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

摘要

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 beta transus temperature. A unique bimodal microstructure consisting of high aspect ratio alpha particles and small colonies with fine secondary lamellar alpha appeared in specimens isothermally held from 950 degrees C to 970 degrees 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 alpha breakdown occurred during isothermal holding treatment because of Ostwald ripening and boundary splitting mechanisms. The formation of small colonies with fine secondary lamellar alpha was also discussed. (C) 2015 Elsevier B.V. All rights reserved.