Thermodynamic modeling and computational predictions of NbC precipitation in Fe-Mn-Si-based shape memory alloys by the classical nucleation and growth theories

NbC precipitation in Fe-Mn-Si-based alloys is an effective method to improve the shape memory effect. In this study, the precipitation behavior was investigated using a thermodynamical model to understand the mechanism and optimize the precipitates for a better performance of Fe-Mn-Si-based shape memory alloys. The influence of alloying elements can be considered in the model by introducing interaction parameters. The precipitate size distribution, mean size, precipitate volume faction, and number density of three typical Fe-Mn-Si-based alloys with different NbC addition amounts were calculated. The results indicated that the mean size could be decreased significantly as the NbC addition increased from 0.5% to 1.0%, while the precipitate volume fraction and number density showed obvious increments. The Fe-28Mn-6Si-5Cr alloys exhibited smaller mean sizes and higher number densities than the Fe-14Mn-6Si-9Cr-5Ni and Fe-21Mn-6Si-9Cr-5Ni alloys. It was also found that the precipitate size distribution showed no evident change as the aging time increased from 0.5 h to 2 h except for the Fe-28Mn-6Si-5Cr-0.5NbC alloy in which the precipitates began to coarsen after about 1.25 h.