Sn-3wt%Pb alloy was directionally solidified without and with a 0.08T transverse magnetic field(TMF),and real-time recorded by in-situ synchrotron X-ray imaging.Results indicate that TMF shortened the distance from the location of nucleation to the advancing interface,and accelerated the growth rate of the equiaxed crystal,which caused the columnar-to-equiaxed transition(CET)finally.The thermoelectromagnetic convection(TEMC)in front of the interface and around the crystal's dendritic branch should respond to changes of the distance and the growth rate.
WANG JiangFAUTRELLE YvesREN Zhong-mingLI XiNGUYEN-THI HenriMANGELINCK-NOEL NathalieSALLOUM ABOU JAOUDE GeorgesZHONG Yun-boKALDRE ImantsBOJAREVICS AndrisBULIGINS Leonid
Solidification characteristics of Ti–46Al–7Nb melts were studied by the electromagnetic levitation technique.A maximum melt undercooling up to 240 K has been achieved. When the undercooling is lower than the critical value DT* = 205 K, the alloy possesses typical hypoperitectic solidification characteristic which can be evidenced by a peritectic layer observed in the as-solidified microstructure. However, the Widmansta¨tten structure can be observed at large undercooling regime of DT C DT*, where peritectic reaction cannot proceed and c lamellar precipitation within a plates is suppressed. Based on the BCT dendrite growth model, the dendrite growth velocities were calculated as a function of undercooling. Theoretical analysis indicates that the growth mechanism of the primary b phase transforms from solutaldiffusion-controlled to thermal-diffusion-controlled in the undercooling range of 188–205 K, which can be attributed to the onset of solute trapping at the critical undercooling. Meanwhile, with increasing undercooling, the solute trapping effect becomes more dominant as a consequence.
In this paper,the textured Si_3N_4 ceramics were prepared by adding seed particles during gel-casting in the magnetic field of 6 T,followed by pressureless sintering.The effect of pH on the stability and dispersibility of Si_3N_4 slurry and the effect of seed particles content on texture formation of Si_3N_4 ceramics were both studied.Those results showed that the slurry with good stability and dispersibility was obtained when pH was about 11.6.The a or b-axis of Si_3N_4particles or crystals was aligned parallel to the direction of the magnetic field in the magnetic field of 6 T.The degree of texture of Si_3N_4 ceramics further increased during sintering.With the increasing of additional β-Si_3N_4 particles in the magnetic field of 6 T,the degree of texture increased from0.19 without seed particles to 0.76 with 9%(mass fraction)seed particles.The increase of seed particles content promoted the texture formation of Si_3N_4 ceramics.
Zhi-Gang YangJian-Bo YuChuan-Jun LiKang DengZhong-Ming RenQiu-Liang WangYin-Ming DaiHui Wang
Rapid solidification of Ti-50 at.%Al peritectic alloy is realized by laser melting technique at diferent conditions of laser power and scanning speed.The temperature field and the cooling rate under the corresponding conditions are derived from the finite element simulation.Comparing the measured pool size with the simulated result,the laser absorptivity of Ti-50 at.%Al peritectic alloy at diferent conditions can be deduced to establish the relationships between the laser absorptivity,the laser power and the scanning speed.The morphology evolution and the phase selection of Ti-50 at.%Al peritectic alloy are described by the temperature gradient and the cooling rate.With the increase of temperature gradient and cooling rate,βphase replacesαphase to become the leading growth phase.And the growth ofαphase experiences the transition from facet to non-facet manner,whileβphase is refined.To understand the underlying mechanism of the competition growth can bring benefit to the industrial application of Ti-Al alloy.
