【作者】 崔崇亮；

【导师】 张密林；

【作者基本信息】 哈尔滨工程大学 ， 材料学， 2013， 博士

【摘要】 作为目前世界上最轻的金属结构材料，Mg-Li合金具有密度低、比强度高、比刚度高、冷热变形能力好，各向异性弱等特点，在航空航天、国防军事以及汽车电子等领域具有广阔的应用前景。然而，Mg-Li合金的强度较低一直是制约其大规模应用的主要原因。在提高合金强度的众多方法中，添加合金化元素以其简便有效的特点受到青睐。作为一种合金化元素，稀土元素常常被应用于镁合金中。本文在α相Mg-Li合金中添加适量的稀土元素Y和Nd，优化这两种稀土元素在Mg-5Li-3Al-2Zn合金中含量，探究稀土元素Y、Nd对Mg-5Li-3Al-2Zn合金显微组织、相组成、力学性能以及不同稀土元素对合金强韧化机理的差异，同时也证实了Y和Nd元素存在复合强韧化合金的作用，并通过对形变织构的分析认识了Mg-Li合金塑性变形的特点，清楚了稀土元素Y和Nd在改变Mg-Li合金变形机制方面的作用。铸态Mg-5Li-3Al-2Zn合金是由基体α-Mg和絮丝状的AlLi相组成。稀土Y的加入会使合金中生成Al2Y，而AlLi相会减少。当Y含量为0.8%时铸态合金的晶粒细化效果最佳，其抗拉强度也达到了最高值（202MPa）。挤压变形后，合金发生了动态再结晶，Y含量为1.2%时，合金的抗拉强度达到了最高值（325MPa），同时，延伸率也都达到了最高值（23.3%）。随着Nd含量的增加，铸态Mg-5Li-3Al-2Zn合金中除了有针状的Al11Nd3，还有块状的Al2Nd金属间化合物。当Nd含量为1.2%时，合金的抗拉强度达到了195.2MPa和延伸率达到17.2%的最佳力学性能。Nd含量为2.0%时，合金的平均晶粒尺寸减小到68μm，比未加稀土合金的平均晶粒尺寸减小了44%。而挤压态合金在Nd含量为0.8%时，抗拉强度达到了最高值（281.9MPa）。在Nd含量为1.6%时，挤压态合金的延伸率达到最高值（27.2%）。Y和Nd元素对铸态Mg-5Li-3Al-2Zn合金进行复合合金化后，合金中存在α-Mg、AlLi、Al11Nd3以及Al2Y。当Y和Nd添加量分别为1.2%和0.8%时，合金的平均晶粒尺寸达到了30μm，复合合金化后晶粒细化效果显著，而该合金的抗拉强度达到了最高值231MPa，而合金的延伸率也达到了最大的16%。Mg-5Li-3Al-2Zn-1.2Y-0.8Nd合金在热挤压过程中发生了完全动态再结晶现象，晶粒细小且呈等轴状，合金中的AlLi相呈条带状的分布。该合金在室温下的抗拉强度为224MPa，延伸率为24.6%，而在100℃下的抗拉强度为166MPa，延伸率为37.9%。当应变速率为10-3s-1时，压缩强度为397.7MPa，应变速率增加到10-2s-1时，压缩强度为406.7MPa。相应应变速率下的屈服强度也有类似的规律。室温下的准静态拉伸过程中，该合金出现了在镁合金中少见的塑性失稳现象。沿着ED(extrusion direction)方向的室温压缩测试表明，合金表现出了屈服强度和最大压缩强度都对应变速率不敏感的特点。对比Mg-5Li-3Al-2Zn合金中加入1.2%Y、0.8%Nd和共同加入1.2%Y+0.8%Nd的这三种合金抗拉强度和延伸率提高比例可知，Y和Nd元素以1.2:0.8的比例添加，其对合金强韧化效果远大于单独添加相应含量的Y和Nd的作用效果之和。证实了稀土Y、Nd间存在复合强韧化合金的效果。Mg-5Li-3Al-2Zn和Mg-5Li-3Al-2Zn-1.2Y-0.8Nd两种铸态合金都没有出现特定的织构类型，复合添加稀土元素的合金其织构表现的更加散漫。在压缩应变为0.17时，Mg-5Li-3Al-2Zn合金的大部分晶粒的C轴与样品法向Nd大致成75°，而Mg-5Li-3Al-2Zn-1.2Y-0.8Nd合金在应变为0.10时出现了较强的柱面织构，即大部分晶_粒<1010>//ND(normal direction)。对Mg-5Li-3Al-2Zn和Mg-5Li-3Al-2Zn-1.2Y-0.8Nd两种合金在不同压缩应变下的主要织构组分分析表明，两种合金在压缩应变小于0.10时，该合金中织构分布较为均匀，在0.17的应变下均出现了织构组分不均匀分布的现象。其中，只有_Mg-5Li-3Al-2Zn-1.2Y-0.8Nd合金的(1120)<0001>织构组分随着应变的增加而增强。两种成分的合金在室温下的压缩过程中都表现出了较好的塑性，其压缩应变均超过了20%。Mg-5Li-3Al-2Zn合金最高的压缩强度为311MPa，加入Y和Nd后合金的压缩强度最高可达到403MPa，强度提高了近30%。加入0.4%的稀土Nd后，热挤压态Mg-5Li-3Al-2Zn合金的(0002)极图中的织构表现出沿TD方向分布的特点，其织构组分出现在ND偏移TD(transverse)左侧大约5°的区域。而当Nd含量为2.0%时，Mg-5Li-3Al-2Zn合金的热挤压织构则是非基面织构类型（ND向TD右侧偏转约30°）。复合添加Y、Nd的Mg-5Li-3Al-2Zn合金，其热挤压织构出现了明显的非基面织构，即在(0002)极图中，极密度最大区域出现在晶粒的C轴偏转ED方向±60～70°。EBSD的取向成像分析数据表明，非基面织构的晶粒取向差为85°左右。晶界分析的结果表明，小角度晶界所占比例较小，大角度晶界的分布比较均匀。挤压变形后合金的晶粒比较细小，其中小于5μm晶粒所占比例为50.1%。在400℃下退火处理了30分钟、60分钟和90分钟后，该合金中的非基面织构发生弱化，与此同时，合金的平均晶粒尺寸也明显长大，合金的强度和塑性均下降。室温拉伸过程中，退火后的该合金仍然有塑性失稳现象出现。对铸态Mg-5Li-3Al-2Zn-1.2Y-0.8Nd合金进行温轧和热轧变形，在变形量超过50%后，两种轧制工艺下的合金均出现了显著的非基面织构类型。即在(0002)基面极图中，在RD两侧出现了对称的双峰。温轧过程中当变形量为28%时，大部分晶粒C轴与RD方向夹角集中在25°附近。当变形量为52%时，该织构得到加强。通过对Mg-5Li-3Al-2Zn-1.2Y-0.8Nd合金压缩、挤压以及不同温度下轧制的织构研究发现，该合金的柱面滑移系比基面滑移系容易启动，并且柱面滑移和锥面孪生机制是该合金主要的变形机制。更多还原

