Study On Fracture Mechanism And Rolling Modification Of AlCoCrFeNi_2.1 High Entropy Alloy

High Entropy Alloy（HEA）,as an emerging multi-principal alloy,has received wide attention due to its excellent comprehensive performance.Different from traditional alloys consisting of one or two principal elements and the performance is improved by adding other elements.The high-entropy alloy consists of five or more elements in an equiatomic ratio or an approximately equiatomic ratio,although the high-entropy alloy consists of a variety of principal components,but it does not form the complex intermetallic compounds predicted by the Gibbs phase,but forms a simple structure,such as body-centered cubic,face-centered cubic,or close-packed hexagonal.Previous studies have shown that face-centered cubic structures have good plasticity,while body-centered cubic structures can provide higher strength.In order to obtain a balance between strength and plasticity,the concept of eutectic high-entropy alloys（EHEA）has been proposed.By combining the face-centered cubic and the body-centered cubic phases to give the alloy a higher strength while maintaining good plasticity.In this paper,the mechanical properties and fracture mechanism of AlCoCrFeNi_2.1eutectic high-entropy alloy were studied by macroscopic tensile test,in-situ tensile test and other experimental equipments and instruments,such as scanning electron microscope and X-ray diffractometer.The results show that the as-cast AlCoCrFeNi_2.1.1 eutectic high-entropy alloy formed by the face-centered cubic phase and the body-centered cubic phase in a lamellar manner has high tensile strength and good fracture plasticity,and its fracture strength and fracture strain are respectively reached 1005MPa and 15.4%.In order to study the fracture mechanism of eutectic high-entropy alloy under uniaxial tensile load,large-arc specimens and small arc-notch specimens were prepared for in-situ tensile experiments.The results show that when the as-cast AlCoCrFeNi_2.1.1 high-entropy alloy is subjected to uniaxial tensile load,the soft and tough surface-centered cubic phase will first be plastically deformed to form a slip zone.At this time,the body-centered cubic phase closely combined with the face-centered cubic phase will not occur plastic deformation produces a large internal stress.Since the interface strength between the body centered cubic and the face centered cubic eutectic phase is higher than the strength of the body centered cubic phase itself,the internal stress formed is released by the fracture of the body centered cubic phase and formed a microcrack,the initial microcrack is at an angle of 45°to the direction of the tensile stress.At the tip of the microcrack,a higher stress field is formed due to the tip effect,and the crack begins to expand under the action of high stress.The body-centered cubic phase near the tip also forms new microcracks under the action of high stress.After the new crack propagates,it merges with the original microcracks to form a main crack,then new microcracks form near the main crack tip due to the tip effect and propagate.The new microcracks merges with the main microcracks to extend the main microcracks a certain distance,this process continues until the final specimen breaks.The mechanical properties of the as-cast AlCoCrFeNi_2.1.1 eutectic high-entropy alloy show that it has good comprehensive mechanical properties,but it is considered that the hard and brittle body-centered cubic phase of the as-cast alloy exists in the form of coarse lamellar form,which limits the alloy.For the improvement of performance,this paper has developed three rolling temperatures of 800°C,1000°C and 1200°C for rolling experiments of as-cast alloys.Through the mechanical properties experiments,X-ray diffraction experiments,microstructure analysis experiments and other related experiments on the rolled materials at different temperatures,it was found that the tensile strength and strain at break of the rolled materials were improved after rolling.And the 800°C rolling has the best performance,in which tensile strength and strain at break reached 1475Mpa and 20.4%respectively,increased by 46.8%and 32.5%compared with the as-cast state;the fracture strain after rolling at1200°C is the highest,reaching 25.1%,tensile strength it is 1193 MPa;the worst performance appear after rolling at 1000°C,in which tensile strength and strain at break are1085 MPa and 18.6%,respectively.The results of XRD analysis of the rolled material showed that the intensity of the diffraction peak of the body-centered cubic phase decreased after rolling at different temperatures,and the intensity of the diffraction peak of the partial face-centered cubic crystal surface increased,indicating that the body-centered cubic phase was refined after rolling.The results of SEM and OM observation on the rolled surface and the rolled side show that after 800°C rolling,the white granular precipitates in the body-centered cubic phase are refined,and the number is increased compared with the as-cast state;After the 1000℃rolling,a large body-centered cubic phase appeared locally on the rolling side of the material,and the white granular precipitated phase only existed on these body-centered cubic phases,and the size increased significantly,but did not appear on the rolled surface.In the rolled surface,a rod-like and spherical precipitated phase precipitated on the face-centered cubic matrix.The precipitated phase has the same composition and structure as the body-centered cubic eutectic phase;When the high-entropy alloy is rolling at 1200°C there is no white precipitated phase appeared on both the rolled surface and the rolled side,and only a spherical body-centered cubic precipitate appeared on the rolled side.In summary,the hot rolling process at different temperatures affects the mechanical properties and phase composition of the AlCoCrFeNi_2.1.1 eutectic high-entropy alloy.