The Experts below are selected from a list of 1920 Experts worldwide ranked by ideXlab platform
Kazuhiko Kuribayashi - One of the best experts on this subject based on the ideXlab platform.
-
Microstructure formation and in situ phase identification from undercooled Co-61.8 at.% Si melts solidified on an electromagnetic levitator and an electrostatic levitator
Acta Materialia, 2008Co-Authors: Kosuke Nagashio, Kazuhiko Kuribayashi, Takehiko Ishikawa, Akitoshi Mizuno, Masayoshi Adachi, Masahito Watanabe, Shinichi Yoda, Yoshinori KatayamaAbstract:Abstract Co–61.8 at.% Si (CoSi–CoSi2) eutectic alloys were solidified on an electromagnetic levitator (EML) and an electrostatic levitator (ESL) at different undercooling levels. The results indicated that there is only a single Recalescence event at low undercooling with the CoSi intermetallic compound as primary phase, which is independent of processing facilities, on either an EML or an ESL. The microstructure, however, is strongly dependent on the processing facility. The interior melt flow behavior in the sphere solidified at the EML differs substantially from that at the ESL, thus yielding different microstructures. On high undercooling, double Recalescence takes place regardless of levitation condition. In situ X-ray diffraction of alloys solidified on the EML demonstrates that the CoSi2 compound becomes the primary phase upon the first Recalescence, and the CoSi intermetallic phase crystallizes during the second Recalescence. In addition to phase identification, real-time diffraction patterns can also provide additional evidence of the fragmentation of the primary phase and the ripening feature in the subsequent cooling process in the semisolid state. The phase competition between the CoSi and CoSi2 compounds is discussed when considering the nucleation barrier. The low interfacial energy of the CoSi2 phase favors a preferential nucleation event over the CoSi phase, which also plays a critical role in non-reciprocity nucleation and thus yields a double Recalescence profile at high undercooling.
-
Dynamic process of dendrite fragmentation in solidification from undercooled Si melt using time-resolved x-ray diffraction
Applied Physics Letters, 2007Co-Authors: Kosuke Nagashio, Kazuhiko Kuribayashi, Kiyoshi Nozaki, Yoshinori KatayamaAbstract:The spontaneous dendrite fragmentation in solidification from undercooled Si melt was analyzed by time-resolved two-dimensional x-ray diffractometry. For the sample solidified at ΔT=261K, several spots appeared at 1ms after Recalescence and the subsequent transition from spots to rings occurred within ∼25ms, which suggests that the fragmentation occurred after Recalescence but just at the initial stage of the plateau period. Although the present experiment supported that the driving force for the fragmentation is a capillarity effect at the plateau period, the time scale for the fragmentation differed from the current quantitative prediction.
-
Growth kinetics of highly undercooled Al2O3 melts
Journal of Applied Physics, 2004Co-Authors: Kazuhiko KuribayashiAbstract:An aeroacoustic levitator was employed to solidify the Al2O3 melt in a containerless condition at different melt undercoolings when a laser beam heating system was utilized. The sample was simulated to crystallize at well-defined temperatures and the Recalescence front was imaged using a high-speed video. Both the observation of solidified microstructures and the theoretical calculation of the hypercooling limit of the undercooled melt indicated that the final microstructure should consist of the primary dendrite formed during rapid Recalescence and the subsequent product yielded after Recalescence near the melting temperature. Microstructure analysis showed that the advancement rate of the Recalescence front should be viewed as the growth velocity of the undercooled melt. The accurate relationship of growth velocity versus melt undercooling was acquired, which was essential in characterizing the growth kinetics of the undercooled melts. Further analysis indicated that the linear kinetic coefficient for t...
