Superplasticity of Directionally Solidified NiAl-15Cr Alloy at High Temperature

Ni-25Al-15Cr (atomic percent, %) alloy was directionally solidified (DS) under argon atmosphere in an Al2O3-SiO2 ceramic mold by standard Bridgman method. The microstructure of the as-fabricated alloy was studied using optical microscope, X-ray diffractometer, and scanting electron microscope. The alloy consisting of dendritic β-NiAl phase, interdendritic γ/γ′ phase, and transient layer γ′ phase, has been investigated. This alloy exhibits superplastic deformation behavior at 1 273–1 373 K over an initial strain rate range of 8. 35×10−4–1. 67×10−2 s−1. The maximum elongation of 280% with strain rate sensitivity index m = 0.22 was obtained at the temperature of 1 323 K and an initial strain rate of 8. 35×10−3 s−1. Transmission electron microscopy (TEM) observations indicate that the superplastic deformation stems from the balance between high resistance (by dislocation sliding) and strain softening (by dynamic recovery and recrystallization).