Phase-Field Simulation of Spinodal Decomposition in Fe-Cr-Co Alloys After Ultra-Rapid Cooling in Powder Bed Fusion Additive Manufacturing

<b>[Introduction]</b> Fe-Cr-Co permanent magnetic alloys are attractive because of the combination of good ductility and tunable magnetic properties. The mechanical and magnetic properties of these alloys are strongly affected by the microstructures of ferromagnetic and paramagnetic phases originating from spinodal decomposition. Lots of experimental investigations and theoretical simulations of the microstructure evolutions in Fe-Cr-Co alloys have been made, and in general, precise multiple-step aging is indispensable to optimize the coercivity and remanence. Recently, the additive manufacturing of the Fe-Cr-Co alloys using powder bed fusion (PBF) has been reported, significantly improving the fabrication efficiency. However, although the segregation of brittle phases can be suppressed commonly in many alloys fabricated by PBF, solidification cellular structures and inter-cellular boundary segregation in stainless steels were observed occasionally ^[1]. Moreover, supersaturated vacancies can be introduced by the ultra-rapid cooling conditions of the PBF process. These characteristics of PBF may affect the spinodal decomposition reaction^[2]. Therefore, the objective of this study is to investigate the influences of vacancies on spinodal decomposition by the phase-field simulation and to optimize the aging conditions using the machine learning method.<br/><b>[Methods]</b> Phase-Field models considering the concentration of vacancies were used to simulate the spinodal decomposition for the Fe-25Cr-12Co mass% alloy. The diffusivity of vacancies was assumed to decay with time. Besides, the influence of the dimension of the simulation model (the 2D or 3D) was also examined. The simulation results under various aging temperatures, aging times, and cooling speeds were analyzed quantitatively, which was used for machine learning.<br/><b>[Results]</b> Although the microstructures were qualitatively like the experimental ones, some discrepancies were found. The discrepancy was reduced by using 3D model. The discrepancy in the initial kinetics can be partly attributed to the supersaturated vacancies. However, it was suggested that the inaccuracy of the thermodynamic parameters is also responsible. Therefore, the thermodynamic parameters were modified. The optimization of aging conditions for the PBF process by phase-field simulation will also be discussed.<br/><b>[References]</b><br/>[1] T. Voisin et al.: “New insights on cellular structures strengthening mechanisms and thermal stability of an austenitic stainless steel fabricated by laser powder-bed-fusion”, Acta Mater. <b>203</b> (2021) 116476.<br/>[2] H. Iwaizako et al.: "Spinodal decomposition in plastically deformed Fe-Cr-Co magnet alloy", ISIJ International, <b>62(6)</b> (2022), 1268-1274.