Study On Strengthening And Toughening Of FeMnNiCr Medium Entropy Alloy

The FCC structured medium/high entropy alloys can exhibit excellent mechanical properties through reasonable composition design and preparation and processing processes.However,the addition of precious metal Co elements increases the price of FCC medium/high entropy alloys,which is very unfavorable for their application in engineering.Therefore,single-phase FCC structured Co-free medium/high entropy alloys（MEAs/HEAs）have been widely investigated for their excellent room temperature/low temperature mechanical properties and relatively low cost.Among them,Fe₅₀Mn₂₅Ni₁₀Cr₁₅ is a typical low-cost,high-performance medium-entropy alloy.However,its yield strength is low,which leads to early deformation failure of the alloy,which is not conducive to making full use of the strength index of the material.In this thesis,Fe₅₀Mn₂₅Ni₁₀Cr₁₅ was toughened mainly by in situ generation of second-phase particles and construction of gradient dislocation structure,in order to obtain a medium-entropy alloy with good strength and toughness coordination.The main results obtained are as follows.（1）The Ti C-reinforced(Fe₅₀Mn₂₅Ni₁₀Cr₁₅)_100-x（Ti C）_x（x=0,1.5,1.0 1.5,at.%）mesoentropic alloy was prepared by generating Ti C second phase in the cobalt-free Fe₅₀Mn₂₅Ni₁₀Cr₁₅ mesoentropic alloy through in situ reaction in a vacuum melting furnace and was rolled and recrystallized and annealed.The results showed that the room temperature yield strength of Fe₅₀Mn₂₅Ni₁₀Cr₁₅ mesoentropic alloy increased from250 MPa to 452 MPa with the addition of 0.5 at.%Ti C,an increase of 80%,while possessing 45.7%elongation after break;the alloy yield strength reached 649 MPa,tensile strength reached 1135 MPa,and elongation after break was 72%at liquid nitrogen temperature.The alloy exhibits excellent strength-plasticity matching relationship at both room temperature and low temperature.The fine grain strengthening,second phase strengthening and dislocation strengthening are the main reasons for the increase in yield strength,while the excellent plasticity comes from the TWIP effect.（2）A gradient structure was prepared on the surface of the cobalt-free Fe₅₀Mn₂₅Ni₁₀Cr₁₅ medium entropy alloy by shot peening to enhance its yield strength.The results showed that the strength of the lightly shot peened specimens was around370 MPa（±10 MPa）,and the yield strength was increased by 54.17%;the yield strength of the vigorously shot peened specimens was increased to 430 MPa（±10 MPa）,and the yield strength was increased by 79.17%compared with the original specimens.After shot peening,the surface tissue dislocations/twinning increased,grain size refinement,lattice distortion,etc.,and with the increase in depth,the gradient structure characteristics were presented,and the synergistic strengthening effect contributed to the increase in yield strength.（3）The gradient structure was constructed in a cobalt free Fe₅₀Mn₂₅Ni₁₀Cr₁₅medium entropy alloy by torsion deformation.The results of mechanical properties show that the yield strength of the specimen before torsion is about 305 MPa,and the elongation after fracture is～28.67%;After 45°torsion,the yield strength of the sample is～415 MPa,and the elongation after fracture is～27.73%;After 90°torsion,the yield strength is～496 MPa,and the elongation after fracture is～24%;The maximum yield strength of the specimen after 180°torsion is 606 MPa.Gradient structure is the reason for the increase of yield strength.