Observation of short-range ordering and formation of Ti3Al particles in Ti 6Al-4V semi-products dependent on variations in chemical composition, cooling rates and operational exposures

Mitropolskaya, Natalia G.

Ti3Al phase (alpha2) is an ordered structure based on the hexagonal close-packed alpha phase. In binary Ti-Al alloys, alpha2 can form at 6 wt.% Al at 500C. The effect of alpha2 on mechanical properties varies with the amount of the phase and the size and shape of the particles but in general is known to increase yield strength and decrease ductility and fatigue performance by inducing intense planar slip bands leading to easy crack nucleation and fast microcrack propagation.

In Ti 6Al-4V, partitioning leads to increased aluminum percentage in the alpha phase (7 to 8 wt.%). Hence, short-range ordering of alpha2 can occur at up to 650C, i.e. in the temperature range of conventional heat treatments, during aging or slow cooling.

Although Ti 6Al-4V is by far the best-studied titanium alloy, variability of production and post-production conditions is high due to its relatively loose chemistry specification requirements and multiple processing routes. This often requires additional studies to avoid unwanted occurrence of alpha2 during fabrication or in operation.

This work includes TEM, XRD, etc. characterization of Ti 6Al-4V specimens to study (1) variations of chemistry and cooling rates, (2) combinations of aging and exposure cycles to understand the boundaries of alpha2 ordering.

Alpha2 particles were observed upon isothermal treatment at 730С and subsequent cooling. Heats with maximal level of impurities were found to be most susceptible to ordering during furnace cooling, while the aluminum concentrations were shown to have less effect.
The onset of alpha2 particles formation was observed after short-term exposures at 450C and growth of their number after 480C. After long-term exposures at 340C and 370C, alpha2 ordering and formation of alpha2 particles was recorded.

Also presented are an overview of the respective changes in mechanical properties, suggested mechanisms for the onset of alpha2 ordering and recommendations for production.