EFFECT OF GROWTH-CONDITIONS ON THE STRUCTURE OF DIRECTIONALLY SOLIDIFIED GAMMA-GAMMA'-DELTA EUTECTIC

γ/γ/'-δ eutectic alloy containing 21.5 wt pct Nb, 2.5 wt pct Al, balance Ni was directionally solidified under a thermal gradient (G) of 500 K/cm and different schedules of growth rate (R). Under steady state growth rate conditions, the critical (G/R) for plane front solidification is 25 K h/cm2. At progressively lower (G/R) the structure becomes richer in γ-phase, i.e. hypoeutectic. Sudden increase in growth speed causes the structure to change from lamellar to cellular and gradual increase in growth speed results in interlamellar spacings that are larger than the extremum values. Sudden decrease in growth speed causes little disturbance in the structure but causes the eutectic grain size to increase. © 1979 American Society for Metals and the Metallurgical Society of AIME.