In situ SEM Observations of the Tensile-Creep Deformation Behavior and Fracture Mechanisms of a γ-TiAl Intermetallic Alloy at Low and High Stresses.

The effect of stress on the deformation and crack nucleation and propagation mechanisms of a γ-TiAl intermetallic alloy (Ti-45Al-2Nb-2Mn (at.%) - 0.8v.%TiB 2 ) was studied by means of in situ tensile (constant strain rate) and tensile-creep (constant load) experiments performed at 973 K inside a scanning electron microscope (SEM). The evolution of the microstructure and the nucleation and propagation of cracks was tracked during the high temperature mechanical tests in the SEM. Colony boundary crack nucleation was found to be activated during the secondary stage in creep tests at 300 MPa and 400 MPa and during the tertiary stage of the creep tests performed at higher stresses and at constant strain rate. Interlamellar ledges were only observed during the high stress tensile-creep tests (σ>400 MPa) and during the constant strain rate test. Quantitative measurements of the nature of the crack propagation path along secondary cracks and along the primary crack were carried out. It was found that colony boundaries were preferential sites for crack propagation under all the conditions investigated. The frequency of interlamellar cracking increased with increasing stress.