The kinetic characteristics of W grain growth operated by diffusion controlled Oswald ripening （DOR） during liquid phase sintering were studied. A liquid phase sintering of W-15wt%Cu was carried out by pushing compa...The kinetic characteristics of W grain growth operated by diffusion controlled Oswald ripening （DOR） during liquid phase sintering were studied. A liquid phase sintering of W-15wt%Cu was carried out by pushing compacts into a furnace at the moment when the temperature increased to 1340℃ for different sintering times. The results show that liquid phase sintering produces the compacts with considerably low relative density and inversely, rather high homogeneity. On the basis of the data extracted from the SEM images, the kinetic equation of W grain growth, G^n = G0^n ＋ kt, is determined in which the grain growth exponent n is 3 and the grain growth rate constant k is 0.15 μm^3/s. The cumulative normalized grain size distributions produced by different sintering times show self-similar. The cumulative distribution function is extracted from the curves by non-linear fitting. In addition, the sintering kinetic characteristics of W-15wt%Cu compacts were also investigated.展开更多
In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel were conducted on a high-temperature laser scanning confocal microscope during continuous heating and subsequent isothermal holding at ...In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel were conducted on a high-temperature laser scanning confocal microscope during continuous heating and subsequent isothermal holding at 850, 1000, and 1100℃ for 30 min. A grain growth model was proposed based on experimental results. It is indicated that the austenite grain size increases with austenitizing temperature and holding time. When the austenitizing temperature is above 1100℃, the austenite grains grow rapidly, and abnormal austenite grains occur. In addition, the effect of heating rate on austenite grain growth was investigated, and the relation between austenite grains and bainite morphology after bainitic transformations was also discussed.展开更多
A new Monte Carlo simulation method for studying three-dimensional microstructures as well as their evolution in polycrystalline materials has been set up. The algorithm is simple and flexible to apply. With the prese...A new Monte Carlo simulation method for studying three-dimensional microstructures as well as their evolution in polycrystalline materials has been set up. The algorithm is simple and flexible to apply. With the present method, kinetics of three-dimensional grain growth is accurately reflected and the simulation efficiency is greatly improved. The simulation can not only be used reliably to analyze quantitatively the microstructures and their evolution, but also be used conveniently to observe microstructures as well as their evolution on the horizontal section and the sections at any angle to the horizontal plane, to measure the characteristic parameters in three dimensions and cross-sections, together with their relationships between the two systems, and to many other aspects.展开更多
Ultra-fine austenite grains with size of i-3 μm were prepared in a Nb-V-Ti steel through repetitive treatment of rapid heating and quenching. A model for the growth kinetics of these ultra-fine austenite grains was s...Ultra-fine austenite grains with size of i-3 μm were prepared in a Nb-V-Ti steel through repetitive treatment of rapid heating and quenching. A model for the growth kinetics of these ultra-fine austenite grains was successfully created through successive 2 processes, and the activation energy Q for growth was estimated to be about 693.2 kJ/mol, which directly shows the inhibition effect of microalloy elements on the growth of ultra-fine austenite grains.展开更多
The microstructures and their kinetics of normal grain growth are simulated using different Monte Carlo （MC） algorithms. Compared with the relative figures and the theoretical normal grain growth exponents of n =0.5...The microstructures and their kinetics of normal grain growth are simulated using different Monte Carlo （MC） algorithms. Compared with the relative figures and the theoretical normal grain growth exponents of n =0.5, the effects of some factors of MC algorithm, i.e. the lattice types, the methods of selecting lattice sites, and the neighbors selection for energy calculations, on the simulation results of grain growth are studied. Two methods of regression were compared, and the three-parameter nonlinear regression is much more suitable for fitting the grain growth kinetics. A better model with appropriate factors included triangular lattice, the attempted site randomly selected, and the first and second nearest neighbors for energy calculations is obtained.展开更多
