【作者】 赵亚忠；

【导师】 潘复生；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2010， 博士

【摘要】 镁合金具有密排六方晶体结构,室温下只有3个几何滑移系和2个独立滑移系,塑性变形能力较差,使变形制品的应用受到了极大的限制。为促进变形镁合金的推广应用,一个方面需要发展提高镁合金塑性的加工技术,另一方面需要开发高塑性变形镁合金,提高镁合金的本质变形能力。我国是镁和稀土的资源大国,研究和开发价格适中的高性能含稀土镁合金具有非常有利的条件。本文拟在镁合金中添加少量稀土元素,改善合金热变形能力,提高生产效率以降低生产成本,同时提高室温塑性,扩大其应用范围,促进变形镁合金的发展。本文采用金相分析、扫描电镜(SEM)、能谱(EDS)分析、X衍射分析(XRD)、差热分析(DSC)、力学性能测试等试验手段,研究了稀土元素铈、钕对ZK20和ZM21镁合金铸态、均匀化退火态、变形态的组织和性能的影响。研究结果表明,钕和铈的添加使铸态合金的组织得到细化,也使合金中出现含稀土合金相。随钕和铈添加量的增加,含稀土合金相增多,其形态由颗粒状经不连续网状向网状转变。网状含稀土合金相的出现,使晶界对变形的协调作用减弱,铸态合金的拉伸断裂方式由穿晶断裂转变为沿晶断裂,使抗拉强度和延伸率均有所下降。研究结果表明,在ZK20+xNd系中,除a-Mg和MgZn相外,随钕添加量的增加先后出现了三元含钕合金相T1(Mg 27.0-33.4, Zn 60.2-66.4,Nd 6.1-7.4)相和T2相((Mg,Zn)11.5Nd);在ZK20+xCe系中随着铈添加量的增加,合金中先后出现了两种三元含铈合金相τ1(CeMg7Zn12)和τ2 (Ce(Mg,Zn)10.1);在ZM21+xNd系中除α-Mg、α-Mn和MgZn相外,随钕添加量的增加也出现了T1和T2相。研究发现稀土对挤压态合金的晶粒尺寸有重要影响。随着稀土钕或铈添加量的增加,挤压态合金中颗粒状含稀土合金相增多的同时,合金晶粒尺寸明显变小。ZK20合金挤压前平均晶粒尺寸为238.8μm,挤压后为23.9μm;ZK20+0.5Nd合金挤压前平均晶粒尺寸为78.2μm,挤压后为4.9μm。添加稀土钕或铈,可使挤压态合金的抗拉强度小幅提高,而延伸率有较大幅度地增加。稀土元素的添加在使晶粒细化、提高塑性和强度的同时,稀土还与Zn元素形成三元合金相,使Zn固溶程度减弱,一定程度上削弱了合金强度。各合金系中挤压态性能最好的合金分别为:ZK20+0.5Nd、ZK20+0.7Ce和ZM21+0.5Nd。ZK20+0.5Nd的抗拉强度为237MPA,延伸率为32.7%,分别比不加铈时增高5%和50%;ZK20+0.7Ce的抗拉强度为261MPa,延伸率为24.6%,分别比不加钕时增高8%和44%;ZM21+0.5Nd的抗拉强度达232.1MPA,延伸率达32.2%,塑性有较大幅度提高。对ZK20和ZK20+0.5Nd合金不同温度均匀化退火的研究表明:均匀化退火能大幅度降低枝晶偏析,使MgZn相及部分Mg-Zn-Nd相溶解到基体中。ZK20合金均匀化退火后挤压,挤压态合金强度与未经均匀化退火相比有所降低;而ZK20+0.5Nd合金经均匀化退火后挤压,挤压态合金强度与未经均匀化相比没有降低,同时合金的塑性明显提高。ZK20和ZK20+0.5Nd合金的优化均匀化退火工艺分别为:390℃×10h和420℃×10h。无论添加稀土元素与否,铸态合金400℃,0.5S-1高温压缩0.7真应变时,均发生不完全动态再结晶,变形合金中残留有被压扁的原始铸态大晶粒,其晶界处存在等轴的再结晶小晶粒。随钕和铈的添加量的增加,合金高温压缩峰值应力及峰值应变均增高。本研究的合金中,ZK20+0.5Nd和ZM21+0.5Nd二种含钕镁合金的挤压态延伸率均达到30%,比常用镁合金具有塑性优势,便于工业上大规模生产。而ZK20+0.7Ce合金在抗拉强度达到了常用镁合金的水平的同时,塑性比常用商业变形镁合金高,在制品成形加工及使用性能上具有一定的优势。更多还原

