| 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. |
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