Phase Transformation and Microstructure Evolution in Near-β Ti-7333 Titanium Alloy during Aging

Abstract

A newly near-β titanium alloy Ti-7Mo-3Cr-3Nb-3Al (Ti-7333) was subjected to β phase solution treatment and ageing in the present work. The characteristics of α phase transformation in ageing treatment were studied. Results show that isothermal aging at a low temperature (350) will result in lots of ω particles with small size homogeneously distributing in the parent phase. These ω particles can act as nucleation sites for α phase and lead to the uniform precipitation of fine α phase within the β grain after further ageing treatment. However, when ageing at a higher temperature, the α phase tends to precipitate direct from the β matrix and the morphology of α phase is determined by the temperature and period of ageing treatment. After aging at 550 for 5min, acicular α phase precipitates in the β grains as well as along β grain boundaries and the size and quantity of α phase increase with the holding time. Note that Ti-7333 alloy has a quick ageing response. When aging at 700 for 1h, coarser α laths precipitate both on the grain boundary and within the grain. Increase the ageing temperature to 800, α phase precipitates within the β grain as short rod-like morphology. It is suggested that the driving force for α phase nucleation and the amount of defects in the intragranular decrease with the increasing of temperature, leading to the grain boundaries become the prior nucleation sites. Substantial α phase precipitate-free regions adjacent to β grain boundaries remained after ageing at 700 for 1h due to the rejection of β-stabilizer from coarse α lath on β grain boundaries. Aging at 800 for 1h resulted in pronounced continuous α-films along β grain boundaries.

Phase Transformation and Kinetics in Metastable β Titanium Alloy During Isothermal Treatment

“…In many cases, there are two transition phases during aging treatment: one is the body-centered tetragonal (bct), solute-lean β 0 phase and the other is the hexagonal, solute-lean ω phase. [20] The transition phases are formed before the precipitation of the α phase when the aging temperature is low. [21] For metastable β-Ti alloys with a relatively low content of β-stabilizing elements, the isothermal ω phase precipitates during low-temperature aging.…”

“…[21] For metastable β-Ti alloys with a relatively low content of β-stabilizing elements, the isothermal ω phase precipitates during low-temperature aging. [20,22] In contrast, when the content of β-stabilizing elements is relatively high, the β 0 phase precipitates, which is known as β phase separation. [23,24] The phase separation involves two solid solutions: the β phase rich in β-stabilizing elements and the β 0 phase with less β-stabilizing elements.…”