Mg-3Sn-lMn alloy profiles with cross-section size of 5 mmx50 mm were prepared by the continuous rheo-forming process, and the microstructures and formation mechanisms of the alloy in the roll-shoe gap were investigated. The results show that the heterogeneous nucleation of the alloy occurs firstly on the surface of roll-shoe. When the roll surface is rough, the heterogeneous nucleation capability of the alloy is stronger on the roll surface, which is favorable to the formation of dendrites. The dendrites are broken up to form free grains under the action of cooling/shearing of the roll. The dendrite fragmentation is caused by the shearing fracture mechanism of dendritic arms and the blowing-out mechanism of dendritic arms. During the growing process of free grains, because of laminar shearing caused by the roll, the free grains are broken up further, the solute distribution around free grains is homogenous, the preferred orientation of heat dissipation is not obvious, the growing chances of grains are equal in all directions, and the grains grow in golobular or equiaxed form.%利用连续流变成形实验机制备断面为5 mm×50 mm的Mg-3Sn-1Mn镁合金型材,研究辊靴型腔中的合金组织及其形成机理.结果表明:合金熔体首先在轧辊和靴子表面异质形核.当轧辊表面比较粗糙时,合金在轧辊表面异质形核能力较强,利于枝晶的形成；在轧辊剪切/冷却作用下,枝晶发生破碎形成自由晶,枝晶破碎主要由枝晶臂剪切断裂机制和枝晶臂熔断机制引起；自由晶在生长过程中,由于合金熔体内部的层流剪切作用,自由晶进一步被破碎,晶粒周围溶质分布趋于均匀,其散热择优方向也不明显,沿各个方向生长机会均等,晶粒以球状晶或等轴晶形式长大.
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