【作者】 郭鹏；

【导师】 郑明毅；

【作者基本信息】 哈尔滨工业大学 ， 材料工程（专业学位）， 2018， 硕士

【摘要】 作为最轻的结构金属材料,镁合金由于其具有的无污染、无毒害、易回收、良好的阻尼减震性能等优势,越来越多的被应用于航空航天领域。然而,较低的绝对强度限制了其进一步的应用。为了克服这一缺陷,本文通过水冷铸造工艺制造了不同成分的高稀土Mg-Gd-Y-Zn-Zr合金,并对其进行均匀化、热挤压以及人工时效处理,研究了不同Gd、Y、Zn含量,不同均匀化工艺以及不同挤压温度对合金显微组织与力学性能的影响,以期得到性能更佳优良的稀土镁合金。LPSO结构能够提高合金强度并改善合金的塑形,是Mg-Gd-Y-Zn-Zr合金均匀化处理后的残余第二相。稀土镁合金在时效过程中会析出大量的纳米尺寸的β’相,在总量相同的情况下,β’相具有比LPSO结构更好的强化效果,但不利于合金塑性。通过控制LPSO结构含量可以调控β’相的析出。LPSO结构会占据合金中的稀土元素,减少时效过程中的β’相的析出。在Mg-Gd-Y-Zn-Zr(Gd+Y=15wt.%)合金中改变Gd、Y比,Y/Gd增大会导致合金能够析出的第二相总量增多,同时均匀化处理后的14H-LPSO结构也会增多。所以Y元素较少时,第二相总量的增多会使合金强度上升。当Y元素过多时,过多的LPSO结构会极大的减少β’相的析出,即便第二相总量增加但合金强度依旧在下降。Gd/Y在2附近性能最佳。一旦Y/Gd大于1综合性能便会很差。超高强韧性能最佳的T5态Mg-9.7Gd-5.8Y-1.6Zn-0.3Zr合金抗拉549MPa,屈服485MPa,延伸率达到8.1%。在Mg-10Gd-5Y-x Zn-0.4Zr合金中改变Zn含量,合金中析出的第二相总量增,但LPSO结构同样急剧增多,β’相的析出减少,当Zn含量增加到4.8wt.%时,几乎没有β’相结构析出。在Mg-Gd-Y-Zn-Zr(Gd+Y=15wt.%)合金中单独增加Gd或Y,结果表明增加Gd更有助于合金屈服强度的提高,因为过多的Y增多了LPSO结构,降低了β’相的析出。相比一级均匀化,二级均匀化处理后残余14H-LPSO结构更少,在200℃时效的过程中能够析出更多的β’相。第二相总量没有改变,合金的时效强化效果得到了加强,T5态合金屈服强度升高、延伸率降低。T5态Mg-9.7Gd-5.8Y-1.6Zn-0.3Zr合金抗拉574MPa,屈服516MPa,延伸率达到4.4%,相比一级均匀化强度提高。更多还原

【Abstract】 As the lightest metal structural materials,Magnesium alloys exhibit many merits including pollution-free,non-toxic,recycling and good damping,are more and more extensively applied in automotive and aerospace industries.However,applications of magnesium alloys are still limited due to the poor strength.To overcome this shortcoming,Mg-Gd-Y-Zn-Zr alloys with high rare earth content were prepared by water cooling casting,and were strengthed by homogenization treatment,extrusion and aging treatment.The effects of microstructures and mechanical properties by different solution processes,different extrusion temperature,differernt Gd,Y,Zn content were analyzed to obtain better comprehensive mechanical properties.LPSO structure can effectively improve the strength and the elongation of the rare earth magnesium alloys.At the same time,the LPSO structure was the main residual second phase after the homogenization treated of Mg-Gd-Y-Zn-Zr alloy.The rare earth magnesium alloy will precipitate a large number of nanoscale β’ phases during aging hardening.The β’ phase has a better strengthening effect than the LPSO structure but will lead to the poor plasticity.The precipitation of β’ phase can be controlled by controlling the LPSO structure content The LPSO structure will occupy rare earth elements,which will reduce the aging precipitation of β ’ phase.The effects of Gd/Y ratios in the Mg-Gd-Y-Zn-Zr(Gd+Y=15wt.%)alloys were studied,the increase of Y/Gd ratios caused the incease the total amount of the second phase while the 14H-LPSO structure after the homogenization treated also increased.Therefore,when the content of Y element is small,the increase of the second phase increases the strength of the alloy.When the content of Y element is too high,too much LPSO structure will greatly reduce the precipitation of the β’ phase.Even if the total amount of the second phase increases,the strength of the alloy is still decreasing.In the Mg-Gd-Y-Zn-Zr(Gd+Y=15wt.%)alloys,Gd/Y ratio has the best performance at about 2.Once Y/Gd is greater than 1,the comprehensive performance will be poor.The peak aged Mg-9.7Gd-5.8Y-1.6Zn-0.3Zr alloy has the excellent UTS,YTS and elongation,i.e.549 MPa,485MPa and 8.1%,respectively.The effects of the Zn content in the Mg-10Gd-5Y-x Zn-0.4Zr alloys were studied,the increase of Zn content caused the incease the total amount of the second phase while the 14H-LPSO structure also increased rapidly.when the Zn content increases to 4.8 wt.At %,almost no β’ phase was precipitated.Adding Gd or Y to the Mg-Gd-Y-Zn-Zr(Gd+Y=15wt.%)alloy alone,it is found that increase of Gd is more effective than increase of Y,because Y element contributes to the formation of LPSO structure,which decreases the precipitation of β’ phase.Compared with the first stage homogenization,the residual 14H-LPSO structure is less after the secondary homogenization treatment,and more β’ phases can be precipitated during the aging at 200°C.The total amount of the second phase did not change,the aging strengthening effect of the alloy was strengthened,and the yield strength of the peak aged alloy increased and the elongation decreased.The peak aged Mg-9.7Gd-5.8Y-1.6Zn-0.3Zr alloy has a tensile strength of 574 MPa,a yield of 516 MPa and an elongation of 4.4%,which is higher than that of the first stage homogenization.更多还原