EVALUATION OF THE NANOSTRAIN CAUSED BY INCOHERENT PARTICLES IN AN ALUMINUM–BASED COMPOSITE

Abstract

Aluminium based composite with dispersed graphite particles was processed by means of mechanical alloying, cold compression, sintering and hot extrusion. Samples for High Resolution Electron Microscopy (HRTEM) were obtained from extruded bars and were prepared using ultrasonic cutting, mechanical grinding, jet electropolishing and ion milling. Significant strain gradients are created in the metal matrix composites because of thermal mismatch between matrix and reinforcement particles. The main objective of this work was to determine by HRTEM in a quantitative way, the strain gradients in the matrix, in order to be able to predict the increments in the flow stress and the modification in the precipitation kinetics in future works. The geometric phase analysis (GPA) technique was employed to measure strain in the vicinity of the graphite particles. In order to avoid commonly observed errors in strain maps generated from HRTEM images we applied the GPA technique to the exit face wave function reconstructed from a focal series of HRTEM micrographs. It was possible to establish that the matrix is subjected to tension strain gradients that extend out to about 5 nm around the graphite particles. It was observed that the strain in transverse direction (xx) was higher than the longitudinal (yy). According to the results obtained the strain was heterogeneous and had a maximum value of 2.2 % in areas close to the particles.