Effects of thermal history on the microstructure evolution of Ti-6Al-4V during solidification

Ti-6Al-4V specimens with different thermal histories were obtained by controlling their solidification and cooling through an automatic temperature-controlled induction melting apparatus. The final microstructure of Ti-6Al-4V was determined by both isothermal holding temperature and time during solidification, especially near the β transus temperature. A unique bimodal microstructure consisting of high aspect ratio α particles and small colonies with fine secondary lamellar α appeared in specimens isothermally held from 950°C to 970°C. Water quenching was conducted after isothermal holding treatment for comparison. X-ray diffraction, transmission electron microscopy, and electron probe microanalysis were used to explore the evolution mechanism of the unique bimodal microstructure. Primary α breakdown occurred during isothermal holding treatment because of Ostwald ripening and boundary splitting mechanisms. The formation of small colonies with fine secondary lamellar α was also discussed.