【作者】 管凯；

【导师】 孟健； 杨强；

【作者基本信息】 中国科学技术大学 ， 无机化学， 2019， 博士

【摘要】 本论文首先研究了稀土Sm元素含量对铸态和挤压态Mg-Sm-Zn-Zr合金微观组织、第二相晶体结构和力学性能的作用,以及不同热处理条件下铸态Mg-Sm-Zn-Zr合金的微观组织演变。以优选的挤压Mg-3.5Sm-0.6Zn-0.5Zr合金为例,分析其强化机制,探究相同温度下挤压比对其微观组织和拉伸性能的影响。然后,我们探究了不同添加量的第二稀土合金化元素Ce对合金微观组织和拉伸性能的影响及相应的强化机理。最后,利用轻、重稀土的协同作用,开发出了新型低稀土高强变形Mg-3.5Sm-2.0Y-1.5Ce-2.0Zn-0.5Zr合金。结果表明:稀土Sm的添加量会影响Mg-Sm-Zn-Zr合金中主要第二相的晶体结构和化学组成,当稀土Sm的添加量小于5.0 wt%时,合金中主要第二相为面心立方结构Mg3Sm(a=0.7371 nm)相,而当稀土Sm的添加量大于5.0 wt%时,合金中主要金属间化合物为体心四方结构Mg41Sm5(a=1.476 nm,c=1.039 nm)相。在对Mg-3.5Sm-0.6Zn-0.5Zr合金进行350 ℃热处理过程中,Mg3Sm相会转变为Mg41Sm5相,Mg41Sm5相与Mg3Sm相的位向关系为:(110)Mg41Sm5//(110)Mg3Sm,[1 1 0]Mg41Sm5//[1 1 1]Mg3Sm。另外,热处理时Mg3Sm相和Mg41Sm5相会同时在Mg基体中析出,位向关系分别为:(110)Mg3Sm//(0001)Mg,[111]Mg3Sm//[1210]Mg;(211)Mg41Sm5//(0001)Mg,[1 20]Mg41Sm5//[1210]Mg。Mg-Sm-Zn-Zr合金经过固溶处理,会在Mg基体中形成多种形貌的小尺寸第二相,经过透射电子显微镜分析,这些第二相主要包括密排六方结构的片状MgZn2相、简单四方结构的纺锤状Zn2Zr3相、纳米颗粒状Zn2Zr3相和棒状相,其中,棒状相包括穿过晶界的棒状Zn2Zr3相、表面附着Mg3Sm相的棒状Zn2Zr3相和表面光滑的面心立方结构棒状Zn2Zr相。另外,研究表明晶粒内部的片状MgZn2相和棒状Zn2Zr3相可以作为Mg3Sm析出相的有效形核位点。Mg-Sm-Zn-Zr合金在挤压过程中会产生Mg3Sm动态析出相,根据粒子激发形核机制,添加Sm元素引入的第二相在动态再结晶中提供异质形核位点,显著提高了合金强度。当Sm元素的添加量为3.5 wt%时,合金表现出最佳拉伸强度,屈服强度约368 MMPa,抗拉强度约383 MPa,与Mg-0.5Znn-0.5Zr合金相比分别提高23.1%和20.8%。时效处理进一步提高了合金的强度,其屈服强度为416 MPa、抗拉强度为427 MPa,达到甚至超过了大部分中等稀土含量的Mg-Gd-Y(-Zn)-Zr系合金,主要强化机制来源于晶界强化和析出强化。研究了不同挤压比Mg-3.5Sm-0.6Zn-0.5Zr合金的组织与性能,当挤压比为10.4和17.6时,合金中第二相发生了明显的相转变,部分Mg3Sm相转变为Mg41Sm5相。随着挤压比的增大,再结晶体积分数增大,基面织构被弱化,同时再结晶晶粒尺寸也逐渐增大,合金的屈服强度也随之减小。添加稀土Ce元素以后,Mg-3.5Sm-0.6Zn-0.5Zr合金中出现了块状Mg12RE相,且随着Ce元素添加量的增加,合金中原有主要金属间化合物Mg3RE相的体积分数逐渐减小,新生相Mg12RE相的体积分数和尺寸逐渐增大。挤压态Mg-3.5Sm-xCe-0.6Zn-0.5Zr合金中均包括尺寸较小的再结晶晶粒和尺寸相对较大的未再结晶区。挤压过程中的动态析出相被确定为Mg3RE相,且随着稀土Ce元素添加量的不断增加,动态析出相的体积分数明显增大。经过时效处理,Mg-3.5Sm-xCe-0.6Zn-0.5Zr合金的屈服强度均有一定程度的提升,合金中大块相的体积分数随稀土Ce添加量的增加而增大,屈服强度随着稀土Ce元素添加量增加而提高的同时,延伸率会有所下降。Mg-3.5Sm-2.0Y-1.5Ce-2.0Zn-0.5Zr合金晶界处的第二相包括面心立方结构的W(Mg3RE2Zn3)相(a=0.683 nm)和体心四方结构的Mg12RE相(a=1.033 nm,cc=0.596 nm),晶粒内部还有条状14H-LPSO相。EBSD分析结果表明,挤压合金中包括细小的再结晶晶粒和相对粗大的未再结晶区,未再结晶区存在大量的小角度晶界,说明位错密度较大,呈现出典型的<1010>纤维织构,还存在较弱的<11214>稀土织构。挤压合金中主要第二相仍为W相和Mg12RE相,在晶界处出现了18R-LPSO相,与Mg12RE相存在确定的位向关系:(0001)18R.LPSO偏离(210)Mg约6.8°,[1210]18R-LPSO//[1 23]Mg12RE,晶粒内部沿基面出现了条状14H-LPSO相。经过时效处理,合金中出现了新的沿基面析出的条状Y相。挤压态Mg-3.5Sm-2.0Y-1.5Ce-2.0Zn-0.5Zr合金的拉伸屈服强度和压缩屈服强度分别为438 MPa和413 MPa,时效态合金的拉伸屈服强度和压缩屈服强度分别增至482 MPa和457 MPa。优异的合金性能来源于多种强化机制,主要包括细晶强化、位错强化和析出强化。更多还原

【Abstract】 In this thesis,effects of samarium(Sm)content(0,2.0,3.5,5.0,6.5 wt%)on microstructure and mechanical properties of Mg-0.5Zn-0.5Zr alloy under as-cast and as-extruded states were thoroughly investigated.Microstructural evolution of as-cast Mg-Sm-Zn-Zr alloy under dififerent heat treatment conditions was studied in detail.Strengthening mechanism of the extruded Mg-3.5Sm-0.6Zn-0.5Zr alloy with excellent properties was explored.Simultaneously,influences of extrusion ratio on the microstructure and mechanical properties of the optimum alloy were studied.In addition,effects of cerium(Ce)content on