Microstructures of directionally solidified ti-45ai-6nb-0. 3b alloys

The Ti-45A1 6Nb 0. 3B alloys were directionally solidified at the growth rates ranging from 10 to 100μm/s under the temperature gradient G= 5×10³ K/m. The microstructures were investigated in both longitudinal and transversal sections of the directionally solidified bars. The micro- structures in quenched mushy zones were also examined. The majority of high-temperature α phases was produced by direct solidification,peri- tectic reaction/transformation and the solid-state transformation βa during the directional solidification (DS) processes at lower, middle and higher growth rates,respectively. The boron addition had not great influence on nucleation of the primary β, but influence on growth of the α phase. Due to the competitive growth of equiaxed a in the interdendritic region,a distinguished transition region with combined equiaxed and columnar grains existed. The boride ribbons were precipitated from the interdendritic regions,and their sizes decreased with increase of the growth rate. The lamellar grains were refined principally by decreasing of the dendritic spacing at higher growth rates. The interlamellar spacing in either interdendritic or dendritic region mostly decreased with increase of the growth rate.