-
Containerless solidification of undercooled oxide and metallic eutectic melts
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2003Co-Authors: Kosuke Nagashio, Kazuhiko KuribayashiAbstract:Abstract A high-speed video was employed to monitor the in situ Recalescence of undercooled oxide Al2O3–36.8 at.% ZrO2 and metallic Ni–18.7 at.% Sn eutectics that were processed on an aero-acoustic levitator and an electromagnetic levitator, respectively. For the oxide eutectic, the entire sample becomes brighter and brighter without any clear Recalescence front during spontaneous crystallization. When the sample was seeded at desired undercoolings, crystallization started from the seeding point and then spread through the entire sample. Microstructures of the oxide solidified via both the spontaneous crystallization and external seeding consist of many independent eutectic colonies at the sample surface, indicating that copious nucleation takes place regardless of melt undercooling and solidification mode. For the metallic eutectics, two kinds of Recalescence are visualized. The surface and cross sectional microstructures reveal that copious nucleation is also responsible for the formation of independent eutectic colonies distributing within the entire sample. It is not possible to measure the growth velocity of a single eutectic colony using optical techniques under the usual magnification. The conventional nucleation concept derived from single-phase alloys may not be applicable to the free solidification of the undercooled double-phase oxide and metallic eutectic systems.
-
Containerless solidification of highly undercooled Al2O3–ZrO2 eutectic melts on an aero-acoustic levitator
Journal of Crystal Growth, 2003Co-Authors: Kosuke Nagashio, Kazuhiko KuribayashiAbstract:Abstract The Al 2 O 3 –ZrO 2 eutectic oxide was levitated in an aero-acoustic levitator and melted by a continuous wave CO 2 laser beam and then solidified under a containerless condition. A high-speed video was operated to image the in situ Recalescence behavior at different undercoolings. Based on the Recalescence frames and ambient microstructures, copious nucleation was proposed to take place when the sample was solidified either spontaneously at the maximum undercooling or externally seeded at lower undercoolings. The Recalescence front was not the reflection of the solid/liquid interface but induced by the simultaneous thermal release of concentrated crystallizing eutectic colonies. The occurrence of copious nucleation should be mainly ascribed to a chain-like successive nucleation reaction in undercooled melts. The temperature distribution inside a spherical sample was considered when applying dimensionless variables of the Biot number and Fourier number so as to elucidate the abnormal eutectic formation near the sample center. Further analysis from the viewpoint of thermal boundary layer indicated that there was a strong thermal interaction among crystallizing eutectic colonies in the flat thermal gradient region near the central part of a sample, which made it impossible to establish a steady thermal flux and thus generating abnormal eutectics after the coarsening process was taken into account.
Kosuke Nagashio - One of the best experts on this subject based on the ideXlab platform.
-
Microstructure formation and in situ phase identification from undercooled Co-61.8 at.% Si melts solidified on an electromagnetic levitator and an electrostatic levitator
Acta Materialia, 2008Co-Authors: Kosuke Nagashio, Kazuhiko Kuribayashi, Takehiko Ishikawa, Akitoshi Mizuno, Masayoshi Adachi, Masahito Watanabe, Shinichi Yoda, Yoshinori KatayamaAbstract:Abstract Co–61.8 at.% Si (CoSi–CoSi2) eutectic alloys were solidified on an electromagnetic levitator (EML) and an electrostatic levitator (ESL) at different undercooling levels. The results indicated that there is only a single Recalescence event at low undercooling with the CoSi intermetallic compound as primary phase, which is independent of processing facilities, on either an EML or an ESL. The microstructure, however, is strongly dependent on the processing facility. The interior melt flow behavior in the sphere solidified at the EML differs substantially from that at the ESL, thus yielding different microstructures. On high undercooling, double Recalescence takes place regardless of levitation condition. In situ X-ray diffraction of alloys solidified on the EML demonstrates that the CoSi2 compound becomes the primary phase upon the first Recalescence, and the CoSi intermetallic phase crystallizes during the second Recalescence. In addition to phase identification, real-time diffraction patterns can also provide additional evidence of the fragmentation of the primary phase and the ripening feature in the subsequent cooling process in the semisolid state. The phase competition between the CoSi and CoSi2 compounds is discussed when considering the nucleation barrier. The low interfacial energy of the CoSi2 phase favors a preferential nucleation event over the CoSi phase, which also plays a critical role in non-reciprocity nucleation and thus yields a double Recalescence profile at high undercooling.
