Temperature and misorientation-dependent austenite nucleation at ferrite grain boundaries in a medium manganese steel: role of misorientation-dependent grain boundary segregation
Authors:
Rama Srinivas Varanasi,
Osamu Waseda,
Faisal Waqar Syed,
Prithiv Thoudden-Sukumar,
Baptiste Gault,
Jörg Neugebauer,
Dirk Ponge
Abstract:
In the current work, we study the role of grain boundary (GB) misorientation-dependent segregation on austenite nucleation in a 50% cold rolled intercritically annealed 10Mn-0.05C-1.5Al (wt. %) medium Mn steel. During intercritical annealing at 500°C, austenite nucleates predominantly at high-angle GBs. At 600°C, austenite nucleates additionally at low-angle GBs, exhibiting a temperature dependanc…
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In the current work, we study the role of grain boundary (GB) misorientation-dependent segregation on austenite nucleation in a 50% cold rolled intercritically annealed 10Mn-0.05C-1.5Al (wt. %) medium Mn steel. During intercritical annealing at 500°C, austenite nucleates predominantly at high-angle GBs. At 600°C, austenite nucleates additionally at low-angle GBs, exhibiting a temperature dependance. Correlative transmission Kikuchi diffraction /atom probe tomography reveals a misorientation-dependent segregation. While GB segregation has been reported to assist austenite nucleation in medium manganese steels (3-12 wt.% Mn), an understanding of the temperature and misorientation dependance is lacking, which is the aim of current work. Since artifacts of atom probe can cause a broadening of the segregation width, we combined experiments with results from density functional theory (DFT) calculations that reveal that the Mn segregation is not limited to the GB plane but confined to a region in the range of approximately 1 nm. Consequently, GB segregation alters both the GB interface energy and the free energy per unit volume corresponding to the transformation. We estimate the local driving force for austenite nucleation accounting for the segregation width of ~ 1 nm. Based on classical nucleation theory, we clarify the effect of GB segregation on the critical radius and activation energy barrier for confined austenite nucleation at the GB.
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Submitted 2 April, 2025;
originally announced April 2025.
Ab-initio based models for temperature-dependent magneto-chemical interplay in bcc Fe-Mn alloys
Authors:
Anton Schneider,
Chu-Chun Fu,
Osamu Waseda,
Cyrille Barreteau,
Tilmann Hickel
Abstract:
Body-centered cubic (bcc) Fe-Mn systems are known to exhibit a complex and atypical magnetic behaviour from both experiments and 0 K electronic-structure calculations, which is due to the half-filled 3d-band of Mn. We propose effective interaction models for these alloys, which contain both atomic spin and chemical variables. They were parameterized on a set of key density functional theory (DFT)…
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Body-centered cubic (bcc) Fe-Mn systems are known to exhibit a complex and atypical magnetic behaviour from both experiments and 0 K electronic-structure calculations, which is due to the half-filled 3d-band of Mn. We propose effective interaction models for these alloys, which contain both atomic spin and chemical variables. They were parameterized on a set of key density functional theory (DFT) data, with the inclusion of non-collinear magnetic configurations being indispensable. Two distinct approaches, namely a knowledge-driven and a machine-learning approach have been employed for the fitting. Employing these models in atomic Monte Carlo simulations enables the prediction of magnetic and thermodynamic properties of the Fe-Mn alloys, and their coupling, as functions of temperature. This includes the decrease of Curie temperature with increasing Mn concentration, the temperature evolution of the mixing enthalpy and its correlation with the alloy magnetization. Also, going beyond the defect-free systems, we determined the binding free energy between a vacancy and a Mn atom, which is a key parameter controlling the atomic transport in Fe-Mn alloys.
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Submitted 5 August, 2020;
originally announced August 2020.