Revealing complex microstructure formation mechanisms in the Al[formula omitted]Cr[formula omitted]Fe[formula omitted]Ni[formula omitted]Ti[formula omitted] compositionally complex alloy

Abstract

The complex phase evolution sequence of a promising new high temperature structural alloy with composition Al20Cr20Fe35Ni20Ti5 (at.%) has been studied in situ using an electromagnetic levitation (EML) setup combined with time-resolved high energy X-ray diffraction (XRD), as well by scanning transmission electron microscopy (STEM) observations during in situ heating. These experiments were supplemented by both scanning electron microscopy (SEM) and high resolution STEM observations of the as-cast alloy microstructure, as well as by energy dispersive X-ray spectroscopy (EDS) composition measurements. Differential scanning calorimetry (DSC) was used to identify liquidus, solidus, A2 ↔ B2, and B2 ↔ L21 transition temperatures. The results obtained by these complementary techniques indicate that the alloy forms primary dendrites of A2-structured body-centered cubic (bcc) phase initially from the melt, followed by ordered bcc of B2-structure in interdendritic regions once compositional segregation has sufficiently enriched the remaining liquid. Ordered B2 precipitates form within the primary A2-structured dendrites and the interdendritic B2 regions undergo a continuous decomposition reaction into a maze-like arrangement of A2 and B2/L21 phases below the solidus. Below approximately 1380 K, the B2 domains in both dendritic and interdendritic regions likely undergo a continuous ordering transition to the L21 phase. The experimental observations of Al20Cr20Fe35Ni20Ti5 are found to be in good agreement with CALPHAD predictions but suggest that the solubility limits of the L21-Heusler phase in the database should be relaxed to include greater Fe solubility for the Ni sublattice and to consider deviations in order parameter from stoichiometric ordering.