-
Dynamic process of dendrite fragmentation in solidification from undercooled Si melt using time-resolved x-ray diffraction
Applied Physics Letters, 2007Co-Authors: Kosuke Nagashio, Kazuhiko Kuribayashi, Kiyoshi Nozaki, Yoshinori KatayamaAbstract:The spontaneous dendrite fragmentation in solidification from undercooled Si melt was analyzed by time-resolved two-dimensional x-ray diffractometry. For the sample solidified at ΔT=261K, several spots appeared at 1ms after Recalescence and the subsequent transition from spots to rings occurred within ∼25ms, which suggests that the fragmentation occurred after Recalescence but just at the initial stage of the plateau period. Although the present experiment supported that the driving force for the fragmentation is a capillarity effect at the plateau period, the time scale for the fragmentation differed from the current quantitative prediction.
-
Containerless solidification of undercooled oxide and metallic eutectic melts
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2003Co-Authors: Kosuke Nagashio, Kazuhiko KuribayashiAbstract:Abstract A high-speed video was employed to monitor the in situ Recalescence of undercooled oxide Al2O3–36.8 at.% ZrO2 and metallic Ni–18.7 at.% Sn eutectics that were processed on an aero-acoustic levitator and an electromagnetic levitator, respectively. For the oxide eutectic, the entire sample becomes brighter and brighter without any clear Recalescence front during spontaneous crystallization. When the sample was seeded at desired undercoolings, crystallization started from the seeding point and then spread through the entire sample. Microstructures of the oxide solidified via both the spontaneous crystallization and external seeding consist of many independent eutectic colonies at the sample surface, indicating that copious nucleation takes place regardless of melt undercooling and solidification mode. For the metallic eutectics, two kinds of Recalescence are visualized. The surface and cross sectional microstructures reveal that copious nucleation is also responsible for the formation of independent eutectic colonies distributing within the entire sample. It is not possible to measure the growth velocity of a single eutectic colony using optical techniques under the usual magnification. The conventional nucleation concept derived from single-phase alloys may not be applicable to the free solidification of the undercooled double-phase oxide and metallic eutectic systems.
-
Containerless solidification of highly undercooled Al2O3–ZrO2 eutectic melts on an aero-acoustic levitator
Journal of Crystal Growth, 2003Co-Authors: Kosuke Nagashio, Kazuhiko KuribayashiAbstract:Abstract The Al 2 O 3 –ZrO 2 eutectic oxide was levitated in an aero-acoustic levitator and melted by a continuous wave CO 2 laser beam and then solidified under a containerless condition. A high-speed video was operated to image the in situ Recalescence behavior at different undercoolings. Based on the Recalescence frames and ambient microstructures, copious nucleation was proposed to take place when the sample was solidified either spontaneously at the maximum undercooling or externally seeded at lower undercoolings. The Recalescence front was not the reflection of the solid/liquid interface but induced by the simultaneous thermal release of concentrated crystallizing eutectic colonies. The occurrence of copious nucleation should be mainly ascribed to a chain-like successive nucleation reaction in undercooled melts. The temperature distribution inside a spherical sample was considered when applying dimensionless variables of the Biot number and Fourier number so as to elucidate the abnormal eutectic formation near the sample center. Further analysis from the viewpoint of thermal boundary layer indicated that there was a strong thermal interaction among crystallizing eutectic colonies in the flat thermal gradient region near the central part of a sample, which made it impossible to establish a steady thermal flux and thus generating abnormal eutectics after the coarsening process was taken into account.
-
on the origin of Recalescence behaviors of undercooled single phase mullite and double phase al2o3 zro2 eutectic melts
Scripta Materialia, 2002Co-Authors: Kosuke Nagashio, Kazuhiko KuribayashiAbstract:Abstract A high-speed video was employed to monitor in situ Recalescence behaviors of deeply undercooled melts of single-phase mullite solid solution and double-phase Al 2 O 3 –ZrO 2 eutectic, showing rather similar Recalescence phenomena. Based on the surface microstructures, it is proposed that the origin of the Recalescence fronts is quite different, in which the bright/dark boundary in mullite can be viewed as the reflection of the solid/liquid interface, whereas that in the Al 2 O 3 –ZrO 2 system results from simultaneous thermal release of concentrated crystallizing eutectic colonies.
