Zn含量及热处理对Mg-Zn-Mn变形镁合金显微组织和力学性能的影响

Effects of Zn Content and Heat Treatment on Microstructures and Mechanical Properties of Mg-Zn-Mn Wrought Magnesium Alloys

摘要:

研究了Zn含量和热处理工艺对挤压态Mg-x%Zn-1%Mn (x=4, 5, 6, 7, 8, 9，质量分数)镁合金组织和力学性能的作用规律。结果表明：Zn含量增加，挤压时动态再结晶趋于完全且小晶粒容易长大，同时第二相流线亦随之增加并阻碍其长大；再结晶晶粒在固溶过程中会长大，但Zn含量越高长大越困难，最终其晶粒更细小。通过透射电子显微镜(TEM)研究发现，时效态合金中主要存在2种亚稳相，即：长轴沿[0001]α的杆状相(β1′相)和(0001)α上的盘状相(β2′相)，它们与基体之间存在共格或半共格界面，杆状β1′ 相对位错运动的阻碍更加强烈；根据X射线衍射(XRD)的分析结果，可知2种相都是具有Laves结构的MgZn2相。双级时效处理在低温预时效阶段从过饱和固溶体析出G.P.区，为第二级时效提供形核核心，从而细化了β1′ 和β2′ 相，增加了其弥散度。高Zn含量合金在180 ℃时效16 h后发生了过时效，通过消耗β1′ 和β2′ 而形成了大块的Mg-Zn化合物。在时效态组织中，Mn以杆状析出，可以作为β1′和β2′相的形核核心，使二者发生粗化。挤压态合金的力学性能对Zn含量的变化不敏感；时效态合金的强度随Zn含量的增加呈抛物线增加，当Zn含量大于6%时，强度增加缓慢，延伸率急剧降低，这与高锌合金容易过时效且存在残余流线有关。故含6%Zn合金具有最佳的力学性能

Abstract:

The roles of Zn content and thermomechanical treatment in affecting microstructures and mechanical properties of Mg-x wt%Zn-1wt%Mn (x=4, 5, 6, 7, 8, 9) wrought Mg alloys were investigated. The microstructure during extrusion was extremely refined by dynamic recrystallization (DRC). As Zn content increasing, the DRC grains tended to grow up; at the same time, more second phase streamlines would be present which restricted their further growth. During solution treatment, the DRC grains would rapidly grow up; however, more Zn content could obstruct the grain boundary expanding which resulted in finer grains. The microstructures of the aged specimens were examined by transmission electron microscopy (TEM). The age-hardening resulted mainly from two metastable phases, known as β1′ and β2′, the interfaces between which and the matrix were coherent or semicoherent. β1′ phases formed as rods with their long axis parallel to the [0001]α direction of the α-Mg matrix could act as a more enormous impediment to the motion of dislocations than β2′ formed as plates on (0001)α. Based on X-ray diffraction, both phases might be the Laves phase MgZn2. Compared with the one-step aging, β1′ and β2′ in two-step aged alloys were much finer and more dispersed, since nano-sized and coherent G.P. zones formed during the pre-aging stage provided more effective nuclei for the two phases during the second stage. Higher Zn content was apt to cause overaging, so large Mg-Zn blocks were formed by consuming β1′ and β2′ after aging at 180 oC for 16 h. Mn was present as rod-shaped precipitates in the aged specimens, which could act as nuclei for β1′ and β2′ and drive them into coarser ones. The mechanical properties of as-extruded samples were not so sensitive to the changes of Zn content. The strength of as-aged samples rose as a parabolic curve with the increasing of Zn content, while Zn content exceeded its solid solution limit in Mg (6.2 wt%), strength rising became slow, and the elongation quickly deteriorated, so a Mg-Zn-Mn alloy with 6wt% Zn possessed the best mechanical properties.