Flow behavior and microstructure evolution of a P/M TiAl alloy during high temperature deformation

Abstract

Hot compression tests of a P/M Ti-47Al-2Cr-2Nb-0.2W-0.15B (at. pct) alloy were carried out on a Gleeble-3800 simulator at the temperatures ranging from 950 degrees C to 1250 degrees C with the strain rates ranging from 10 s(-1) to 10(-3) s(-1). Optical microscope, electron backscatter diffraction technique, and transmission electron microscope were employed to investigate the microstructure evolution and nucleation mechanisms of dynamic recrystallization. It was found that the flow behavior is a function of the deformation temperature and strain rate. The dependence of the peak stress on the deformation temperature and strain rate can be expressed by a hyperbolic-sine type equation. The activation energy for the alloy is calculated to be 315 kJ/mol. The size of the dynamically recrystallized grains decreased with increasing the value of parameter Z. However, the size of dynamically recrystallized grains almost remains constant with increasing deformation strain. At the early stage the dominant nucleation mechanism of dynamic recrystallization in the alloy is the discontinuous dynamic recrystallization, which is characterized by the bulging of the original grain boundaries and the deformation twinning. As the deformation strain increased, the continuous dynamic recrystallization characterized by progressive subgrain rotation occurred. Twinning was observed under all deformation conditions. The spheroidization of the alpha(2) took place at the compression temperature 950 degrees C and the strain rate 10(-3) s(-1). (C) 2011 Elsevier B.V. All rights reserved.