The effect of cerium(Ce)on the solidification microstructure of Cr_(4)Mo_(4)V bearing steel was investigated via a combined experimental and theoretical method.With a trace amount(0.056 wt%)of Ce addition,the coarse c...The effect of cerium(Ce)on the solidification microstructure of Cr_(4)Mo_(4)V bearing steel was investigated via a combined experimental and theoretical method.With a trace amount(0.056 wt%)of Ce addition,the coarse columnar grains in as-cast microstructure transform into equiaxed ones,and the average diameter is reduced from 56 to 27μm.The network-like and bulky primary MC and M2C carbides at the interdendritic regions become disconnected and refined,and their volume percentage decreases from4.15 vol%to 2.1 vol%.Ce-inclusions acting as heterogeneous nucleation agents of prior-austenite grains and Ce atoms segregating at grain boundaries,both contribute to the refinement of grains.Thermodynamic calculations reveal that primary carbides are precipitated afterγ-austenite forms near the end of the solidification process.The modification of primary carbides in size and amount is mainly attributed to the isolated remaining melt separated by refinedγ-austenite grains in which the nucleation of carbides is promoted,while the growth is restrained owing to the less segregation of alloying elements.展开更多
Refinement of grains and intermetallic phases in the as-solidified alloy structure offers uniform struc-tural properties,eliminates or minimizes common solidification defects,including segregation and hot cracking,and...Refinement of grains and intermetallic phases in the as-solidified alloy structure offers uniform struc-tural properties,eliminates or minimizes common solidification defects,including segregation and hot cracking,and improves thermomechanical processing of wrought alloys.Melt processing by an external field is an efficient process for achieving refinement of the solidification structure of Al and Mg alloys without altering the alloy composition.A wide range of melt processing methods and solidification stud-ies(conventional,directional,and in-situ approaches)have been reported in the literature that explore the mechanism of refinement.Identifying the dominant grain refinement mechanism has been a focus of most investigations because significant variations exist according to the casting conditions and the type of applied external treatments.The origin of fine grains occurs through either one or a combination of heterogenous nucleation,fragmentation of dendrites and grains formed and then separated from the surface of the melt and mould wall under vibration or agitation.The first part of this review describes the prominent external field techniques and the mechanisms proposed for the origin of fine grains.The second part critically compares the current understanding of these grain refinement mechanisms to de-termine differences and commonalities to identify the factors that promote the formation of equiaxed zones occupying a large volume fraction of the casting.展开更多
Eutectic solidification in near-eutectic Al-13 wt pct Si casting alloys and the effect of trace addition of boron or strontium on it have been investigated using thermal analysis and microstructural characterization. ...Eutectic solidification in near-eutectic Al-13 wt pct Si casting alloys and the effect of trace addition of boron or strontium on it have been investigated using thermal analysis and microstructural characterization. In unmodified alloy, dual eutectic structure has been observed. The coarse eutectic （dendrite-like Al＋ coarse Si flakes） is formed above the equilibrium temperature of eutectic （Al＋Si） reaction （577℃）. The coarse eutectic （CE） grains nucleate from the primary silicon particles formed earlier due to local enrichment of silicon solute and grow in a divorced mode between the dendritic Al phase and large silicon flakes. The fine eutectic （FE） grains nucleate later on other potential sites activated by melt undercooling and grow in coupled-growing mode with the silicon crystals as fine flakes. The formation of the FE grains is favored in the alloys containing boron because of a great number of potential nucleation sites being added from boron-containing particles. Addition of strontium to the alloys restrains completely the formation of primary silicon particles and hence limits the nucleation of the CE. This is because the eutectic point has moved far enough to make the alloy, at this composition （Al-13 wt pct Si）, hypo-eutectic. Local cooling rate during solidification has an important influence on competition formation of these two eutectics.展开更多
