Directional solidification of Ni 3Al

Abstract

A nickel aluminide intermetallic material (IC-50) was directionally solidified under various growth rates utilizing a modified Bridgeman apparatus. The microstructural features of interest, namely: primary dentrite arm spacing (PDAS); secondary dendrite arm spacing (SDAS); and dendrite tip radius of curvature (ϱ) were measured as functions of the growth rate. Under the experimental conditions used in the present study, the critical growth rate for the cellular-dendritic transition occurs in the 2.7–6.1 μm/s range. This range was noted to be in excellent agreement with the value of 2.9 μm/s predicted by the Kurz and Fisher model. The primary dendrite arm spacing (PDAS) decreases proportionally with the growth rate to a power of −0.17. Although this growth rate exponent is smaller than that anticipated from theoretical models (−0.25), the present results are in good agreement with those reported elsewhere for nickel base superalloys. The secondary dendrite arm spacing (SDAS) and the tip radius (ϱ) vary with the growth rate approximately in the same manner. The present results suggest a scaling law between SDAS and ϱ of the following type: λ 2 = 3.0 ϱ; the numerical coefficient is higher than that anticipated theoretically (2.1). The discrepancy between the present data and the results anticipated from the Kurz and Fisher model are discussed in terms of the assumptions involved in the derivation of the model.