For further knowledge about the refining performance of AlTiC master alloys, Al5.5Ti0.25C and Al6.5Ti0.5C master alloys containing high Ti and C content were prepared and used in grain refining experiments of 99.8% co...For further knowledge about the refining performance of AlTiC master alloys, Al5.5Ti0.25C and Al6.5Ti0.5C master alloys containing high Ti and C content were prepared and used in grain refining experiments of 99.8% commercial pure aluminum(CPAl). Their performance was compared with two types of Al5Ti1B refiners whose performance was nowadays considered to be the best. These two types of master alloys show similar refining efficiency at the addition level of 0.2%. However, at the addition level of 0.5%, there still exists great performance difference between AlTiC and Al5TiB alloys in grain refinement of 99.98% and 99.995% high purity aluminum(HPAl). The growth of columnar grains is fully suppressed due to the refinement of AlTiC at the addition level of 0.5%. Also, at the same addition level, the grain refining experiments of Al3Ti0.15C and Al5Ti0.2C master alloys which have found initial commercial applications are conducted in the above-mentioned three types of pure aluminum. According to the experimental results, these two refiners of different compositions are both nonideal. The second phase particles extracted from each refiner were observed through TEM, while the nuclei of grains after grain refinement were observed through SEM. The results were analyzed through computation and comparison of the constitutional-supercooling parameter and the growth-restriction parameter whose values were determined by solute element in aluminum melt with different purity. Apparently, AlTiC master alloys with high content of Ti and C element have great refining potential.展开更多
The grain refining process of an AZ91D Mg alloy by Sr addition was studied and the heterogeneous nucleating particles of α-Mg were investigated by electron probe microanalysis （EPMA）. With 0.6 wt% Sr addition, the ...The grain refining process of an AZ91D Mg alloy by Sr addition was studied and the heterogeneous nucleating particles of α-Mg were investigated by electron probe microanalysis （EPMA）. With 0.6 wt% Sr addition, the mean grain size of AZ91D alloy was refined from 235.4μm to 52.5 μm at the one-half radius of the ingot. The morphology of primary crystal changed from a sixford symmetrical shape to a petallike shape, Mg-Sr-Al-Fe-Mn heterogeneous nucleating particles were observed at the grain centers and Sr solute atoms presented segregation along the grain boundaries. Grain refinement was facilitated by both the Mg-Sr-Al- Fe-Mn nucleating particles and the Sr solute atoms, and the former played a dominate role in the process.展开更多
Here we reported a heterogeneous fiberous structured Mg-5.6Zn-0.6Zr(wt%)alloy obtained by conventional extrusion method,which exhibited high yield strength of∼345 MPa,ultimate tensile strength of∼370 MPa,and high te...Here we reported a heterogeneous fiberous structured Mg-5.6Zn-0.6Zr(wt%)alloy obtained by conventional extrusion method,which exhibited high yield strength of∼345 MPa,ultimate tensile strength of∼370 MPa,and high tensile strain of∼20.5%,superior to most of the Mg-Zn based alloys reported so far.The extraordinarily high mechanical properties were mainly attributed to the heterogeneous fiberous structure consisting of alternating coarse-and fine-grain layers.Grains in the different layers grew into the neighboring layers,ensuring a good layer bonding.A high Schmid factor and geometric compatibility factor for pyramidal slip led to full slip transfer between the neighboring coarse grains and fine grains,which could help to release the stress concentration and avoid early fracture.The profuse acti-vated<c+a>glide dislocations could render the unprecedented high tensile strain.The constraint by the hard fine-grain domains made the soft coarse-grain domains strong like the hard fine-grain domains,as well as the nanoscale precipitates pinning dislocations,contributed to the high strength.The hetero-geneous microstructure design was shown to have synergistic improvement in strength-ductility balance,which could be an inspiring strategy to improve mechanical properties of hexagonal close-packed(hcp)metals.展开更多
The distinctive intrinsic heat treatment(IHT)originating from cyclic reheating in the laser-directed energy deposition(LDED) has attracted growing attention in recent years.In this investigation,simulations and experi...The distinctive intrinsic heat treatment(IHT)originating from cyclic reheating in the laser-directed energy deposition(LDED) has attracted growing attention in recent years.In this investigation,simulations and experimental characterizations were performed to examine the impact of IHT on the micros true ture and mechanical properties of LDED-fabricated CrCoNi medium-entropy alloy(MEA).The results show that the intensity of the IHT is proportional to the utilized laser energy density(LED).As the LED increased,significant dynamic recrystallization and grain refinement occurred within the alloy due to the enhanced intensity of IHT.However,the high LED leads to severe hot cracking within the as-built MEA,resulting in inferior ductility.By decreasing LED,the hot cracking was effectively eliminated.Meanwhile,low LED weakened the intensity of IHT and consequently inhibited kinetic conditions of dynamic recrystallization,resulting in a heterogeneous grain structure characterized by multi-scale-sized grains.This structure provides significant hetero-deformation-induced hardening during plastic deformation,enabling the alloy to have a sustainable work-hardening capacity.We expect that this work will have implications in taking full advantage of the unique IHT of the LDED process to fabricate ME As with excellent metallurgical quality and mechanical performance.展开更多