microstructure and mechanical properties of Mg-3.5Sm-0.5Zn-0.5Zr alloy under as-cast and as-extruded states were investigated systematically.The last but not the least,a novel high-strength Mg-3.5Sm-2.0Y-1.5Ce-2.0Zn-0.5Zr alloy with low RE content has been developed by the synergistic effect of light and heavy RE elements.The research results are as follows:The chemical composition and crystal structure of the dominant intermetallic phase will be affected by different content of Sm addition.The dominant intermetallic phase changes from Mg3Sm(face-center cubic,a = 0.7371 nm)to Mg4:Sm5(body-centered tetragonal,a=1.476 nm c= 1.039 nm)till Sm content exceeds 5.0 wt%.Mg3Sm phase was transformed into Mg41Sm5 phase during the heat treatment of Mg-3.5Sm-0.6Zn-0.5Zr alloy at 350 ℃。The orientation relationship between Mg41 Sm5 phase and Mg3Sm phase was revealed as:(110)Mg41Sm5//(110)Mg3Sm,[1 1 0]Mg41Sm5//[1 1 1]Mg3Sm.This indicates that Mg41Sm5 phase was precipitated along the(110)plane of Mg3Sm phase.In addition,both Mg3Sm phase and Mg41Sm5 phase were precipitated along the(0001)Mg plane.The orientation relationships between them and Mg matr:ix were revealed as:(110)Mg3Sm//(0001)Mg,[111]Mg3Sm//[1 2 10]Mg and(211)Mg41Sm5//(0001)Mg,[1 20]Mg41Sm5//[1 2 10]Mg.The intermetallic phases formed during solution treatment in Mg-5.0Sm-0.6Zn-0.5Zr alloy were studied using transmission electron microscopy.Four types,namely the plate phase,the spindle-like phase,the nano-scale granuliform phase and the rod-like phase were revealed.The first one is MgZn2(close-packed hexagonal)phase,the other three ones are Zn-Zr phase.In addition,the granuliform phase presents quadrate-shaped or lath-shaped and was identified as Zn2Zr3(primitive tetragonal).Furthermore,three kinds of rod-like phases were observed:one across the grain boundary but only coherent with one grain,one covered by jagged Mg3Sm precipitates and one with no surficial phase.The former twos are Zn2Zr3 while the later one is Zn2Zr(face-center cubic).Finally,this work indicates that only the plate MgZn2 phase and the rod-like Zn2Zr3]phase in the grain interior will act as heterogeneous nucleation sites for the Mg3Sm precipitates,thus influencing the precipitation.The dynamically precipitated intermetallic phase during hot-extrusion in all Sm-containing alloys is Mg3Sm.The intermetallic particles induced by Sm addition could act as heterogeneous nucleation sites for dynamic recrystallization during hot extrusion.Sm addition can significantly enhance the strength of the as-extruded Mg-0.5Zn-0.5Zr alloy at room temperature,with the optimal dosage of 3.5 wt%.The optimal yield strength and ultimate tensile strength are 368 MPa and 383 MPa,which were enhanced by approximately 23.1%and 20.8%compared with the Sm-free alloy,respectively.Numerous nano-scale plate β’particles and lamellae β"particles homogeneously precipitated in the