Abstract:

Beta flecks are one of the most common defects occur in some α + β and β titanium alloys. In this study, formation of beta flecks in Ti-17 alloy was investigated by directional solidification experiments. Samples were directionally solidified under a constant temperature gradient of 2 × 10 ⁴ K/m and a wide range of withdrawal rates (R) from 3 mm/h to 150 mm/h. We find that macrostructure of the directionally solidified Ti-17 samples can be characterized by “four zones and two lines” after the heat treatment. Profile of the solid-liquid interface transits from planar to cellular to dendritic shape with solidification rate increasing from 3 mm/h to 150 mm/h. The critical rates for planar to cellular (R_c1) transition and cellular to dendritic (R_c2) transition can be well predicted based on the traditional solidification theory. Dark and light contrast areas in macrostructure are directly related to elemental segregation. Dark contrast areas are rich of Cr, Zr but lean of Mo, while no apparent segregation is found in light contrast areas and the mean level of Cr, Zr is lower and Mo is higher in this area than that in dark contrast areas. We conclude that β-flecks in Ti-17 titanium alloy are induced by segregation of alloying elements with k<1 and their shape and size are determined by solidification conditions. Based on the findings of the present article and other literatures, three types of β-flecks are proposed and their formation mechanisms are discussed.

Key words: Titanium, Alloy, Beta flecks, Directional solidification, Elemental segregation, Microstructure heterogeneity