抽拉速率对车用Mg-12.85Gd-1.25Zn-0.45Y镁合金的定向凝固组织演化的影响

选择Mg-12.85Gd-1.25Zn-0.45Y镁合金作为测试材料, 通过电子背散射方法测试了各抽拉速率条件下其凝固组织内生成的初生-Mg相晶粒结构及其择优取向特点。研究结果表明:无论在何种换热系数下镁合金温度均随凝固时间的延长表现出单调降低的变化规律, 在300 s之后降温速度明显增加, 得到试样模型的对流换热系数为1600 W/ (m²·K) 。所有生长工艺条件下得到的Mg-12.85Gd-1.25Zn-0.45Y镁合金中定向凝固组织都只存在a与Mg₅Gd这类两种组织结构。当抽拉速率增大后, 镁合金基体中形成了相对圆滑的晶粒, 同时枝晶的间距也开始变小。形成的晶界数量较少, 表明大部分晶粒偏差角都在15°以内, 镁合金达到了一致性较高的晶粒生长取向性。随着抽拉速率的增加, 取向偏差表现出先降低后增加的变化规律, 在抽拉速率100μm/s时取得最小值。{0001}面与定向凝固热流方向一致, {1010}面形成了与{1120}面相近的极图。

Effect of Pull-Speed on Microstructures of Unidirectionally Solidified Mg-Ga Alloy

We experimentally addressed the microstructures of Mg-Ga alloys, exemplified by Mg-12.85 Gd-1.25 Zn-0.45 Y, fabricated by unidirectional solidification.The influence of the pull-speed on the microstructures and preferential growth orientation of primary α-Mg grains was investigated with X-ray diffraction and electron backscattered diffraction (EBSD).The results show that the pull-speed has a major impact.For example, α and Mg₅Gd phases co-exist independent of the solidification conditions;as the pull-speed increased, fairly smooth grains formed in Mg-matrix and the dendrite spacing decreases;a small number of grain boundary indicated that the preferential grain-growth orientations were highly consistent, with an orientation deviation within 15°;moreover, the deviation changed in an decrease-increase manner, minimizing at a pull-speed of 100 mμ/s.{0001}plane oriented in the solidification heat-flow direction, {1010}and{1120}planes formed almost identical polar diagrams.