Mg-6.75Zn-0.57Zr-0.4Y-0.18Gd(wt.%)sheet with typical basal texture was produced by cross rolling and annealing.Room temperature tensile tests were subsequently conducted along rolling direction(RD),transverse directio...Mg-6.75Zn-0.57Zr-0.4Y-0.18Gd(wt.%)sheet with typical basal texture was produced by cross rolling and annealing.Room temperature tensile tests were subsequently conducted along rolling direction(RD),transverse direction(TD),and diagonal direction(RD45).Deformation mechanism and orientation evolution during the tension were investigated by quasi-in-situ electron backscatter diffraction observation and in-grain misorientation axis analysis.The results indicate that the activation of deformation mechanism mainly depends on the initial grain orientation.For RD sample,prismatic<a>slip plays an important role in the deformation of grains with<0001>axis nearly perpendicular to the RD.With the<0001>axis gradually tilted towards the RD,basal<a>slip becomes the dominant deformation mode.After the tensile fracture,the initial concentrically distributed{0001}pole is split into double peaks extending perpendicular to the RD,and the randomly distributed{1010}pole becomes parallel to the RD.The evolution in{0001}and{1010}poles during tension is related to the lattice rotation induced by basal<a>slip and prismatic<a>slip,respectively.TD and RD45 samples exhibit similar deformation mechanism and orientation evolution as the RD sample,which results in the nearly isotropic mechanical properties in the annealed cross-rolled sheet.展开更多
Single-crystal rods with different diameters and deviation angles with respect to the solidification direction were produced by Bridgman rapid solidification method at withdrawal rates of 3 and 6 mm·min^(-1) and ...Single-crystal rods with different diameters and deviation angles with respect to the solidification direction were produced by Bridgman rapid solidification method at withdrawal rates of 3 and 6 mm·min^(-1) and used as grain continuators.The crystallographic orientation of the rods,which cross-sections were perpendicular to the solidification direction at different solidification heights,was measured by electron backscattered diffraction,while the corresponding microstructures were observed by optical microscopy.The mushy zone morphology and the distribution of the temperature gradient were simulated by the finite element analysis software ProCAST.The experimental results indicate that the crystallographic orientation of the single-crystal rods corresponds to the statistical average value of all the dendrite orientations in cross-section.The crystallographic orientation of the primary and secondary dendrites of each single-crystal rod at different cross-sections fluctuates irregularly within a small range(less than 4°).The crystallographic orientation of the dendrite in each single-crystal rod is not exactly consistent with each other and is affected by their branching mode of dendrites in the solidification space.In addition,the simulation results show that the mushy zone shapes and the temperature gradient of single-crystal rods change with the increase of solidification height during the solidification process.Finally,the evolution mechanism of the crystallographic orientations and the corresponding influence factors were analyzed and discussed.展开更多
A dynamic compression test was performed on α+β dual-phase titanium alloy Ti20C using a split Hopkinson pressure bar.The formation of adiabatic shear bands generated during the compression process was studied by com...A dynamic compression test was performed on α+β dual-phase titanium alloy Ti20C using a split Hopkinson pressure bar.The formation of adiabatic shear bands generated during the compression process was studied by combining the proposed multi-scale crystal plasticity finite element method with experimental measurements.The complex local micro region load was progressively extracted from the simulation results of a macro model and applied to an established three-dimensional multi-grain microstructure model.Subsequently,the evolution histories of the grain shape,size,and orientation inside the adiabatic shear band were quantitatively simulated.The results corresponded closely to the experimental results obtained via transmission electron microscopy and precession electron diffraction.Furthermore,by calculating the grain rotation and temperature rise inside the adiabatic shear band,the microstructural softening and thermal softening effects of typical heavily-deformed α grains were successfully decoupled.The results revealed that the microstructural softening stress was triggered and then stabilized(in general)at a relatively high value.This indicated that the mechanical strength was lowered mainly by the grain orientation evolution or dynamic recrystallization occurring during early plastic deformation.Subsequently,thermal softening increased linearly and became the main softening mechanism.Noticeably,in the final stage,the thermal softening stress accounted for 78.4% of the total softening stress due to the sharp temperature increase,which inevitably leads to the stress collapse and potential failure of the alloy.展开更多
On the basis of research method in FTIR imaging, we made a heterogeneous thin film of isotactic polypropylene （iPP） that contains a few large spherulites （-150 μm） which are surrounded by small spherulites （-15 ...On the basis of research method in FTIR imaging, we made a heterogeneous thin film of isotactic polypropylene （iPP） that contains a few large spherulites （-150 μm） which are surrounded by small spherulites （-15 μm） for tensile testing. The evolution processes of crystalline and amorphous orientations of iPP are monitored with its characteristic peaks at 998 and 973 cm^-1, respectively. By introducing the correlation images, the analysis demonstrates the relationships between the orientation evolutions of crystalline and amorphous phases in a space of 250 μm × 250 μm detecting area. During the plastic deformation, crystalline orientation is higher than amorphous orientation outside the large spherulite, while that is opposite inside the region. In addition, the evolutions of crystalline and amorphous orientations almost keep a positive correlation.展开更多
