Microstructural characterization of a directionally-solidified Ni–33 (at.%) Al–31Cr–3Mo eutectic alloy as a function of withdrawal rate

The Ni–33 (at.%)Al–31Cr–3Mo eutectic alloy was directionally-solidified (DS) at different rates, VI, varying between 2.5 to 508 mm h−1. Detailed qualitative and quantitative metallographic and chemical analyses were conducted on the directionally-solidified rods. The microstructures consisted of eutectic colonies with parallel lamellar NiAl/(Cr,Mo) plates for solidification rates at and below 12.7 mm h−1. Cellular eutectic microstructures were observed at higher solidification rates, where the plates exhibited a radial pattern. The microstructures were demonstrated to be fairly uniform throughout a 100 mm length of the DS zone by quantitative metallography. The average cell size, d̄, decreased with increasing growth rate to a value of 125 μm at 508 mm h−1 according to the relation d̄ (μm) ≈ 465 VI−0.22, where VI is in mm h−1. Both the average NiAl plate thickness, Δ̄NiAl, and the interlamellar spacing, λ̄, were observed to be constant for VI ⩽ 50.8 mm h−1 but decreased with increasing growth rate above this value as Δ̄NiAl (m)=61.2 VI−0.93 and λ̄ (μm)=47.7 VI−0.64, respectively. The present results are detailed on a microstructural map.