Niobium has an important effect on the transformation behaviour,grain size refinement and precipitation strengthening during hot rolling and subsequent cooling in low carbon steels,with even a low content of niobium h...Niobium has an important effect on the transformation behaviour,grain size refinement and precipitation strengthening during hot rolling and subsequent cooling in low carbon steels,with even a low content of niobium having a strong effect on the transformation rate from austenite to ferrite.However,the effects of niobium on transformation behaviour have not been fully characterised and understood to date.This paper examines in detail austenite grain growth as a function of austenitisation time in high strength low alloy (HSLA) steels with three different niobium contents,together with the effect of niobium on the isothermal transformation kinetics from austenite to ferrite as a function of temperature.It is shown that austenite has the slowest grain growth rate in the steel with the highest niobium content.When austenite grain sizes are consistent,the steel with the highest niobium content was found to have the slowest transformation rate from austenite to ferrite.展开更多
The Nd2Fe14 B grain growth kinetics in sintered NdFeB magnets with nominal compositions of Nd30+xDy1.5Fe67.08-xAl0.4B1.02 (%, x = 0, 3) was studied. The grain size parameters were determined by means of the linear int...The Nd2Fe14 B grain growth kinetics in sintered NdFeB magnets with nominal compositions of Nd30+xDy1.5Fe67.08-xAl0.4B1.02 (%, x = 0, 3) was studied. The grain size parameters were determined by means of the linear intercept method on SEM secondary electron images. It is observed that the grain growth process is more sensitive to sintering temperature than to sintering time although the grain size rises with both sintering temperature and time. It is also found that magnets prepared from the pre-alloy powder with a higher oxygen content exhibit a lower grain growth rate and magnets made from the pre-alloy powder with a broader particle size distribution demonstrate a higher grain growth rate. It is believed that the presence of appropriate amounts of Nd oxides effectively impedes the grain growth process and a larger difference in sizes between pre-alloy powder particles significantly accelerates the grain growth process. On the basis of experimental results, the grain growth exponent and the corresponding activation energy were obtained. A possible grain growth mechanism in NdFeB magnets during sintering was briefly discussed.展开更多
Monodispersed spheroidal SnO2 nanocrystals with the grain size of 8-30 nm were synthesized by the precipitation method using SnCl4·5H2O （stannic chloride hydrate） as raw materials.Differential scanning calorime...Monodispersed spheroidal SnO2 nanocrystals with the grain size of 8-30 nm were synthesized by the precipitation method using SnCl4·5H2O （stannic chloride hydrate） as raw materials.Differential scanning calorimetry/thermogravimetry （DSC/TG）,X-ray diffraction （XRD） and transmission electron microscope （TEM） were used to characterize the structure of SnO2 nanocrystals.The influences of the calcination temperature and time on the lattice constant,the lattice distortion and the grain size of SnO2 nanocrystals were discussed based on the XRD results.The grain growth kinetics of SnO2 nanocrystals during calcination process was simulated with a conventional grain growth model which only took into account of diffusion and with a new isothermal model proposed by our group,which took into account of both diffusion and surface reactions.Using conventional model,the grain growth rate constant of SnO2 crystals is 1.55×104nm5/min with a pre-exponential factor of 5 and an activation energy of 108.62 kJ/mol.Compared with the convention model,the new isothermal model is more realistic in reflecting the grain growth behavior of SnO2 nanocrystals during the calcination process.This indicates that the grain growth of SnO2 nanocrystals is controlled by both diffusion and reaction factors,and the effect of surface reactivity on the grain growth of SnO2 nanocrystals could not be ignored.A combined activation energy estimated with the new isothermal model is 53.46 kJ/mol.展开更多
