Hardness and modulus of the lamellar microstructure in PST-TiAl studied by nanoindentations and AFM

A nanoindenting atomic force microscope (NI-AFM) allows quantitative measurements of the modulus of elasticity and the hardness on a very local scale. This technique was used to study the elastic and plastic deformation properties of different lamellae in a polysynthetically twinned TiAl crystal. The results show, that the hardness determined at a maximum indentation force of 1000 μN is higher in α2 lamellae than in γ lamellae. Hardness variations were also observed between γ lamellae of different orientation. The shape of impressions left from indentations in γ lamellae deviate clearly from the shape of triangular Berkovich indenters. The irregular shape of these indents, as imaged with the atomic force microscope, indicate a strong anisotropic plastic deformation mode in γ domains. Pop-ins or yield-points, which mark the transition from elastic to plastic deformation, were observed frequently on γ lamellae and less frequently on α2 lamellae. The modulus in the investigated direction of α2 lamellae is significantly lower than the modulus of γ lamellae. This corresponds with calculations of the indentation modulus from the elastic stiffness constants of these phases.