L12 short-range order microstructure and ordered solid solution model of K-state in NiCrAlFe alloy

Abstract

Short-range order (SRO) structures are commonly found in various solid solution alloys such as NiCr, FeAl, medium/high entropy alloys, etc., which significantly affect the mechanical and functional properties of alloys. The K-state in many alloys is a typical inhomogeneous solid solution with SRO. However, the local atomic structure of the K-state is still unclear and the quantitative characterization of the SRO remains a formidable challenge. In this study, the microstructure of the K-state in the NiCrAlFe alloy was characterized by spherical aberration-corrected transmission electron microscopy, and based on crystallography and elastic distortion theory, three-dimensional reconstruction of the microscopic crystal structure was performed. From the results, the K-state crystal structure model of NiCr alloys is defined as Ni₁₉Cr₁₃, and the occupation of Ni and Cr atoms in short-range ordered solid solutions are clarified. The essence of the K-state is L1₂ SRO arranged along the 〈110〉 direction of the face-centered cubic (FCC) matrix. The L1₂ SRO domains with sizes of 1–5 nm are semi-coherent with a FCC matrix through a large number of edge dislocations, and the crystallographic orientation relationship between the FCC matrix and L1₂ SRO domains is ${\left(100\right)}_{\mathrm{BCT}}//{\left(110\right)}_{\mathrm{FCC}}$, ${\left(100\right)}_{\mathrm{BCT}}//{\left(100\right)}_{\mathrm{FCC}}$ (BCT = body-centered tetragonal). The “local state” and “defect state” caused by the SRO of the K-state are the fundamental reasons for the change of the physical properties of the NiCrAlFe alloy. The proposed strategy can be generally used to investigate short-range ordering phenomena in different materials with the K-state and medium/high entropy alloys.