Examination of Novel Titanium-639 Alloy as a Means of Balancing Strength and Ductility through Molybdenum Addition Rather than Prolonged Aging Heat Treatment

Abstract

Manufacturing titanium alloys with simultaneous enhancement in strength and ductility has motivated extensive research into various strategies for regulating the arrangement and texture of α and β phases. The present study explores a novel α + β titanium alloy, TIMETAL 639 (Ti-639), produced by replacing a portion of vanadium in Ti-64 with molybdenum. The low diffusivity and β-stabilizing effects of molybdenum help retain bimodal characteristics within solution heat-treated Ti-639 microstructures. EBSD and TEM were used to examine β-phase evolution after thermal processing and recrystallization of new globular α grains within pre-existing colonies in a depleted bimodal microstructure. These depleted bimodal colonies in solution heat-treated Ti-639 also led to lower misorientation spreads and dislocation densities within neighboring primary α grains. Quasi-static compression along the plate normal direction demonstrated the ability of the depleted bimodal microstructure to simultaneously enhance strength and ductility in Ti-639 (~90 MPa stronger, ~6% higher failure strain) versus identically processed Ti-64. Only one solution heat-treatment step (1 h at 900 °C) is needed to achieve these properties in Ti-639, whereas comparable properties in Ti-64 required prolonged aging heat treatment (24 h at 600 °C) after the same solution heat-treatment step, making Ti-639 a viable α + β alloy candidate.

Keywords

• Ti alloys
• recrystallization
• EBSD
• microstructure formation mechanism
• misorientation