【Abstract】 As the lightest metallic structural materials with low density, high specific stiffness andstrength, weaken anisotropy, Mg-Li alloy is very attractive in defense, aerospace, automotiveand electronics industry. However, the poor strength limits the application of Mg-Li alloy.Adding alloying elements is an effective and simple method to improve the strength of thealloy among the method of strength improving. Rare earth elements are the main elementsused in magnesium alloys as alloying elements.Y and Nd were added into α phase Mg-Li alloy in the study to optimizing the content of thetwo rare earth elements, researching the effects of Y and Nd on the microstructure, phasescomposition, mechanical properties of the alloy and the differences of strengthening andtoughening mechanism of Nd and Y and confirming that Nd and Y have the compositestrengthening and toughening in Mg-5Li-3Al-2Zn alloy, studying the characteristics throughanalyze the deformation textures, realizing the effects of changing plastic deformationmechanism with Y and Nd’s addition in alloy.The as-cast Mg-5Li-3Al-2Zn alloy is composed of α-Mg substrate and filamentous AlLiphase. After adding Y, Al2Y generates in the alloy, and the amount of AlLi reduces. When thecontent of Y is0.8%, the grain refining effect of as-cast alloy reaches the best, and the tensilestrength reaches the highest level of202MPa. The dynamic recrystallization arising afterextrusion, as the content of Y is1.2%, the alloy has the highest tensile strength and elongation,325MPa and23.3%, respectively.With the increase of Nd content, needle-like Al11Nd3and massive Al2Nd intermetalliccompounds exist in the alloy. When the content of Nd is1.2%, the alloy achieves the bestmechanical properties,195.2MPa of tensile strength and17.2%of elongation. When the Ndcontent is2.0%, the average grain size of the alloy reduces to68μm, smaller than that of alloywithout Nd addition by44%. For the extruded alloys, the alloy with0.8%Nd possesses thehighest tensile strength of281.9MPa. When the Nd is1.6%, the elongation of the extrudedalloy reaches the maximum value of27.2%.The α-Mg, AlLi, Al11Nd3and Al2Y exist in Mg-5Li-3Al-2Zn alloy with the addition of Yand Nd. When the content of Y and Nd is1.2%and0.8%, respectively, the effect of grainrefining is the best, while the average grain size is of30μm, and the tensile strength achieves the highest value of231MPa and the elongation also reaches a maximum value of16%.The dynamic recrystallization arises in the Mg-5Li-3Al-2Zn-1.2Y-0.8Nd alloy during hotextrusion, resulting in the equiaxed fine grains. The tensile strength at room temperature is224MPa and the elongation is24.6%. At the temperature of100℃,the tensile strength andelongation are166MPa and37.9%, respectively. When the strain rate is10-3s-1, thecompressive strength is397.7MPa, and the compressive strength is406.7MPa when the strainrate is10-2s-1. The yield strength under the corresponding strain rates also has a similar law.The plastic instability phenomenon appears in the alloy during the quasi-static tensile processat room temperature. The compression test along ED at room temperature shows that the yieldstrength and the maximum compressive strength are both insensitive to the strain rate.Comparing the tensile strength and elongation of Mg-5Li-3Al-2Zn-1.2Y,Mg-5Li-3Al-2Zn-0.8Nd