高熔点金属区域熔炼中晶体生长角和凝固速率对熔区稳定性的影响

EFFECT OF GROWTH ANGLE AND SOLIDIFICATION RATE ON THE FLOATING ZONE STABILITY FOR PROCESSING OF HIGH-TEMPERATURE PURE METALS

对高熔点金属Nb,W,Ta,Mo及Ir电子束区熔熔区高度进行了稳定性分析,发现在区熔相同尺寸试样时,能够稳定熔区高度大小排序依次为Nb〉Mo〉W〉Ta〉Ir.计算获得了这5种金属的晶体生长角在8°~13°之间,发现生长角不为零对大尺寸试样熔区高度起主导作用,同时金属的实际晶体生长角与界面生长机制有关.当为粗糙界面生长机制时,生长角随区熔凝固速率增加变化不大;而为位错生长机制时,生长角随区熔凝固速率增加而减少;如为小面生长机制时,生长角在低速下会大幅度减小,并随凝固速率增加而增大.采用较大的凝固速率（约1 mm/min）有利于控制Ir和Mo晶体生长角变化和熔区高度,这一点与Mo区熔单晶生长实验结果基本吻合.

The height of floating zone and molten zone instability for five pure metals including Nb, W, Ta,Mo, and Ir with high melting points is investigated using electron beam floating zone method（EBFZM）. The results show that the height level of floating zone for these five metals are in order with the sequence of Nb〉Mo〉W〉Ta〉Ir. The crystal growth angles for these metals are in the range of 8°~13° and the sample in large size can be developed by EBFZM as the growth angle is found not to be zero. Meanwhile, the actual growth angles are related with the interface growth mechanism. For continuous growth mechanism, the growth angles vary slightly with the solidification rate for rough interface, and for dislocation growth mechanism, the growth angles decrease with increasing the solidification rate. If faceting growth mechanism prevails, the growth angles drop remarkably at a low solidification rate and further increase with increasing the solidification rate. Additionally, by employing EBFZM growth of Ir and Mo pure metals, a solidification rate approaching 1 mm/min is available for controlling the growthangle and the height of floating zone. These calculations fit well with the experimental results of Mo single crystal prepared by EBFZM.