F. Liu - One of the best experts on this subject based on the ideXlab platform.
-
Effects of initial undercooling on microstructure formation and recrystallisation of undercooled melts
Materials Science and Technology, 2017Co-Authors: Hua Hou, F. LiuAbstract:ABSTRACTThe critical undercoolings for the two grain refinement events and the onset of recrystallisation event are determined by detailed analysis of the microstructure evolution of bulk undercooled Ni–20 at.-%Cu alloy melts. The first grain refinement event occurred in the low undercooling range was explained by dendrite remelting. The second grain refinement event occurred in the high undercooling range was due to the combined effects of dendrite remelting stress-induced dendrite breakup during Recalescence and recrystallisation during the near-equilibrium solidification stage after Recalescence. The micro-stress induced by the solidification contraction during Recalescence in the so called ‘first mushy zone’ would lead to distortion and breakup of primary dendrites. The stress-induced broken-up dendrites have sufficient driving force for recrystallisation.
-
Investigation on Recalescence temperatures of deeply undercooled melts
Journal of Crystal Growth, 2016Co-Authors: F. Liu, Hua Hou, Yuhong Zhao, Wang Shuai, F. YanAbstract:Abstract According to the theory of classic thermodynamics, any transformation is driven by the decrease of Gibbs free energy of the system. Solidification pertains to the first order transformation and obeys this basic law. The Gibbs free energy of the condensed phases of metals and alloys is closely related to the temperature and composition of the system. Thus we can describe rapid solidification process in a more precise way by using quantitative thermodynamic calculation. In combination with solidification kinetics theory, we calculated the evolution of the thermodynamic parameters during rapid solidification process. On this basis, we proposed a criterion for the end point of Recalescence process and built a physical model for describing rapid solidification process and predicting Recalescence temperatures of undercooled melts. Good agreement can be achieved between the present model prediction and experimental data.
-
Grain growth and thermal stability accompanying recrystallization in undercooled Ni-3at.%Sn alloy
Journal of Alloys and Compounds, 2015Co-Authors: Z. Chen, C J Shen, Q. Chen, F. LiuAbstract:Abstract The grain growth and thermal stability after recrystallization in as-solidified highly undercooled Ni-3at.%Sn alloy melt were investigated. As for undercooled Ni-3at.%Sn alloy, a transition from dendritic to granular crystals occurred when ΔT≥ΔT*, which was induced by the plastic deformation of dendrites and subsequent recrystallization. On this basis, the subsequent grain growth and solute segregation accompanying Recalescence were calculated by a recently proposed thermo-kinetic model, which showed close agreement with the experimental results. It is concluded that the grain growth process was interrelated to Recalescence, solute trapping and solute segregation of Sn atoms captured by solute trapping, which was responsible for the reduction of grain boundary energy and improvement of thermal stability.
-
The effect of non-equilibrium δ/γ transition on the formation of metastable “dendrite core” in undercooled Fe–Cu alloy
Journal of Crystal Growth, 2012Co-Authors: Z. Chen, F. Liu, Xiaoqin Yang, Ning Liu, C J ShenAbstract:Abstract The effect of non-equilibrium δ/γ transition on the formation of metastable “dendrite core” in undercooled Fe–Cu alloy was studied. Three kinds of solidified paths were adopted following Recalescence, i.e., quenching at Recalescence moment, quenching after Recalescence plateau and natural cooling. In comparison of the three paths, it was evidenced that the metastable “dendrite core” resulted in the incomplete transition of the primary δ dendrite to γ phase. In combination with the JMA solid-state transition kinetic theory, the formation of “dendrite core” was dominated by volume diffusion controlled-δ/γ transition. Two typical morphologies, i.e., small granules and dendrite trucks in “dendrite cores”, have been detected. It may be attributed to the volume fraction of δ/γ transition.
