Exceptional strength-ductility combination of additively manufactured high-entropy alloy matrix composites reinforced with TiC nanoparticles at room and cryogenic temperatures

Abstract

The cryogenic mechanical properties and deformation behaviour of additively manufactured (AM) medium/high-entropy alloys (MEA/HEA) and their composites are seldom studied. This work fills this gap by systematically studying the mechanical properties, deformation behaviour and strengthening mechanism of the additively manufactured CoCrFeMnNi high-entropy alloy (HEA) and 2 wt% TiC/CoCrFeMnNi composite (HEC) at room (293 K) and cryogenic (93 K) temperatures. The TiC nanoparticles promote the densification behaviour, and they are uniformly distributed around and in interior of the cellular substructures of the HEC sample. The tensile tests reveal that the strength and ductility can be simultaneously increased in both HEA and HEC samples when the temperature decreases from 293 K to 93 K. The TiC nanoparticles significantly promote a remarkable combination of strength and ductility in the HEC at both 293 K and 93 K, and the cryogenic tensile strength of HEC (1506 ± 6.6 MPa) is the highest value reported for AM-fabricated MEA/HEAs. For the HEA sample, the significant increase in strength and ductility is attributed to an additional hardening contribution induced by multiple twin systems at 93 K, which results in a steady and even increased strain hardening rate. When the temperature decreases from 293 K to 93 K, a remarkable yield strength increment (344.4 MP) is found in the HEC sample, which is significantly higher than that of HEA (229.0 MPa). The difference in the increase in yield strength between HEA and HEC sample is ascribed to the difference in dislocation strengthening and Orowan strengthening, which are related to the temperature-dependent thermal expansion coefficient and elastic modulus, respectively. However, the HEC sample has a smaller ductility increase (10.3%) than the HEA sample (26.4%), since no additional hardening mechanism is generated during the cryogenic deformation in the HEC sample. This research provides a new route to design materials with a good combination of strength and ductility for cryogenic applications.