【作者】 张栋栋；

【导师】 曹占义；

【作者基本信息】 吉林大学 ， 材料学， 2021， 博士

【摘要】 镁合金是目前最轻的结构金属材料,因此在汽车工业、航空航天、电子通讯以及国防军工等行业的轻量化应用方面具有显著的优势。但是相对于商用铝合金,传统镁合金的强度低严重限制了其进一步应用,因此开展高强度镁合金的研发对于拓宽镁合金的应用具有重要意义。目前高强度镁合金研究主要集中在高稀土添加(>12wt.%)的Mg-Gd基合金,虽然其强度可达500MPa以上,但是高稀土添加导致高的成本,限制了其广泛应用。本文采用常规的低温挤压技术制备出了低稀土高强度Mg-4Sm-2Yb-0.6Zn-0.4Zr(SY42)镁合金,其屈服强度高达470MPa以上,超过了大部分高稀土添加的Mg-Gd基变形合金,同时可媲美T8处理的2024变形铝合金。本文详细研究了低稀土高强度SY42变形镁合金的微观组织、强化机制以及蠕变行为等;并且首次提出了变形镁合金蠕变过程中门槛应力的起源以及采用高密度纳米间距基面层错调控拉伸-压缩屈服不对称性的方法。主要研究内容如下:首先,研究了挤压SY42合金的微观组织和强化机制。通过调控挤压工艺,屈服强度可达450～470MPa左右。挤压SY42合金展示典型的双峰组织,即具有随机取向的亚微米级再结晶晶粒和具有强基面织构的粗大未再结晶晶粒,同时基体中存在大量的多相多尺度Mg-RE颗粒,未再结晶区存在变形带和亚晶界等亚结构。200℃峰时效之后,基体中引入了许多基面γ"沉淀相,使合金的屈服强度进一步提升。挤压SY42合金超高的拉伸屈服强度主要归因于亚微米级再结晶晶粒的细晶强化、粗大未再结晶晶粒的织构强化和高密度多相多尺度Mg-RE颗粒的弥散强化。由于强基面织构的存在,挤压SY42合金表现出明显的室温拉伸-压缩屈服不对称性,但是时效析出的基面γ"相在一定程度上改善了拉压屈服不对称性。其次,探究了挤压SY42合金的高温拉伸蠕变行为。挤压SY42合金表现出延长的第三蠕变阶段,不同于常规金属的蠕变行为,这主要与可动位错的密度增加引起的应变软化有关。同时挤压合金展示了高的应力指数和蠕变激活能,n=9.3,Q=233 kJ/mol。采用了门槛应力的方法修正幂律关系,得到修正的应力指数为5,表明门槛应力的方法适用于本研究的挤压合金。另外,热稳定性良好的Mg-RE颗粒以及稀土原子与位错交互作用导致其蠕变激活能显著高于镁晶格自扩散能。蠕变过程中,未再结晶区部分的γ"沉淀相转变为γ’,而再结晶区未发生相转变。蠕变机制似乎与应力水平关系不大。未再结晶区的蠕变机制总是<c+a>位错交滑移,无论应力大小,虽然在中等应力水平也发现了位错攀移的迹象。在中、低应力水平,部分可动的<c+a>位错发生了锥面-基面转变,形成基面分解的<c+a>位错固定结构,有益于提升蠕变抗力,而在高应力水平则没有。再结晶晶粒的蠕变机制强烈依赖于它们的取向。然后,分析了挤压SY42合金的蠕变各向异性。与拉伸蠕变不同,压缩蠕变表现出短暂的初始蠕变阶段和近乎恒定的稳态蠕变阶段,但在任何应力水平均无第三蠕变阶段的迹象。特别地,蠕变第二阶段出现了反常蠕变速率降低现象,这主要归因于蠕变试样在压缩模式下的尺寸效应、基面分解的<c+a>位错固定结构、动态析出的γ’’/γ’相的强化和局部背应力的综合作用结果。沿挤压方向不同角度的试样表现出了明显的压缩蠕变各向异性,并且发现基面织构越强,蠕变抗力越大、门槛应力值也越大;然而传统的载荷转移机制和颗粒-位错交互机制均无法解释挤压合金中门槛应力起源。因此,我们提出门槛应力和蠕变抗力的差异主要是由于不同强度基面织构导致优先启动滑移系的不同造成的。虽然挤压SY42合金的室温压缩屈服强度明显低于拉伸屈服强度,但压缩蠕变性能却优于拉伸蠕变,即压缩蠕变最小蠕变速率明显低于拉伸蠕变,这主要是因为压缩模式下易形成不可动的基面<c+a>位错固定结构,有益于降低蠕变速率。最后,为了进一步改善挤压SY42合金的室温拉伸-压缩屈服不对称性,我们采用2%Y替换1%Sm和1%Yb调控合金的微观结构。Y替换导致挤压SY42合金的未再结晶区内从弥散的多尺度Mg-RE颗粒转变为高密度的纳米间距的基面层错,其次是近一半的再结晶晶粒内部也形成了纳米间距的基面层错,最后是弱化了合金的基面织构。因此,Y替换导致挤压SY42合金的拉伸屈服强度提升20MPa以上,压缩屈服强度提升高达110MPa以上,从而显著改善了挤压SY42合金的拉伸-压缩屈服不对称性,即CYS/TYS从0.71提升至0.92。拉伸屈服强度的提升主要归因于高密度的纳米间距的基面层错的强化与即Y原子的固溶强化;而压缩屈服强度显著提升的主要原因是未再结晶区高密度的纳米间距的基面层错在压缩过程中有效抑制了孪晶形核和长大,从而显著提升了压缩屈服强度、改善了挤压合金的拉伸-压缩屈服不对称性。更多还原

