定向凝固Ti—43Al—3Si合金的组织演化规律 Ⅱ.稳态生长区组织演化规律

STRUCTURE EVOLUTION OF DIRECTIONALLY SOLIDIFIED Ti-43Al-3Si ALLOY Ⅱ.Microstructure Evolution in the Steady-State Growth Region

分析了定向凝固Ti-43Al-3Si(原子分数,%)合金在3—90μm/s的生长速度下的稳态生长区组织.在定向凝固过程中经历下列反应:L→Ti_5Si_3,L→α+Ti_5Si_3,α→α_2(Ti_3Al)+γ(TiA1),α_2→γ+Ti_5Si_3,其中,α与Ti_5Si_3共晶是合金最显著的凝固行为.当生长速度大于20μm/s时,还出现L→γ+Ti_5Si_3.随着生长速度增大,稳态组织逐渐由粗胞晶向细胞晶、胞状枝晶及枝晶转变,起稳定α相作用的Ti_5Si_3相由低速时分布于α相中逐渐向高速时分布于凝固γ相中转变,不利于该合金的引晶.选择10μm/s的初始生长速度,既能减少到达稳态生长的距离,又能保证引晶效果.

Microstructure evolutions in the steady-state growth region of directionally solidified Ti-43Al-3Si （atomic fraction, %） alloy were studied at growth rates of 3-100 μm/s. There are four main phase transitions during directional solidification： L→TisSi3, L→α＋Ti5Si3, α→α2（Ti3Al）＋γ（TiAl） and α2→γ＋Ti5Si3, especially, the eutectic reaction of α and Ti5Si3 is the typical solidification behavior. When the growth rate exceeds 20 μm/s, another phase transition of L→γ＋Ti5Si3 also happens. With increase of growth rate, the steady-state growth microstructure changes from coarse cells to fine cells, to cellular dendrites and then to dendrites. The TisSia phase stabilizing α phase distributes in the α matrix at a low growth rate, and in the solidified γmatrix at a relatively high growth rate, which is unfavorable in seeding. When the growth rate is 10 μm/s, the distance needed to reach steady-state region becomes shorter, which is favourable in seeding for the present alloy.