Microstructure and mechanical properties of ultra-high strength TiCp/Mg-1.4Zn-2.6Ca-0.5Mn nanocomposite after hot extrusion
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摘要: 采用超声波辅助半固态搅拌铸造法制备TiCp/Mg-1.4Zn-2.6Ca-0.5Mn纳米复合材料，实现纳米TiCp的均匀分布，并分析热挤压前后复合材料的组织与力学性能。结果表明：挤压前第二相密集区晶粒尺寸小于第二相贫瘠区，第二相为Ca2Mg6Zn3相；不同温度（350 ℃、310 ℃和270 ℃）挤压后复合材料均发生了动态再结晶（DRX），随挤压温度的降低，DRX晶粒尺寸及其体积分数趋于减小，而析出相体积分数则略有增加，超细晶（约0.34 μm）和大量MgZn2析出相出现在270 ℃挤压态复合材料中；复合材料晶粒细化不仅与DRX有关，还与纳米级的α-Mn颗粒、TiCp和MgZn2析出相的钉扎效应有关；经270 ℃/0.1 mm•s−1挤压后，复合材料的屈服强度（YS）、极限抗拉强度（UTS）和伸长率（EL）分别约为439.7 MPa、460.2 MPa和1.73%；屈服强度提高主要与细晶强化、Orowan强化、热错配强化和位错强化有关，其中细晶强化的贡献率最大超过60%。Abstract: A uniform distribution of TiCp nanoparticles was realized in the TiCp/Mg-1.4Zn-2.6Ca-0.5Mn nanocomposite fabricated by the method of ultrasonic-assisted semisolid stirring. Microstructure and mechanical properties of the nanocomposite before and after extrusion were investigated. The results show that the grains in the dense area of the second phase were smaller than those in the barren area, and the second phase was Ca2Mg6Zn3. Dynamic recrystallization (DRX) occurred in the nanocomposites after extrusion at different temperatures (350 °C, 310 °C and 270 °C). Both the sizes and volume fraction of DRX grains and precipitates size were obviously refined as the extrusion temperature decreased, while the volume fraction of precipitates increased. Ultrafine recrystallized grain structure (≈0.34 μm) with a substantial of fine precipitates appeared in the nanocomposite extruded at 270 °C. The refined grain structure was not only due to DRX, but also the synergistic pinning effect of nano-TiCp, precipitated MgZn2 and α-Mn particles. The optimum tensile strength was achieved in the nanocomposites extruded at 270 °C/0.1 mm•s–1, and the yield strength (YS), ultimate tensile strength (UTS) and elongation to failure (EL)were ≈439.7 MPa、≈460.2 MPa and ≈1.73%, respectively. The grain refinement strengthening with the contribution ratio over 60% to YS increment was much higher relative to thermal expansion effect, Orowan strengthening and dislocation strengthening.
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图 2 铸态和均匀化处理后TiCp/ZXM纳米复合材料的OM图像和SEM图像 （a）铸态；（b）、（c）、（d）均匀化处理
Figure 2. OM and SEM images of as-cast and as-homogenized TiCp/ZXM nanocomposites （a） as-cast；（b），（c），（d）as-homogenized nanocomposite
图 3 铸态TiCp/ZXM纳米复合材料的TEM像（a）沿晶界分布的粗大块状Ca2Mg6Zn3相（图3（a）中的插图为块状相的选区电子衍射，表明为Ca2Mg6Zn3）；（b）晶粒内部的细小片状Ca2Mg6Zn3相及沿晶界分布的TiCp
Figure 3. TEM micrographs of as-cast TiCp/ZXM nanocomposites （a） block Ca2Mg6Zn3 phase along grain boundary（Insert in Fig. 3（a） shows the selected area diffraction pattern from the block phase，shown to be consistent with Ca2Mg6Zn3）；（b） fine Ca2Mg6Zn3 phase within grain and TiCp along grain boundary
图 4 TiC/pZXM以0.1 mm/s挤压速率在不同温度挤压后OM像和DRX晶粒尺寸分布 （a）、（b）、（c）TiCp/ZXM-350；（d）、（e）、（f）TiCp/ZXM-310；（g）、（h）、（i）TiCp/ZXM-270
Figure 4. OM images and its corresponding DRX grain size distribution of TiCp/ZXM extruded with the speed of 0.1 mm/s at different extrusion temperatures （a），（b），（c）TiCp/ZXM-350；（d），（e），（f）TiCp/ZXM-310；（g），（h），（i）TiCp/ZXM-270
图 5 TiCp/ZXM在0.1 mm/s经不同温度挤压后的析出相尺寸和体积分数
Figure 5. Average size and volume fraction of precipitates in TiCp/ZXM nanocomposite extrude at 0.01 mm/s with different extrusion temperatures
图 6 TiCp/ZXM在270 ℃以0.1 mm/s挤压后的TEM明场像 （a）TiCp颗粒分布；（b）第二相分布和DRX晶粒
Figure 6. Bright field TEM images of TiCp/ZXM extruded at 270 ℃ by 0.1 mm/s （a） distribution of TiCp；（b）distribution of second phases and DRX grains
图 7 不同温度挤压后TiCp/ZXM纳米复合材料的室温拉伸性能 （a）工程应力-应变曲线（插图为基于YS和d-1/2拟合的TiCp/ZXM纳米复合材料的Hall-Petch斜率）；（b）YS、UTS和EL
Figure 7. Room tensile properties of TiCp/ZXM with different extrusion temperatures （a）tensile stain-stress curves（Insert shows the Hall-Petch plots of the yield strength against d-1/2 for TiCp/ZXM nanocomposites）；（b） YS，UTS and EL
图 8 各强化机制及实验值和理论值对比 （a）强化机制；（b）实验值和理论值
Figure 8. Comparison strengthening mechanisms and comparison of YS among the experimental data and predicted data（a）strengthening mechanisms；（b）experimental data and predicted data
图 9 TiCp/ZXM 纳米复合材料经270 ℃/0.1 mm•s−1挤压后拉伸断口SEM像 （a）低倍；（b）高倍
Figure 9. SEM fracture surface morphologies of TiCp/ZXM-1.2 nanocomposites extruded at 270 ℃/0.1 mm•s−1 （a）low magnification；（b）high magnification
表 1 本工作TiCp/ZXM纳米复合材料和其他挤压态镁基纳米复合的拉伸性能对比
Table 1. Tensile properties of developed TiCp/ZXM nanocomposites，and their comparison with previous studies
Material Deformation YS/MPa YS/MPa EL/% References TiCp/ZXM As-cast 92.8 ± 2.3 133.6 ± 13.1 2.1 ± 0.7 [Present work] TiCp/ZXM-350 EX 333.4 ± 15.9 338.8 ± 3.5 6.2 ± 1.3 [Present work] TiCp/ZXM-310 EX 398.2 ± 8.6 405.1 ± 6.6 3.1 ± 0.6 [Present work] TiCp/ZXM-270 EX 439.7 ± 10.6 460.2 ± 1.8 1.73 ± 0.4 [Present work] N-SiCp/AZ91 EX ≈255 ≈345 ≈12 [10] N-SiCp/AZ31 EX ≈320 ≈385 ≈6 [6] N-TiCpZK60 EX ≈184 ≈309 ≈11.6 [14] 
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