V. Soni

Designing and characterizing a complex concentrated gamma/gamma prime ‘superalloy’

Complex concentrated alloys (CCAs), a more general class of high entropy alloys (HEAs) are often near equi-atomic alloys with at-least five different principal components [1,2]. The recently exploding interest in HEAs/CCAs has led to various complex alloy compositions being prepared, characterized using different techniques, and mechanically tested. Though HEAs were initially proposed to have only a single random solid solution phase, in many cases these were found to have ordered intermetallic phases that could in fact be potentially useful for high temperature applications of these complex systems [3]. Hence the motivation of this study is to design HEAs/CCAs containing strengthening ordered L12 precipitates in an fcc matrix (the architecture of nickel or cobalt base super-alloys) [4]. This study focuses on a detailed atom probe tomography (APT) using Imago LEAP 3000X HR Atom Probe Microscope, coupled with high resolution transmission electron microscopy (FEI Tecnai G2 F20 HRTEM) investigation of fcc/L12 HEA systems. Two systems will be discussed, Al0.3CoCrFeNi and Al0.3CuCrFeNi2. These alloys have been melt-processed and subsequently heat-treated to develop an appropriate gamma + gamma prime (fcc+L12) microstructure [4].

Influence of Cr Substitution and Temperature on Hierarchical Phase Decomposition in the AlCoFeNi High Entropy Alloy

While the AlCoFeNi high entropy alloy exhibits a single ordered B2 phase at high temperature, both the substitution of ferromagnetic Co with antiferromagnetic Cr, and lower annealing temperatures lead to a tendency for this system to decompose into a two-phase mixture of ordered B2 and disordered BCC solid solution. The length scale of this decomposition is determined by the combination of composition and annealing temperature, as demonstrated in this investigation by comparing and contrasting AlCoFeNi with the AlCo0.5Cr0.5FeNi alloy. The resulting phase stability has been rationalized based on solution thermodynamic predictions. Additionally, it is shown that replacement of Co by Cr in the AlCoFeNi alloy resulted in a substantial reduction in saturation magnetization and increase in coercivity. The microhardness is also strongly influenced by the composition and the length scale of B2 + BCC decomposition in these high entropy alloys.

Microstructural Design for Improving Ductility of An Initially Brittle Refractory High Entropy Alloy

Typically, refractory high-entropy alloys (RHEAs), comprising a two-phase ordered B2 + BCC microstructure, exhibit extraordinarily high yield strengths, but poor ductility at room temperature, limiting their engineering application. The poor ductility is attributed to the continuous matrix being the ordered B2 phase in these alloys. This paper presents a novel approach to microstructural engineering of RHEAs to form an “inverted” BCC + B2 microstructure with discrete B2 precipitates dispersed within a continuous BCC matrix, resulting in improved room temperature compressive ductility, while maintaining high yield strength at both room and elevated temperature.