Integrally directional solidification of an Nb-Ti-Si based ultrahigh temperature alloy was performed in an ultrahigh temperature and high thermal gradient furnace with the use of ceramic crucibles. The microstructural...Integrally directional solidification of an Nb-Ti-Si based ultrahigh temperature alloy was performed in an ultrahigh temperature and high thermal gradient furnace with the use of ceramic crucibles. The microstructural evolution with the withdrawing rate increasing during directional solidification was revealed. The integrally directionally solidified microstructure was composed of couple grown lamellar (Nbss+(Nb,X)5Si3) eutectic colonies and a few hexagonally cross-sectioned (Nb,X)5Si3 columns (X represents Ti and Hf elements). All the directionally solidified microstructure was straightly aligned along the longitudinal axis of the specimens. With increasing of the withdrawing rate, the average diameter of the eutectic cells and inter lamella spacings in the eutectic cell decreased. The near-planar solid/liquid interface appeared when the withdrawing rate was 1μm/s, and the cellular solid/liquid interface formed when the withdrawing rate was 5 μm/s.展开更多
Two-phaseγ-TiAl/α_(2)-Ti_(3)Al lamellar intermetallics have attracted considerable attention because of their excellent strength and plasticity.However,the exact deformation mechanisms remain to be investigated.In t...Two-phaseγ-TiAl/α_(2)-Ti_(3)Al lamellar intermetallics have attracted considerable attention because of their excellent strength and plasticity.However,the exact deformation mechanisms remain to be investigated.In this paper,a solidified lamellar Ti-Al alloy with lamellar orientation at 0°,17°,and 73°with respect to the loading direction was stretched by utilizing molecular dynamics(MD)simulations.The results show that the mechanical properties of the sample are considerably influenced by solidified defects and tensile directions.The structure deformation and fracture were primarily attributed to an intrinsic stacking fault(ISF)accompanied by the nucleated Shockley dislocation,and the adjacent extrinsic stacking fault(ESF)and ISF formed by solidification tend to form large HCP structures during the tensile process loading at 73°.Moreover,cleavage cracking easily occurs on theγ/α_(2)interface under tensile deformation.The fracture loading mechanism at 17°is grain boundary slide whereas,at 73°and 0°,the dislocation piles up to form a dislocation junction.展开更多
A microscopic phase-field model was used to investigate a directional coarsening mechanism caused by the anisotropic growth of long period stacking and different effects of phases on precipitation in Ni-Al-V alloy.The...A microscopic phase-field model was used to investigate a directional coarsening mechanism caused by the anisotropic growth of long period stacking and different effects of phases on precipitation in Ni-Al-V alloy.The results show that DO22 mainly coarsens along its short axis,which may press the neighboring L12,leading to the interaction among atoms.Diffusion channels of Al are formed in the direction where the mismatch between γ＇ and γ reduces;the occupation probabilities are anisotropic in space;and direction coarsening of L12 occurs finally.With a rise of ageing temperature,phases appear later and DO22 is much later at a higher temperature,the average occupation probabilities of Al and V reduce,and Al changes more than V.展开更多
The microstructures and crystal growth directions of permanent mould casting and directionally solidified Al-Mg al oys with different Mg contents have been investigated. The results indicate that the effect of Mg cont...The microstructures and crystal growth directions of permanent mould casting and directionally solidified Al-Mg al oys with different Mg contents have been investigated. The results indicate that the effect of Mg content on microstructure is basical y same for the al oys prepared by these two methods. The primary grains change from cel ular crystals to developed columnar dendrites, and then to equiaxed dendrites as the Mg content is increased. Simultaneously, both the cel ular or columnar grain region and the primary trunk spacing decrease. Al of these changes are mainly attributed to the constitutional supercooling resulting from Mg element. Comparatively, the cellular or columnar crystals of the directionally solidified alloys are straighter and more paral el than those of the permanent mould casting al oys. These have straight or wavy grain boundaries, one of the most important microstructure characteristics of feathery grains. However, the transverse microstructure and growth direction reveal that they do not belong to feathery grains. The Mg seemingly can affect the crystal growth direction, but does not result in the formation of feathery grains under the conditions employed in the study.展开更多
基金Supported by the National Natural Science Foundation of China (No 500671081) the Doctorate Foundation of Northwestern Polytechnical University (No CX200605)
