Effect of alloying and aging on morphological changes from lamellar to equiaxed microstructure of α 2+γ titanium aluminides

Abstract

The stability of a lamellar structure consisting of ␣ 2 and ␥ phases in alloys has been studied as a function of aging time and temperature. The alloys were solution treated (1400 °C, 30 min, and air-cooled (AC)) and aged at 1000 °C and 1100 °C for 1, 4, and 16 hours, respectively. The results indicate that the kinetics of lamellae to equiaxed transformation depends on alloy chemistry, aging time, and temperature. The Nb decreases and Mo increases the kinetics of transformation. The combined effect of Nb and Mo results in the highest volume fraction of equiaxed microstructure at a given aging time and temperature. The results have been discussed in relation to microstructural features and have been compared with those reported in other ␣ 2 ϩ ␥ alloys.

Microstructure and transformation characteristics of a TiAlNb alloy with Cr and Mo addition

“…The Ti-46Al-4Nb-2Mo had a finer colony size than the Ti-46Al-4Nb-2Cr alloys; however, the Ti-46Al-4Nb-2Cr alloy exhibited the highest volume fraction of transformed equiaxed microstructure at the grain boundaries. Sreenivasulu et al 5 reported that decreasing Nb and increasing Mo increased the volume fraction of transformed equiaxed microstructures compared with Ti-46Al-4Nb-2Cr; therefore, the kinetics of lamellar to equiaxed transformation are decreased by Nb and increased by Mo. The addition of Mo resulted in increased stability in the β phase.…”

“…The microstructure and mechanical properties of TiAl alloys are strongly influenced by both the chemical composition and the heat-treatment procedures employed. Regarding the chemical composition, alloying with a ternary or quaternary element (e.g., Nb, Mo, Cr, V, B, or Si) can have significant effects on the microstructure and the mechanical properties of the alloys 4,5 . The microstructure is ultimately controlled by the heat treatment, and the cooling rate is particularly important in determining the microstructure of titanium aluminides because phase transformation mechanisms control the final nature of the microstructure 6 .…”

The microstructures and mechanical properties of powder metallurgy (PM) Ti-48A1-2Cr-2Nb-(0-1)W

“…Different alloying additions also affect the degree of discontinuous coarsening. Work from Sreenivasulu et al [91] shows that in a ternary system, Nb reduces the amount of discontinuous coarsening while Mo increases the amount of coarsening. When both are combined in a quaternary system, the amount of discontinuous coarsening is much higher than in either of the ternary systems [91].…”

“…Work from Sreenivasulu et al [91] shows that in a ternary system, Nb reduces the amount of discontinuous coarsening while Mo increases the amount of coarsening. When both are combined in a quaternary system, the amount of discontinuous coarsening is much higher than in either of the ternary systems [91]. This makes it difficult to predict how alloying elements will interact with each other when basing assumptions on results generated from different ternary compositions.…”