The eutectic Ag-Cu alloys exhibiting fine Ag-Cu lamellar eutectic structure formed upon rapid solidification have great potentials being used in various engineering fields.However,the desired fine primary lamellar eut...The eutectic Ag-Cu alloys exhibiting fine Ag-Cu lamellar eutectic structure formed upon rapid solidification have great potentials being used in various engineering fields.However,the desired fine primary lamellar eutectic structure(PLES)is usually replaced by a coarse anomalous eutectic structure(AES)when the undercooling prior to solidification exceeds a certain value.The forming mechanism of AES in the undercooled eutectic Ag-Cu alloy has been a controversial issue.In this work,the undercooled Ag-39.9 at.% Cu eutectic alloy is solidified under different cooling conditions by using techniques of melt fluxing and copper mold casting.The results show that the coupled eutectic growth of this alloy undergoes a transition from a slow eutectic-cellular growth(ECG)to a rapid eutectic-dendritic growth(EDG)above a undercooling of 72 K,accompanying with an abrupt change of the distribution and amount of AES in as-solidified microstructures.Two kinds of primary lamellar eutectic structures are formed by ECG and EDG during recalescence,respectively.The destabilization of PLES that causes the formation of AES is ascribed to two different mechanisms based on the microstructural examination and theoretical calculations.Below 72 K,the destabilization of PLES formed by slow ECG is caused by the mechanism of"termination migration"driven by interfacial energy.While above 72 K,the destabilization of PLES formed by rapid EDG is attributed to the unstable perturbation of interface driven by interfacial energy and solute supersaturation.展开更多
Second-phase particle pinning has been well known as a mechanism impeding grain boundary （GB） migration, and thus, is documented as an efficient approach for stabilizing nanocrystalline （NC） materials at elevated ...Second-phase particle pinning has been well known as a mechanism impeding grain boundary （GB） migration, and thus, is documented as an efficient approach for stabilizing nanocrystalline （NC） materials at elevated temperatures. The pinning force exerted by interaction between small dispersed particles and GBs strongly depends on size and volume fraction of the particles. Since metallic oxides, e.g. Al2O3, exhibit great structural stability and high resistance against coarsening at high temperatures, they are expected as effective stabilizers for NC materials. In this work, NC composites consisting of NC Fe and Al2O3 nanoparticIes with different amounts and sizes were prepared by high energy ball milling and annealed at various temperatures （Tann） for different time periods （tann）. Microstructures of the ball milled and annealed samples were examined by X-ray diffraction and transmission electron microscopy. The results show that the addition of Al2O3 nanoparticles not only enhances the thermal stability of NC Fe grains but also reduces their coarsening rate at elevated temperatures, and reducing the particle size and/or increasing its amount enhance the stabilizing effect of the Al2O3 particles on the NC Fe grains.展开更多
Microstructures of nanoporous Pd are essentially important for its physical and chemical properties.In this work,we show that the microstructures of nanoporous Pd can be tuned by adjusting compositions of the precurso...Microstructures of nanoporous Pd are essentially important for its physical and chemical properties.In this work,we show that the microstructures of nanoporous Pd can be tuned by adjusting compositions of the precursor alloys,and dealloying and heat treatment parameters.Both the ligament and pore sizes decrease with increasing the electrochemical potential upon dealloying and the concentration of noble component in the precursor alloys.Heat treatment causes coarsening of the nanoporous structure.Above a critical temperature,the nanoporous structures are subjected to significant coarsening.Below the critical temperature,surface diffusion is believed to dominate the coarsening process.Above the critical temperature,the nanoporous structure coarsens remarkably at a rather high rate,which is ascribed to a multiple-mechanism controlled process.展开更多
基金the National Natural Science Foundation of China(Nos.51771153,51371147,51790481 and 51431008)the Innovation Guidance Support Project for Taicang Top Research Institutes(No.TC2018DYDS20)the Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University(CX201825)。
文摘The eutectic Ag-Cu alloys exhibiting fine Ag-Cu lamellar eutectic structure formed upon rapid solidification have great potentials being used in various engineering fields.However,the desired fine primary lamellar eutectic structure(PLES)is usually replaced by a coarse anomalous eutectic structure(AES)when the undercooling prior to solidification exceeds a certain value.The forming mechanism of AES in the undercooled eutectic Ag-Cu alloy has been a controversial issue.In this work,the undercooled Ag-39.9 at.% Cu eutectic alloy is solidified under different cooling conditions by using techniques of melt fluxing and copper mold casting.The results show that the coupled eutectic growth of this alloy undergoes a transition from a slow eutectic-cellular growth(ECG)to a rapid eutectic-dendritic growth(EDG)above a undercooling of 72 K,accompanying with an abrupt change of the distribution and amount of AES in as-solidified microstructures.Two kinds of primary lamellar eutectic structures are formed by ECG and EDG during recalescence,respectively.The destabilization of PLES that causes the formation of AES is ascribed to two different mechanisms based on the microstructural examination and theoretical calculations.Below 72 K,the destabilization of PLES formed by slow ECG is caused by the mechanism of"termination migration"driven by interfacial energy.While above 72 K,the destabilization of PLES formed by rapid EDG is attributed to the unstable perturbation of interface driven by interfacial energy and solute supersaturation.
基金the National Key R&D Program of China (Project No. 2017YFB0703001)the National Natural Science Foundation of China (Nos. 51371147, 51101121, 51125002, 51134011, 51771153 and 51431008)+2 种基金the Research Fund of the State Key Lab. of Solidification Processing (NWPU) (No. 146-QZ2016)the Fundamental Research Funds for the Central Universities (No. 3102017jc03008)the Shaanxi Young Stars of Science and Technology (No. 2016KJXX-44) for financial supports
文摘Second-phase particle pinning has been well known as a mechanism impeding grain boundary （GB） migration, and thus, is documented as an efficient approach for stabilizing nanocrystalline （NC） materials at elevated temperatures. The pinning force exerted by interaction between small dispersed particles and GBs strongly depends on size and volume fraction of the particles. Since metallic oxides, e.g. Al2O3, exhibit great structural stability and high resistance against coarsening at high temperatures, they are expected as effective stabilizers for NC materials. In this work, NC composites consisting of NC Fe and Al2O3 nanoparticIes with different amounts and sizes were prepared by high energy ball milling and annealed at various temperatures （Tann） for different time periods （tann）. Microstructures of the ball milled and annealed samples were examined by X-ray diffraction and transmission electron microscopy. The results show that the addition of Al2O3 nanoparticles not only enhances the thermal stability of NC Fe grains but also reduces their coarsening rate at elevated temperatures, and reducing the particle size and/or increasing its amount enhance the stabilizing effect of the Al2O3 particles on the NC Fe grains.
基金supported financially by the National Natural Science Foundation of China(Nos.51771153,51371147,51790481 and 51431008)the Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University(No.CX201825)。
文摘Microstructures of nanoporous Pd are essentially important for its physical and chemical properties.In this work,we show that the microstructures of nanoporous Pd can be tuned by adjusting compositions of the precursor alloys,and dealloying and heat treatment parameters.Both the ligament and pore sizes decrease with increasing the electrochemical potential upon dealloying and the concentration of noble component in the precursor alloys.Heat treatment causes coarsening of the nanoporous structure.Above a critical temperature,the nanoporous structures are subjected to significant coarsening.Below the critical temperature,surface diffusion is believed to dominate the coarsening process.Above the critical temperature,the nanoporous structure coarsens remarkably at a rather high rate,which is ascribed to a multiple-mechanism controlled process.
