Microstructure and enhanced tensile properties of AlCoxCrFeNi high entropy alloys with high Co content fabricated by laser melting deposition

AlCoxCrFeNi (x = 2.2, 2.8) high entropy alloys (HEAs) were successfully prepared by multi-layer and multi-channel laser melting deposition (LMD). The tensile properties of the LMD-fabricated AlCoxCrFeNi HEAs were investigated. The phase evolution of these alloys was examined by X-ray diffraction and compared with existing models. The microstructure of the alloys was characterized using scanning electron microscopy and electron backscatter diffraction. It is found that Co element can promote the phase transformation from BCC phase to FCC phase in the as-deposited AlCoxCrFeNi HEAs, and the volume fraction of FCC phase increases from 51.4% to 74.6% as the Co content increases from 36.2 at% to 40.8 at%. With the increase of Co content, the grain size of BCC phase in the alloys decreases and a larger amount of fine needle-like BCC phase appears in the FCC matrix. Tensile testing shows that higher Co content in the deposited AlCoxCrFeNi alloy can enhance its plasticity without significantly compromising its ultimate strength. As the Co content increases, the fracture strain increases from 5.9% to 15.4%, while the yield strength reduces from 450 MPa to 360 MPa and the ultimate tensile strength increases from 734 MPa to 739 MPa. The variations in tensile properties of the AlCoxCrFeNi alloy result from phase structure changes and microstructure evolution. Through this research, it is demonstrated that enhancement of the tensile properties of the LMD-fabricated AlCoCrFeNi HEAs can be realized by increasing the content of Co element