The Research On Microstructure And Mechanical Properties Of Multi-component Medium-Entropy Alloy

High-entropy alloys and medium-entropy alloys are developed on the basis of exploring the entropy for stable chemical disordered solid solution structures or structurally disordered amorphous structures.The number of atoms of each element in the second generation high-entropy alloys with non-equiatomic ratios and multiphases is mixed in different proportions,the design freedom of the alloy is greater,the"cocktail"effect on the performance is more fully exerted,and it usually has high strength,high temperature thermal stability and high corrosion resistance,etc.,and makes up for the room temperature brittleness of the metastable material as well as the lack of metastable crystallization,its development and research work has received much attention in recent years.In this paper,the medium-entropy alloys Fe_77.3-x7.3-x Mn_x Si_9.1Cr_9.8C_3.8?x=14³7,at.%?are taken as the research object,the magnetic suspension melting+vacuum suction casting method is used to prepare rod-shaped samples,and the relevant parameters are calculated from the thermodynamic model And alloy layer fault energy,analyze the effect of Mn content change on the microstructure and mechanical properties of the alloy at room temperature,and study the effect of low temperature treatment and high temperature heat treatment on the microstructure and mechanical properties of medium-entropy alloy.First,according to Boltzmann's statistical thermodynamic theory,the mixed entropy of Fe_77.3-x7.3-x Mn_x Si_9.1Cr_9.8C_3.8?x=14³7?alloy is calculated to be between 1.14R and 1.31R,and increases with the increase of Mn content;the mixed enthalpy value of the alloy is calculated between-20.67kJ/mol and-19.61kJ/mol,but it decreases with increasing Mn content;the atomic size difference?and the electronegativity??increases with the increase of Mn content,and the valence electron concentration VEC value decreases with the increase of Mn content,in which the atomic size difference is between 6.72%and 6.87%,and the electronegativity difference is between 0.18 and 0.21,The electron concentration varies from 6.92 to 7.14.It can be seen that the atomic size difference,electronegativity difference and valence electron concentration of the entropy alloy in this system are small,and the alloy tends to form a solid solution structure;It is calculated that the entropy alloys in this system have lower stacking fault energy,the stacking fault energy value is between 9.70mJ/m² and17.12mJ/m²,and gradually increases with the increase of Mn element content.The alloy with the Mn content of x=14 has the lowest stacking fault energy.On this basis,the microstructure and mechanical behavior of the entropy alloy system in Fe_77.3-x7.3-x Mn_x Si_9.1Cr_9.8C_3.8?x=14³7?are studied.When the Mn content is x=14,the microstructure of the as-cast alloy sample is a single FCC???austenite phase;with the increase of Mn content,thermally induced HCP???martensite gradually precipitates in the alloy.When x=14,the alloy structure after fracture is FCC???+HCP???+BCT????,and with the increase of Mn content,the BCT????martensite phase gradually decreases,and HCP???martensite phase gradually increases.When x=23.2,the structure of the alloy after fracture is FCC???+HCP???,and there is no BCT????martensite phase;for alloy samples of x?23.2,the structure after fracture is FCC???+HCP???.With the increase of Mn content,the yield strength of the material gradually increased,but the fracture strength and plasticity decreased gradually.When x=37,the alloy showed complete brittle fracture.When the Mn content is x=14,the alloy has the best comprehensive mechanical properties,and the yield strength and fracture strength are 1300MPa and 2380MPa,respectively,and the compression rate is 16.7%;when the Mn content is x=23.2,the alloy yield strength and fracture strength are 1526MPa and 2332MPa,respectively,and the compression rate is 16.0%,the river-like patterns of the fractures of these two alloys are more dense and have a large number of tough dens.Comparing the Vickers hardness of alloys with different Mn contents,it is found that the hardness of alloys with Mn content of x=23.2 is significantly higher than that of x=14,this is due to the precipitation of the thermally induced HCP???martensite phase with greater hardness in the as-cast structure of the alloy.Select the samples with excellent Mn content of x=14 and x=23.2 with comprehensive mechanical properties at room temperature,and study the mechanical behavior of the samples after liquid alloy low temperature treatment and high temperature quenching heat treatment.After low temperature treatment,the alloy structure still maintains a single austenite structure when x=14,and the alloy structure is FCC???+HCP???phase when x=23.2,which is no different from the untreated sample structure,but the mechanical properties have great changes.After low temperature treatment,the medium-entropy alloy with x=14 has the best mechanical properties when the strain rate is^??=2*10^-4s^-1,the yield strength and fracture strength are 1180MPa,2550MPa,and the compression rate is 19.2%.However,the alloy with x=23.2?FCC+HCP structure?has a severe decrease in mechanical properties after low temperature treatment,showing cold brittleness.For the alloys with x=14 and x=23.2,after quenching heat treatment at 1100?for different time?10⁶0min?,the structure returns to the alloy with single FCC structure,and after high temperature heat treatment,the mechanical properties of the alloy are significantly improved?The fracture strength and compression rate of the alloy with x=14 increased by 36.5%and 46.7%after heat treatment,respectively;the fracture strength and compression rate of the alloy with x=23.2 increased by 4.2%and 55.5%after heat treatment,respectively?,two alloys with different compositions have the best mechanical properties when the holding time is t=20min.From the fracture morphology,after the low temperature treatment and high temperature heat treatment of the Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8.8 medium-entropy alloy,the cleavage steps in the fracture morphology of the sample basically disappear,the river patterns are dense and evenly distributed,and the number of tough dens are greatly increased.It shows that the mechanical properties of Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8.8 medium entropy alloy is improved after low temperature treatment and heat treatment.