Determination of the matrix in situ flow stress of a continuous fibre reinforced metal matrix composite using instrumented indentation

Nanoindentation is employed to determine the flow stress of a pure aluminium matrix of a composite reinforced with continuous alumina fibres. The stress for a given representative strain, which depends on the geometry of the indenter, can be deduced using the reverse method established by Bucaille et al. [Acta. Mater. 51 (2003) 1663] assuming that the normal force is proportional to the square of the penetration depth: F = Ch(2). During the first stage of this work, nanoindentation experiments were conducted on annealed and cold-worked aluminium using the Berkovich and cube corner pyramids. The apparent hardness of these materials was found to decrease with increasing penetration depth. The evolution of the normal force as a function of penetration depth was therefore modelled by a polynomial equation allowing extrapolation of the mechanical behaviour to the macroscopic scale. Good agreement was found between the flow stress of cold-worked aluminium obtained in this manner and the values obtained from tensile testing. However, for the annealed aluminium, the present method largely overestimates the values. It is suggested that both geometrically necessary dislocations and the microstructure are the main factors responsible for the indentation size effect (ISE). Several scales must therefore be considered in order to extrapolate the behaviour of aluminium from the nano to the macroscale. In the second stage of this work, nanoindentation tests were carried out in the aluminium matrix of the composite at small penetration depths to avoid any influence of the fibres. By comparison with previous results on bulk aluminium, the flow stress of the matrix was determined and its plastic behaviour was found to be close to that of cold-worked aluminium, in agreement with other studies. This implies that the aluminium matrix is strongly modified by the presence of fibres. during the fabrication of the composite. (C) 2003 Elsevier B.V. All rights reserved.