Statistical analysis of lamellar and Widmannstätten structures obtained in Nb-rich γ-TiAl alloy with varied cooling rate and annealing duration

Abstract

Nb-rich γ-TiAl alloys are of interest for enhanced mechanical properties and oxidation resistance. The influence of cooling rate and annealing duration on microstructural evolution of Nb-rich γ-TiAl alloy (Ti-45Al-8Nb-0.2B) has been meticulously investigated, with emphasis on phase volume fraction, grain size, and lamellar spacing. The alloy was annealed at 1320 °C for 15 min, and then furnace-cooled, oil-quenched or cryogenic quenched, whereas annealing for 2 h and 26 h was followed by water quenching. The post-anneal microstructures were characterized using X-ray diffraction, optical and scanning electron microscopy, along with energy dispersive spectroscopy and electron backscattered diffraction. Across all samples, a fully lamellar microstructure comprising α₂ and γ phases has been consistently achieved, along with sporadically observed small β precipitates, and elongated ribbon-like TiB₂. Widmannstätten colonies (WC) have been observed with diverse morphologies, most of which exhibited an angle of around 64° to regular lamellae, supporting the theory of $\left\{\bar{1}\bar{1}22\right\}$ twinning in the α-phase for WC formation. The γ phase fraction has decreased with increasing cooling rate and annealing time. Grain size and lamellar spacing have also declined with cooling rate but increased with annealing time. Additionally, the mean size of equiaxed γ grains has enlarged with annealing duration.