【作者】 陈彬；

【导师】 卢晨； 林栋樑；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2007， 博士

【摘要】 镁合金具有比重轻、比强度、比刚度高、导热导电性好、阻尼减振、电磁屏蔽、易于加工成形和容易回收等优点,在汽车、电子通信、航空航天和国防军事等领域具有及其重要的应用价值和广阔的应用前景。但在镁合金研究与应用中存在强度低的基础性问题,制约着高性能镁合金材料的开发与应用。本文研究了合金元素Y、Zn和Zr对Mg-Y-Zn合金组织、性能的影响;分析了凝固过程的冷却速度、固溶温度、固溶后的冷却方式、挤压温度、二次挤压温度等不同工艺对Mg₉₇Y₂Zn₁合金组织性能的影响;并以等通道角挤压法加工制取了高强度Mg-Y-Zn合金;利用热模拟技术研究了Mg₉₇Y₂Zn₁镁合金的中高温变形行为;研究了长周期有序结构对Mg-Y-Zn合金的强化作用。研究发现,Y、Zn和Zr合金元素对Mg-Y-Zn合金组织、性能有很大的影响。Mg₉₈Y₁Zn₁、Mg₉₇Y₂Zn₁、Mg₉₆Y₃Zn₁、Mg_95.5Y₃Zn_1.5、Mg₉₄Y₄Zn₂合金铸态组织都呈现典型的树枝晶结构,Y元素含量或Y和Zn元素总量越高,树枝晶越细,第二相数量增加。XRD分析表明,Mg₉₈Y₁Zn₁合金含有W-Mg₃Zn₃Y₂相和X-Mg12ZnY相,而Mg₉₇Y₂Zn₁、Mg₉₆Y₃Zn₁、Mg_95.5Y₃Zn_1.5、Mg₉₄Y₄Zn₂则只含有X-Mg12ZnY相。Mg₉₇Y₂Zn₁、Mg₉₆Y₃Zn₁、Mg_95.5Y₃Zn_1.5合金在400℃经过挤压后,在原始晶界和第二相附近发生了部分再结晶,三种合金在室温和高温都具有较高的强度。Zr元素的加入,不影响合金的相组成,却大大细化了Mg₉₇Y₂Zn₁合金的组织,也提高了合金的力学性能。本文研究了凝固过程的冷却速度、固溶温度、固溶后的冷却方式、挤压温度、二次挤压温度等不同工艺对Mg₉₇Y₂Zn₁合金组织、性能的影响。冷却速度越快,树枝晶越精细,力学性能越好。对Mg₉₇Y₂Zn₁合金进行不同温度的固溶处理,发现只有在高于X相熔点温度的固溶处理对组织有较大影响。415℃固溶的Mg₉₇Y₂Zn₁合金在400℃进行挤压后发生完全再结晶,而560℃固溶的合金在400℃进行挤压后只发生部分再结晶。挤压前经过560℃固溶处理合金在挤压和等通道角挤压后的强度都要高于415℃固溶处理过的合金,但延伸率要低于415℃固溶处理过的合金。对Mg₉₇Y₂Zn₁合金在不同温度下进行一次挤压和二次挤压研究发现,在较低温度下挤压后获得较细的组织,随着挤压温度提高,晶粒尺寸逐渐增大,强度下降,而延伸率上升。一次挤压得到的合金只发生部分再结晶,而二次挤压得到的合金发生完全再结晶,但在相同温度下,两者的晶粒尺寸相近。对Mg-Y-Zn合金进行等通道角挤压实验发现,随着等通道角挤压道次数的增加,第二相被逐渐细化,挤压态已发生再结晶区域的晶粒尺寸被进一步细化,而未发生再结晶的区域却仍然没有发生再结晶,只是沿着剪切方向形成一些剪切带。随着等通道角挤压道次数的增加,已再结晶区向未再结晶区域逐渐发展。经过一道次的等通道角挤压后,就有大量的超细晶粒形成。在随后道次的挤压过程中,晶粒虽然有所细化,但细化效果远不如第一道次挤压明显。经过等通道角挤压后的Mg-Y-Zn合金的晶粒尺寸要远远小于其它镁合金,其超细晶的形成和Mg-Y-Zn合金含有大量的X相和长周期有序结构有关。Mg₉₇Y₂Zn₁合金的晶粒尺寸被细化到200-500nm,Mg_95.5Y₃Zn_1.5合金的晶粒尺寸被细化到约200nm。经过等通道角挤压后,Mg-Y-Zn合金的强度得到了提高,Mg₉₇Y₂Zn₁合金的最高屈服强度和抗拉强度分别达到406.2MPa和455.2MPa,Mg_95.5Y₃Zn_1.5合金的最高屈服强度和抗拉强度分别达到444.6MPa和472.7MPa。由于等通道角挤压的剪切力会使X相内部产生微裂纹,因此材料的延伸率随着道次数的增加而降低。利用Gleeble热模拟试验机对Mg₉₇Y₂Zn₁镁合金进行了高温压缩实验研究,在同一应变速率下,随变形温度的升高,Mg₉₇Y₂Zn₁镁合金的流动应力水平逐渐下降;在同一变形温度下,随应变速率增加,Mg₉₇Y₂Zn₁镁合金的流动应力水平逐渐升高。Mg₉₇Y₂Zn₁具有较大的应力指数值。研究发现,在Mg-Y-Zn合金进行变形时,位错会在X相、长周期有序结构和基体的界面上塞积,基面滑移受到了抑制。长周期有序结构可以和孪晶发生交互作用,使孪晶界在长周期有序结构区域发生偏离。由于Mg-Y-Zn合金的Y、Zn元素含量高,层错能低,在变形过程中容易形成层错,位错容易在层错区域塞积。而位错的滑移运动也可以使层错产生扭曲和变形。长周期结构和位错的交互作用对Mg-Y-Zn合金的强化产生影响。更多还原

