Phase transformation mechanism and hardness during ageing of an austenitic Fe-30Mn-10.5Al-1.1C-3Mo lightweight steel

Abstract

The effects of an ageing treatment on the microstructural evolution and mechanical properties of austenite-based Fe-30Mn-10.5Al-1.1C-3Mo lightweight steel were investigated through a transmission electron microscopy (TEM) analyses and an atom probe tomography (APT) analyses. The solution-treated sample was aged at 675 °C for various holding periods up to 5 × 105 s. The initial microstructure of the solution-treated sample consisted of γ-austenite and ordered κ-(Fe,Mn)3AlC carbides, and it was found that the phase transformation sequence during ageing was as follows: γ + κ → γ + κ + D03 → γ + κ + D03 + β-Mn → γ + κ + (D03) + β-Mn + B2 → γ + κ + β-Mn + B2 → γ + κ + β-Mn + B2 + M6C. In the early stage of ageing, ordered D03 phases formed at the austenite grain boundaries (AGB), twin boundaries and austenite grain interior due to the decreased austenite stability caused by the precipitation of κ-carbides. After ageing for longer than 5000sec, β-Mn phases began to precipitate at the AGB and γ/D03 interphase, and then grew into the γ and D03 phases encroaching upon them. As the β-Mn phase grew, B2 phases newly formed in two ways. First, when the β-Mn phase grew into the D03 phase via the D03 to β-Mn transformation, the discontinuous precipitation of the B2 phase formed in the Al enriched zone caused by low solubility of Al in the β-Mn phase. Next, the B2 phase formed along the AGB inside the β-Mn phase regardless of the D03 phase, i.e., when the β-Mn phase nucleated at the AGB and grew into the austenite interior, the B2 phases precipitated in the Al enriched zone along the PAGB at the β-Mn interior. Finally, Mo-enriched M6C carbides formed around the B2 phases. Vickers hardness tests of solution-treated and aged samples were carried out. At the early stage of ageing, the Vickers hardness increased gradually due to the precipitation of the κ-carbides and D03 phases. With the formation of the β-Mn phase after ageing for more than 10000sec, the Vickers hardness increased dramatically. © 2019 Elsevier B.V.