利用落管研究微重力对TC20合金凝固行为的影响

Solidification Behavior of TC20 Alloy under Microgravity Researched Using Drop Tube

利用50m长真空落管研究了TC20合金在微重力下与重力下的籽晶外延凝固行为。采用金相显微镜观察了样品在微重力和重力下凝固组织，用图像分析软件统计分析了重熔区纵截面晶粒数量、形态和尺寸，用电子探针（EPMA）对重熔区轴向成分分布进行了分析。结果显示，合金重熔凝固组织均为等轴晶组织，但其数量、尺寸和形态不同。相对于微重力环境，重力环境下凝固的组织中晶粒数量较多、轴径比更大且尺寸多分布在小尺寸区间。样品凝固初期形成细小等轴晶，后期则形成较多拉长晶粒。其中重力环境形成一些沿径向水平生长的晶粒，而微重力下则多为倾斜向上生长的晶粒。此外，重力和微重力样品中合金元素沿生长方向分布都很均匀，未发生宏观偏析。结果表明，合金样品整体凝固速度较快，使重力引发的对流效应和溶质输运效应对凝固组织影响微弱，凝固过程主要受热扩散控制。

The epitaxial solidification behavior from seed crystal of TC20 alloy under both microgravity and normal gravity was investi- gated by using a 50 m long drop tube. The solidification microstructure of samples solidified under microgravity and normal gravity were observed under optical microscope. The grain amounts, morphologies together with sizes on longitudinal section of the remohen regions were measured and analyzed with the assistance of image analysis software. Moreover, the axial element distributions in the remolten regions were studied by electron probe microanalysis （EPMA）. The results showed that the solidification microstructure obtained under microgravity and normal gravity was both composed of equiaxed grains, but the amount, size and morphology of the grains were differ- ent. Compared with that obtained under microgravity environment, the amount and diameter ratio of the grains obtained under normal gravity were larger, and grain sizes were distributed more in small size range. Fine equiaxed grains formed at early solidification stage while more elongated grains arose later on. Some of the long grains grew along radial direction under normal gravity but mostly were ob- liquely upward under mierogravity. In addition, the solute elements were distributed homogeneously along the growth direction of under both normal gravity and microgravity, no macrosegregation occurred. The results revealed that the solidification speed of the samples was rapid, and consequently convection effect and solute tran^oort effect caused by gravity had little influence on the solidification mi- crostructure. Therefore, the solidification process was mainly controlled by thermal diffusion.