DEFORMATION AND FRACTURE IN DIRECTIONALLY SOLIDIFIED CO-COAL EUTECTIC

The effect of growth defects known as lamellar terminations on the yie lding and fracture behaviour of Co-CoAl eutectic single crystals was s tudied using tensile tests and finite-element modelling. The yield str ength and strain to fracture were found to decrease with increasing te rmination density. Observations of deformed surfaces and serial sectio ning experiments on fractured tensile specimens revealed that crack in itiation during the fracture process was enhanced by the presence of l amellar terminations. The fracture surfaces were found to have a stair case-type appearance, which indicated that the final fracture process was discontinuous with a step-wise propagation from one CoAl lamella t o adjacent CoAl lamellae. A computer simulation was conducted to deter mine the stress distributions about lamellar terminations in model mic rostructures, since the experimental results suggested that the lamell ar terminations behaved as stress concentrations in the microstructure . The finite-element calculation confirmed that lamellar terminations can influence the yielding process; the stress at which the first slip system was activated was found to decrease with increasing terminatio n density.