and Mg-5Li-3Al-2Zn-1.2Y-0.8Nd, it shows that the increasing ratiofor the1.2Y+0.8Nd is greater than that for the sum of adding of1.2Y and0.8Nd separately,confirming the effect of the composite strengthening of Y and Nd in Mg-5Li-3Al-2Zn.There is no specific type of texture arising in as-cast alloys, the texture of the alloy with Yand Nd shows more scatter. In the compressive strain of0.17, the C-axis of most of the grainsis toward ND about75°in Mg-5Li-3Al-2Zn. For Mg-5Li-3Al-2Zn-1.2Y-0.8Nd, a strongcylindrical texture appear when the compressive strain is0.10, most of the grains distributing_with the orientation of <1010>//ND.The texture components analysis between Mg-5Li-3Al-2Zn-1.2Y-0.8Nd andMg-5Li-3Al-2Zn under different compressive strain showed that when the strain is less than0.10, the texture components of the two alloys distribute both evenly. When the strain is0.17,_the texture components of the two alloys distribute both unevenly.(1120)<0001> texture ofMg-5Li-3Al-2Zn-1.2Y-0.8Nd increases with the strain increasing. The two alloys show goodplasticity under compression process at room temperature, exceeding20%. The highestcompressive strength of Mg-5Li-3Al-2Zn is311MPa, after adding Y and Nd, it can reach403MPa, about increasing by30%.In as-extruded Mg-5Li-3Al-2Zn-0.4Nd alloy, the (0002) pole figure exhibites the texturealong TD, appearing in ND deviating leftwards TD about5°. When the content of Nd is2.0%,the texture is non-basal texture (ND deviating rightwards TD about30°).Obvious non-basal texture appears in as-extruded Mg-5Li-3Al-2Zn-1.2Y-0.8Nd, the maximum density area appears from C-axis toward ED60~70°of the (0002) pole figure.EBSD shows that the grain orientation of the non-basal texture is about85°. The grainboundary analysis of the result show that the small-angle grain boundaries take a smallproportion, while the large-angle grain boundaries distribute more evenly. The grain of thealloy after deformation becomes small, the proportion of grains smaller than5μm is50.1%.After annealing treatment at400℃for30minutes,60minutes and90minutes, the non-basaltexture weakens, while the average grain size of the alloy also significantly increases, thestrength and ductility of the alloy decline. The plastic instability phenomenon appears in thetensile test at room temperature.The significant non-basal texture appears in Mg-5Li-3Al-2Zn-1.2Y-0.8Nd afterwarm-rolled and hot-rolled with a reduction percentage of larger than50%. Symmetricaldouble-peak on both sides of RD appears in (0002) pole figure. When the reductionpercentage reaches28%under warm rolling, the C-axis of most of grains is toward RD of25°.The texture is strengthened when the reduction percentage reaches52%.The studies of the textures of compressed, extruded, warm-rolled and hot-rolled ofMg-5Li-3Al-2Zn-1.2Y-0.8Nd alloy shows that the prismatic slip systems is easier to beactivated than the basal slip system, and the prismatic slip and pyramidal twinning are themain deformation mechanisms of this alloy.更多还原