In Situ Synchrotron Studies of Reversible and Irreversible Non-elastic Strain in a Two-Phase TiAl Alloy

This paper contrasts the cyclic tensile loading of two-phase titanium aluminide Ti-44Al-8Nb-1B microstructures, namely fully lamellar (FL) and duplex (DP). The former, in contrast to the latter, shows premature yielding and hysteresis loops on cycling. These phenomena were studied by in situ cyclic loading of these specimens using high energy synchrotron X-ray diffraction. The results show significant differences in the micromechanics of deformation. Load transfer between the γ and α ₂ phases has been identified at stresses below the macroscopic yield point in the FL microstructure, while the DP microstructure showed no signs of such load transfer before its well-defined macroscopic yield point. A partially reversible pseudo-plastic deformation mechanism seems to be operating at relatively low stresses in the FL specimen. This mechanism is believed to be twinning/partial reversible twinning of the γ phase. The presence of twins at relatively low applied stresses has been confirmed for the FL microstructure by electron channeling contrast imaging. Further support of twinning/partial reversible twinning in FL but not DP microstructures below the macroscopic yield point was obtained by following the evolution of the integrated intensity of particular diffraction peaks measured during the in situ synchrotron X-ray experiments.