【作者】 郭晶；

【导师】 常丽丽；

【作者基本信息】 山东大学 ， 工程硕士（材料工程）（专业学位）， 2019， 硕士

【摘要】 镁锂合金具有较高的比强度、比刚度以及低温抗冲击性能,广泛应用于航空、航天、汽车和电子等领域。然而镁锂合金的绝对强度偏低、延展性一般、常温下塑性变形能力差限制了其应用,合金化是改善镁锂合金综合力学性能的有效途径,而挤压工艺有助于合金组织的细化,拓宽合金的应用领域。本文设计并通过真空熔炼制备了含有合金化Y或Sn元素的三个系列镁锂铝合金成分(α-Mg相、α+β相和β-Li相),借助光学显微镜、扫描电子显微镜、透射电子显微镜、X射线、背散射电子衍射技术和拉伸实验等方法,研究了 Y或Sn元素合金化对Mg-Li-Al合金微观组织和力学性能的影响规律,主要研究结果如下:在Mg-4Li-3Al-xY(x=0,0.5,1,1.5,2)合金中加入Y有助于细化晶粒,A12Y相弥散分布在基体内析出强化的方式强化合金。在挤压过程中,A12Y相通过粒子激发形核(PSN)机制促进晶粒细化进而以细晶强化方式提高合金的综合力学性能。挤压态合金呈现<10-10>纤维织构,随着Y含量的逐渐增加,合金的<10-10>纤维织构沿<10-10>-<11-20>弧段的漫散越来越明显,并出现了[0002]//TD的新织构组分,合金各向异性的弱化有助于合金延展性的改善。当添加0.5 wt.%Y时,挤压态合金的抗拉强度最大(约259MPa),当Y含量达到1.5 wt.%时,合金延伸率最高为27.1%。退火后,合金的第二相颗粒数量明显减少,存在再结晶晶粒发生长大现象,Mg-4Li-3Al合金出现[0002]//TD的新织构组分,但添加Y元素的合金织构类型未发生明显变化。在挤压态 Mg-9Li-3Al-0.5Y、Mg-9Li-3Al-2Y、Mg-9Li-3Al-2Sn 和Mg-9Li-3Al-lSn-lY合金中,β-Li相发生动态再结晶过程,均匀分布在基体上的MgLi2Sn和Al2Y相通过析出相强化的方式强化了 Mg-9Li-3Al合金,分布在相界附近和在β-Li相晶粒内部的MgLi2Al相能够有效阻碍晶界移动,细化晶粒。在Mg-9Li-3Al合金添加Y,合金的α-Mg相体积分数降低,基面织构强度被弱化,促进<c+a>锥面滑移,β-Li相呈现出较强α-纤维织构,明显改善了合金的延展性。Sn的添加强化了合金α-Mg相的基面织构,通过细晶强化和析出相强化的方式使合金的强度得到大幅度的提高。Mg-13Li-3Al-xY(x=0,0.5,1,1.5,2)合金经过热挤压变形后,合金完成动态再结晶过程,与铸态组织相比,晶粒内部和沿晶界析出的第二相颗粒数量和大小明显减少,随着Y含量的逐渐增加,A12Y相的数量也逐渐增多,但合金的织构类型未发生显著变化,当添加0.5 wt.%Y时,挤压态合金的抗拉强度最大(约239MPa),延伸率为25.2%,具有良好的综合力学性能。更多还原

【Abstract】 Mg-Li alloys with high specific strength,specific stiffness and low temperature impact resistance have been applied in aerospace,automotive,electronics device industries and so on.However,their application is limited by low absolute strength,general ductility and poor plastic deformation ability at room temperature.Alloying is an effective way to improve comprehensive mechanical properties of Mg-Li alloys,and extrusion process is helpful to refine microstructure and broaden the application field of Mg-Li alloys.In this work,three series of Mg-Li-Al alloys with Y or Sn addition were prepared(α-Mg phase,α+β duplex phase and β-Li phase).Microstructure evolution and mechanical properties of Mg-Li-Al based alloys were characterized by using optical microscopy,scanning electrical microscopy,X-ray diffraction,electron back-scatter diffraction,transmission electrical microscopy and tensile test.The effects of alloying Y or Sn on microstructure and mechanical properties of Mg-9Li-3Al based alloys were discussed.The main results were as follows:Y addition contributed to refining the grains in Mg-4Li-3Al-xY(x=0,0.5,1,1.5,2 wt.%)and Al2Y dispersed in the matrix and strengthens the alloys by precipitation hardening.During hot-extrusion process,Al2Y particles facilitated grain refinement by particle stimulated nucleation(PSIN),improving the comprehensive mechanical properties by fine grain strengthening.Hot-extruded Mg-4Li-3Al alloy presented<10-10>fiber texture.As Y content increased,the dispersion of<10-10>fiber texture along the arc of<10-10>-<11-20>became more and more obvious and new texture components of[0002]//TD appeared.The weakening of anisotropy helped improve the ductility.When adding 0.5 wt.%Y,extruded Mg-4Li-3Al alloy achieved the highest tensile strength(～259MPa).When 1.5 wt.%Y was added,extruded Mg-4Li-3Al alloy has the best elongation(～27.1%).After annealing,the amount of second phase particles decreased obviously and the recrystallized grains grew.Mg-4Li-3Al alloy presented an obvious texture component of[0002]//TD,but the texture type of alloys with Y addition didn’t change significantly.In hot-extruded Mg-9Li-3AI-0.5Y,Mg-9Li-3Al-2Y,Mg-9Li-3Al-2Sn and Mg-9Li-3Al-1Sn-1 Y alloys,dynamic recrystallization occurred in β-Li phase.Uniform dispersing MgLi2Sn and Al2Y particles in the matrix resulted in strength improvement in Mg-9Li-3Al alloys by precipitates hardening.MgLi2Al precipitates locating at phase interfaces and inside P-Li phase impeded grain boundary migration,leading to grain refinement during dynamic recrystallization.The volume fraction of a phase and the intensity of basal texture in Mg-9Li-3Al alloys were decreased by Y addition,promoting pyramidal<c+a>slip in a-Mg phase.The increasing Y addition into Mg-9Li-3Al alloys leads to stronger a-fiber textures,providing a strengthening effect on the alloys.While the maximum texture intensity of a phase is obtained by Sn addition.Strength was significantly improved mainly by the addition of Sn due to grain refining and precipitation strengthening.After hot extrusion,dynamic recrystallization occurred in Mg-13Li-3Al-xY(x=0,0.5,1,1.5,2 wt.%)alloys.Compared with cast alloys,the amount and size of second phase particles distributing inside and along grain boundaries were decreased.With Y content increasing,the number of Al2Y particles was increased,but the texture type of alloys did not change significantly.When 0.5 wt.%Y was added,extruded Mg-13Li-3Al alloy achieved the highest tensile strength(-239MPa),and its elongation was 25.2%,presenting good comprehensive mechanical properties.更多还原