Revealing the dynamic recrystallization and deformation instability of TiC reinforced AZ61 composites with uniform structure and bimodal grain structure

Although microstructural configuration significantly enhances composites' comprehensive mechanical properties, its influence on hot deformation remains unclear. To investigate the effects of microstructural configurations on hot workability, dynamic recrystallization (DRX) mechanisms and instability mechanisms, hot compression tests were conducted on two TiC/AZ61 composites (bimodal/uniform structures) with constitutive modeling, processing maps, and microstructure observations. The results show that uniform composite exhibits better hot workability, lower deformation activation energy (Q) and smaller instability regions than bimodal composite. The uniform composite primarily undergoes continuous DRX (CDRX), while the bimodal composite involves both CDRX and discontinuous DRX (DDRX) mechanisms. At low temperatures (T) and high strain rates ( $\dot{\epsilon }$), the uniform composite achieves sufficient DRX, whereas the bimodal composite experiences only partial DRX accompanied by twinning. At high T and low $\dot{\epsilon }$, the uniform composite is prone to grain boundary sliding (GBS) due to fine grains, causing to micro-voids formation; the bimodal composite undergoes abnormal grain growth (AGG), leading to instability. The favorable hot-processing regions for the bimodal composite are (260–320 ℃, 0.0009–0.03 s^-1) and (325–370 ℃, 0.0001–0.0003 s^-1), while those for the uniform composite are (260–310 ℃, 0.01–0.1 s^-1) and (250–300 ℃, 0.0001–0.0005 s^-1). This work offers in-depth insights into DRX and deformation instability in magnesium matrix composites with different initial structures. It provides a guidance for their selection of hot working processes.