激光原位制备TiB/Ti-6Al-4V复合涂层中TiB管状结构的形成机制

Formation Mechanism of the Tubular TiB in Situ Formed in TiB/Ti-6Al-4V Composite Coatings by Laser Cladding

利用透射电镜(TEM)、电子背散射衍射(EBSD)技术,系统观察、分析了激光原位制备TiB/Ti-6Al-4V复合涂层中TiB生长形态、分布以及晶体学取向,以探讨管状结构TiB的形成机制。结果表明:涂层中TiB的形态主要有棒状和针状2种,受控于生长时间和元素浓度2个因素。棒状TiB形核于过共晶成分区域,针状TiB形核于共晶成分和亚共晶成分区域。当TiB的直径和生长速度之积超过临界值时,(010)_(B27)和(001)_(Bf)晶面的生长将不再稳定。界面边缘区域更易获得硼原子,生长速度快于中心区域,而边缘区域对硼元素的消耗进一步抑制了中心区域的生长,从而产生了管状结构。随着TiB直径的增加,管状结构出现的概率增加。

The morphology, the distribution and crystallographic orientation of TiB in situ formed in TiB/Ti-6Al-4V composite coatings by laser were observed by TEM and EBSD, in order to explore the formation mechanism of tubular TiB. The results show that TiB grains exhibit two morphologies： rod and needle, and which are controlled by growth time and element concentration. Rod-like TiB nucleate in the hypereutectic region, while needle-like TiB nucleate in the eutectic and hypoeutectic region. The growth of （010）_（B27） and （001）_（Bf） planes would no longer be stable, when the diameter and the growth rate of TiB exceed their critical values. The growth rate in the edge region of TiB is faster than that in the center due to the concentration of boron. TiB grains eventually form a tubular structure because boron element is consumed by the growth of edges that further inhibits the growth of the central region. With the increase of diameter of TiB, the tubular structure appears more frequently.