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

A nanoindenting atomic force microscope (NI-AFM) allows quantitative measur ements of the modulus of elasticity and the hardness on a very local scale. This technique was used to study the elastic and plastic deformation prope rties of different lamellae in a polysynthetically twinned TiAl crystal. Th e results show, that the hardness determined at a maximum indentation force of 1000 muN is higher in alpha (2) lamellae than in gamma lamellae. Hardne ss variations were also observed between gamma lamellae of different orient ation. The shape of impressions left from indentations in gamma lamellae de viate clearly from the shape of triangular Berkovich indenters. The irregul ar shape of these indents, as imaged with the atomic force microscope, indi cate a strong anisotropic plastic deformation mode in gamma domains. Pop-in s or yield-points, which mark the transition from elastic to plastic deform ation, were observed frequently on gamma lamellae and less frequently on al pha (2) lamellae. The modulus in the investigated <11-20> direction of alph a (2) lamellae is significantly lower than the modulus of gamma lamellae. T his corresponds with calculations of the indentation modulus from the elast ic stiffness constants of these phases. (C) 2001 Acta Materialia Inc. Publi shed by Elsevier Science Ltd. All rights reserved.