Evading strength-ductility trade-off in a metastability engineered layered metallic composite

• New composite design strategy is proposed based on the metastability engineering of alloys. • The plastic instability was delayed due to additional strain hardening. • Enhanced TWIP and TRIP are attributed to the greater properties. • Biaxial stresses at the interface are responsible for enhanced TWIP and TRIP. This investigation demonstrates a unique layered metallic composite (LMC) design strategy which exploits the metastability tunability of the transformative complex concentrate alloys (CCAs). Metastability engineered LMC (ME-LMC) was prepared by sandwiching a relatively less metastable Fe 38.5 Mn 20 Co 20 Cr 15 Si 5 Cu 1.5 CCA (SFE = 12 mJ/m 2 ) between the two layers of the highly metastable Fe 40 Mn 20 Co 20 Cr 15 Si 5 CCA (SFE = 6 mJ/m 2 ). In ME-LMC, plasticity instability of highly metastable alloy got delayed resulting in slight increase in the ultimate tensile strength (UTS) while maintaining comparable ductility compared to the monolithic CCAs. Superior properties of the ME-LMC are attributed to the enhanced activation of transformation and twin systems in the HCP phase due to the generation of biaxial state of stresses originating from the CCA interface affected zones. Enhanced transformation and twinning led to the greater dynamic refinement of the microstructure providing higher strain hardening enabling greater ductility while benefitting from the dynamic Hall-Petch strengthening. A dislocation density evolution based modelling framework is developed to elucidate the enhancement in mechanical properties.