【Abstract】 Magnesium alloys are the lightest structural materials with high specific strength, good electric conduction, thermal conduction, damping capacity, electromagnetic shielding, formability, as well as easy recycled. It has great application prospect on automobile, electro-communication, aviation, and military industry. However, an important disadvantage of Mg alloys is their low strength, which limited their development and application. The paper investigated the effects of the yttrium, zinc and zirconium additions on microstructure and mechanical properties of Mg-Y-Zn alloys. The effects of cooling rates during solidification, solid solution temperatures and cooling ways, extrusion and two-step extrusion temperatures on microstructure and mechanical properties of Mg-Y-Zn alloys were also investigated. The high strength Mg-Y-Zn alloys can be prepared by equal channel angular pressing （ECAP）. Moreover, the hot simulation was conducted on Mg₉₇Y₂Zn₁ alloy to investigate its hot deformation behavior. The strengthening effects of long period stacking ordered structure on Mg-Y-Zn alloys were also analyzed.It is found that the addition of yttrium, zinc and zirconium play very important role in improving microstructure and mechanical properties of Mg-Y-Zn alloys. The as-cast Mg₉₈Y₁Zn₁, Mg₉₇Y₂Zn₁, Mg₉₆Y₃Zn₁, Mg_95.5Y₃Zn_1.5 and Mg₉₄Y₄Zn₂ present a typical dendrite structure. The XRD pattern of as-cast alloys indicates that the Mg₉₈Y₁Zn₁ alloy is consists ofα-Mg, W-Mg₃Zn₃Y₂ and X-Mg12ZnY while Mg₉₇Y₂Zn₁, Mg₉₆Y₃Zn₁, Mg_95.5Y₃Zn_1.5 and Mg₉₄Y₄Zn₂ alloy are consist ofα-Mg and X-Mg12ZnY. After extrusion at 400℃, the dynamic recrystallization （DRX） occurs around secondary phase particles and distorted grains boundaries in Mg₉₇Y₂Zn₁, Mg₉₆Y₃Zn₁ and Mg_95.5Y₃Zn_1.5 alloys, but the DRX process is incomplete. The extruded Mg-Y-Zn alloys exhibit excellent mechanical properties both at ambient temperature and elevated temperature. The addition of zirconium has no effect on phase constituents, but the microstructure of Mg₉₇Y₂Zn₁ alloy is refined and the mechanical properties of Mg₉₇Y₂Zn₁ alloy are improved by the addition of zirconium.The paper investigated the effects of cooling rates of solidification, solid solution temperatures and cooling ways, extrusion and two-step extrusion temperatures on microstructure and mechanical properties of Mg-Y-Zn alloys. It was found that the higher cooling rate of solidification, the finer dendrite and higher mechanical properties for Mg-Y-Zn alloys. It was found that at the temperatures above the melting temperature of X-phase eutectic pockets, the solid solution can has significant effects on microstructure. After extrusion at 400℃, the DRX process is completed in Mg₉₇Y₂Zn₁ alloy solution treated at 415℃, but the DRX process is incomplete in the alloy solution treated at 560℃. After extrusion and equal channel angular pressing （ECAP）, the alloy solution treated at 560℃exhibits higher strengths and lower elongation than that of alloy solution treated at 415℃. After extrusion and two-step extrusion, the lower extrusion temperature results in the finer grain size. With the extrusion temperature increasing, the grain size is increasing. Both extrusion and two-step extrusion have similar grain size. The DRX process is completed in extruded Mg₉₇Y₂Zn₁ alloy and the DRX process is incomplete in two-step extruded Mg₉₇Y₂Zn₁ alloy.After ECAP, the secondary phases were broken into uniformly distributed sections. The ultrafine grains were observed after ECAP. The grain size of Mg₉₇Y₂Zn₁ alloy was refined to 200-500nm,and the grain size of Mg_95.5Y₃Zn_1.5 alloy was refined to about 200nm。It is found that the LPS structure contributes to the formation of ultrafine grain size of ECAP processed Mg₉₇Y₂Zn₁ alloy. The ECAP processed Mg-Y-Zn alloy exhibits excellent mechanical properties. The high strengths with the yield strength （YS） of 406.2MPa and the ultimate tensile strength （UTS） of 455.2MPa were obtained for ECAP processed Mg₉₇Y₂Zn₁ alloy. The YS and UTS of ECAP processed Mg_95.5Y₃Zn_1.5 alloy are 444.6MPa and 472.7MPa correspondingly. The elongation is decreased with pass number increasing. It is due to that the process of ECAP introduced micro-cracks in the X-phase, which accelerated the growth and coalescence of the cracks during tensile test.The hot simulation was conducted on Mg₉₇Y₂Zn₁ alloy to investigate its hot deformation behavior. With the increasing of strain rate and decreasing of temperature, the flow stress is increasing gradually. And Mg₉₇Y₂Zn₁ alloy has high stress exponent value.It is also found that, the dislocations accumulated at the interfaces of X phase, the LPS structure and the matrix. The inhibition of the basal slip in Mg-Y-Zn alloys is due to the formation of X phase and the LPS structure. The deformation twin is deflected in the LPS structure region. The stacking fault energy of Mg-Y-Zn alloy is quite low due to high Y, Zn additions. During the deformation, the stacking faults can be easily introduced in and the dislocation would be accumulated at the front of stacking fault region. The dislocation glide can cross stacking faults and leads them to be bent and form steps. Therefore, it is concluded that interaction between LPS structure and dislocation contributes to the strengthening of Mg-Y-Zn alloy.更多还原