Revealing the Effects of Friction Stir Processing on the Microstructural Evolutions and Mechanical Properties of As‐Cast Interstitial FeMnCoCrN High‐Entropy Alloy

The present study aims to investigate the effect of friction stir processing (FSP) on the microstructural evolutions and mechanical properties of a nonequiatomic interstitial high‐entropy alloy (HEA). To achieve this, an as‐cast FeMnCoCrN interstitial HEA undergoes FSP to modify the as‐cast microstructure and investigate the resulting effects on mechanical properties. The grain size, sub‐boundaries, martensite fraction, and accommodated strain exhibit a gradient from the base metal (BM) to the stir zone (SZ). As a result of FSP, the average grain size in the upper SZ is significantly decreased from ≈700 μm in BM to 1.5 μm, which is around 500 times finer than that of the BM. Furthermore, the martensitic transformation in a single metastable face‐centered‐cubic phase specifically occurs in the thermomechanical‐affected and heat‐affected zones. Continuous and geometric dynamic recrystallizations triggered by FSP lead to the development of a refined equiaxed microstructure. This gradient microstructure, in turn, plays a pivotal role in achieving significantly improved mechanical properties when compared to the as‐cast microstructure. Remarkably, the yield strength doubles, the ultimate tensile strength increases from 548 to 852 MPa, and ductility increases by 16%.