Directional solidification of Ni [sbnd]Al [sbnd]Cr [sbnd]Fe alloy

Abstract

During directional solidification of engineering alloys, the formation of cellular and dendritic structures with concomitant microsegregation and microporosity is of great interest since these solidification features significantly influence mechanical properties. The critical growth rate V{sub c} at which the planar solid-liquid interface reorganizes to a periodic array of cells can be well predicted by the linear analysis of Mullins and Sekerka. This analysis was recently extended by Trivedi and Kurz for high thermal Peclet number conditions and concentrated alloys. Several theoretical models were also reported on the cellular and dendritic spacings in binary systems. According to the model of Hunt, the cellular spacing sharply increases from zero to a maximum as the growth rate increases from V{sub c} to 2V{sub c}. At higher growth rates, the cellular spacing decreases with increasing the growth rate and no discontinuity in spacing occurs at the cellular-dendritic transition. The Kurz and Fisher`s model predicts a minimum in cellular spacing near the cellular-dendritic transition that is expected to occur at the transition growth rate V{sub t} = V{sub c}/k, where k is the solute distribution coefficient. At the growth rates V > V{sub c}/k, the primary dendrite arm spacing {lambda}{sub 1} varies with the growthmore » rate and temperature gradient G according to the relationship {lambda}{sub 1} {proportional_to} V{sup {minus}0.25} G{sup {minus}0.5}. On the contrary, the model of Trivedi predicts a maximum in cellular spacing to occur near the cellular-dendritic transition and cellular structures are found to grow with smaller spacings than dendritic structures under identical growth conditions. The aim of this paper is to investigate the influence of directional solidification on the microstructure evolution in Ni-12.7Al-6.8Cr-1.8Fe (at.%) alloy. Experiments have been carried out at a constant temperature gradient over a wide range of growth rates.« less