The grain density,Nv,in the solid state after solidification of AZ91/SiC composite is a function of maximum undercooling,ΔT,of a liquid alloy.This type of function depends on the characteristics of heterogeneous nucl...The grain density,Nv,in the solid state after solidification of AZ91/SiC composite is a function of maximum undercooling,ΔT,of a liquid alloy.This type of function depends on the characteristics of heterogeneous nucleation sites and number of SiC present in the alloy.The aim of this paper was selection of parameters for the model describing the relationship between the grain density of primary phase and undercooling.This model in connection with model of crystallisation,which is based on chemical elements diffusion and grain interface kinetics,can be used to predict casting quality and its microstructure.Nucleation models have parameters,which exact values are usually not known and sometimes even their physical meaning is under discussion.Those parameters can be obtained after mathematical analysis of the experimental data.The composites with 0,1,2,3 and 4wt.% of SiC particles were prepared.The AZ91 alloy was a matrix of the composite reinforcement SiC particles.This composite was cast to prepare four different thickness plates.They were taken from the region near to the thermocouple,to analyze the undercooling for different composites and thickness plates and its influence on the grain size.The microstructure and thermal analysis gave set of values that connect mass fraction of SiC particles,and undercooling with grain size.These values were used to approximate nucleation model adjustment parameters.Obtained model can be very useful in modelling composites microstructure.展开更多
Heterogeneous nucleation is an effective way to promote the dispersion and precipitation of second-phase particles in steel and refine the grain size of the solidification structure.Not only refining as-cast structure...Heterogeneous nucleation is an effective way to promote the dispersion and precipitation of second-phase particles in steel and refine the grain size of the solidification structure.Not only refining as-cast structure grain size,but TiN in ferritic stainless steel can also pin grain boundaries and restrain the overgrowth of grains during rolling.The interface characteristics between TiN and heterogeneous phases(high-melting inclusions and ferrite phase)were studied based on the wetting angles between molten steel with different compositions and TiN substrate,and on the matching degree between TiN and ferrite lattice.It was found that,for the molten steel with the same composition,the wetting angle with the TiN substrate was significantly smaller than the contact angles with the other three substrates,while the wetting angle between ferrite phase and TiN was the smallest.The lattice matching was compared among MgAl_(2)O_(4),TiN andδmatrix by means of a high-resolution transmission electron microscope,which revealed that a coherent or semi-coherent interface was formed between the crystal plane(400)of MgAl_(2)O_(4)and the crystal plane(200)of TiN,as well as between the crystal plane(200)of TiN and the crystal plane(110)ofδmatrix,with a lattice misfit of 5.1%and 3.4%,respectively.Finally,these two characteristics between TiN and ferrite phase were both explained from the perspective of interfacial energy.The microstructure refinement mechanism from high temperature to room temperature can be better reflected by the proposed wetting–lattice misfit theory.展开更多
Cold rolling and post-deformation annealing(PDA)heat treatments were used to produce heterogeneous grain structures(HGS)in a single-phase face-centered cubic(fcc)Cr_(10)Co_(30)Fe_(30)Ni_(30)high-entropy alloy(HEA).The...Cold rolling and post-deformation annealing(PDA)heat treatments were used to produce heterogeneous grain structures(HGS)in a single-phase face-centered cubic(fcc)Cr_(10)Co_(30)Fe_(30)Ni_(30)high-entropy alloy(HEA).The microstructural evolution and microstructure-property relationship of the HEA were systematically studied under different states.HGS could be achieved in PDA-treated samples at 875℃for 20 s and at 900℃for 20 s(PDA-900-20 s).PDA-900-20 s sample exhibits the most excellent combination of strength and ductility,showing a tensile yield strength of~590 MPa,an ultimate strength of~706 MPa and a total elongation of~23.9%.Additionally,compared with the homogenized counterpart exhibiting homogenous grains,PDA-900-20 s sample displays a notable increment of~413%in yield strength and simultaneously maintains a good ductility.The dominated strengthening mechanisms in PDA-900-20 s sample are grain-boundary strengthening and heterogeneous deformation-induced(HDI)strengthening,whereas the good ductility is mainly resulted from the HDI ductility.Accordingly,the present study provides an effective and simple pathway to overcome the strength-ductility tradeoff of typical fcc HEAs through heterogeneous microstructure.展开更多
基金Project (51074033) supported by the National Natural Science Foundation of China
文摘For further knowledge about the refining performance of AlTiC master alloys, Al5.5Ti0.25C and Al6.5Ti0.5C master alloys containing high Ti and C content were prepared and used in grain refining experiments of 99.8% commercial pure aluminum(CPAl). Their performance was compared with two types of Al5Ti1B refiners whose performance was nowadays considered to be the best. These two types of master alloys show similar refining efficiency at the addition level of 0.2%. However, at the addition level of 0.5%, there still exists great performance difference between AlTiC and Al5TiB alloys in grain refinement of 99.98% and 99.995% high purity aluminum(HPAl). The growth of columnar grains is fully suppressed due to the refinement of AlTiC at the addition level of 0.5%. Also, at the same addition level, the grain refining experiments of Al3Ti0.15C and Al5Ti0.2C master alloys which have found initial commercial applications are conducted in the above-mentioned three types of pure aluminum. According to the experimental results, these two refiners of different compositions are both nonideal. The second phase particles extracted from each refiner were observed through TEM, while the nuclei of grains after grain refinement were observed through SEM. The results were analyzed through computation and comparison of the constitutional-supercooling parameter and the growth-restriction parameter whose values were determined by solute element in aluminum melt with different purity. Apparently, AlTiC master alloys with high content of Ti and C element have great refining potential.
