Directional Solidification Microstructure And Mechanical Properties Of CoCrFeNi Series High Entropy Alloy

The thermal stability and sluggish diffusion effect of high-entropy alloy make it have considerable application prospects as high-temperature structural material.However,the high-temperature structure application potential of high-entropy alloys has not been fully exploited due to few studies on high-entropy alloy single crystals.The directional solidification experiments were used to investigate the growth behavior of FCC structure FeCoCrNi high-entropy alloy.The solid-liquid interface morphology was studied by means of interface quenching during the process of the directional solidification.The alloying elements Al and Mo affect on the directional solidification behavior was investigated,such as the solid-liquid interface morphology,solute redistribution and crystal growth mode.The configuration entropy on the directional solidification behavior of high-entropy alloys was studied,and the entropy was changed by controlling the number of principal elements to form single-phase,homogeneous,and the same crystal structure of high-entropy alloys.The mechanical properties of the single crystal high-entropy alloys were analyzed,and the single crystal material was strengthened by cold rolling deformation and annealing secondary hardening.The results show that the alloying Al and Mo are the solute atoms in Al_0.3CoCrFeNi and Mo_0.2CoCrFeNi high-entropy alloys with the non-homogeneous FCC single-phase.The solutes significantly reduce the stability of interface during the process of the directional solidification.The more configuration entropy,the greater interfacial tension for homogeneous CoCrNi and CoCrFeNi high-entropy alloys.CoCrFeNi has excellent solid-liquid interface stability,resulted in a small corner radius of the dendrite tip.The stability of the branched structure is better than the tip bifurcation structure.The formation of the secondary dendrite trended to be emerging during the process of the directional solidification.For the AlCoCrFeNi high-entropy alloy with homogeneous and single-phase BCC structure,as the drawing speed is higher than 50?m·s^-1,the original dendrites growth is delay.The front edge area of the interface appears a large number of free nucleation,which block the growth channel of the original dendrites,and therefore the solidification pattern changed from crystal growth to free nucleation and growth.The growth mode of dendrite has a great influence on its competitive behavior.Dendrites in alloys with strong dendrite tip bifurcation instability can be deflected in a cyclic mode of tip splitting-competitive growth-splitting.Finally the primary trunk of dendrite is gradually parallel to the direction of heat flow.The forks can replace the primary crystal axis to participate in a competitive growth of the dendrites.The change of growth angle caused by the bifurcation of the dendrites results in a favorable competitive position in two directions for the dendrites and it can be more likely to cause abnormal elimination.Under the guidance of directional solidification behavior,Al_0.3CoCrFeNi,Mo_0.2CoCrFeNi,Al_0.2Mo_0.2CoCrFeNi and Al_0.4CoCrFeNi four high-entropy alloy single crystals were prepared,and tensile mechanical properties were tested.Al_0.2Mo_0.2CoCrFeNi and Al_0.3CoCrFeNi two high-entropy alloys have the best plasticity and the elongation reached 170%.The strength of four high-entropy alloy single crystals,treated by cold rolling at strain rate of 78.6%and annealing at 500?for 120 minutes,increased by nearly 10 times.Especially,Al_0.2Mo_0.2CoCrFeNi has the highest tensile strength,up to 2.2 GPa,and the mode of tensile fracture for the four high-entropy alloys is brittle fracture.Organizational analysis results show that a small number of ordered FCC phases and a large number of nano-sub-grains have been formed in the single-phase FCC crystal of Al_0.2Mo_0.2CoCrFeNi,and a small amount of L1₂ ordered phase is emerged in the Al_0.4CoCrFeNi alloy matrix.The number of low-angle interfaces in the Al_0.3CoCrFeNi and Al_0.4CoCrFeNi high-entropy alloy single crystals increases after cold rolling and annealing,but they still maintain a original single crystal state.A large number of high-angle interfaces in Mo_0.2CoCrFeNi and Al_0.2Mo_0.2CoCrFeNi high-entropy alloy single crystals are formed after cold rolling and annealing.A large number of[111]orientation textures are produced in the Mo_0.2CoCrFeNi high-entropy alloy,and a large number of[111]and[100]orientation textures are produced in the Al_0.2Mo_0.2CoCrFeNi high-entropy.As a result,the original single crystal state can be failed to maintain.