Perfect cellular eutectic growth in directionally solidified NiAl–Cr(Mo) hypereutectic alloy

Abstract

Cellular eutectic microstructures with fully lamellar morphology were observed in the directionally solidified Ni–31Al–32Cr–6Mo (at%) hypereutectic alloy at withdrawal rates of 15, 25 and 50μm/s, but the morphologies of cellular microstructures did not change consecutively with increasing withdrawal rate. The growth interfaces were deep cellular at withdrawal rates of 15 and 50μm/s, but it changed to be shallow cellular at rate of 25μm/s. The reason is that the interface undercooling comes to minimum at the middle rate of 25μm/s. If the interface undercooling decreases, the tendency of constitutional undercooling will be weaken. The small constitutional undercooling will increase the interface stability, so that the interface morphology changes from deep cellular to shallow cellular. The shallow cellular growth interface led to a perfect cellular eutectic microstructure, which was analogous to the planar eutectic microstructure. In this case, the widths of the intercellular regions were narrowest, no coarse or irregular plates existed at the cell boundaries, and the thicknesses of the lamellae were almost uniform. The properties of the alloy may be markedly improved.