| High entropy alloys have considerable potential applications in the fields of industrial transportation and aerospace because of their outstanding properties which are different from traditional alloys.In order to improve the mechanical properties of high entropy alloys,regulating the elements and doping concentration of alloys became a research focus.In addition,the different processing methods can also influence the microstructure and properties of alloys significantly.Nevertheless,the comprehensive theoretical system of composition design has not been established,and the relationship between processing methods and the evolution of microstructures as well as mechanical properties was still not clear.Based on the existing problems of high entropy alloys,the effects of element content and processing method on the microstructures and mechanical properties were discussed.In this work,arc melting and directional solidification were applied to prepare AlxCoCrFeNi high entropy alloys.Microstructures,element segregation and compressive properties were investigated to reveal the effect of Al content and directional solidification on AlxCoCrFeNi high entropy alloys.The main conclusions are as follows:(1)With the increase of Al content,the FCC solid solutions in as-cast AlxCoCrFeNi high entropy alloys were transformed into BCC solid solutions,the microstructures evolved from cellular to dendrite and then to equiaxed dendrite.For the Al0.6CoCrFeNi and Al1.2CoCrFeNi high entropy alloys,A1 and Ni were enriched in the inter-dendritic,Co、Cr and Fe were enriched in the dendritic.(2)For as-cast AlxCoCrFeNi high entropy alloys,the ultimate compressive strengths increased and the compressive strain decreased as Al content increased.The Al0.3CoCrFeNi high entropy alloys had excellent ductility.The Al1.2CoCrFeNi high entropy alloys showed the maximum ultimate compressive strength of 2183.46MPa,and Al0.6CoCrFeNi high entropy alloys showed the maximum strain of 47.96%.The fracture mechanisms changed from ductile fraction to brittle cleavage fracture with the increase of Al content.(3)As the Al content increased,the phase in directionally solidified AlxCoCrFeNi high entropy alloys was transformed from FCC solid solutions to BCC solid solutions.Solid-liquid interface morphology is dendrite,and there was serious elements segregation existing in interface zone.The solidification microstructures changed from dendrite to equiaxed dendrite with the increase of Al element content.For the Al0.3CoCrFeNi and Al0.6CoCrFeNi high entropy alloys,Al and Ni were enriched in the inter-dendritic,Co、Cr and Fe were enriched in the dendritic.For the Al0.9CrFeNi and Al1.2CoCrFeNi high entropy alloys,Al and Ni were segregated in the precipitate,Co、Cr and Fe were segregated in the matrix.(4)With the increase of Al content,the compressive strain decreased,and the ultimate compressive strengths increased at first and then decreased.The strength of Al0.9CoCrFeNi high entropy alloys prepared by directional solidification at the withdrawal rate of 150μm/s and 70μm/s were 2114.57MPa and 2265.17MPa,respectively.The fracture mechanisms evolved from ductile fraction to brittle cleavage fracture with the increase of Al content.(5)Directional solidification didn’t change the structure of Al0.6CoCrFeNi and Al1.2CoCrFeNi high entropy alloys.The directionally solidified grains were arranged neatly and had preferred orientation,the size of grains coarsened.Directional solidification promoted the growth of the third dendrite in Al1.2CoCrFeNi high entropy alloys.Because the withdrawal rate was large,the composition segregation of directionally solidified alloy was similar to that of the as-cast alloys.(6)The compressive strain of Al0.6CoCrFeNi and Al1.2CoCrFeNi high entropy alloys decreased after directional solidification.The ultimate compressive strength showed different variations.The strength of Al0.6CoCrFeNi high entropy alloys increased from 1675.32MPa to 1903.42MPa,and the strength of Al1.2CoCrFeNi high entropy alloys decreased from 2183.46MPa to 1463.36MPa.The difference in strength has been suggested to be the result of micropores in the matrix. |