-
Use of Recalescence behavior analysis for the prediction of grain refinement in undercooled Cu–Ni alloy
Journal of Materials Science, 2010Co-Authors: Wu Yang, F. Liu, Gaolin YangAbstract:Considering non-equilibrium solidification and its influence on subsequent near-equilibrium solidification together, the description of Recalescence behavior in bulk undercooled Cu70Ni30 alloy was adopted to predict the corresponding microstructure transition. The thermal plateau time for near-equilibrium solidification can be deduced directly with the calculation of non-equilibrium solid fraction formed in Recalescence. On the basis of quantitative description for Recalescence behavior, the non-equilibrium solid fraction, residual liquid fraction, dendrite broken-up time, and thermal plateau time can be determined as functions of initial undercooling. Then, a simple and accurate application of dendrite fragmentation model was performed as the grain refinement at both low- and high-undercooling originates from dendrite breakup. The predicted undercooling regions for the double grain refinement agree well with the experimental observation. Moreover, the change of grain morphology for the second grain refinement can be ascribed to the occurrence of recrystallization produced by the enhanced residual stress upon highly undercooled solidification.
Douglas M. Matson - One of the best experts on this subject based on the ideXlab platform.
-
Metastable solidification pathways of undercooled eutectic CoSi–CoSi2 alloys
Acta Materialia, 2019Co-Authors: Sangho Jeon, O. Shuleshova, Matthias Kolbe, Victoria Kaban, Gabrielle String, Andre Cleaver, Ivan Kaban, Jianrong Gao, Douglas M. MatsonAbstract:Abstract Solidification and growth behavior of undercooled eutectic CoSi–CoSi2 melts was observed using containerless levitation and in situ high-energy synchrotron X-ray diffraction techniques. Three metastable solidification pathways of eutectic CoSi–CoSi2 were determined as a function of undercooling. Upon double Recalescence at low undercoolings the primary and secondary phases are CoSi and CoSi2, while at high undercoolings the phase formation sequence is reversed. At intermediate undercoolings a single Recalescence was observed and attributed to a crossover of the nucleation barriers for the two phases. Scanning electron microscopy combined with electron back-scattering diffraction measurements revealed changes of the morphological characteristics and orientation of CoSi and CoSi2 phases for different solidification pathways.
-
Formation of Cellular Structure on Metastable Solidification of Undercooled Eutectic CoSi-62 at. %
Crystals, 2017Co-Authors: Sangho Jeon, Douglas M. MatsonAbstract:The relationship between emissivity, delay time, and surface growth for metastable solidification of CoSi-62 at. % eutectic alloys is reported from undercooling experiments conducted using electrostatic levitation. A fraction of the undercooled melt is first solidified to CoSi2 with subsequent nucleation in the mushy-zone of CoSi after an observed delay time. During this double Recalescence event, the temperature of the secondary Recalescence exceeds the liquidus, indicating that the spectral emissivity has changed. This emissivity change increases with longer delay times during solidification and is linked to the growth of cellular structure on the sample surface. Density measurements showed that the cellular structure begins to grow rapidly at a certain time during metastable solidification. This phenomenon is likely associated with the constitutional undercooling of the remaining melt.
-
Growth Competition During Double Recalescence in Fe-Cr-Ni Alloys
MRS Proceedings, 1998Co-Authors: Douglas M. MatsonAbstract:AbstractThe rapid solidification of a Fe- 12wt%Cr- 16wt%Ni alloy was investigated under containerless processing conditions using both ground-based electromagnetic levitation equipment and aboard the shuttle Columbia using the TEMPUS facility. A high-speed digital video technique was used to image growth of the metastable ferritic phase and the stable austenitic phase into the undercooled melt. Above a critical undercooling, the metastable phase nucleates first. After a delay, a second thermal rise is observed during transformation to the stable phase. Double Recalescence events were observed at temperatures consistent with the To, temperature of the bcc phase thus defining a value of the critical undercooling for metastable nucleation which is significantly lower than previously predicted. For a given liquid temperature the velocity of the stable fcc phase is greater than that of the metastable bcc phase. The velocity for growth of the stable phase into the semi-solid which forms during primary metastable Recalescence was also measured and found to be independent of the initial undercooling. A model based on competitive growth of the two phases successfully predicts the limit where double Recalescence events may be detected.
