The Kinetics of Primary Alpha Plate Growth in Titanium Alloys

The kinetics of primary alpha-Ti colony/Widmanstatten plate growth from the beta are examined in Ti-6246, comparing a simple quasi-analytic model to experiment. The plate growth velocity depends sensitively both on the diffusivity D(T) of the rate-limiting species and on the supersaturation around the growing plate. These result in a maxima in growth velocity around 40 K below the transus, once sufficient supersaturation is available to drive the plate growth. In Ti-6246, the plate growth velocity was found to be around 0.32 mu m min(-1) at 850 degrees C, which was in good agreement with the model prediction of 0.36 mu m min(-1). The solute field around the growing plates, and the plate thickness, was found to be quite variable, due to the intergrowth of plates and soft impingement. This solute field was found to extend to up to 30 nm, and the interface concentration in the beta was found to be around 6.4 at. pct Mo. It was found that the increasing O content from 500 to 1500 wppm will have minimal effect on the plate lengths expected during continuous cooling; in contrast, Mo approximately doubles the plate lengths obtained for every 2 wt pct Mo reduction. Alloys using V as the beta stabilizer instead of Mo are expected to have much faster plate growth kinetics at nominally equivalent V contents. These findings will provide a useful tool for the integrated design of alloys and process routes to achieve tailored microstructures.

在Ti - 6246中研究了从β相生长的初生α - Ti集落/魏氏板的动力学，将一个简单的准解析模型与实验进行了比较。板的生长速度对限速物质的扩散率D(T)以及生长板周围的过饱和度都非常敏感。一旦有足够的过饱和度来驱动板的生长，这会导致在相变点以下约40K处生长速度达到最大值。在Ti - 6246中，发现850℃时板的生长速度约为0.32μm/min，这与模型预测的0.36μm/min非常吻合。由于板的共生和软碰撞，发现生长板周围的溶质场以及板厚变化很大。发现该溶质场可延伸至30nm，并且β相中的界面浓度约为6.4at.% Mo。研究发现，氧含量从500增加到1500wppm对连续冷却过程中预期的板长影响极小；相比之下，每减少2wt% Mo，板长大约会加倍。预计使用V作为β稳定元素而非Mo的合金，在名义上等效的V含量下，板的生长动力学会快得多。这些发现将为合金和工艺路线的集成设计提供一个有用的工具，以实现定制的微观结构。