The complex grain fragmentation mechanisms of coarse grains in titanium alloys under multi-directional forging(MDF)directly influence the optimization and control of primary hot working processes.This study conducted ...The complex grain fragmentation mechanisms of coarse grains in titanium alloys under multi-directional forging(MDF)directly influence the optimization and control of primary hot working processes.This study conducted MDF experiments onβ-phase as-cast Ti-6554 alloy and simulated non-uniform deformation during cyclic multi-directional compression through macro-and micro-deformation modeling.The results revealed that friction and surface cooling caused low strain and tensile stress concentration at billet edges,leading to mixed grain structures.In contrast,high strain and triaxial compressive stress at billet centers facilitated uniform grain refinement.After 14 compressions and 4 intermediate reheating processes,coarse grains of the billet were refined from 2-5 mm to 0.25-0.50 mm,achieving uniform grain sizes across different regions.For the first time,the orientation evolution of grains with different morphologies during multi-directional compressions was visualized microscopically.Columnar grains were found to be more easily subdivided than equiaxed grains due to local strain accumulation.Under cumulative compressions,grain orientations gradually rotated from uniform to random,driving continuous dynamic recrystallization(CDRX).Slip system interactions and concentrated misorientation led to the formation and extension of transition and shear bands,inducing grain fragmentation dominated by transgranular subdivided CDRX.Smooth grain boundaries transformed into serrated ones after multiple passes,providing additional nucleation sites for discontinuous dynamic recrystallization(DDRX)and facilitating boundary expand CDRX.The interaction of diverse DRX mechanisms was the fundamental cause of grain refinement.This study clarified the principles of refining and homogenizing millimeter-grade coarse grains under increasing forging strain,offering valuable insights for the development of primary hot processing techniques for as-castβtitanium alloys.展开更多
基金the financial support from the National Natural Science Foundation of China(NSFC)with projects Nos.51874367,51574291.
文摘Mg-6.75Zn-0.57Zr-0.4Y-0.18Gd(wt.%)sheet with typical basal texture was produced by cross rolling and annealing.Room temperature tensile tests were subsequently conducted along rolling direction(RD),transverse direction(TD),and diagonal direction(RD45).Deformation mechanism and orientation evolution during the tension were investigated by quasi-in-situ electron backscatter diffraction observation and in-grain misorientation axis analysis.The results indicate that the activation of deformation mechanism mainly depends on the initial grain orientation.For RD sample,prismatic<a>slip plays an important role in the deformation of grains with<0001>axis nearly perpendicular to the RD.With the<0001>axis gradually tilted towards the RD,basal<a>slip becomes the dominant deformation mode.After the tensile fracture,the initial concentrically distributed{0001}pole is split into double peaks extending perpendicular to the RD,and the randomly distributed{1010}pole becomes parallel to the RD.The evolution in{0001}and{1010}poles during tension is related to the lattice rotation induced by basal<a>slip and prismatic<a>slip,respectively.TD and RD45 samples exhibit similar deformation mechanism and orientation evolution as the RD sample,which results in the nearly isotropic mechanical properties in the annealed cross-rolled sheet.
基金supported by the National Natural Science Foundation of China(No.51674235)the National Key Research and Development Program of China(Nos.2017-VI0001-0070 and 2017-VI-0019-0091)。
文摘Single-crystal rods with different diameters and deviation angles with respect to the solidification direction were produced by Bridgman rapid solidification method at withdrawal rates of 3 and 6 mm·min^(-1) and used as grain continuators.The crystallographic orientation of the rods,which cross-sections were perpendicular to the solidification direction at different solidification heights,was measured by electron backscattered diffraction,while the corresponding microstructures were observed by optical microscopy.The mushy zone morphology and the distribution of the temperature gradient were simulated by the finite element analysis software ProCAST.The experimental results indicate that the crystallographic orientation of the single-crystal rods corresponds to the statistical average value of all the dendrite orientations in cross-section.The crystallographic orientation of the primary and secondary dendrites of each single-crystal rod at different cross-sections fluctuates irregularly within a small range(less than 4°).The crystallographic orientation of the dendrite in each single-crystal rod is not exactly consistent with each other and is affected by their branching mode of dendrites in the solidification space.In addition,the simulation results show that the mushy zone shapes and the temperature gradient of single-crystal rods change with the increase of solidification height during the solidification process.Finally,the evolution mechanism of the crystallographic orientations and the corresponding influence factors were analyzed and discussed.