In this paper, the microstructural evolution, grain refinement and mechanical properties of as-cast Mg-6Zn-0.4Zr alloys with varying Sm contents（0, 2 wt.%, 4 wt.% and 6 wt.%） were investigated by using an optical mi...In this paper, the microstructural evolution, grain refinement and mechanical properties of as-cast Mg-6Zn-0.4Zr alloys with varying Sm contents（0, 2 wt.%, 4 wt.% and 6 wt.%） were investigated by using an optical microscope（OM）, a scanning electron microscope（SEM） equipped with energy dispersive spectroscope（EDS）, an X-ray diffractometer（XRD） and mechanical tests at room temperature, respectively. The experimental results indicated that the addition of Sm could obviously refine the as-cast grains, modify the eutectic morphology and affect the mechanical properties of the alloys. The main phases in Mg-6Zn-xSm-0.4Zr alloys included matrix α-Mg, Mg2Zn3, Mg（41）Sm5 and MgZ nS m. With Sm content increasing to 4%, the MgZ nS m phase was created, meanwhile, the morphology of some eutectic phases revealed apparently lamellar structure, which had a bad effect on the mechanical properties. In addition, the maximum values of ultimate tensile strength（UTS, 214 MPa） and elongation（EL, 7.42%） were simultaneously obtained from the alloy with 2% Sm. However, Sm addition had no obvious effects on the fracture behavior of the alloys, namely, the fracture pattern of Mg-6Zn-0.4Zr alloy belonged to inter-granular and brittle modes while the fracture regimes of all the Sm-containing alloys were dominated by the mixture of inter-granular and trans-granular modes.展开更多
Martensitic microstructure in quenched and tempered 17CrNiMo6 steel with the prior austenite grain size ranging from 6 μm to 199 μm has been characterized by optical metallography （OM）, scanning electron microsco...Martensitic microstructure in quenched and tempered 17CrNiMo6 steel with the prior austenite grain size ranging from 6 μm to 199 μm has been characterized by optical metallography （OM）, scanning electron microscopy （SEM） and transmission electron microscopy （TEM）. The yield strength and the toughness of the steel with various prior austenite grain sizes were tested and correlated with microstructure characteristics. Results show that both the prior austenite grain size and the martensitic packet size in the 17CrNiMo6 steel follow a HalI-Petch relation with the yield strength. When the prior austenite grain size was refined from 199 μm to 6 μm , the yield strength increased by 235 MPa, while the Charpy U-notch impact energy at 77 K improved more than 8 times, indicating that microstructure refinement is more effective in improving the resistance to cleavage fracture than in increasing the strength. The fracture surfaces implied that the unit crack path for cleavage fracture is identified as being the packet.展开更多
基金Project supported by the National Natural Science Foundation of China(52031013,52173305,52233017)the National Key Research and Development Program(2018YFA0702900)。
文摘The effect of cerium(Ce)on the solidification microstructure of Cr_(4)Mo_(4)V bearing steel was investigated via a combined experimental and theoretical method.With a trace amount(0.056 wt%)of Ce addition,the coarse columnar grains in as-cast microstructure transform into equiaxed ones,and the average diameter is reduced from 56 to 27μm.The network-like and bulky primary MC and M2C carbides at the interdendritic regions become disconnected and refined,and their volume percentage decreases from4.15 vol%to 2.1 vol%.Ce-inclusions acting as heterogeneous nucleation agents of prior-austenite grains and Ce atoms segregating at grain boundaries,both contribute to the refinement of grains.Thermodynamic calculations reveal that primary carbides are precipitated afterγ-austenite forms near the end of the solidification process.The modification of primary carbides in size and amount is mainly attributed to the isolated remaining melt separated by refinedγ-austenite grains in which the nucleation of carbides is promoted,while the growth is restrained owing to the less segregation of alloying elements.
基金support provided by the Australian Research Council Research Hub for Advanced Manufacturing of Medical Devices (No.IH150100024)the ARC Discovery (No.DP140100702)and the ARC linkage project (No.LP150100950).
文摘Refinement of grains and intermetallic phases in the as-solidified alloy structure offers uniform struc-tural properties,eliminates or minimizes common solidification defects,including segregation and hot cracking,and improves thermomechanical processing of wrought alloys.Melt processing by an external field is an efficient process for achieving refinement of the solidification structure of Al and Mg alloys without altering the alloy composition.A wide range of melt processing methods and solidification stud-ies(conventional,directional,and in-situ approaches)have been reported in the literature that explore the mechanism of refinement.Identifying the dominant grain refinement mechanism has been a focus of most investigations because significant variations exist according to the casting conditions and the type of applied external treatments.The origin of fine grains occurs through either one or a combination of heterogenous nucleation,fragmentation of dendrites and grains formed and then separated from the surface of the melt and mould wall under vibration or agitation.The first part of this review describes the prominent external field techniques and the mechanisms proposed for the origin of fine grains.The second part critically compares the current understanding of these grain refinement mechanisms to de-termine differences and commonalities to identify the factors that promote the formation of equiaxed zones occupying a large volume fraction of the casting.