基金Funded by the National Natural Science Foundation of China (No. 50171037) Key Project of Science and Technology Research of Ministry of Education of China (No. 01105)
文摘The grain refining process of an AZ91D Mg alloy by Sr addition was studied and the heterogeneous nucleating particles of α-Mg were investigated by electron probe microanalysis （EPMA）. With 0.6 wt% Sr addition, the mean grain size of AZ91D alloy was refined from 235.4μm to 52.5 μm at the one-half radius of the ingot. The morphology of primary crystal changed from a sixford symmetrical shape to a petallike shape, Mg-Sr-Al-Fe-Mn heterogeneous nucleating particles were observed at the grain centers and Sr solute atoms presented segregation along the grain boundaries. Grain refinement was facilitated by both the Mg-Sr-Al- Fe-Mn nucleating particles and the Sr solute atoms, and the former played a dominate role in the process.
基金support of the National Natural Science Foundation of China(No.51901174)the 111 Project 2.0 of China(BP2018008)the China Postdoctoral Science Foun-dation(No.2020M673383).
文摘Here we reported a heterogeneous fiberous structured Mg-5.6Zn-0.6Zr(wt%)alloy obtained by conventional extrusion method,which exhibited high yield strength of∼345 MPa,ultimate tensile strength of∼370 MPa,and high tensile strain of∼20.5%,superior to most of the Mg-Zn based alloys reported so far.The extraordinarily high mechanical properties were mainly attributed to the heterogeneous fiberous structure consisting of alternating coarse-and fine-grain layers.Grains in the different layers grew into the neighboring layers,ensuring a good layer bonding.A high Schmid factor and geometric compatibility factor for pyramidal slip led to full slip transfer between the neighboring coarse grains and fine grains,which could help to release the stress concentration and avoid early fracture.The profuse acti-vated<c+a>glide dislocations could render the unprecedented high tensile strain.The constraint by the hard fine-grain domains made the soft coarse-grain domains strong like the hard fine-grain domains,as well as the nanoscale precipitates pinning dislocations,contributed to the high strength.The hetero-geneous microstructure design was shown to have synergistic improvement in strength-ductility balance,which could be an inspiring strategy to improve mechanical properties of hexagonal close-packed(hcp)metals.
基金supported by the National Key Research and Development Program of China (No.2022YFE0134400)the National Natural Science Foundation of China (Nos.51971247,51971190)the Natural Science Foundation of Hunan Province,China (No.2022JJ30712).
基金financially supported by the funding from Guangdong Province Basic and Applied Research Key Projects (No.2020B0301030001)National Key R&D Programme,Ministry of Science and Technology of China (Nos. 2018YFB1105200 and 2019YFA0209904)the National Natural Science Foundation of China (Nos.52371106 and 52371025)。
文摘The distinctive intrinsic heat treatment(IHT)originating from cyclic reheating in the laser-directed energy deposition(LDED) has attracted growing attention in recent years.In this investigation,simulations and experimental characterizations were performed to examine the impact of IHT on the micros true ture and mechanical properties of LDED-fabricated CrCoNi medium-entropy alloy(MEA).The results show that the intensity of the IHT is proportional to the utilized laser energy density(LED).As the LED increased,significant dynamic recrystallization and grain refinement occurred within the alloy due to the enhanced intensity of IHT.However,the high LED leads to severe hot cracking within the as-built MEA,resulting in inferior ductility.By decreasing LED,the hot cracking was effectively eliminated.Meanwhile,low LED weakened the intensity of IHT and consequently inhibited kinetic conditions of dynamic recrystallization,resulting in a heterogeneous grain structure characterized by multi-scale-sized grains.This structure provides significant hetero-deformation-induced hardening during plastic deformation,enabling the alloy to have a sustainable work-hardening capacity.We expect that this work will have implications in taking full advantage of the unique IHT of the LDED process to fabricate ME As with excellent metallurgical quality and mechanical performance.