matrix during artificial aging treatments.Therefore,the studied alloy exhibits very high ultimate tensile strength and yield strength,whose maximum values at room temperature are approximately 427 MPa and 416 MPa,respectively,the dominant strengthening mechanisms are revealed to be grain boundary strengthening and precipitation strengthening.The microstructure and mechanical properties of Mg-3.5Sm-0.6Zn-0.5Zr alloy with different extrusion ratio were also investigated.The results indicate that the dominant phase was changed from Mg3Sm to Mg41Sm5 when the extrusion ratio was 10.4 and 17.6.With the increase of extrusion ratio,the fraction of recrystallization region increased,the grain size also increased gradually,and the base texture was weakened effectively,the corresponding yield strength was decreased finally.After Ce addition into Mg-3.5Sm-0.6Zn-0.5Zr alloy,blocky Mg12RE phase was observed.With the increase of Ce content,the volume fraction of Mg3RE decreased gradually,at the same time,both the volume and size of Mg12RE increased obviously.The extruded Mg-3.5Sm-xCe-0.6Zn-0.5Zr alloy contains fine recrystallized grains and relatively coarse non-recrystallized regions.The volume fraction of dynamic precipitates Mg3RE increased with the increasing of the amount of Ce.After aging treatment,the yield strength of extruded alloys was improved to a certain extent.The volume fraction of bulk phase increased with the increase of Ce addition.The yield strength was enhanced with the increase of Ce addition,while the elongation decreased.W phase and Mg 12RE phase distributed at grain boundary were observed in as-cast Mg-3.5Sm-2.0Y-1.5Ce-2.0Zn-0.5Zr alloy,also lamellar 14H-LPSO phase in grain.The extruded alloy consists of fine recrystallized grains and relatively coarse non-recrystallized zone.Non-recrystallization zone contains lots of small angle grain boundaries,indicating high density dislocations in it.The results of EBSD analysis indicated typical<1010>fiber texture and relatively weak<1124>rare earth texture were in extruded alloy.Apart from W phase and Mg12RE phase,18R-LPSO phase appeared at grain boundaries in extruded alloy,which has a certain orientation relationship with Mg12RE:(0001):8R-LPSO about 6.80°from(210)Mg12RE,[1210]18R-LPSO//[1 2 3]Mg12RE.Numerous lamellar 14H-LPSO phase were observed in the inter:ior of the grains,and the orientation relationship with Mg matrix was:(0001)14H-LPSO//(0001)Mg,[1210]14H-LPSO//[1210]mg.After aging treatment,numerous lamellar y precipitates aligned on base planes.The tensile yield strength and compressive yield strength of extruded alloy are 438 MPa and 413 MPa,respectively.After ageing treatment,they are 482 MPa and 457 MPa,respectively.The excellent properties of alloys come from various strengthening mechanisms,including fine grain strengthening,dislocation strengthening and precipitation strengthening.更多还原