Objective Recently, with the development of laser technology, increasingly complex components of high entropy alloy (HEA) can be prepared using laser three-dimensional (3D) printing technology. However, HEA prepared using this method exhibit low strength and plasticity. Therefore, AlCoCrFeNi_2.5 HEA with high plasticity is introduced in laser melting deposition (LMD) technology, which is a laser 3D printing technology. Herein, the microstructure and mechanical properties of AlCoCrFeNi_2.5 HEA prepared using LMD are studied. We aim to fabricate HEA with excellent mechanical properties using the laser 3D forming method.

Methods LMD has been developed to synthesize AlCoCrFeNi_2.5 HEA. The laser process parameters are as follows: laser power, scanning speed, powder feeding speed, shielding gas flow rate, spot diameter, defocusing amount, and lifting amount are 700 W, 400 mm/min, 8 g/min, 5 L/min, 2 mm, 11 mm, and 0.25 mm, respectively. The material used for LMD is AlCoCrFeNi_2.5 high entropy prealloyed powder (sphericity ≥90%), and the range of the alloy particle size measured using the laser particle size analyzer is 45--105 μm. The alloy powder is placed in a vacuum drying oven, heated to 120 ℃, and retained for 2 h. Then, it is cooled to room temperature in a vacuum environment, poured into a powder feeder, and placed in a feeding barrel for standby. Further, a 316L stainless steel plate with dimensions of 100 mm×60 mm×10 mm is selected as the base plate, and the oxide layer on the surface is removed using a grinder. Additionally, an electric spark cutting machine is used to cut the AlCoCrFeNi_2.5 HEA samples into different sizes based on the test requirements. A heat setting machine is used to inlay the samples that required grinding and polishing. The samples are polished with 240 ^#, 400 ^#, 800 ^#, 1200 ^#, 2500 ^#, 4000 ^# metallographic sandpaper and SiC polishing solution with particle size of 0.05 μm and 0.02 μm, respectively The appropriate amount of aqua regia is prepared to corrode the polished samples. The X-ray diffractometer (XRD) is used to perform phase analysis of the sample, and the metallographic microscope (OM) and scanning electron microscope (SEM) are used to observe the structure and fracture morphology of the sample. Moreover, an energy spectrometer (EDS) is used to perform surface analysis of the alloy samples scan to obtain the element distribution, and the electron backscatter diffraction device (EBSD) is employed to conduct crystallographic analysis of the alloy sample. The mechanical properties of the plate-shaped tensile sample are investigated using a tensile testing machine.

Results and Discussions The surface of the AlCoCrFeNi_2.5 HEA sample prepared using the LMD technology shows metallic luster without macro or microcracks. Compositional analysis revealed that AlCoCrFeNi_2.5 HEA prepared using LMD exhibit epitaxy columnar dendrite textures, which are primarily composed of the face-centered cubic structure (FCC) at the primary and secondary dendrites and body-centered cubic structure (BCC) at the dendrite gap, respectively. The columnar dendrites grow along the maximum temperature gradient direction in the molten pool, which is parallel to the direction of the laser deposition (DD). The FCC phase located at the trunk of the dendrite grows preferentially along the <100> crystallographic direction. Stretching results show that the tensile strength and elongation of the alloy are 1428 MPa and 25.8%, respectively, along DD. In the laser scanning direction (SD), the yield strength, tensile strength, and elongation at break of the alloy are 586 MPa, 1288 MPa, and 16.1%, respectively. Because columnar dendrites grow epitaxially along DD, DD shows fewer dendrite walls and phase boundaries than SD. Further, fewer “obstacles” are encountered by the dislocation slip during the stretching process, and it can store more dislocations to provide more plasticity and work-hardening ability; thus, the alloy shows more excellent mechanical properties in DD than in SD. The fracture morphology analysis revealed abundant dislocation slippages in the FCC phase region. The BCC phase located in the dendrite clearance effectively hinders the propagation of slippage during the deformation process, thereby further increasing the dislocation density in the FCC phase. Thus, the tensile sample undergoes continuous work hardening in the middle and late stages of deformation. Therefore, the high strength and ductility of AlCoCrFeNi_2.5 HEA are primarily ascribed to the coupling synergy between the FCC and BCC phases.

Conclusions Plate-like AlCoCrFeNi_2.5 HEA samples with excellent comprehensive mechanical properties are prepared using the LMD technology. The alloy prepared using this method exhibits a uniformly distributed structure, no component segregation, and excellent comprehensive mechanical properties. The addition of the Ni element to the AlCoCrFeNi_2.1 eutectic HEA (EHEA) leads to the uniform precipitation of the BCC hard phase only in the dendrite gap, thus ensuring high strength and good plasticity of the alloy. The tensile strength and elongation of the alloy reach 1428 MPa and 25.8%, respectively. The solidification structure of the plate-like AlCoCrFeNi_2.5 HEA sample prepared using LMD shows columnar dendrite with epitaxial growth. The columnar dendrites grow along the maximum temperature gradient direction in the molten pool, which is parallel to DD. The FCC phase at the dendrite stem grows preferentially along the <100> crystallographic direction. This study provides a new strategy for controlling the microstructure of dual phase HEAs and preparing HEA with high strength and plasticity.