The thermal stabilities of In situ composites in a temperature gradient—I. The Al-CuAl 2 eutectic

Abstract

A study has been made of the thermal stability of the aligned lamellar Al-CuAl 2 eutectic when subjected to a temperature gradient of about 10 3Kmm −1. After only a few hours instabilities formed on those lamellae that were roughly perpendicular to the heat-flow direction, and which were above a temperature of ~0.9 T e . These instabilities rapidly pinched off from the parent phase and subsequently migrated through the adjacent eutectic plates. At a more macroscopic level, extensive diffusion of Al up the temperature gradient, with attendant thinning of α lamellae, was observed. Experiments involving the orthodox isothermal annealing of strain free and pre-strained specimens confirmed the role of thermal diffusion as responsible for the present observations. It is anticipated that, over typical turbine-component lifetimes of several thousand hours, such effects will be important at much lower homologous temperatures and temperature gradients. Practical in situ composites of the lamellar type may thus need closer scrutiny before they can be unreservedly recommended for typical aero-engine applications.