Synergy effect of multi-strengthening mechanisms in FeMnCoCrN HEA at cryogenic temperature

Abstract

High-entropy alloys (HEAs) have attracted great research interest owing to their good combination of high strength and ductility at both room and cryogenic temperatures. However, expensive raw materials are always added to overcome the strength-ductility trade-off at low temperatures, leading to an increased production cost for the cryogenically used alloys. In this work, a series of nitrogen-doped FeMnCoCr HEAs have been processed by homogenization annealing, cold rolling and recrystallization annealing followed by water quenching. The microstructural evolution and mechanical properties of the alloys are studied systematically. The Fe49Mn30Co10Cr10N1 alloy shows excellent mechanical properties at both 293 K and 77 K. Particularly, the yield and ultimate tensile strength of 1078 and 1630 MPa are achieved at the cryogenic temperature, respectively, while a satisfactory uniform elongation of 33.5 % is maintained. The ultrahigh yield strength results from the microstructure refinement caused by the activation of athermal martensitic transformation and mechanical twinning that occur in the elastic regime together with the increased lattice friction due to the cryogenic environment. In the plastic regime, the dynamic Hall-Petch effect caused by twinning, martensitic transformation, and reverse transformation together with the high barrier to dislocation motion jointly contribute to the ultrahigh tensile strength. Simultaneously, the transformation induced plasticity (TRIP) and the twinning induced plasticity (TWIP) effects jointly contribute to the ductility. The design strategy for attaining superior mechanical properties at low temperatures, i.e. by adjusting stacking fault energy in the interstitial metastable HEAs, guides the development of high-performance and low-cost alloys for cryogenic applications.