructure seriously affect the microstructural homogeneity and deteriorate the properties. In this study, it is found that by isothermal compression at 1230 °C and 1250 °C, the lamellar colonies of Ti-43.5Al-4Nb-1Mo-0.1B (TNM) alloys can be completely broken. This is attributed to the weakened anisotropic deformation behavior of the lamellar colonies due to the isothermal holding treatment before deformation. The deformation behavior at 1230 °C was investigated by quasi-in-situ experiments. It is observed that the regions near lamellar colony boundaries first undergo dynamic recrystallization at small strain, while the lamellar colonies gradually break down with increasing strain. The adequate fragmentation of lamellar colonies mainly depends on the recrystallization of α lamellae (αL). The isothermal holding at 1230 °C leads to an increase in the content and thickness of αL, which allows it to assume more deformation and promotes its recrystallization by reaching critical strain. The interrupted γ lamellae (γL) formed by decomposition during isothermal holding facilitates the occurrence of α recrystallization within the lamellar colonies by hindering dislocation movement. In addition, recrystallized γ grains (γR) are gradually dissolved by the formation of α precipitates inside them through the γ → α phase transformation and the subsequent consumption of α precipitates by the recrystallized α grains.