Andrew M. Mullis - One of the best experts on this subject based on the ideXlab platform.
-
Evidence for dendritic fragmentation in as-solidified samples of deeply undercooled melts
Journal of Crystal Growth, 2020Co-Authors: Andrew M. Mullis, Nafisul HaqueAbstract:Abstract The congruently melting, single phase intermetallic β-Ni3Ge has been subject to rapid solidification via drop-tube processing. We establish that the rapidly solidified material growing during the Recalescence phase of solidification can be distinguished from the post-Recalescence material in the as-solidified sample by the degree of chemical ordering displayed. This can in turn be used to visualize the material from the Recalescence phase of solidification. At intermediate cooling rates this Recalescence material consists of fragmented dendrites. The occurrence of fragmentation is compared against established theoretical models based on the growth of Rayleigh instabilities with excellent agreement being found. EBSD mapping is used to establish the relationship between these dendritic fragments and the final grain size distribution. The dendritic fragments are found to be poor nuclei for new grains and the fragmented dendrites do not consistently give rise to classical grain refined structures.
-
Determination of the Origin of Anomalous Eutectic Structures from In Situ Observation of Recalescence Behaviour
Materials Science Forum, 2014Co-Authors: Andrew M. Mullis, C. R. Clopet, Robert F. CochraneAbstract:A melt encasement (fluxing) method has been used to undercool Ag-Cu alloy at its eutectic composition. The Recalescence of the undercooled alloy has been filmed at high frame rate. At low undercooling lamellar eutectic is observed to grow, giving way to a mixed anomalous-lamellar structure at higher undercooling. In situ observation of the spot brightness reveals, as expected, that the lamellar eutectic grows via a planar front mechanism, while the anomalous eutectic grows via a more complex process characterised by a double Recalescence. The first Recalescence is non-space-filling (dendritic) in character and is followed shortly afterwards by a second Recalescence which appears to be of the planar front type. Moreover, the first Recalescence event appears to be to a temperature in excess of the equilibrium eutectic temperature. This is strongly suggestive that the anomalous eutectic morphology arises due to the growth and subsequent partial remelting of a dendritic morphology, probably a two-phase (eutectic) dendrite, followed by planar front growth of a lamellar eutectic into the residual liquid.
-
The origin of anomalous eutectic structures in undercooled Ag–Cu alloy
Acta Materialia, 2013Co-Authors: C. R. Clopet, Robert F. Cochrane, Andrew M. MullisAbstract:Abstract A melt encasement (fluxing) method was used to undercool Ag–Cu alloy at its eutectic composition. The Recalescence of the undercooled alloy was filmed at a high frame rate. For undercoolings
-
The formation of regular αNi-γ(Ni31Si12) eutectic structures from undercooled Ni–25 at.% Si melts
Intermetallics, 2012Co-Authors: R. Ahmad, Robert F. Cochrane, Andrew M. MullisAbstract:Abstract Undercooling experiments have been performed on an Ni–25.2 at.% Si alloy using a flux encasement technique, with a maximum undercooling of 160 K having been achieved. Double Recalescence was observed at all undercoolings, with crystal growth velocities being measured for the first Recalescence event. These velocities were extremely low, with a maximum value of 0.018 m s−1 being recorded. At all undercoolings a eutectic structure was observed, comprising alternating lamellae of single phase γ(Ni31Si12) and Ni-rich lamellae containing of a fine (200–400 nm) dispersion of βNi3Si and αNi. This is contrary to the equilibrium phase diagram from which direct solidification to βNi3Si would be expected for undercoolings in excess of 53 K. We postulate that the direct solidification of βNi3Si from the melt is suppressed, with an α–γ eutectic being formed instead. The α appears to be supersaturated in Si and undergoes a eutectoid decomposition to α and βNi3Si, giving rise to the observed second Recalescence.
