The formation mechanism of eutectic microstructures in NiAl–Cr composites

Abstract

NiAl-based eutectic alloys, consisting of an ordered bcc matrix (B2) and disordered bcc fibers (A2), have been a subject of intensive efforts aimed at tailoring the properties of many of the currently used nickel-based superalloys. A thermodynamic phase field model was developed on a thermodynamic foundation and fully integrated with a thermo-kinetic database of the Ni-Al-Cr ternary system to elucidate the resulting peculiar eutectic microstructure. Invoking a variation of the liquid/solid interfacial thickness with temperature, we simulated the characteristic sunflower-like eutectic microstructures in the NiAl-Cr composites, consistent with experimental observations. The mechanism that governs the formation of the peculiar eutectic morphology was envisioned from the modeled evolutions associated with six sequential steps. Our calculations show that the conditional spinodal decomposition occurring in sequence could further trim and revise the microstructure of the eutectics by generating fine-domain structures, thereby providing an additional method to explore the novel NiAl-based eutectic composites with tunable properties at elevated temperatures.

Microstructural verification of the theoretically predicted morphologies of the NiAl–Cr pseudo-binary alloy systems and NiAl–Cr eutectic structure modification by Mo addition

“…The big amounts of Cr into B2 (NiAl) phase (approximately 9%) and respectively of Ni and Al into Cr rich phase are remarkable. According to Tang et al [1], for the composition of Alloy A, the T0-B2 (practically the maximum temperature necessary for the onset of the B2 (NiAl) phase nucleation in a partitionless way) is much higher than that of the T0-A2 (Cr phase) corresponding one. As the primary dendrites grow, heat is released, due to recalescence, ahead of their front, which alters the solidification kinetics.…”

“…Prior to any attempt for the explanation of the received microstructures and since the work is based on the work of Tang et al [1], a graphical representation of their NiAl-Cr pseudo-binary phase diagram is graphically presented in Fig. 1.…”

“…According to the equilibrium NiAl-Cr pseudo-binary phase diagram [1], alloy A is of hypo-eutectic composition and as such, primary NiAl phase followed by the formation of eutectic micro-constituent should be observed. Indeed, as shown in Fig.…”

“…During the last decade, significant research effort has been conducted in the field of NiAl based alloy and composite systems. NiAl having a density of 5.9 g/cm 3 (stoichiometric NiAl), a coefficient of thermal expansion comparable to that of Ni-base superalloys [3] and a high degree of thermodynamic stability as evident from its very large negative enthalpy of mixing − 22 kJ/ mol [3] (thus easy to fabricate in various forms), made NiAl alloys potential and attractive candidates for high temperature applications in an effort to substitute various categories of existed super-alloys [1,4,5]. The NiAl-Cr system, as part of this effort, has attracted special interest [6].…”

“…Tang et al [1] presented an exceptional research effort on clarifying the possible microstructural configurations of the NiAl-Cr system at various compositions, based on thermodynamic and kinetic calculations. This is exactly where this current report aims to contribute: The authors attempted an experimental verification of the theoretical predictions proposed by Tang et al [1] at off equilibrium conditions. They produced the alloys by arc melting technique where the solidification rate is high as it takes place on a water-cooled cooper-base crucible.…”