采用选晶法在改进的Bridgman定向凝固炉上进行镍基单晶高温合金DD3定向凝固实验,研究了选晶法制备镍基单晶高温合金过程中晶粒组织演化和螺旋选晶行为, 采用电子背散射衍射(EBSD)技术对晶粒的竞争过程进行分析. 结果表明,选晶器引晶段的主要作用是优化晶粒取向, 获得取向良好的<001>定向凝固组织. 在距激冷板36.1 mm处, 90%的<001>取向晶粒与轴向偏离小于10°, 只有很少的晶粒取向与轴向偏离大于12°. 螺旋选晶段的主要作用是确保只有一个晶粒进入型腔, 在选晶段沿螺旋方向攀旋一周时就可获得单晶组织. 选晶过程中, 远离选晶器螺旋通道内侧的晶粒由于受到螺旋通道较强的几何约束, 进而被靠近螺旋通道内侧的晶粒所淘汰. 螺旋选晶器的狭窄通道对枝晶生长的阻碍作用是实现螺旋选晶的主要原因.
Ni–based single–crystal superalloy DD3 was prepared by using the modified Bridgman apparatus with spiral grain selector. Grain structure evolution and grain selection in spiral selector have been investigated. Electron back scattered diffraction (EBSD) technique was used to characterize the grain texture evolution. The main function of starter block is to optimize the grain orientations. At the height of 36.1 mm from the copper chill plate, the misorientations of nearly 90% grains are less than 10?, and few grains’ orientations deviate from solidification direction more than 12?. Finally, only one grain is allowed to grow into the cast by grain selcting with one pitch in spiral part. Moreover, due to the geometric constraint of spiral selector, the grains near the outer wall are gradually overgrown by the grains close to the inner wall of spiral passage. The grain selection in the spiral selector can be mainly attributed to the geometric constraint of narrow passage.
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