Deformation behavior of Ti-6Al-4V microstructures under uniaxial loading: Equiaxed Vs. transformed-β microstructures

Abstract

The dual-phase titanium alloy Ti-6Al-4V can be thermomechanically treated to produce a variety of microstructures to obtain desired mechanical properties. The extreme microstructure morphologies were developed by heat treatment of mill annealed microstructure to equiaxed, and transformed-β microstructures (lamellar) of coarser α-lath and α′-laths (martensite). The uniaxial tensile test shows the highest elongation in the equiaxed microstructure followed by coarse α-lath lamellar, while the α′-lath morphology has the least elongation. The higher ductility in the equiaxed microstructure is due to smaller slip length compared to coarse α-lath lamellar. On the other hand, the poor ductility of the α′-lath is due to premature crack initiation. Both equiaxed and coarse α-lath lamellar microstructures mostly show prismatic and pyramidal slip <a>/ <c + a>. In addition to this, though less prevalent, these microstructures exhibit twinning as the other deformation mechanism, which is uncommon in Ti-6Al-4V. The twin boundary interaction with the grain boundary led to the damage nucleation, causing the intra-grain crack. In the coarse lath lamellar (furnace cooled) morphology, the crack was mostly observed at the junction of α-colonies as well at the α-layer grain boundary/α-colony interface due to strain localization. However, in the lamellar (water quenched), the primary α′-lath shows the void nucleation at the junction of the primary and secondary-α′ interface, which coalesce to form microcrack and further grow instantly to fracture. The void nucleation is generally observed in a basal orientation along the primary α′-lath, oriented 45° to the loading axis, and having dominant pyramidal slip (<a>/<c + a>). Thus, the deformation mechanisms slip, twin, and fracture depend on the microstructure morphology in Ti-6Al-4V.