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AZ31镁合金超低温轧制和室温轧制的微观组织演变

浏览:40 发表时间:2020-08-22 09:58:42

01


研究背景

通过轧制、挤压等加工方式可以消除镁合金的铸造缺陷并显著细化晶粒等,使其具有更高的力学性能。与在室温轧制相比,超低温轧制(-196℃)的纯钛、纯铜、纯铝等板材可形成超细晶结构,使其具有优异的强度和延展性。然而,迄今为止,对镁合金超低温轧制的相关研究仍然较少,并且尚未深入研究镁合金室温轧制和超低温轧制的显微组织差异。
近日,来自韩国庆北大学的Sung Hyuk Park博士等人,通过在25℃(即室温)和-196℃(即超低温)下轧制AZ31镁合金,比较了两种轧制后板材的显微组织特征,研究了超低温轧制对镁合金的显微组织的影响,以及随后退火过程中其显微组织和硬度的变化。研究发现,在轧制过程中,由于合金的初始晶粒有利于{10-11}压缩孪晶的形成,因此,在两种轧制后均形成了大量的{10-11}压缩孪晶和{10-11}-{10-12}双孪晶。对比AZ31板材的室温轧制和超低温轧制,可以发现:1. 两种轧制均发生了边裂,但超低温轧制产生的边裂更多,表明超低温轧制时AZ31的可轧性能变差;2. 超低温轧制时合金的塑性变形更不稳定,导致孪晶沿剪切带集中分布,而室温轧制后的孪晶分布更均匀,数量更多,位错密度也更高;3.室温轧制时合金储存的应变能较高,因此退火后形成了更加细小均匀的微观结构,合金的硬度更高,而在超低温轧制样品中,在高密度孪晶区域发生了局部再结晶,退火后形成了明显的双峰结构。综上所述,室温轧制镁合金比超低温轧制镁合金具有更好的可轧制性、更均匀的微观组织、更细小的晶粒和更大的硬度。因此,对AZ31镁合金而言,室温轧制工艺比超低温轧制工艺更适合。



图1 AZ31板材的微观组织,a-c:室温轧制;d-f: 超低温轧制

02


文章发表

该研究成果发表于《Journal of Magnesium and Alloys》2020年第八卷第2期:

[1] S.W. Lee, S.-H. Kim, S.H. Park, Microstructural characteristics of AZ31 alloys rolled at room and cryogenic temperatures and their variation during annealing, Journal of Magnesium and Alloys 8(2) (2020) 537-545.

03


中文摘要

本论文研究了AZ31镁合金在室温(RT)和超低温(CT)下轧制的显微组织特征,以及随后在退火过程中显微组织和硬度的变化。由于材料在超低温下适应变形的能力变差,从而导致其在超低温轧制时形成了更多的边裂纹。轧制时,材料的初始晶粒有利于{10-11}孪晶的形成,因此在室温和超低温轧制时,都形成了大量{10-11}压缩孪晶和{10-11}-{10-12}双孪晶。相比于超低温轧制,室温轧制的材料具有更高的位错密度以及更多的孪晶分布。因此,室温轧制的材料在后续退火过程中的动态再结晶更加明显,形成了高度再结晶的均匀组织。而超低温轧制材料的孪晶则主要沿着剪切带分布,退火后形成了含有大量的粗大未再结晶晶粒的不均匀双峰结构。由于室温轧制的材料在退火后具有更加细小的晶粒组织,因此具有更高的硬度。

04


Abstract

This study investigates the microstructural characteristics of AZ31 Mg alloys rolled at room temperature (RT) and cryogenic temperature (CT) and the variation in their microstructure and hardness during subsequent annealing. Cryorolling induces the formation of more side cracks than does RT rolling, because of the reduction in the ability of the material to accommodate deformation at CT. Numerous {10–11} contraction and {10–11}-{10–12} double twins are formed in both the material rolled at RT and that rolled at CT, because the grains of the initial material are favorably oriented for {10–11} twinning under rolling. The RT-rolled material has a higher dislocation density than the cryorolled material, and more twins are uniformly distributed throughout the former material. As a result, static recrystallization during subsequent annealing is more pronounced in the RT-rolled material, which results in the formation of a highly recrystallized homogeneous microstructure after annealing. In contrast, the formed twins are predominantly present along the shear bands in the cryorolled material, as a result of which this material has an inhomogeneous bimodal structure containing a large amount of coarse unrecrystallized grains after annealing. The hardness of the annealed RT-rolled material is higher than that of the annealed cryorolled material owing to the finer grain structure of the former.



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