Primary dendrite arm spacing（PDAS） of α phase in directionally solidified Pb-26%Bi（mass fraction） hypo-peritectic alloys was measured by considering the effect of melt convection in cylindrical samples with diffe...Primary dendrite arm spacing（PDAS） of α phase in directionally solidified Pb-26%Bi（mass fraction） hypo-peritectic alloys was measured by considering the effect of melt convection in cylindrical samples with different diameters.The experimental results show the measured PDAS increases with increasing diameter of the sample.At the growth velocity of 5 μm/s,its value changes from 161.5 μm for the sample with 1.8 mm in diameter to 240.4 μm for the sample with 7 mm in diameter.The strong melt convection in large diameter samples causes a high bulk alloy composition and a high concentration gradient in peritectic β phase,resulting in a larger PDAS.Simultaneously,the high concentration gradient could effectively promote the peritectic transformation,enhancing the dissolution of the thin α dendrite.展开更多
Directional solidification experiments on Pb-Bi peritectic alloys were carried out at very low growth rate （v=0.5 μm/s） and high temperature gradient （G=35 K/mm） in an improved Bridgman furnace. The banding struc...Directional solidification experiments on Pb-Bi peritectic alloys were carried out at very low growth rate （v=0.5 μm/s） and high temperature gradient （G=35 K/mm） in an improved Bridgman furnace. The banding structures were observed in both hypoperitectic and hyperperitectic compositions （Pb-xBi, x=26%, 28%, 30% and 34%）. Tree-like primary α phase in the center of the sample surrounded by the peritectic β phase matrix was also observed, resulting from the melt convection. The banding microstructure, however, is found to be transient after the tree-like structure and only the peritectic phase forms after a few bands. Composition variations in the banding structure are measured to determine the nucleation undercooling for both α and β phases. In a finite length sample, convection is shown to lead only to the transient formation of bands. In this transient banding regime, only a few bands with a variable width are formed, and this transient banding process can occur over a wide range of compositions inside the two-phase peritectic region.展开更多
基金Project(50395100)supported by the National Natural Science Foundation of ChinaProject(NCET-07-0692)supported by the New Century Talents Program of the Ministry of Education,ChinaProject(34-TP-2009)supported by Open Project of State Key Laboratory of Solidification Processing,China
文摘Primary dendrite arm spacing（PDAS） of α phase in directionally solidified Pb-26%Bi（mass fraction） hypo-peritectic alloys was measured by considering the effect of melt convection in cylindrical samples with different diameters.The experimental results show the measured PDAS increases with increasing diameter of the sample.At the growth velocity of 5 μm/s,its value changes from 161.5 μm for the sample with 1.8 mm in diameter to 240.4 μm for the sample with 7 mm in diameter.The strong melt convection in large diameter samples causes a high bulk alloy composition and a high concentration gradient in peritectic β phase,resulting in a larger PDAS.Simultaneously,the high concentration gradient could effectively promote the peritectic transformation,enhancing the dissolution of the thin α dendrite.
基金Project (20110491492) supported by the China Postdoctoral Science FoundationProject (20114BAB216017) supported by the Natural Science Foundation of Jiangxi Province, ChinaProject (GJJ12035) supported by the Science Foundation of the Educational Department of Jiangxi Province, China
文摘Directional solidification experiments on Pb-Bi peritectic alloys were carried out at very low growth rate （v=0.5 μm/s） and high temperature gradient （G=35 K/mm） in an improved Bridgman furnace. The banding structures were observed in both hypoperitectic and hyperperitectic compositions （Pb-xBi, x=26%, 28%, 30% and 34%）. Tree-like primary α phase in the center of the sample surrounded by the peritectic β phase matrix was also observed, resulting from the melt convection. The banding microstructure, however, is found to be transient after the tree-like structure and only the peritectic phase forms after a few bands. Composition variations in the banding structure are measured to determine the nucleation undercooling for both α and β phases. In a finite length sample, convection is shown to lead only to the transient formation of bands. In this transient banding regime, only a few bands with a variable width are formed, and this transient banding process can occur over a wide range of compositions inside the two-phase peritectic region.