文摘Integrally directional solidification of an Nb-Ti-Si based ultrahigh temperature alloy was performed in an ultrahigh temperature and high thermal gradient furnace with the use of ceramic crucibles. The microstructural evolution with the withdrawing rate increasing during directional solidification was revealed. The integrally directionally solidified microstructure was composed of couple grown lamellar (Nbss+(Nb,X)5Si3) eutectic colonies and a few hexagonally cross-sectioned (Nb,X)5Si3 columns (X represents Ti and Hf elements). All the directionally solidified microstructure was straightly aligned along the longitudinal axis of the specimens. With increasing of the withdrawing rate, the average diameter of the eutectic cells and inter lamella spacings in the eutectic cell decreased. The near-planar solid/liquid interface appeared when the withdrawing rate was 1μm/s, and the cellular solid/liquid interface formed when the withdrawing rate was 5 μm/s.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.51761004,51661005,and 11964005)Industry and Education Combination Innovation Platform of Intelligent Manufacturing and Graduate Joint Training Base at Guizhou University(Grant No.2020520000-83-01-324061)+2 种基金the Guizhou Province Science and Technology Fund,China(Grant Nos.ZK[2021]051,[2017]5788,and J[2015]2050)High Level Creative Talent in Guizhou Education Department of Chinathe Cooperation Project of Science and Technology of Guizhou Province,China(Grant No.LH[2016]7430)。
文摘Two-phaseγ-TiAl/α_(2)-Ti_(3)Al lamellar intermetallics have attracted considerable attention because of their excellent strength and plasticity.However,the exact deformation mechanisms remain to be investigated.In this paper,a solidified lamellar Ti-Al alloy with lamellar orientation at 0°,17°,and 73°with respect to the loading direction was stretched by utilizing molecular dynamics(MD)simulations.The results show that the mechanical properties of the sample are considerably influenced by solidified defects and tensile directions.The structure deformation and fracture were primarily attributed to an intrinsic stacking fault(ISF)accompanied by the nucleated Shockley dislocation,and the adjacent extrinsic stacking fault(ESF)and ISF formed by solidification tend to form large HCP structures during the tensile process loading at 73°.Moreover,cleavage cracking easily occurs on theγ/α_(2)interface under tensile deformation.The fracture loading mechanism at 17°is grain boundary slide whereas,at 73°and 0°,the dislocation piles up to form a dislocation junction.
基金Projects(51075335,10902086,50875217) supported by the National Natural Science Foundation of ChinaProject(JC201005) supported by the Northwestern Polytechnical University Foundation for Fundamental Research,ChinaProject(CX201007) supported by the Doctorate Foundation of Northwestern Polytechnical University,China
文摘A microscopic phase-field model was used to investigate a directional coarsening mechanism caused by the anisotropic growth of long period stacking and different effects of phases on precipitation in Ni-Al-V alloy.The results show that DO22 mainly coarsens along its short axis,which may press the neighboring L12,leading to the interaction among atoms.Diffusion channels of Al are formed in the direction where the mismatch between γ＇ and γ reduces;the occupation probabilities are anisotropic in space;and direction coarsening of L12 occurs finally.With a rise of ageing temperature,phases appear later and DO22 is much later at a higher temperature,the average occupation probabilities of Al and V reduce,and Al changes more than V.
基金financially supported by the National Natural Science Foundation of China(Grant No.51061010)the Program for New Century Excellent Talents in University of China(Grant No.NCET-10-0023)the Program for Hongliu Outstanding Talents of Lanzhou University of Technology
文摘The microstructures and crystal growth directions of permanent mould casting and directionally solidified Al-Mg al oys with different Mg contents have been investigated. The results indicate that the effect of Mg content on microstructure is basical y same for the al oys prepared by these two methods. The primary grains change from cel ular crystals to developed columnar dendrites, and then to equiaxed dendrites as the Mg content is increased. Simultaneously, both the cel ular or columnar grain region and the primary trunk spacing decrease. Al of these changes are mainly attributed to the constitutional supercooling resulting from Mg element. Comparatively, the cellular or columnar crystals of the directionally solidified alloys are straighter and more paral el than those of the permanent mould casting al oys. These have straight or wavy grain boundaries, one of the most important microstructure characteristics of feathery grains. However, the transverse microstructure and growth direction reveal that they do not belong to feathery grains. The Mg seemingly can affect the crystal growth direction, but does not result in the formation of feathery grains under the conditions employed in the study.
