Characterization of beta flecks and local solute segregation in Ti-17 alloy ingots produced by vacuum arc remelting

Abstract

As a melt defect, beta flecks (β-flecks) may exist in many metastable β and α + β titanium alloys. In this work, the main characteristics of β-flecks, including detectability, macroscopic morphology, microstructural characteristics, and chemical composition, were investigated in an industrial-scale Ti-17 ingot produced by the vacuum arc remelting (VAR) process. The results show that β-flecks can be identified as dark areas in the ingot head through a specific heat treatment. Enrichment of Cr and Zr, along with deficiency of Mo, is found in the β-flecks, resulting in a lowered β transformation temperature (T_β) and increased β phase stability in local areas. When soaked at temperatures from 45 to 15°C below T_β, β-flecks can be distinctly visualized by adjusting the volume of α phase. Additionally, β-flecks exhibit continuous fluctuations in both chemical composition and microstructure, closely related to the size of the β-flecks. Based on the findings, it can be concluded that β-flecks may originate from residual segregated liquid during the final stage of solidification processes, forming as local defects in the final solidified microstructure. Analytical and numerical modeling provide insights into how solute elements segregate and evolve into β-fleck defects.