Relationship between microstructure and tensile strength in the directionally solidified (23-27) at. % Al-Ni alloys

Abstract

Directional or single crystal technique was applied to enhance the ductility, and two phases of gamma (Ni) phase or beta (NiAl) phase in gamma'(Ni3Al) matrix were also considered to increase the strength and ductility. In this study, directionally solidified rods were prepared at the solidification rate of 50 mu m/s in 23-27 at.% Al-Ni alloys, and tensile strengths of these rods were analyzed at room temperature. Directionally solidified samples showed the gamma dendrite fibers formed in the Ni3Al matrix in the hypo eutectic composition of 23 at.% Al, the gamma' single phase in the eutectic composition of 24.5 at. % Al, and the beta dendrite fibers in the gamma' matrix in the hyper eutectic compositions of 25, 26, 27 at.% Al. The hypoeutectic alloy including gamma dendrites with gamma' matrix exhibited a large elongation of over 70% with ductile transgranular fracture at room temperature. With increasing Al contents, the gamma dendritic microstructure changed to the beta dendrite in the gamma' matrix, which resulted in decreasing the elongation by increasing the volume fraction of the brittle beta dendrites in the ductile gamma' matrix.