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

Abstract The Ni–33 (at.%)Al–31Cr–3Mo eutectic alloy was directionally-solidified (DS) at different rates, V I , 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, d , decreased with increasing growth rate to a value of 125 μm at 508 mm h −1 according to the relation d (μm) ≈ 465 V I −0.22 , where V I is in mm h −1 . Both the average NiAl plate thickness, Δ NiAl , and the interlamellar spacing, λ , were observed to be constant for V I ⩽ 50.8 mm h −1 but decreased with increasing growth rate above this value as Δ NiAl (m)=61.2 V I −0.93 and λ (μm)=47.7 V I −0.64 , respectively. The present results are detailed on a microstructural map.