利用定向凝固方法研究了在1580和1650℃加热温度下Ti-47Al-2Cr-2Nb合金不同片层取向的晶粒竞争生长规律. 显微组织分析表明, 合金初生相为β相,随后发生包晶反应形成α相, 并在β→α转变时α相依附已有β相形核, 12种α变体中仅形成了一种特定取向的α变体,两相区平均温度梯度由40 K/cm增大至160 K/cm, 从而使凝固界面形态由柱状树枝晶转变为胞状树枝晶. 在排除切割面影响的前提下, 对γ片层取向的演变过程进行了分析. 结果表明, 在1 mm/min的抽拉速度下, 温度梯度为40 K/cm(加热温度为1580 ℃) 时, 与生长方向呈74°左右的柱状晶逐渐淘汰其他与生长方向约呈45°的晶粒; 提高温度梯度至160 K/cm (加热温度为1650 ℃) 时, 与生长方向呈74°左右的柱状晶逐步淘汰其他与生长方向约呈90°的晶粒而生长占优. 晶体取向计算表明, 在本实验条件下β枝晶倾向于沿<110>β方向择优生长, 提高温度梯度 使β枝晶沿<110>β方向择优生长趋势增强, <001>β等其他取向枝晶被更快淘汰.
Directional solidification experiments under heating temperatures of 1580℃ and 1650℃ were performed on Ti–47Al–2Cr–2Nb alloy in order to obtain the evolution of lamellar grains. From microstructural analysis in the mushy zone of directional solidified ingots, β phase was firstly solidified, then α phase was formed through peritectic reaction; α phase depended on the pre–existed β phase on which it nucleated, and only one of the 12 α orientation variants was selected during solid state β→ α transformations. As the average temperature gradient of the mushy zone was increased from 40 K/cm to 160 K/cm, the solidification interface morphologies were changed from columnar dendrite to celluar dendrite. Eliminating the influence of cutting plane to the γ–lamella orientation, it was shown that the columnar lamellar grains with an angle of approximately 74? to growth direction gradually overgrew the ones with the angle of nearly 45? to the growth direction at the withdrawal rate of 1 mm/min and temperature gradient of 40 K/cm. Increasing temperature gradient to 160 K/cm, the grains with the angle of about 74? progressively rejected other with the angle of nearly 90? to the growth direction. Calculation of the β dendrite preferred growth orientation indicated that β dendrites tend to grow along the <110>β orientation at the present solidification experiments. Increasing temperature gradient, the preferred growth tendency of dendrite became more drastically, other orientations, such as the <001>β directional orientation, could rapidly be replaced.
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