Hot deformation behavior originated from dislocation activity and β to α phase transformation in a metastable β titanium alloy

Profound and comprehensive knowledge on hot deformation of metastable beta titanium alloy is essential to process optimization and microstructure control. In this work, the hot compression behavior of a beta quenched metastable beta titanium alloy is thoroughly studied by examining microstructural evolution to resolve the related deformation mechanisms. It is demonstrated that the deformation of the alloy presents three characteristic stress-strain stages: a linear stage (Stage I), a discontinuous yielding (Stage II) and a steady-state (Stage III). Before and during Stage I, the beta to alpha phase transformation happened intensively along the beta grain boundaries with more than 90% of the boundaries occupied by grain boundaries alpha (alpha(GB)). The linear behavior deviates from the Young's modulus when the linear deformation proceeds to the late stage, indicating the onset of plastic deformation. This behavior results from dislocation slip near beta grain boundary regions. The discontinuous yielding of Stage II is originated from the fragmentation of the alpha(GB) and the intensive formation of mobile dislocations near the beta grain boundary areas. The steady-state of Stage III arises from two orientation dependent deformation modes of the beta grains. For those in favorable activation orientation for their {110}(beta) < 1<(1)over bar>1 >(beta) systems with respect to the external compressive load (type I), the deformation is realized by the dislocation slip and by the formation of slip bands; whereas for those with their (1 (1) over bar(2) over bar)(beta) < <(1)over bar>1 (1) over bar > beta systems-the major lattice deformation for structure change from beta to alpha-in favorable activation orientations (type II), the deformation is achieved by intensive phase transformation by forming intragranular alpha precipitates. The present results provide new information on hot deformation mechanisms of metastable beta titanium alloys, especially the phase transformation associated lattice deformation as an important plastic deformation mode.