Effect Of Si Addition On The Microstructure And Properties Of Equimolar AlCoCrFeNi High Entropy Alloy

In recent years,high entropy alloys have developed rapidly,their novel design characteristics of multi-principal components,simple solid solution phase structure and unique four effects make them exhibit excellent comprehensive properties unparalleled by traditional alloy materials.Among many high entropy alloy systems,Al Co Cr Fe Ni high entropy alloy has excellent low and high temperature properties.Thus,in this study,five kinds of HEAs with different Si contents of Al Co Cr Fe Ni Si_x（x=0,0.2,0.5,0.8,1.0）were prepared by vacuum arc melting.The effects of the addition and content of Si elements on the microstructure,and high and low temperature performance of Al Co Cr Fe Ni Si_x HEAs were studied by means of XRD,SEM,EDS,TEM analysis and testing.The following conclusions were drawn:（1）In Al Co Cr Fe Ni Si_x（x=0,0.2,0.5,0.8,1.0）HEAs,after adding Si,the alloys were mainly consist of BCC and Si-rich precipitates,and both the size and quantity of the Si-rich precipitates increase with increase in the Si content.As a result lattice distortion effect caused by the Si addition and the production of intermetallic silicides,the strength and hardness of the alloy increase,while the plasticity decrease.The dynamic polarization test results in 3.5 wt.%Na Cl salt solution show that the addition of Si elements will lead to the enrichment of intermetallic silicides such as Cr₃Si at grain boundaries,which will affect the distribution of Cr in the alloy and deteriorate the corrosion resistance of the alloy.At room temperature,the wear resistance of the alloy increases significantly with the increase of Si content.（2）The oxidation results of Al Co Cr Fe Ni Si_x（x=0,0.2,0.5,0.8,1.0）HEAs at 1100℃show that:The addition of 0.2 at.%Si promotes the formation of a dense single Al₂O₃scale and improves the high temperature oxidation resistance of the alloy by reducing the critical concentration of solute atoms to form a protective Al₂O₃ scale.While excessive Si addition leads to the formation of an easily exfoliated oxide scale structure.The oxide scale consists of Al₂O₃outer layer and mixed oxide inner layer,which adversely affects the oxidation resistance of the alloy.Moreover,the excessive addition of Si leads to the large growth stress of the Al₂O₃ scale,which causes the oxide scale to crack and wrinkle.As the surface oxide scale is destroyed and the O^2-diffuses to the inner layer,a large number of mixed oxides are formed in the inner oxide layer,which leads to the decrease of the high temperature oxidation resistance of the alloy.（3）The hot corrosion results of Al Co Cr Fe Ni Si_x（x=0,0.2,0.5,0.8,1.0）HEAs at950℃in mixed salt of 75%Na₂SO₄+25%Na Cl show that:After 100 h of hot corrosion,due to the selective oxidation of Al,the phase composition of the alloy surface changes from BCC to FCC.The addition of an appropriate amount of Si will promote the rapid formation of a single Al₂O₃ scale on the surface of the alloy and improve the hot corrosion resistance of the alloy.But with the increase of Si content,a mixed oxidation product composed of Al₂O₃ and Cr-Fe-Co-Si-rich spinel oxide is formed on the surface,and under the synergistic effect of the internal stress of the oxide scale and the corrosive medium（oxidation-sulfurization-chlorination）,the structure is loose and easy to fall off,and the alloy undergoes serious internal oxidation and internal vulcanization.