基金supported by the National Natural Science Foundation of China under Grant No. 50771031the Research Foundation of Southeast University under Grant No. XJ0612238
文摘Eutectic solidification in near-eutectic Al-13 wt pct Si casting alloys and the effect of trace addition of boron or strontium on it have been investigated using thermal analysis and microstructural characterization. In unmodified alloy, dual eutectic structure has been observed. The coarse eutectic （dendrite-like Al＋ coarse Si flakes） is formed above the equilibrium temperature of eutectic （Al＋Si） reaction （577℃）. The coarse eutectic （CE） grains nucleate from the primary silicon particles formed earlier due to local enrichment of silicon solute and grow in a divorced mode between the dendritic Al phase and large silicon flakes. The fine eutectic （FE） grains nucleate later on other potential sites activated by melt undercooling and grow in coupled-growing mode with the silicon crystals as fine flakes. The formation of the FE grains is favored in the alloys containing boron because of a great number of potential nucleation sites being added from boron-containing particles. Addition of strontium to the alloys restrains completely the formation of primary silicon particles and hence limits the nucleation of the CE. This is because the eutectic point has moved far enough to make the alloy, at this composition （Al-13 wt pct Si）, hypo-eutectic. Local cooling rate during solidification has an important influence on competition formation of these two eutectics.
基金Project supported by the National Nature Science Foundations of China(51464032)the National Basic Research Program of China(2010CB635106)
文摘In this paper, the microstructural evolution, grain refinement and mechanical properties of as-cast Mg-6Zn-0.4Zr alloys with varying Sm contents（0, 2 wt.%, 4 wt.% and 6 wt.%） were investigated by using an optical microscope（OM）, a scanning electron microscope（SEM） equipped with energy dispersive spectroscope（EDS）, an X-ray diffractometer（XRD） and mechanical tests at room temperature, respectively. The experimental results indicated that the addition of Sm could obviously refine the as-cast grains, modify the eutectic morphology and affect the mechanical properties of the alloys. The main phases in Mg-6Zn-xSm-0.4Zr alloys included matrix α-Mg, Mg2Zn3, Mg（41）Sm5 and MgZ nS m. With Sm content increasing to 4%, the MgZ nS m phase was created, meanwhile, the morphology of some eutectic phases revealed apparently lamellar structure, which had a bad effect on the mechanical properties. In addition, the maximum values of ultimate tensile strength（UTS, 214 MPa） and elongation（EL, 7.42%） were simultaneously obtained from the alloy with 2% Sm. However, Sm addition had no obvious effects on the fracture behavior of the alloys, namely, the fracture pattern of Mg-6Zn-0.4Zr alloy belonged to inter-granular and brittle modes while the fracture regimes of all the Sm-containing alloys were dominated by the mixture of inter-granular and trans-granular modes.
文摘Martensitic microstructure in quenched and tempered 17CrNiMo6 steel with the prior austenite grain size ranging from 6 μm to 199 μm has been characterized by optical metallography （OM）, scanning electron microscopy （SEM） and transmission electron microscopy （TEM）. The yield strength and the toughness of the steel with various prior austenite grain sizes were tested and correlated with microstructure characteristics. Results show that both the prior austenite grain size and the martensitic packet size in the 17CrNiMo6 steel follow a HalI-Petch relation with the yield strength. When the prior austenite grain size was refined from 199 μm to 6 μm , the yield strength increased by 235 MPa, while the Charpy U-notch impact energy at 77 K improved more than 8 times, indicating that microstructure refinement is more effective in improving the resistance to cleavage fracture than in increasing the strength. The fracture surfaces implied that the unit crack path for cleavage fracture is identified as being the packet.
