On the Stability of Reversely Formed Austenite and Related Mechanism of Transformation in an Fe-Ni-Mn Martensitic Steel Aided by Electron Backscattering Diffraction and Atom Probe Tomography

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Physical metallurgy and materials science, 2017-11, Vol.48 (11), p.5244-5257
• Main Authors: Koohdar, Hamidreza, Nili-Ahmadabadi, Mahmoud, Habibi-Parsa, Mohammad, Jafarian, Hamid Reza, Bhattacharjee, Tilak, Tsuji, Nobuhiro
• Format: Article
• Language: English
• Subjects: Ambient temperature; Annealing; Austenite; Characterization and Evaluation of Materials; Chemistry and Materials Science; Dilatometry; Dual phase steels; Duplex stainless steels; Electron backscatter diffraction; Grain refinement; Heating rate; Manganese; Martensite; Martensitic stainless steels; Martensitic transformations; Materials Science; Metallic Materials; Metallurgy; Microstructure; Nanotechnology; Nickel; Stability; Structural Materials; Surfaces and Interfaces; Thin Films; Tomography; Transition temperature; X-ray diffraction
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/s11661-017-4288-2

The stability of reversely formed austenite and related mechanism of transformation were investigated against temperature and time in an Fe-9.6Ni-7.1Mn (at. pct) martensitic steel during intercritical annealing at a dual-phase ( α  +  γ ) region. Dilatometry, electron backscattering diffraction (EBSD), atom probe tomography (APT), and X-ray diffraction (XRD) were used to characterize the mechanism of reverse transformation. It was found that under intercritical annealing at 853 K (580 °C), when the heating rate is 20 K/s (20 °C/s), reverse transformation takes place through a mixed diffusion control mechanism, i.e. , controlled by bulk diffusion and diffusion along the interface, where Ni controls the diffusion as its diffusivity is lower than that of Mn in the martensite and austenite. Increasing the intercritical annealing to 873 K (600 °C) at an identical heating rate of 20 K/s (20 °C/s) showed that reverse transformation occurs through a sequential combination of both martensitic and diffusional mechanisms. The transition temperature from diffusional to martensitic transformation was obtained close to 858 K (585 °C). Experimental results revealed that the austenite formed by the diffusional mechanism at 853 K (580 °C) mainly remains untransformed after cooling to ambient temperature due to the enrichment with Ni and Mn. It was also found that the stability of the reversely formed austenite by martensitic mechanism at 873 K (600 °C) is related to grain refinement.