Microstructure Evolution of a New Precipitation-Strengthened Fe–Al–Ni–Ti Alloy down to Atomic Scale

Abstract

Ferritic materials consisting of a disordered matrix and a significant volume fraction of ordered intermetallic precipitates have recently gained attention due to their favorable properties regarding high-temperature applicability. Alloys strengthened by Heusler-type precipitates turned out to show promising properties at elevated temperatures, e.g., creep resistance. The present work aims at developing a fundamental understanding of the microstructure of an alloy with a nominal composition of 60Fe–20Al–10Ni–10Ti (in at. %). In order to determine the microstructural evolution, prevailing phases and corresponding phase transformation temperatures are investigated. Differential thermal analysis, high-temperature X-ray diffraction, and special heat treatments were performed. The final microstructures are characterized by means of scanning and transmission electron microscopy along with hardness measurements. Atom probe tomography conducted on alloys of selected heat-treated conditions allows for evaluating the chemical composition and spatial arrangement of the constituent phases. All investigated sample conditions showed microstructures consisting of two phases with crystal structures A2 and L21. The L21 precipitates grew within a continuous A2 matrix. Due to a rather small lattice mismatch, matrix–precipitate interfaces are either coherent or semicoherent depending on the cooling condition after heat treatment.

Keywords

• ferritic alloys
• microstructure
• Heusler phases
• X-ray diffraction (XRD)
• transmission electron microscopy (TEM)
• atom-probe tomography (APT)