The RAFM（reduced activation ferritic/martensitic）steels containing different tantalum contents（0wt.%,0.027wt.%,0.073wt.%）were designed and cast.Differential scanning calorimetry and optical microscopy were employe...The RAFM（reduced activation ferritic/martensitic）steels containing different tantalum contents（0wt.%,0.027wt.%,0.073wt.%）were designed and cast.Differential scanning calorimetry and optical microscopy were employed to explore the influence of tantalum content on the austenitic transformation of RAFM steels.The austenitic transformation kinetics was described by aphase-transformation model.The model,involving site saturation nucleation,diffusion-controlled growth and impingement correction,was established based on the classical Johnson-Mehl-Avrami-Kolmogorov model.The phase-transformation kinetics parameters,including D_0（pre-exponential factor for diffusion）and Q_d（activation energy for diffusion）,were calculated by fitting the experimental data and the kinetic model.The results indicated that the average grain size is decreased with the increase of tantalum.The values of A_（c1） and A_（c3） （onset and finish temperature of austenitic transformation,respectively）are increased by increasing the tantalum content.The increase of tantalum caused the decrease of D_0.However,Q_d is increased with the increase of tantalum.In addition,as a carbides forming element,tantalum would reduce the carbon diffusion coefficient and slow down the austenitic transformation rate.展开更多
In order to control the grain size during hot forming,grain growth behavior of a pre-extruded Mg-6Zn magnesium alloy and its correlation with solute and second phase distribution were investigated.Isothermal annealing...In order to control the grain size during hot forming,grain growth behavior of a pre-extruded Mg-6Zn magnesium alloy and its correlation with solute and second phase distribution were investigated.Isothermal annealing was conducted on a Gleeble-1500 thermo-mechanical simulator.The mean grain size Dg of each annealed specimen was measured by the quantitative metallography technique.The grain growth kinetics of the Mg-6Zn alloy annealed at 473-623 K was obtained as Dg^4- Dg0^4=2.25 ×10^11 exp（-95450）by the least square linear regression method.The deviation of grain growth exponent n = 4 from the theoretical value of 2 may be attributed to the presence of solute zinc and second phases which will retard the boundary migration.Microscopic observations show that the non-uniform distribution of grain size for samples pre-extruded or annealed at low temperatures is closely related to the non-uniform distribution of fine and dispersed second phases but not to the non-uniform distribution of solute zinc.This indicates that second phase pinning effect plays an important role in microstructure refinement.展开更多
基金This work was financially supported by the National Natural Science Foundation of China (No. 50174007).
文摘The kinetic characteristics of W grain growth operated by diffusion controlled Oswald ripening （DOR） during liquid phase sintering were studied. A liquid phase sintering of W-15wt%Cu was carried out by pushing compacts into a furnace at the moment when the temperature increased to 1340℃ for different sintering times. The results show that liquid phase sintering produces the compacts with considerably low relative density and inversely, rather high homogeneity. On the basis of the data extracted from the SEM images, the kinetic equation of W grain growth, G^n = G0^n ＋ kt, is determined in which the grain growth exponent n is 3 and the grain growth rate constant k is 0.15 μm^3/s. The cumulative normalized grain size distributions produced by different sintering times show self-similar. The cumulative distribution function is extracted from the curves by non-linear fitting. In addition, the sintering kinetic characteristics of W-15wt%Cu compacts were also investigated.
基金the National Natural Science Foundation of China(No.51274154)the National High-Tech Research and Development Program of China(No.2012AA03A504)+1 种基金the State Key Laboratory of Development and Application Technology of Automo-tive Steels(Baosteel Group)the Key Project of Hubei Education Committee(No.D20121101)
文摘In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel were conducted on a high-temperature laser scanning confocal microscope during continuous heating and subsequent isothermal holding at 850, 1000, and 1100℃ for 30 min. A grain growth model was proposed based on experimental results. It is indicated that the austenite grain size increases with austenitizing temperature and holding time. When the austenitizing temperature is above 1100℃, the austenite grains grow rapidly, and abnormal austenite grains occur. In addition, the effect of heating rate on austenite grain growth was investigated, and the relation between austenite grains and bainite morphology after bainitic transformations was also discussed.
