Study On Microstructure And Mechanical Properties In Multi-phase Al-Cr-Fe-Ni-Mo High Entropy Alloys

Unlike traditional alloys,which are composed of 1 or 2 main components,high-entropy alloys developed in recent years contain 5 or more components,and the content of each component is between 5%and 35%.This novel design concept provides a new research direction for the development of metallurgy and a broad space for the design of alloy components.At present,there are hundreds of high entropy alloy systems studied,among which the strength and plasticity are difficult to achieve a good match.For example FCC metals have good plasticity but low strength,BCC metals are just the opposite.The precipitation of intermetallic compounds and their distribution in the alloy have different effects on the strength and plasticity of the alloy.Therefore,it is necessary to study the structure and mechanical properties of multi-phase high-entropy alloys.In this paper,two series of high-entropy alloys,AlCrFe₂Ni₂?Mo,Nb?_x,Al_xCrFe₂Ni₂Mo_0.2.2 were designed.The structure of the alloys were detected and analyzed by X-ray diffraction,scanning electron microscopy,electron probe microanalysis and so on.The compression,tensile properties and hardness of the alloys were tested.The conclusions are as follows:?1?In the AlCrFe₂Ni₂Mo_x alloy system,there are always FCC and BCC?B2?phases,but with the addition of Mo element to x=0.3,the FeCrMo type?phase begins to precipitate in the alloys.And the content of BCC phase gradually increases.The hardness of this series of alloys changes linearly before the precipitation of?phase,and then fluctuates up and down.The compression test results show that after the?phase precipitates,the compressive yield strength of the alloys showed an upward trend.The AlCrFe₂Ni₂Mo_0.2 with better comprehensive compression performance is selected for tensile test.And it is found that the yield strength and fracture strength of the alloy reach to 919 MPa and 1495 MPa,respectively,but the elongation is only 6.7%.?2?AlCrFe₂Ni₂Nb_x alloys are mainly composed of AlNi BCC phase,FeCr FCC phase and?Fe,Cr?₂Nb Laves phase.With the addition of Nb elements,the content of Laves phase and eutectic structure in the alloys increase obviously.The hardness of the alloys increased steeply before x=0.2,but after x>0.2,the hardness fits a linear function of y=271.79x+415.32,and the growth was relatively slow.The compressive yield strength of the alloys increased from 989.1MPa to 1824.7 MPa with the addition of Nb elements.Both the compressive fracture strength and the compressibility of AlCrFe2Ni2Nb0.1 are the highest,reaching 2946.6 MPa and 43.3%,respectively.?3?The microstructure with dual phases of FCC+BCC is formed in Al_xCrFe₂Ni₂Mo_0.2with the x value from 0.6 to 0.9.And solidified structures change from dendrite to columnar grain then back to dendrite again.With the increase of the Al content,the volume fraction of the BCC phase increases,and the distribution of two-phase microstructure becomes more uniform.The woven mesh structure can be observed in the entire Al0.9CrFe2Ni2Mo0.2,and the width of the two phases is only about 200 nm.Results of tensile experiments show that the yield strength increases from 341 MPa to 657 MPa,but the plasticity is severely reduced from 51%at Al_0.6CrFe₂Ni₂Mo_0.2 to 8%at Al_0.9CrFe₂Ni₂Mo_0.2.The increasing BCC phase and its nearly uniform distribution in Al_0.8CrFe₂Ni₂Mo_0.2 and Al_0.9CrFe₂Ni₂Mo_0.2 generate more phase interfaces,which increases the resistance of deformation and thus improves the strength of both alloys.?4?The strain hardening rate of Al_xCrFe₂Ni₂Mo_0.2 alloy is increasing from x=0.6 to x=0.9.Al0.6CrFe2Ni2Mo0.2 has the most stable strain hardening rate in the uniform deformation stage.The hardening ability of Al_0.6CrFe₂Ni₂Mo_0.2 and Al_0.7CrFe₂Ni₂Mo_0.2 during deformation is improved,while Al_0.8CrFe₂Ni₂Mo_0.2 and Al_0.9CrFe₂Ni₂Mo_0.2 alloys have stable work hardening indexs.The improvement of work hardening ability makes the product of strength and elongation of Al0.6CrFe2Ni2Mo0.2 reach up to 38.6 GPa%which is higher than most of reported as-cast high-entropy alloys.