Effect of Heat Treatment on the Microstructure and Microhardness of (α + β) Wrought and 3D-Printed Ti-5Al-3Mo-1V Alloys
摘要
The microstructure and phase composition of the Ti-5Al-3Mo-1V alloy with different initial states and, namely, the as-received forged rod and the as-built 3D-printed alloys were studied after heat treatment including quenching from 900 °C followed by aging at 500 °C for 16 h. The X-ray diffraction analysis and the transmission electron microscopy with the energy-dispersive analysis of the elemental composition were used. Regardless of the alloy initial state, two types of microstructure were formed as a result of the heat treatment, namely, irregularly shaped elongated α-grains with a dislocation structure and regions surrounding the α-grains with α′-, α″-martensite laths and the β-phase structure. The irregularly shaped elongated α-grains were formed during heating at 900 °C during recrystallization. The microstructure in the areas with the α′-, α″-martensite laths and the β-phase were formed due to the phase transformations β → α′ → α″ during the heat treatment. The incompleteness of the β → α phase transformation was conditioned by the high (1014 m-2) dislocation density in the initial states of the as-received forged rod and the as-built 3D-printed alloys. An increase in the volume fraction of the martensitic α″-phase during aging was accompanied by a decrease in the elastic microstrain ε of the α-phase crystal lattice or in the elastic microstresses. An increase in the microhardness after the heat treatment is due to the barrier effect of the lath boundaries of the α-, α′- and α″-phases, as well as the dislocation hardening. An additional contribution to strengthening in the 3D-printed alloy was made by the elastic microstresses and the nanocrystalline α″-phase in the α-grains.