通过分析bcc结构金属Mo和fcc结构金属Ir电子束区熔中3种不同取向[100]、[110]和[111]晶体生长,发现金属Mo在平界面凝固下会以[110] 取向择优生长,而在凸界面下可能存在[100]和[110] 2种择优生长方向,取决于界面能各向异性参数a₁和a₂的相对大小,说明电子束区熔中金属Mo单晶通常呈现[110]择优取向,而不是通常认为的[100]择优取向。无论是在平界面还是凸界面下,fcc结构高熔点金属Ir始终以[100]择优取向生长,并通过Ir单晶生长实验得到验证。电子束区熔下需综合考虑晶体生长曲率过冷度和动力学过冷度的影响,小尺寸试样(晶粒尺寸在毫米量级及以下)中晶向选择和淘汰取决于曲率过冷度;大尺寸试样(晶粒尺寸在厘米量级及以上)的晶向选择和淘汰主要依靠动力学过冷度。

The preparation of single crystal involves the grain orientation competitive growth. For high melting point metals, Mo is a typical bcc crystal structure and its preferred growth orientation of single crystal was revealed to be the [110] direction, different from the known preferred growth orientation [100] for bcc metals during solidification. This disagreement remains unclear. For the high melting point metal Ir that is a fcc crystal structure, its preferred growth orientation of single crystal remains unknown. Based on electron beam floating zone melting (EBFZM), the orientation competitive growth of these two metals Mo and Ir with three different directions [100], [110] and [111] has been analyzed using solidification theory. It shows that the preferred growth orientation of Mo is the [110] direction at a planar front interface. When introducing the interface energy anisotropy, the preferred growth orientation of Mo could be the [100] or the [110] direction, depending on the magnitude of interface energy anisotropy parameters a₁ and a₂. This result matches well with the experimental results of Mo single crystal prepared by EBFZM. For the fcc structure Ir, its preferred growth orientation always keeps the [100] direction, agreeing with the experimental results of Ir single crystal prepared by EBFZM in this study. Besides, the effects of interface curvature undercooling and kinetic undercooling on the growth behavior of single-crystal metals prepared by EBFZM have been discussed. It demonstrates that when the grain size in the specimen is in the order of about millimeter or less, the curvature undercooling would dominate the grain orientation competitive growth. As the grain size becomes in the order of about centimeter or larger, the kinetic undercooling would prefer the grain competitive growth process.