Towards optimizing the microstructure of magnesium alloy during hot forming: Pre-deformation at cryogenic temperature

The preparation of massive twins by pre-deformation at cryogenic temperature (-196 degrees C) is expected to induce dynamic recrystallization (DRX) during hot forming, thereby optimizing the microstructure of hot-deformed alloys. In this study, Mg-5.5Gd-3Y-0.5Zr alloy was pre-deformed at room and cryogenic temperatures, respectively. Then, hot compression was carried out on the pre-deformed samples. The results indicate that the alloy fabricated by room-temperature pre-deformation (RT-PD) and then hot deformation (RT-HD) contains many initial grains which have not been refined, while the microstructure and mechanical properties of the alloy prepared through cryogenic-temperature pre-deformation (CT-PD) and then hot deformation (CT-HD) are improved significantly. There are only a few {1012} tension twins (TTWs) formed in RT-PD sample, so the sites for twin-induced dynamic recrystallization (TDRX) grains during subsequent hot deformation are limited. However, the number of twins in the alloy after CT-PD increases significantly, including {1011}-{1012} double twins (DTWs) and (1012)-(0112) twin-twin interactions, which facilitates the promotion of the TDRX and reduction of average grain size in the microstructure. In addition, a weak texture can be generated in CT-HD sample, primarily owing to the orientation inheritance of TDRX grains from various twins in CT-PD sample.