In situ observation of eutectic and primary TiB formation and their thermodynamic and kinetic mechanisms during the solidification of titanium alloys

Abstract

To investigate the thermodynamic and dynamic formation mechanism of eutectic and primary TiB particle morphologies, a Ti-4Al-6Cr-5Mo-8Nb alloy with varying boron contents was prepared, and in-situ observations were conducted during solidification. The phase morphology, distribution, transformation during solidification, and thermodynamic calculations were systematically studied, and the underlying mechanisms were revealed. The results show that when the boron content is 1.2 wt%, both eutectic and primary TiB phases are present, whereas at a boron content of 0.4 wt%, only the eutectic TiB phase is observed. The eutectic TiB appears as fine, short rods predominantly located at β grain boundaries, while primary TiB forms large, elongated needles distributed across β grains. The average lengths of the eutectic TiB and primary TiB are 6.16 μm and 37.87 μm, respectively, with both exhibiting a similar width of approximately 2.65 μm. Compared to 0.4B alloy, the β grain size in 1.2B alloy decreased by 65 %. Thermodynamic calculations determined the effective phase transformation driving force for the formation of primary TiB to be 119.6 J/mol, with a theoretical size of 34.55 μm. The theoretical size of eutectic TiB at a born content of 0.4 wt% was calculated to be 5.26 μm, based on a maximum growth rate of 3.75 × 10⁻¹¹ m/s during the eutectic reaction. A paired t-test confirmed that the error between the calculated and experimental values was within acceptable limits. The conclusion of this article can provide a theoretical and experimental foundation for predicting the sizes of TiB in boron-modified titanium alloys.