基金financially supported by the National Natural Science Foundation of China(No.51571031)。
文摘A dynamic compression test was performed on α+β dual-phase titanium alloy Ti20C using a split Hopkinson pressure bar.The formation of adiabatic shear bands generated during the compression process was studied by combining the proposed multi-scale crystal plasticity finite element method with experimental measurements.The complex local micro region load was progressively extracted from the simulation results of a macro model and applied to an established three-dimensional multi-grain microstructure model.Subsequently,the evolution histories of the grain shape,size,and orientation inside the adiabatic shear band were quantitatively simulated.The results corresponded closely to the experimental results obtained via transmission electron microscopy and precession electron diffraction.Furthermore,by calculating the grain rotation and temperature rise inside the adiabatic shear band,the microstructural softening and thermal softening effects of typical heavily-deformed α grains were successfully decoupled.The results revealed that the microstructural softening stress was triggered and then stabilized(in general)at a relatively high value.This indicated that the mechanical strength was lowered mainly by the grain orientation evolution or dynamic recrystallization occurring during early plastic deformation.Subsequently,thermal softening increased linearly and became the main softening mechanism.Noticeably,in the final stage,the thermal softening stress accounted for 78.4% of the total softening stress due to the sharp temperature increase,which inevitably leads to the stress collapse and potential failure of the alloy.
基金financially supported by the China Postdoctoral Science Foundation(No.2012M521233)the Fundamental Research Funds for the Central Universities(No.WK2310000031)+1 种基金the National Natural Science Funds for Distinguished Young Scholar(No.51325301)973 program of MOST(No.2010CB934504)
文摘On the basis of research method in FTIR imaging, we made a heterogeneous thin film of isotactic polypropylene （iPP） that contains a few large spherulites （-150 μm） which are surrounded by small spherulites （-15 μm） for tensile testing. The evolution processes of crystalline and amorphous orientations of iPP are monitored with its characteristic peaks at 998 and 973 cm^-1, respectively. By introducing the correlation images, the analysis demonstrates the relationships between the orientation evolutions of crystalline and amorphous phases in a space of 250 μm × 250 μm detecting area. During the plastic deformation, crystalline orientation is higher than amorphous orientation outside the large spherulite, while that is opposite inside the region. In addition, the evolutions of crystalline and amorphous orientations almost keep a positive correlation.
基金supported by the National Key Research and Development Program of China(No.2022YFB3706901)the National Natural Science Foundation of China(No.52274382)。
文摘The complex grain fragmentation mechanisms of coarse grains in titanium alloys under multi-directional forging(MDF)directly influence the optimization and control of primary hot working processes.This study conducted MDF experiments onβ-phase as-cast Ti-6554 alloy and simulated non-uniform deformation during cyclic multi-directional compression through macro-and micro-deformation modeling.The results revealed that friction and surface cooling caused low strain and tensile stress concentration at billet edges,leading to mixed grain structures.In contrast,high strain and triaxial compressive stress at billet centers facilitated uniform grain refinement.After 14 compressions and 4 intermediate reheating processes,coarse grains of the billet were refined from 2-5 mm to 0.25-0.50 mm,achieving uniform grain sizes across different regions.For the first time,the orientation evolution of grains with different morphologies during multi-directional compressions was visualized microscopically.Columnar grains were found to be more easily subdivided than equiaxed grains due to local strain accumulation.Under cumulative compressions,grain orientations gradually rotated from uniform to random,driving continuous dynamic recrystallization(CDRX).Slip system interactions and concentrated misorientation led to the formation and extension of transition and shear bands,inducing grain fragmentation dominated by transgranular subdivided CDRX.Smooth grain boundaries transformed into serrated ones after multiple passes,providing additional nucleation sites for discontinuous dynamic recrystallization(DDRX)and facilitating boundary expand CDRX.The interaction of diverse DRX mechanisms was the fundamental cause of grain refinement.This study clarified the principles of refining and homogenizing millimeter-grade coarse grains under increasing forging strain,offering valuable insights for the development of primary hot processing techniques for as-castβtitanium alloys.
基金supported by the National Key R&D Program of China(2020YFB1505802)the Ministry of Science and Technology(2017YFA0208200)+2 种基金the National Natural Science Foundation of China(22025108,U21A20327,and 22121001)Guangdong Provincial Natural Science Fund for Distinguished Young Scholars(2021B1515020081)the start-up support from Xiamen University and Guangzhou Key Laboratory of Low-Dimensional Materials and Energy Storage Devices(20195010002)。
