On the Aging Behavior of Ti-1.0 wt pct Fe Alloy With an Equiaxed α + β Initial Microstructure
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science
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 (ωath) 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 ωath gradually decreased with increasing aging temperature, until they were totally replaced by secondary α precipitates (αs) at 500 °C. With further increase of the aging temperature (500 °C ~ 700 °C), the size of αs 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 ωath from reverse-transformed β phase during water quench was identified. Due to an extremely high Fe concentration of these ωath that inherited from reverse-transformed β phase, their lattice constants were much smaller than those in the initial microstructure. The existence of ωath inside β phase promoted a homogeneous precipitation behavior of αs precipitates during aging, by providing much more potential intragranular nucleation sites. This led to the formation of plate-shaped αs 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 αs precipitates directly transformed from β phase. Moreover, it was also revealed that the transformation of nano-sized ωath into αs precipitates as well as their subsequent coarsening led to a continuous decrease of the nano-hardness of prior β areas.
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