文摘A new Monte Carlo simulation method for studying three-dimensional microstructures as well as their evolution in polycrystalline materials has been set up. The algorithm is simple and flexible to apply. With the present method, kinetics of three-dimensional grain growth is accurately reflected and the simulation efficiency is greatly improved. The simulation can not only be used reliably to analyze quantitatively the microstructures and their evolution, but also be used conveniently to observe microstructures as well as their evolution on the horizontal section and the sections at any angle to the horizontal plane, to measure the characteristic parameters in three dimensions and cross-sections, together with their relationships between the two systems, and to many other aspects.
基金supported by the National Natural Science Foundation of China(No.50527402)
文摘Ultra-fine austenite grains with size of i-3 μm were prepared in a Nb-V-Ti steel through repetitive treatment of rapid heating and quenching. A model for the growth kinetics of these ultra-fine austenite grains was successfully created through successive 2 processes, and the activation energy Q for growth was estimated to be about 693.2 kJ/mol, which directly shows the inhibition effect of microalloy elements on the growth of ultra-fine austenite grains.
基金the International Science & Technology Cooperation Project of Shandong Province(2006)the Natural Science Foundation of Shandong Province(Y2007F06).
文摘The microstructures and their kinetics of normal grain growth are simulated using different Monte Carlo （MC） algorithms. Compared with the relative figures and the theoretical normal grain growth exponents of n =0.5, the effects of some factors of MC algorithm, i.e. the lattice types, the methods of selecting lattice sites, and the neighbors selection for energy calculations, on the simulation results of grain growth are studied. Two methods of regression were compared, and the three-parameter nonlinear regression is much more suitable for fitting the grain growth kinetics. A better model with appropriate factors included triangular lattice, the attempted site randomly selected, and the first and second nearest neighbors for energy calculations is obtained.
文摘Niobium has an important effect on the transformation behaviour,grain size refinement and precipitation strengthening during hot rolling and subsequent cooling in low carbon steels,with even a low content of niobium having a strong effect on the transformation rate from austenite to ferrite.However,the effects of niobium on transformation behaviour have not been fully characterised and understood to date.This paper examines in detail austenite grain growth as a function of austenitisation time in high strength low alloy (HSLA) steels with three different niobium contents,together with the effect of niobium on the isothermal transformation kinetics from austenite to ferrite as a function of temperature.It is shown that austenite has the slowest grain growth rate in the steel with the highest niobium content.When austenite grain sizes are consistent,the steel with the highest niobium content was found to have the slowest transformation rate from austenite to ferrite.
文摘The Nd2Fe14 B grain growth kinetics in sintered NdFeB magnets with nominal compositions of Nd30+xDy1.5Fe67.08-xAl0.4B1.02 (%, x = 0, 3) was studied. The grain size parameters were determined by means of the linear intercept method on SEM secondary electron images. It is observed that the grain growth process is more sensitive to sintering temperature than to sintering time although the grain size rises with both sintering temperature and time. It is also found that magnets prepared from the pre-alloy powder with a higher oxygen content exhibit a lower grain growth rate and magnets made from the pre-alloy powder with a broader particle size distribution demonstrate a higher grain growth rate. It is believed that the presence of appropriate amounts of Nd oxides effectively impedes the grain growth process and a larger difference in sizes between pre-alloy powder particles significantly accelerates the grain growth process. On the basis of experimental results, the grain growth exponent and the corresponding activation energy were obtained. A possible grain growth mechanism in NdFeB magnets during sintering was briefly discussed.
