Insights into Defect-Mediated Nucleation of Equilibrium B2 Phase in Face-Centered Cubic High-Entropy Alloys

Though a fine-scale second-phase distribution is a potent strengthening mechanism for alloys, achieving a high precipitate density is often difficult owing to sluggish precipitation kinetics and limited nucleation sites. More specifically, in case of transition-element-based complex concentrated alloys (CCAs) or high-entropy alloys (HEAs), precipitation of the equilibrium strengthening phase, such as the ordered B2 phase, can be limited due to its high nucleation barrier for homogeneous precipitation within the face-centered cubic (FCC) matrix. This can lead to competing homogeneous nucleation of a metastable ordered L12 phase, which has a substantially lower nucleation barrier since it is isostructural with the FCC matrix. Using three different CCAs/HEAs as examples, thermomechanical processing has been employed to introduce a large number density of homogeneously distributed heterogeneous nucleation sites within the FCC matrix, to manipulate the phase fraction, morphology, and distribution of B2 precipitates. This approach of tailoring the microstructure is widely applicable to other multicomponent alloys.