【作者】 徐世伟；

【导师】 郑明毅；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2006， 硕士

【摘要】 本文采用等通道角挤压(ECAP)对挤压态Mg-5.0wt%Zn-0.9wt%Y-0.2wt%Zr合金在175℃至300℃温度范围内进行了不同道次的塑性变形。采用光学显微镜、扫描电子显微镜和透射电子显微镜研究了挤压态镁合金ECAP变形前后的显微组织变化,采用中子衍射和EBSD(电子背散射)对ECAP变形前后镁合金的织构演变进行了分析,并对ECAP变形前后镁合金的室温拉伸性能进行了测试。在室温和高温下对挤压态镁合金和200℃8道次ECAP变形后的超细晶镁合金进行了不同应变速率的拉伸和压缩试验,并对其变形机理进行了分析。挤压态合金的起始晶粒尺寸约为5μm,准晶相沿挤压方向呈带状分布。在ECAP变形过程中准晶相破碎弥散分布于基体合金中,一些纳米尺寸的准晶相从基体中析出。ECAP变形可以显著细化合金晶粒,在200℃经过8道次ECAP变形后,平均晶粒尺寸细化至0.5μm,达到亚微米超细晶粒水平。随着ECAP变形道次的增加,合金的屈服强度下降,但室温延伸率却得到了显著的提高,在300℃经过8道次ECAP变形以后,合金的延伸率达到了35%,是原始挤压态合金的三倍,这主要是晶粒细化和ECAP变形过程中织构的变化所致。压缩实验表明,由于晶粒的细化以及准晶相在基体合金中的破碎和析出可以有效地阻止晶粒长大,使得ECAP超细晶镁合金与挤压态镁合金相比,具有更好的高温压缩变形性能。在150℃压缩时,ECAP超细晶镁合金在所有的应变速率下都表现出均匀的塑性变形,而挤压态镁合金却过早的发生断裂,只有在较低的应变速率1.67×10-3s-1下压缩时才表现出均匀的塑性变形。与挤压态镁合金相比,ECAP超细晶镁合金没有明显的加工硬化现象,这主要是因为ECAP变形导致镁合金晶粒显著细化,{0002}基面和< 1010>方向倾向与挤压方向约成45o角分布,使得{1012}孪晶很难产生,因此由孪晶引入的加工硬化也减弱。200℃8道次ECAP变形后的超细晶镁合金表现出低温高应变速率拉伸超塑性,在200℃应变速率为1.67×10-3s-1拉伸时,得到最大的延伸率860%,在200℃应变速率为1.67×10-2s-1拉伸时,延伸率为200%;在150℃应变速率为1.67×10-3s-1拉伸时,延伸率为120%。而挤压态镁合金只在250℃应变速率为1.67×10-3s-1拉伸时才表现出超塑性,延伸率仅为160%。更多还原

【Abstract】 Equal channel angular pressing (ECAP) was performed on the extruded Mg-5.0wt%Zn-0.9wt%Y-0.2wt%Zr magnesium alloy containing quasicrystallines at the temperature from 175℃to 300℃for different passes. The microstructure of the as-extruded magnesium alloy and ECAPed ultra-fine grained magnesium alloy were examined by means of optical microscopy (OM), scanning electronic microscopy (SEM) and transmission electron microscopy (TEM). Texture development of the as-extruded magnesium alloy and ECAPed ultra-fine grainrd magnesium alloy was investigated by neutron diffractometer and EBSD(Electron back scattered diffraction). Tensile deformation behavior of the as-extruded magnesium alloy and ECAPed ultra-fine grained magnesium alloy at ambient temperature were investigated. Both the compressive and tensile deformation behavior of the as-extruded magnesium alloy and the 200℃-8pass ECAPed ultra-fine grained magnesium alloy at ambient temperature and elevated temperatures under different strain rates were investigated and the deformation mechanism was discussed.The as-extruded Mg-Zn-Y-Zr alloy had an initial grain size of about 5μm and bands of quasicrystal phase parallel to the extrusion direction. During ECAP, the quasicrystalline phases were broken and dispersed in the matrix and some nano-quasicrystallines precipitated from the matrix. After the extruded alloy was subjected to 8-pass ECAP at 200℃, the grain size was refined to about 0.5 um. With increasing of ECAP passes, the yield stress of the alloy was decreased, while the elongation to failure at ambient temperature was effectively improved. After 8-pass ECAP at 300℃, the elongation to failure of the alloy reached 35%, which was three times of that of as-extruded alloy and was considered to be resulted from the grain refinement and the texture modification during ECAP.The compressive deformation showed that due to the significant refinement of the grain size and the broken and dispersed quasicrystalline phases could effectively prevented the grain from growing, so the ECAPed ultra-fine grained magnesium alloy exhibited better deformability than the as-extruded magnesium alloy. During the compressive test at 150oC, the ECAPed alloy specimens更多还原