Study on deformation mechanism of Ti-2Al-2.5Zr alloy tube in the flattening test

Abstract

The flattening test, using one pair paralleled plates to flatten tube structure radially, is generally employed to examine the ductility of tube used in once through steam generator (OTSG). This study focuses on the stress condition and deformation mechanism analysis on the concentrated stress regions of Ti-2Al-2.5Zr alloy tube during the flattening test. Firstly, the finite element analysis was performed using the commercial software ABAQUS to determine the stress condition. Secondly, the Electron Back-Scattered Diffraction (EBSD) was implemented to observe the microstructure evolution of Ti-2Al-2.5Zr alloy. Finally, Schmid law was employed to analyze the activated deformation mechanism under condition of the complex stress. It was found that the condition of the complex stress in stress concentration regions, including tension and compression regions, can be simplified into two directional stresses condition. In grains whose c-axis is nearly towards TD and ND, the strain is mainly accommodated by prismatic slip, whereas in grains whose c-axis deviates about 45° from ND to TD mainly by basal slip. The {10–12} extensive twin and corresponding parent grain orientation mainly relied on the stress condition. Additionally, the intergranular twin pairs connected at common grain boundaries (GBs) with high strain compatibility were found at low angle GBs, which were activated under a combination of macro stress and strain compatibility effect.