液态Ti-Al合金的深过冷与快速枝晶生长

Substantial undercooling and rapid dendrite growth of liquid Ti-Al alloy

采用电磁悬浮和自由落体两种试验技术研究了液态Ti-25 wt.%Al合金的亚稳过冷能力、晶体形核机制和枝晶生长过程.试验发现,即使电磁悬浮无容器状态下仍难以消除润湿角θ≥60°的异质晶核,合金熔体过冷度可达210 K（0.11T_L）.β-Ti相形核的热力学驱动力随过冷度近似以线性方式增大,其枝晶生长速度高达11.2 m/s,从而在慢速冷却条件下实现了快速凝固.理论计算表明,随着过冷度的逐步增大,β相枝晶生长从溶质扩散控制转变为热扩散控制.当过冷度超过100 K时,非平衡溶质截留效应可使合金熔体发生无偏析凝固.然而,单靠深过冷状态不足以抑制β相的后续固态相变.对于落管中快速凝固的直径77—1048μm合金液滴,其冷却速率最高达1.05×10~5K/s,深过冷与快速冷却的耦合作用能更有效地调控凝固组织形成过程.

It is highly desirable to undercool titanium based alloy melts and modulate their dendritic solidification process due to the relevant applications in aerospace engineering.But the serious chemical reactivities of this category of alloys result in potent heterogeneous nucleation and suppress remarkable undercoolings in the course of normal material processing.This paper shows that such a challenge can be solved by containerless processing approach.Liquid Ti-25 wt.%Al alloy is highly undercooled and rapidly solidified under containerless state by both electromagnetic levitation and drop tube techniques.Its metastable undecoolability,crystal nucleation mechanism and dendrite growth process are examined experimentally and analyzed theoretically.Those heterogeneous nuclei with wetting angles above 60°are found to be quite difficult to eliminate even during levitation processing,thus reducing the undercoolability of this alloy.The maximum undercooling of bulk alloy melt reaches 210 K（0.11 T_L）.The thermodynamic driving force to initiate the nucleation of β-Ti phase increases almost linearly with the enhancement of undercooling.The β phase dendrite displays a growth velocity up to 11.2 m/s,indicating that the rapid solidification is realized at the relatively slow cooling rate of levitated alloy melt.With the increase of undercooling,β phase dendrite experiences a kinetic transition from solute diffusion controlled to thermal diffusion controlled growth.Once undercooling exceeds 100 K,the nonequilibrium solute trapping effect brings about the practically desirable segregationless solidification.Nevertheless,the single condition of substantial undercooling is insufficient to suppress the solid state transformation of β phase.It is decomposed into α_2-Ti_3Al phase plus a small amount of γ-Ti Al compound after containerless solidification at levitated state.A more efficient approach to controlling and modulating the solidification microstructures is to utilize the coupled effects of high undercooling and rapid quenching,which proves to be feasible through the rapid solidification of alloy droplets inside drop tube.For those alloy droplets with diameters ranging from 77 to 1048 μm,their cooling rates attain a maximum of1.05 × 10~5K/s,and the predicted maximum undercooling is 227–778 K.In this case,β phase dendrites are well refined and kept in a metastable state until ambient temperature.The heat transfer calculations indicate that the thermal radiation is the dominant cooling mechanism for the large alloy droplets above 690 μm,whereas thermal convection becomes the major cooling mechanism for the small alloy droplets below 690 μm.The microgravity condition during free falling does not show apparent effect on the microstructural formation of these alloy droplets.