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 muN is higher in alpha (2) lamellae than in gamma lamellae. Hardness variations were also observed between gamma lamellae of different orientation. The shape of impressions left from indentations in gamma 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 gamma domains. Pop-ins or yield-points, which mark the transition from elastic to plastic deformation, were observed frequently on gamma lamellae and less frequently on alpha (2) lamellae. The modulus in the investigated <11-20> direction of alpha (2) lamellae is significantly lower than the modulus of gamma lamellae. This corresponds with calculations of the indentation modulus from the elastic stiffness constants of these phases. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.