Microstructural Evolution and Mechanical Behavior of Heat Treated Ti-6Al-4V Powders

Plasma-atomized Ti-6Al-4V powder is widely used in a variety of powder-based manufacturing technologies due to its attractive properties. In many cases there is a desire to modify the microstructure of the as-atomized powder via heat treatment, to improve manufacturing characteristics (e.g., deformability in the case of cold spray deposition). Microstructure characterization of as-received and heat treated plasma-atomized Ti-6Al-4V powder was performed using optical microscopy and x-ray diffraction. Additionally, heat treated powder was characterized using back-scatter electron imaging and energy-dispersive x-ray spectroscopy. The as-received powder was characterized by martensitic alpha grain structures, while the heat treated powder was characterized by either equiaxed alpha or lamellar alpha phase grains, with intergranular beta phase structures. Bulk Ti-6Al-4V was heat treated similar to powders to establish a control, for effective comparison. Microstructure characterization of bulk Ti-6Al-4V using optical microscopy revealed a bright oxygen-rich region, also known as alpha case, near the surface with microstructures resembling heat treated powder. Hardness of the powder and bulk sample was evaluated using Vickers microhardness testing. The hardness values for the alpha case in bulk Ti-6Al-4V correlated with the hardness of heat treated powder and were twice as hard when compared to as-received powder. Overall, this study demonstrated the conversion of powders to alpha case, even during heat treatment in argon with close oxygen control.