Effects of prior microstructures and deformation parameters on the ultra-refining uniformity of Ti-Mo ferritic steel

Samples with different prior microstructures, quasi-polygonal ferritic (QPF) matrix and polygonal ferritic (PF) matrix, were compressed at 500 ℃ with different strain rates. As for the QPF matrix, its microstructure more tend to be compressed and refined under small strain rate, while it is opposite for the PF matrix. Increase of deformation temperature to 600 ℃ and 700 ℃ illustrates that during cDRX process, extending of geometric necessary boundaries (GNBs) increases the flow stress whereas the rearrangement of geometric necessary dislocations (GNDs) mainly causes the softening phenomenon. Additionally, extending of GNBs in prior grains firstly facilitates the increase of texture intensity on certain orientations, then the texture intensity decreases with further conduct of continuous dynamic recrystallization (cDRX) process, during which the subgrains mainly rotate along < 001 > //SD or < 111 > //SD and make orientations distributed more randomly within fiber textures. Deformation at 600 ℃ shows better refining effect and the average grain size can be refined to submicron scale, however, deformation at 700 ℃ presents better ultra-refining uniformity that the area fraction of ultra-refined regions can reach 94.8% with an average grain size of 1.1 µm. TEM observation clarifies that with the increase of deformation temperature, low angle boundaries (LABs) will replace dislocation walls dividing the prior grains and show better extending ability. Activation energy of the cDRX process also been estimated and it is about 768 KJ/mol.