基金supported financially by the European Community under Marie Curie Transfer of Knowledge grant No. MTKD-CT-2006-042468 (AGH No.27.27.170.304)Polish Ministry of Science and Higher Education for financial support under grant No. N507-44-66-34 (AGH No.18.18.170.325)
文摘The grain density,Nv,in the solid state after solidification of AZ91/SiC composite is a function of maximum undercooling,ΔT,of a liquid alloy.This type of function depends on the characteristics of heterogeneous nucleation sites and number of SiC present in the alloy.The aim of this paper was selection of parameters for the model describing the relationship between the grain density of primary phase and undercooling.This model in connection with model of crystallisation,which is based on chemical elements diffusion and grain interface kinetics,can be used to predict casting quality and its microstructure.Nucleation models have parameters,which exact values are usually not known and sometimes even their physical meaning is under discussion.Those parameters can be obtained after mathematical analysis of the experimental data.The composites with 0,1,2,3 and 4wt.% of SiC particles were prepared.The AZ91 alloy was a matrix of the composite reinforcement SiC particles.This composite was cast to prepare four different thickness plates.They were taken from the region near to the thermocouple,to analyze the undercooling for different composites and thickness plates and its influence on the grain size.The microstructure and thermal analysis gave set of values that connect mass fraction of SiC particles,and undercooling with grain size.These values were used to approximate nucleation model adjustment parameters.Obtained model can be very useful in modelling composites microstructure.
基金the National Natural Science Foundation of China(Grant Nos.51774208,52074186,51804205 and U1860205).
文摘Heterogeneous nucleation is an effective way to promote the dispersion and precipitation of second-phase particles in steel and refine the grain size of the solidification structure.Not only refining as-cast structure grain size,but TiN in ferritic stainless steel can also pin grain boundaries and restrain the overgrowth of grains during rolling.The interface characteristics between TiN and heterogeneous phases(high-melting inclusions and ferrite phase)were studied based on the wetting angles between molten steel with different compositions and TiN substrate,and on the matching degree between TiN and ferrite lattice.It was found that,for the molten steel with the same composition,the wetting angle with the TiN substrate was significantly smaller than the contact angles with the other three substrates,while the wetting angle between ferrite phase and TiN was the smallest.The lattice matching was compared among MgAl_(2)O_(4),TiN andδmatrix by means of a high-resolution transmission electron microscope,which revealed that a coherent or semi-coherent interface was formed between the crystal plane(400)of MgAl_(2)O_(4)and the crystal plane(200)of TiN,as well as between the crystal plane(200)of TiN and the crystal plane(110)ofδmatrix,with a lattice misfit of 5.1%and 3.4%,respectively.Finally,these two characteristics between TiN and ferrite phase were both explained from the perspective of interfacial energy.The microstructure refinement mechanism from high temperature to room temperature can be better reflected by the proposed wetting–lattice misfit theory.
基金financially supported by the National Natural Science Foundation of China (No.52103360)the Basic and Applied Basic Research Foundation of GuangdongProvince (No.2020A1515111104)the Key-Area Research and Development Program of Guangdong Province (No. 2018B090905002)
文摘Cold rolling and post-deformation annealing(PDA)heat treatments were used to produce heterogeneous grain structures(HGS)in a single-phase face-centered cubic(fcc)Cr_(10)Co_(30)Fe_(30)Ni_(30)high-entropy alloy(HEA).The microstructural evolution and microstructure-property relationship of the HEA were systematically studied under different states.HGS could be achieved in PDA-treated samples at 875℃for 20 s and at 900℃for 20 s(PDA-900-20 s).PDA-900-20 s sample exhibits the most excellent combination of strength and ductility,showing a tensile yield strength of~590 MPa,an ultimate strength of~706 MPa and a total elongation of~23.9%.Additionally,compared with the homogenized counterpart exhibiting homogenous grains,PDA-900-20 s sample displays a notable increment of~413%in yield strength and simultaneously maintains a good ductility.The dominated strengthening mechanisms in PDA-900-20 s sample are grain-boundary strengthening and heterogeneous deformation-induced(HDI)strengthening,whereas the good ductility is mainly resulted from the HDI ductility.Accordingly,the present study provides an effective and simple pathway to overcome the strength-ductility tradeoff of typical fcc HEAs through heterogeneous microstructure.
