Measurement of Young's modulus and internal friction of an in situ Al-Al3Ni functionally gradient material

Abstract

An in situ Al-Al3Ni functionally gradient material (FGM) was produced by centrifugally casting an Al-20 mass% Ni alloy into a thick-walled tube. Four specimens, 90 mm long, with rectangular cross-sections (width × thickness) of 6 × 6, 6 × 5, 6 × 4 and 6 × 3 mm2 were machined from the tube such that the thickness direction of the specimens was in the radial direction of the tube. The microstructure of the FGM tube consisted of granular morphology Al3Ni as a second phase distributed within the aluminium matrix with an increasing volume fraction gradient from the inside to the outside of the tube. Thus, the thicker the specimen, the greater was the composition gradient and the thinner the specimen, the greater was the volume fraction of Al3Ni. The dependence of the Young's modulus and internal friction on the composition gradient of the FGM was determined by a flexural forced-resonance technique from the resonant frequency and the resonance peak width, respectively, as a function of nominal specimen thickness. The Young's modulus of the Al3Ni second phase was determined from a correlation plot of assumed Al3Ni Young's modulus values against the calculated resonant frequency values corresponding to the associated FGM Young's modulus values. The latter were calculated using a rule of mixtures with a fixed matrix Young's modulus and a gradient volume fraction of Al3Ni for each specimen thickness. By plotting the experimental FGM specimen resonant frequencies on this plot, the average Al3Ni Young's modulus was found to be 140 GPa. The Young's modulus of the FGM was found to vary between 81.5 and 100.8 GPa across the 6 mm tube-wall thickness from the inner to outer surface, reflecting the 15.2 and 43.2 vol % Al3Ni second phase, respectively. The measured internal friction increased with the volume fraction of Al3Ni, and owing to the relatively large Al3Ni particle size, was thereby dependent on the resultant increase in the second phase-matrix interface number density rather than the dislocation density.