Bi-lamellar microstructure in Ti-6Al-4V: Microstructure evolution and mechanical properties

Some characteristic microstructures in Ti-6Al-4V alloy, including lamellar microstructure, martensite microstructure, bimodal microstructure and equiaxed microstructure, have been intensively investigated in the last 30 years. Nevertheless, the bi-lamellar microstructure, which was first defined by G. Lutjering, has not attracted much attention, despite of its significant benefits on various mechanical properties. In the present study, the bi-lamellar microstructure having fine secondary alpha lamellae (similar to 300 nm) in-between coarse primary alpha lamellae was obtained after intercritical annealing treatments in the alpha+beta region of a lamellar microstructure. The effects of intercritical annealing temperature (800-960 degrees C) and cooling rate (water quench similar to 100 degrees C/min) on the bi-lamellar microstructure as well as their yield strength at room temperature were systematically studied. It was found that the thickness (and volume fraction) of transformed beta (beta(trans)) regions including fine secondary alpha lamellae with various orientations increased with the increase of the annealing temperature. The thickness of the secondary alpha lamellae increased with the decrease of the cooling rate. Yield strength of the bi-lamellar microstructure was significantly improved compared with that of the lamellar microstructure composed of coarse primary alpha plates. Appearance of the peak yield strength of the bi-lamellar microstructure at intermediate annealing temperature was found for the first time. By means of TEM observation, slip line analysis as well as elemental analysis of Al, this peak yield strength was attributed to the combined effects of both increased volume and thickness of beta(trans) as well as decreased solid solution strengthening of primary a lamellae by Al with the increase of the annealing temperature. The yield strength decreased monotonically with the decrease of the cooling rate. It was believed that the reduced volume and thickness of beta(trans) and similarity in orientation between primary and secondary alpha lamellae led to the gradual drop of strength with decreasing the cooling rate.