【Abstract】 Wrought magnesium alloys have low ductility compared with aluminum alloys because of their hexagonal close pack crystal structure which has limited slip systems. Wrought magnesium alloys take 10% only in total magnesium alloys that is less than the cast magnesium alloy. To develop the plastic working technology of magnesium alloy is an important positive factor, and another important positive factor for accelerating the application of wrought magnesium alloy in industries is to develop new alloys. Therefore, it is very important to develop new magnesium alloys with perfect ductility to meet the needs of their deformation and application. China has the most plentiful magnesium and rare earth resources in the world, which provides advantages in researching and developing high properties magnesium alloys containing rare earth elements with the acceptable cost. In this work,magnesium alloys containing rare earth elements were developed for the increase of ductility and deformation ability of alloys.In the present work, the microstructures, mechanical properties of as-cast and as-extruded ZK20+xNd alloys, ZK20+xCe alloys and ZM21+xNd alloys had been investigated systematically by optical microscope (OM), scanning electron microscope (SEM), X-ray diffractometer (XRD), mechanical properties testing, etc.Results showed that the microstructure of as-cast alloys gradually defined with the increase of the Nd or Ce addition, and Mg-Zn-Re ternary phases appeared. The morphology of ternary Mg-Zn-Re phases at intragranular changed from partical,to broken-network,and then to network with the increase of the Nd or Ce addition. Broken-network or network Mg-Zn-Re phase caused the decrease of alloy properties and a change in the fracture mechanism from cleavage fracture to intergranular fracture.Results showed that Mg-Zn-Nd ternary phases T1(Mg 27.0-33.4, Zn 60.2-66.4, Nd 6.1-7.4)and T2((Mg,Zn)11.5Nd)appeared in ZK20+x Nd alloys; Mg-Zn-Ce ternary phasesτ1 ( CeMg7Zn12 ) andτ2 (Ce(Mg,Zn)10.1) appeared in ZK20+xCe alloys; Mg-Zn-Nd ternary phases T1 and T2 appeared as well as in ZK 20+x Nd alloys.The grain size of as-extruded alloys gradually decreased with the increase of Nd or Ce addition from 0 to 0.7 wt. %. The average grain size of ZK20 alloy was 23.9μm after extrusion,but it was 238.8μm before extrusion; the average grains size of ZK20+0.5Nd alloy was 4.9μm after extrusion,but it was 78.2μm before extrusion.In as-extruded ZK20+xNd alloys、ZK20+xCe alloys and ZM21+xNd alloys, Nd or Ce addition caused the slight increase of alloy strength, but it caused alloy ductility to increase significantly. It led to a little increase of alloy strength by the Nd or Ce addition that Zn amount inα-Mg solid solutions decreased because of the formation of Mg-Zn-Re ternary phases.The as-extruded alloys ZK20+0.5Nd, ZK20+0.7Ce and ZM21+0.5Nd showed higher properties respectively. The UTS and El. of as-extruded ZK20+0.5Nd alloy were 237 MPa and 32.8% respectively, with the increase of 5% and 50% on those of ZK20 alloy respectively. The UTS and El. of as-extruded ZK20+0.7Ce alloy were 261MPa and 24.6% respectively, with the increase of 8% and 44% on those of ZK20 alloy. The UTS and El. of as-extruded ZM21+0.5Nd alloy were 232.1 MPa and 32.2%, of which the elongation increased significantly.The effects of ingot homogenizing annealing at different temperatures on properties of as-extruded ZK20 and ZK20+0.5Nd magnesium alloy were investigated. The results showed Mg-Zn and Mg-Nd-Zn compounds which were observed in the cast ingot disappeared gradually with the increase of annealing temperature. The strength of as-extruded ZK20 alloy after homogenizing annealing decreased than that of the alloy without annealing, while the strength of ZK20+0.5Nd remained unchanged. It can be concluded that the optimal homogenizing annealing process were 390℃×10 h for ZK20, and 420℃×10 h for ZK20+0.5Nd.Incomplete dynamic recrystallization had occurred in alloys at compression deformation at 400℃,0.5S-1. Peak stress and peak strain increased with the increase of Nd or Ce addition.ZK20+0.5Nd alloy and ZM21+0.5Nd alloy showed the advantage in deformation with the better ductility. ZK20+0.7Ce showed the advantage in application, since its strength hit the level of commercial magnesium alloys and its ductility was better than common commercial magnesium alloys.更多还原