On the Aging Behavior of Ti-1.0 wt pct Fe Alloy With an Equiaxed α + β Initial Microstructure

The aging behavior (300 °C to 700 °C) of Ti-1.0 wt pct Fe alloy was investigated in this study, by means of various microstructural characterization techniques and micromechanical testing methods. The initial microstructure consisted of equiaxed α and β grains as well as athermal ω precipitates (ω_ath) with several tens of nanometers inside β grains. During the aging process, significant microstructural modifications took place within the β grains. At relatively lower aging temperatures (300 °C  to 500 °C), the volume fraction of ω_ath gradually decreased with increasing aging temperature, until they were totally replaced by secondary α precipitates (α_s) at 500 °C. With further increase of the aging temperature (500 °C ~ 700 °C), the size of α_s precipitates substantially increased while the volume fraction gradually decreased, indicating a partial α to β reverse phase transformation. At aging temperatures higher than 600 °C, the re-precipitation of ω_ath from reverse-transformed β phase during water quench was identified. Due to an extremely high Fe concentration of these ω_ath that inherited from reverse-transformed β phase, their lattice constants were much smaller than those in the initial microstructure. The existence of ω_ath inside β phase promoted a homogeneous precipitation behavior of α_s precipitates during aging, by providing much more potential intragranular nucleation sites. This led to the formation of plate-shaped α_s precipitates with multiple crystallographic variants, in sharp contrast to the conventional α lamellar/colony structure with limited crystallographic variants obtained by thermomechanical processing in which α_s precipitates directly transformed from β phase. Moreover, it was also revealed that the transformation of nano-sized ω_ath into α_s precipitates as well as their subsequent coarsening led to a continuous decrease of the nano-hardness of prior β areas.