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

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-beta microstructures (lamellar) of coarser alpha-lath and alpha'-laths (martensite). The uniaxial tensile test shows the highest elongation in the equiaxed microstructure followed by coarse alpha-lath lamellar, while the alpha'-lath morphology has the least elongation. The higher ductility in the equiaxed microstructure is due to smaller slip length compared to coarse alpha-lath lamellar. On the other hand, the poor ductility of the alpha'-lath is due to premature crack initiation. Both equiaxed and coarse alpha-lath lamellar microstructures mostly show prismatic and pyramidal slip / . 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 alpha-colonies as well at the alpha-layer grain boundary/alpha-colony interface due to strain localization. However, in the lamellar (water quenched), the primary alpha'-lath shows the void nucleation at the junction of the primary and secondary-alpha' 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 alpha'-lath, oriented 45 degrees to the loading axis, and having dominant pyramidal slip (/ ). Thus, the deformation mechanisms slip, twin, and fracture depend on the microstructure morphology in Ti-6Al-4V.

Automated summary

The dual-phase titanium alloy Ti-6Al-4V can be thermomechanically treated to produce various microstructures affecting its mechanical properties. The equiaxed microstructure exhibits the highest elongation, while the alpha'-lath morphology shows the lowest elongation. The deformation mechanisms of slip, twin, and fracture depend on the microstructure morphology in Ti-6Al-4V.