EUTECTIC FORMATION IN THE NI-AL SYSTEM

Directional solidification studies have been carried out on Ni-Al alloys at the eutectic and at off-eutectic compositions. This system has a peritectic reaction at a composition and temperature very close to the eutectic reaction and hence three different solid phases beta, gamma, and gamma', can form from the liquid at near the same temperature. Apparently, as a result of this fact, several very unusual types of coupled eutectic growth structures were found in this system. Both the equilibrium eutectic, beta + gamma' and a metastable eutectic, beta + gamma, form in this system under just slightly different conditions. In addition, at low volume fractions of the beta phase an unusual form of rod eutectic forms. In this structure beta rods are arrayed around the cell boundaries of gamma' cells with a spacing along the cell boundaries that matches the spacing for the normal rod eutectic that occurs as close packed arrays at higher volume fractions of beta. This eutectic is confined to the gamma' cell boundaries and may be called a cell boundary eutectic. A second type of cell boundary eutectic was observed which is a metastable beta + gamma eutectic. Here the eutectic is made up of alternating blades of the beta and gamma phases along the cell boundaries of gamma' cells. The spacing versus solidification rate of the various eutectic structures are measured and analyzed with reference to the Jackson-Hunt theory of coupled eutectic growth.