基金National Key R&D Program(2018YFB1307902)National Natural Science Foundation of China(U1710113)+5 种基金China Post-doctoral Science Foundation(2017M622903)Shanxi Key R&D Project(201703D111003)Shanxi Province Graduate Joint Talent Training Project(2018JD33)Shanxi Graduate Education Innovation Project(2019SY482)Outstanding Young Academic Leader of Colleges and Universities in Shanxi Province(2019045)Shanxi Province Outstanding Achievement Cultivation Project(2019KJ027)。
基金Funded by the International Cooperation of Science and Technology Ministry PRC (2005DFBA028)the Nation Undergraduate Innovation Experimentation Plan of Education Ministry PRC (LA08025)
文摘Monodispersed spheroidal SnO2 nanocrystals with the grain size of 8-30 nm were synthesized by the precipitation method using SnCl4·5H2O （stannic chloride hydrate） as raw materials.Differential scanning calorimetry/thermogravimetry （DSC/TG）,X-ray diffraction （XRD） and transmission electron microscope （TEM） were used to characterize the structure of SnO2 nanocrystals.The influences of the calcination temperature and time on the lattice constant,the lattice distortion and the grain size of SnO2 nanocrystals were discussed based on the XRD results.The grain growth kinetics of SnO2 nanocrystals during calcination process was simulated with a conventional grain growth model which only took into account of diffusion and with a new isothermal model proposed by our group,which took into account of both diffusion and surface reactions.Using conventional model,the grain growth rate constant of SnO2 crystals is 1.55×104nm5/min with a pre-exponential factor of 5 and an activation energy of 108.62 kJ/mol.Compared with the convention model,the new isothermal model is more realistic in reflecting the grain growth behavior of SnO2 nanocrystals during the calcination process.This indicates that the grain growth of SnO2 nanocrystals is controlled by both diffusion and reaction factors,and the effect of surface reactivity on the grain growth of SnO2 nanocrystals could not be ignored.A combined activation energy estimated with the new isothermal model is 53.46 kJ/mol.
基金financially sponsored by the China National Funds for Distinguished Young Scientists (Granted No.51325401)the National Natural Science Foundation of China(Granted No.51501126)the National Magnetic Confinement Fusion Energy Research Program(Granted No.2015GB119001)
文摘The RAFM（reduced activation ferritic/martensitic）steels containing different tantalum contents（0wt.%,0.027wt.%,0.073wt.%）were designed and cast.Differential scanning calorimetry and optical microscopy were employed to explore the influence of tantalum content on the austenitic transformation of RAFM steels.The austenitic transformation kinetics was described by aphase-transformation model.The model,involving site saturation nucleation,diffusion-controlled growth and impingement correction,was established based on the classical Johnson-Mehl-Avrami-Kolmogorov model.The phase-transformation kinetics parameters,including D_0（pre-exponential factor for diffusion）and Q_d（activation energy for diffusion）,were calculated by fitting the experimental data and the kinetic model.The results indicated that the average grain size is decreased with the increase of tantalum.The values of A_（c1） and A_（c3） （onset and finish temperature of austenitic transformation,respectively）are increased by increasing the tantalum content.The increase of tantalum caused the decrease of D_0.However,Q_d is increased with the increase of tantalum.In addition,as a carbides forming element,tantalum would reduce the carbon diffusion coefficient and slow down the austenitic transformation rate.
基金the financial support from the National Natural Science Foundation of China(Grant No.51105328)the Natural Science Foundation of Jiangsu Province of China(No.BK20130447)the Colleges and Universities in Jiangsu Province Natural Science Foundation of China(Grant No.13KJB430026)
文摘In order to control the grain size during hot forming,grain growth behavior of a pre-extruded Mg-6Zn magnesium alloy and its correlation with solute and second phase distribution were investigated.Isothermal annealing was conducted on a Gleeble-1500 thermo-mechanical simulator.The mean grain size Dg of each annealed specimen was measured by the quantitative metallography technique.The grain growth kinetics of the Mg-6Zn alloy annealed at 473-623 K was obtained as Dg^4- Dg0^4=2.25 ×10^11 exp（-95450）by the least square linear regression method.The deviation of grain growth exponent n = 4 from the theoretical value of 2 may be attributed to the presence of solute zinc and second phases which will retard the boundary migration.Microscopic observations show that the non-uniform distribution of grain size for samples pre-extruded or annealed at low temperatures is closely related to the non-uniform distribution of fine and dispersed second phases but not to the non-uniform distribution of solute zinc.This indicates that second phase pinning effect plays an important role in microstructure refinement.
