Microstructural analysis of rapidly solidified TiAlX powders
Microstructural analysis of rapidly solidified TiAlX powders
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DOI：
10.1016/0921-5093(90)90337-3
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发表时间：
1990-04
期刊：
影响因子：
6.4
通讯作者：
C. Mccullough;J. Valencia;C. Levi;R. Mehrabian
中科院分区：
文献类型：
--
作者：
C. Mccullough;J. Valencia;C. Levi;R. Mehrabian
Ti-48 at.%的显微组织演变通过Ti-48 at.%悬浮熔融凝固实验，对添加钽和碳的Al粉进行了分析Al和Ti-45 at.%添加和不添加碳的铝合金，其中过冷可以量化并与观察到的显微组织相关。二元合金的过冷温度可达258 K，在所有情况下，初生凝固相均为β相，γ偏析相的相对数量随ΔT的增加而减少。通过等离子体旋转电极工艺（PREP）和名义上相同的组合物的XSR制备的粉末的分析揭示了主要相选择的主要差异。PREP粉末在所有粒度下均显示出β形成的证据，而XSR粉末在较粗的粉末粒度中绝大多数为α，初级β的比例随着粒度的减小而增加。这种影响主要归因于XSR工艺中引入的碳污染。碳似乎改变了相平衡，因此α是三元合金中稳定的主相。如果碳含量大于约1at.%，则有Ti 2AlC形成的证据(0.3重量%），但是碳化物在相选择过程中似乎具有边缘作用。即使在较高的碳含量，相选择恢复到初级β在高过冷度，大概是由于动力学偏好后者相。α 2相的分解--由初生枝晶的固态转变--变成α2+γ板条显微组元，似乎被钽的添加和与粉末粒度减小相关的冷却速率的增加所抑制。还观察到马氏体在无偏析的粉末中形成，推测是因为颗粒完全固化为单相β。当形成马氏体时，室温组织为无序α，而不是枝晶粉末中常见的α 2或α2+γ。
Microstructural evolution in Ti-48at.%Al powders with additions of tantalum and carbon is analyzed with the aid of levitation melting-solidification experiments on Ti-48at.%Al and Ti-45at.%Al alloys with and without additions of carbon, where the supercooling can be quantified and related to the observed microstructures. Supercoolings up to 258 K were achieved in the binary alloys; in all cases the primary solidification phase was β, and the relative amount of γ segregate decresed with increasing ΔT. Analysis of powders prepared by the plasma-rotating-electrode process (PREP) and XSR of nominally identical compositions revealed major differences in the primary phase selection. The PREP powders showed evidence of β formation at all particle sizes, while the XSR powders were overwhelmingly α in the coarser powder sizes, with the proportion of primary β increasing with decreasing particle size. This effect was ascribed primarily to the carbon contamination introduced in the XSR process. Carbon appears to shift the phase equilibria so that α is the stable primary phase in the ternary alloys. There is evidence of Ti2AlC formation if the carbon content is larger than about 1 at.% (0.3 wt.%), but the carbide appears to have a marginal role in the phase selection process. Even at the higher carbon contents, the phase selection reverts to primary β at high supercoolings, presumably owing to a kinetic preference for the latter phase. The decomposition of theα2phase—resulting from solid state transformations of the primary dendrites—into theα2+γlath microconstituent, appears to be suppressed by tantalum additions and the increased cooling rate associated with reduction in powder particle size. It was also observed that martensite forms in segregate-free powders, presumably because the particles solidify completely as single-phase β. When martensite is formed, the room temperature microstructure is disordered α, rather than theα2orα2+γcommonly observed in the dendritic powders.