Superplastic Behavior of B- and Gd-Containing β-Solidifying TiAl Based Alloy

Mechanical behavior and microstructure evolution of the cast Ti-43.2Al-1.9V-1.1Nb-1.0Zr-0.2Gd-0.2B alloy were studied at temperatures from 1100 to 1250°С and strain rates in the range 0.001-1 s-1. Following phase fields (α2+γ), (α+γ), (α) and (α+β) during heating of alloy were revealed. Microstructure analysis after deformation and mechanical behavior allowed defining main processes of structure formation. Two temperature-strain rate conditions with pronounced superplastic behaviour were found: the first one corresponded to the (α2+γ)-phase field (1100°C), where the microstructure had mainly a lamellar morphology, and the second was associated with the (α+β)-phase field (1250°C), in which the α-phase dominated. At T=1100°C and έ=0.05 s-1 the maximum strain rate sensitivity m was of 0.40. At T=1250°C and έ=0.5 s-1 the maximum strain rate sensitivity m was of 0.59. In the (α2+γ)-phase field, superplastic behavior was associated with the transformation of the lamellar structure into globular one. In the (α+β)-phase field, it was due to the formation of a homogeneous refined microstructure during dynamic recrystallization. The relationship between coefficient m value and microstructure formed was discussed.