The influence of C addition on microstructure and compressive properties of Al-Cr-Fe-Ni high entropy alloys

Al-Cr-Fe-Ni high-entropy alloys (HEAs) have garnered significant research attention due to their exceptional properties and cost-effectiveness. Coherent microstructures with ordered nanoprecipitates in AlCrFeNi HEAs represent an effective strategy for enhancing mechanical properties. However, most studies on Al-Cr-Fe-Ni HEAs with coherent microstructures have primarily focused on alloys with low aluminum content. In this study, carbon atoms were incorporated into Al-Cr-Fe-Ni HEAs with a high aluminum content to achieve a coherent microstructure. The phases remained as BCC/B2 with no changes upon carbon addition confirmed via XRD . The microstructure of these HEAs transformed from a weave-like structure to blocks and particles. An appropriate amount of carbon atoms was found to refine the grain size of these HEAs. Additionally, nanosphere phases, approximately 50 nm in size, were observed in the B2 phase when the carbon content was 0.8 at% and 1.2 at%. These nanosphere phases, identified as BCC by TEM , precipitated through spinodal decomposition . The lattice misfit between the BCC and B2 phases decreased from 1.58 % to 0.51 % with the addition of 0.8 at% carbon. The Al-Cr-Fe-Ni HEAs with 1.2 at% C addition exhibit the highest hardness, compressive strength , and ductility, with values of 626.77 ± 27.1 HV, 2019.3 ± 65.9 MPa, and 24.7 ± 0.6 %, respectively. These represent improvements of 12.2 %, 25.4 %, and 27.3 % compared to the carbon-free samples, which exhibited values of 558.0 ± 40.1 HV, 1610.6 ± 85.6 MPa, and 19.4 ± 1.4 %, respectively. All samples exhibited brittle fracture behavior , although the fracture surfaces of the C0.8 and C1.2 alloys displayed a more rugged morphology with pronounced deformation steps. During compression, dislocation slip predominantly occurred within the BCC phase in the carbon-free HEAs, whereas dislocations transferred from the BCC phase to the B2 phase in the 0.8 at% C addition Al-Cr-Fe-Ni HEAs due to the reduced lattice misfit.