Mg-Bi合金中超-细混晶结构的构筑及其强韧化机制

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

摘要:

为制备出由超细晶（<1 μm）和细晶（1~10 μm）构成的超-细混晶结构镁合金，基于低温等径角挤压（ECAP）加工在制备超-细混晶结构金属材料方面的巨大潜力以及细晶Mg-Bi基合金优异的低温塑性变形性能，本研究选取细晶Mg-6Bi合金为坯料，提出逆温度场等径角挤压（ITF-ECAP）技术，实现了细晶Mg-6Bi合金的低温（<100 ℃）多道次强塑性变形加工。微观组织与力学性能结果表明，细晶Mg-6Bi合金在多道次ITF-ECAP加工过程中，优先在初始晶界处发生动态再结晶，同时有大量亚微米级Mg₃Bi₂相析出。4道次ITF-ECAP加工后，Mg-6Bi合金中形成了由平均晶粒尺寸（AGS）约为 600 nm的超细晶区和AGS约为2 μm的细晶区构成的超-细混晶结构，其超细晶区体积分数约占72.5%。归因于细晶强化、沉淀强化、位错强化与背应力强化共同作用的结果，超-细混晶结构Mg-6Bi合金兼具优异的强度和塑性，其屈服强度和延伸率分别达到(315.6±3.6) MPa与(22.3±1.0)%。

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.