A study on TIMETAL 639 titanium alloy and the effects of cyclic rapid heat treatment (CRHT) on the microstructure and mechanical response of alpha + beta titanium alloys
AdvisorEl Kadiri, Haitham
Oppedal, Andrew L.
Embargo Lift Date8/15/2022
Manufacturing titanium alloys with simultaneous improvement in strength and ductility poses a challenge which exceeds that of purely cubic metals. Various thermal and thermomechanical strategies have been proposed to regulate the geometrical arrangement of alpha + beta phases and texture of the alpha phase in an effort to impart the microstructure with concurrent strengthening and toughening. This study explores two potential methods of achieving mechanical enhancement. The first involves the substitution of molybdenum for a portion of vanadium in Ti-64 (alpha + beta, 6.0 Al, 4.0 V weight %, balance Ti), thereby creating a new alloy, termed Ti-639 (alpha + beta, 6.5 Al, 1.7 Mo, 1.7 V, 0.3C, 0.19O, 0.3 Si weight %, balance Ti). Electron backscatter diffraction(EBSD)and transmission electron microscopy are used to study the preservation of the as-received microstructure of TIMETAL 639, owing to the low diffusivity of molybdenum. EBSD texture analysis of solution heat treated TIMETAL 639 shows a marked effect of beta phase on the recrystallization of new globular alpha grains within preexistent colonies, leading to the generation of a distinct depleted bimodal microstructure. Quasi-static compression mechanical testing indicates a measurable increase in mechanical response of the depleted bimodal microstructure,about 90MPa higheryield, with a 6% higher strain at failure, when compared to identically heat treated Ti-64 samples. This alloy design strategy enhances the performance of alpha + beta titanium alloys while foregoing additional prolonged aging heat treatments associated with Ti-64.The second method involves a processing strategy to impart near-net shape structureswith ultra-fine grain microstructures without resorting to severe mechanical deformation. The proposed strategy relies on utilizing a cyclic rapid heat treatment (CRHT) process to generate refined martensitic microstructures in aerospace grade Ti-64. Analysis of resultant microstructures using EBSDreveals trends in microstructure refinement during successive CRHT iterations. For the given heat treatment parameters, three CRHT cycles yielded an optimum microstructure refinement, by effectively harnessing discontinuous dynamic recrystallization,while diminishing the occurrence of dynamic recovery and continuous dynamic recrystallization.