Microstructure and failure mechanism in As-deposited, vacuum plasma-sprayed Ti-6Al-4V alloy

The microstructure, phase composition, and chemical composition of vacuum plasma-sprayed Ti-6Al-4V alloys were examined in detail using a variety of techniques, including x-ray diffraction, x-ray photoelectron spectroscopy, and transmission electron microscopy. The observed chemistry and structure were related to the conditions under which the deposit was formed and the phase equilibria in the Ti alloy system. The porosity of the deposit was in the range of 3 to 5%. A slight decrease in the Al content and a slight increase in the amount of oxygen and hydrogen was found relative to the starting powder. Within individual splats, a columnar solidification structure can be seen. However, the as-deposited material is ≥90% α′ martensite that is present in the form of fine lathes on the order of 500 nm in width surrounded by residual β-phase. This herringbone structure obscures to some extent the preexisting columnar structure of the as-solidified β-phase. The material fails at low elongations (∼1%) when tested in tension, with a macroscopic stress-strain curve, which appears to be quite brittle. Examination of the fracture surface, however, reveals a ductile failure mode within individual splats, which is consistent with the structure described above. Sections perpendicular to the fracture surface show that failure occurs at the weak splat boundaries through the development and growth of voids between splats.