Construction of Bimodal-Grained Microstructure of Mg-Bi Alloy Composed of Ultrafine Grains and Fine Grains and Its Strengthening and Toughening Mechanisms

Abstract:

Low-temperature equal channel angular pressing (ECAP) processing technology has great potential in fabricating bimodal-grained alloys composed of ultrafine grains and fine grains. Besides, fine-grained Mg-Bi based alloys demonstrate excellent low temperature plastic deformation performance. Based on this, a new inverse temperature field ECAP (ITF-ECAP) processing method was developed to realize the severe plastic processing of a fine grained Mg-6Bi (B6) alloy at low temperature (<100 ℃) to construct a bimodal-grained microstructure composed of ultrafine (<1 μm) and fine grains (1?10 μm). The microstructure and mechanical properties characterization results show that dynamic recrystallization preferentially occur at the initial grain boundaries of the fine-grained B6 alloy during the multi-pass ITF-ECAP processing. In addition, a large amount of submicron sized Mg₃Bi₂ phase precipitates during ITF-ECAP processing. As a result, bimodal-grained microstructure consisting of ultrafine grains with an average grain size (AGS) of about 600 nm and fine grain region with an AGS of about 2 μm was successfully constructed in B6 alloy through 4-pass ITF-ECAP processing. The volume fraction of the ultrafine grain region accounts for about 72.5%. Due to the combined effects of grain-boundary strengthening, precipitation strengthening, dislocation strengthening, and back stress strengthening, the bimodal-grained B6 alloy exhibits excellent strength and ductility, i.e. yield strength and elongation reach (315.6±3.6) MPa and (22.3±1.0)%, respectively.