Phase Composition, Microstructure, and Mechanical Properties of Spark Plasma Sintered AlCrFeNiX (X = Co, Cu, and CoCu) High Entropy Alloys

Phase composition, microstructure, and mechanical properties of spark plasma sintered (SPSed) AlCrFeNiX (X = Co, Cu, and CoCu) high entropy alloys (HEAs) were investigated. The results show that gas‐atomized AlCrFeNiCo and AlCrFeNiCu HEAs exhibit the single body‐centered cubic (BCC) phase, while a dual‐phase (face‐centered cubic (FCC) + BCC) structure is observed in AlCrFeNiCoCu HEA. After spark plasma sintering, an FCC phase with different lattice parameters can be observed in all three HEAs except for the BCC phase. FCC1 phase enriched in Fe‐Cr and FCC2 phase enriched in Cu appear in SPSed AlCrFeNiCo and AlCrFeNiCu HEAs, respectively, while both FCC1 and FCC2 phases are precipitated in SPSed AlCrFeNiCoCu HEA. The SPSed AlCrFeNiCoCu HEA has the largest fraction of the FCC phase (33%), while the lowest fraction of the FCC phase (12%) is observed in the SPSed AlCrFeNiCu HEA. Furthermore, the brittle and hard σ phase is also precipitated in SPSed AlCrFeNiCo HEA. The SPSed AlCrFeNiCo HEA possesses the highest hardness of 465 HV and compressive yield strength of 1175 MPa, which is attributed to the high volume fraction of BCC phase and σ phase, while the SPSed AlCrFeNiCoCu HEA shows the lowest hardness of 363 HV and compressive yield strength of 1024 MPa due to the highest volume fraction of FCC phase. Besides the body‐centered cubic phase, the FCC1 phase appears in the spark plasma sintered (SPSed) AlCrFeNiCo alloy, and the face‐centered cubic (FCC)2 (Cu) phase forms in AlCrFeNiCu alloy, while both FCC1 and FCC2 (Cu) phases are observed in AlCrFeNiCoCu alloy. The AlCrFeNiCo alloy possesses the highest hardness compressive yield strength, while the AlCrFeNiCoCu alloy shows the lowest hardness compressive yield strength.