Synergistic effect of precipitation strengthening and multi-heterostructure on the improvement of strength and ductility in NbC-reinforced FeMnCoCr high entropy alloys

An appropriate multi-heterostructure NbC-reinforced Fe₄₈Mn₃₀Co₁₀Cr₁₀NbC (at%) high entropy alloy (HEA) was obtained by adjusting the annealing parameter. Nb addition can lead to static recrystallization retardation, additionally, shear bands and micro-sized NbC as nucleation sites led to the heterogeneous of recrystallization nucleation, which is conducive to the regulation of multi-heterostructure. The multi-heterostructure contained ultrafine recrystallized grains, micron-sized recrystallized grains, large un-recrystallized grains with dense dislocations, and micron-, submicron-, and nano-sized NbC particles, with the grain size exhibiting a bimodal distribution. The sample with 41.8% (volume fraction) ultrafine recrystallized grains exhibited excellent comprehensive mechanical properties with yield strength of 843 MPa and uniform elongation of 30.8%. The high yield strength was attributed to precipitation strengthening and multi-heterostructure, which is about 1.5 times that of other HEAs with similar uniform elongation. The samples with high recrystallization volumes (>70%) exhibited three-stage work hardening behavior, and the increase in the work hardening rate was primarily related to dislocation strengthening and the transformation induced plasticity (TRIP) effect. Twinning induced plasticity (TWIP) effect preferentially occurred in the coarse grain, and lower dislocation density in the un-recrystallized grain was beneficial to TRIP effect. This new design strategy of the multi-heterostructure NbC-reinforced HEA provides a theoretical basis for the development of high-performance HEAs.