The Study Of Microstructures And Mechanical Properties Of AlCoCrFeNiTi_x High-entropy Alloys

High-entropy alloys, a newly developed alloy system, are composed of a variety of principal elements and the concentration of each element is almost equal, this breaks the traditional alloy design frame work which is designed only one major alloy element. In addition, it is available to get high strength, high hardness, high corrosion resistance, and high temperature oxidation resistance and other excellent performance by heat treatment or changing composition of alloys. Because of its broad application prospects, the study of its phase formation and transformation, microstructure and mechanical properties becomes very significant. In this paper, effects of the heat treatment and Ti addicton on the microstructures and mechanical properties of AlCoCrFeNiTix high-entropy alloys (HEAs) were studied.The as-cast AlCoCrFeNiTix (x=0,0.1,0.2,0.3,0.4and0.5) alloys obtained by high vacuum arc-melting annealed at temperatures of500,700, and900℃respectively for2hours. The microstructures, morphologies and chemical compositions of alloys of the as-cast and heat-treated samples were characterized by x-ray diffraction (XRD) and scanning electron microscope (SEM) equipped with an energy dispersive spectrometry (EDS). Vickers hardness and compressive tests were performed with a material test system servohydraulic mechanical testing machine. the experimental results are as follows:1. AlCoCrFeNiTix (x=0、0.1、0.2、0.3、0.4and0.5) HEAs, which shows typical dendrites morphology, has a transformation of single-phase solid solution (bcc) to two-phase solid solution (bcc) with the increase of the Ti contents and heat-treatment temperature, and meanwhile eutectic morphology and modulated structure emerge during the transformation process. Furthermore, obvious elemental segregation can be detected in the dendrite and interdendrite regions for the alloys. Cr, Fe and Ti spread outside the dendrites with the increase of temperature, leading to the dendrite region enriched with Al and Ni while the interdendrite region enriched with Cr and Fe.2. AlCoCrFeNiTix (x=0、0.1、0.2.0.3、0.4and0.5) HEAs, which exhibit excellent mechanical properties. With the increase of heat-treatment temperature, the average hardness and yield strengths increase at first and then decreases, while the plasticity decreases initially and then increases with the increase of heat-treatment temperature. With the increase of the Ti contents, the average hardness and yield strengths are greatly improved, and the plasticity decreases initially and then increases, AlCoCrFeNiTi0.5alloy annealed at700"C exhibits the highest Vickers hardness with a value of583HV0.1, and the AlCoCrFeNiTio alloy annealed at500℃exhibits the highest yield strength while the alloy annealed at900℃shows the best plasticity, about2.07GPa and30.4%, respectively. 3. AlCoCrFeNiTio.3alloy showes excellent dynamic compression mechanical properties, and its yield strength and strain rate reach about2000MPa, and10%, respectively.