【Abstract】 Magnesium alloys are the lightest structural metallic material so far,thus which have significant advantages in in terms of lightweight applications in the automotive industry,aerospace,electronic communications,national defense and military industries.However,compared with commercial aluminum alloys,the low strength of traditional magnesium alloys severely limits them further extensive application.Therefore,developing high-strength magnesium alloys is of great significance for expanding the application of magnesium alloys.Up to now,high-strength magnesium alloys mostly focus on Mg-Gd based alloy in which the strength can reach up to 500 MPa,but high RE additions give rise to high cost of raw material and processing.In present work,we adopted conventional low-temperature extrusion technology to fabricate low-RE-alloyed high-strength Mg-4Sm-2Yb-0.6Zn-0.4Zr(SY42)alloys,with yield strength over 470 MPa,superior to the majority of Mg-Gd based alloy with high RE additions,even comparable to T8-treated 2024 Al alloys.Microstructure,strengthening mechanisms and creep behavior of low-RE-alloyed high-strength wrought Mg alloys were thoroughly investigated and origin of the threshold stress and the methods for improving tension-compression yield asymmetry by high-density of basal stack faults are proposed for the first time in this work.And the main studied contents were followed:Firstly,microstructure and strengthening mechanisms of extruded SY42 alloy were investigated.The yield strength can reach up to about 450～470MPa by tailoring extrusion processes.It shows typical bimodal microstructure consist of submicron recrystallized grains with random orientations and coarse hot-worked grains with strong basal texture.Also,amounts of multiphase and multiscale Mg-RE particles formed in Mg matrix,and the hot-worked grains remain some substructure such as deformation bands,subgrain boundaries.After peak-ageing at 200℃,a lot of basal γ’’ phases were introduced in Mg matrix,which further enhances the yield strength of the alloys.The ultrahigh tensile yield strength of the alloy is principally attributed to the combined results of refinement strengthening of submicro recrystallized grains,texture strengthening of coarse hot-worked grains,and dispersion strengthening of dense multiscale Mg-RE particles.On the contrary,strong basal texture results in an obvious yield asymmetry,but which is improved by basal γ’’ phases to some extent.Secondly,tensile creep behavior of extruded SY42 alloy was studied.It exhibited the extended tertiary creep stage,which is clearly different from that of conventional metals.This possibly associates with the strain softening caused by an increase of density of mobile dislocations.Also,the alloy showed high stress exponent and creep activation energy,i.e.,n= 9.3,Q = 233 kJ/mol.Therefore,adopting threshold stress approach was used to modify the power-law equation,leading to a modified stress exponent of 5,which indicates that this method is suitable for the extruded alloy in this work.In addition,the interaction between thermostable Mg-RE particles and RE atoms and glided dislocations is underlying reason of creep activation energy higher than lattice self-diffusion energy of Mg.During creep,partialγ" phase transformed into γ’ in the hot-worked regions but not in recrystallized regions.Cross-slip of <c + a> dislocations always is dominated creep mechanism in hot-worked regions regardless of the magnitude of applied stress.In case of low and medium stress,partial mobile <c + a> dislocations decompose into the sessile basal-dissociated <c + a>dislocations structure while no in case of high stress.However,creep mechanism in recrystallized grains strongly depends on their orientation.Then,we investigated creep asymmetry of the extruded SY42 alloy.Compared with tensile creep behavior,creep curves in compression were dominated by a steady-state creep stage and a transient primary creep stage,but no any sign of tertiary creep stage.An abnormal decrease of creep rate is observed in the later period of the steady-state creep stage,particularly the creep at high stress levels more significant,which can be attributed to the combined effect of the geometric effect of creep samples in compression mode,basal-dissociated immobile dislocation structures,strengthening of dynamic precipitatedγ’’/γ’ phase,and the local backstress.The samples tilted angle of 0,45 and 90o with extrusion direction showed an obvious compressive creep asymmetry.And we found that the stronger basal texture caused the greater the creep resistance and the larger the threshold stress value,but which is not explained by traditional loads transmission mechanism and the interaction between dislocations and particles.Accordingly,we proposed that discrepancy of creep resistance and threshold stress is mainly due to the difference of the preferentially activated slips system caused by basal texture with different intensity.Compressive yield strength is apparently lower than that of tension of the alloy,but compressive creep resistance is superior to than that of tensile creep,which is greatly because immobile <c+a> dislocations basal structure readily formed under compressive mode,conducive to reduce creep rate.Finally,we adopted the substitution of 1%Sm and 1%Yb by 2%Y to modify microstructure of the alloy.Y substitution results in a transition from dense multiscale Mg-RE particles to profuse nano-spaced basal plane stacking faults(SFs),introduction of nano-spaced SFs in nearly half of the recrystallized grain,and weakening basal texture.As a consequence,Y substitution leads to that tensile yield strength of extruded SY42 alloy is increased by more than 20 MPa,the increment of the compressive yield strength over 110 MPa,thus significantly improving its yield asymmetry,namely CYS/TYS of 0.71 increases to 0.92.The improvement of ensile yield strength of extruded SY42 alloy is attributed to the strengthening of nano-spaced SFs.Meanwhile,those of profuse nano-spaced SFs within hot-worked grains can effectively suppress nucleation and propagation of twin during compression,thereby significantly enhancing compressive yield strength and thus modifying tension-compression yield asymmetry.更多还原