Abstract
In this study, phase transformations occurring during ageing of Timetal LCB (Ti-6.8Mo-4.5Fe-1.5Al in wt.%) and their influence on mechanical properties were investigated. The initial condition was prepared by solution treating above beta-transus directly followed by heat treatment in alpha/beta temperature regime.
This treatment resulted in precipitation of alpha phase on beta/beta grain boundaries. The subsequent ageing response of this material was studied during isothermal ageing at 400 degrees C, 450 degrees C and 500 degrees C.
The phase transformations were studied by X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Mechanical properties were investigated using microhardness measurements.
During ageing at 400 degrees C, the omega phase was detected in all studied conditions. Due to its presence in the material, very fine and homogeneously distributed alpha particles were formed, leading to a high microhardness.
At 450 degrees C, the omega phase disappeared already after a few hours of ageing. The highest microhardness values at this temperature were reached in samples aged for a few hours after the dissolution of omega particles.
No omega was detected in samples aged at 500 degrees C. The alpha phase morphology after ageing at 500 degrees C consisted of large plate-shaped precipitates which coarsened rapidly with increasing ageing time.
This coarse microstructure corresponded to a low microhardness.