Study On Mechanisms Of Strengthening-toughening And Continuous Plastic Deformation Of Fe35Ni35Cr20Mn10 Multi-principal Element Alloy

Multi-principal element alloys（MPEAs）with face-centered cubic（FCC）structure have attracted extensive attention,due to their excellent strength-toughness at cryogenic temperature.Most of them have a low yield strength at room temperature and contain cobalt element which is an expensive and strategic matter.The strength-ductility trade-off is a great challenge of the development of FCC-MPEAs.These factors limit their applications as a practical structural material.Therefore,in this study,a novel non-equiatomic Fe-Ni-Cr-Mn MPEA which is expected to have a good combination of strength and plastic toughness,excellent structural stability and corrosion resistance was designed and prepared to enrich the Fe-Ni-Cr-Mn MPEA system.In order to solve the challenge of strength-toughness trade-off,the effects of alloying and thermo-mechanical processing on the microstructure and mechanical properties of Fe35Ni35Cr20Mn10 MPEA were studied,and the mechanisms of strengthening-toughening were discussed.The continuous plastic forming ability of industrial Fe35Ni35Cr20Mn10 MPEA was evaluated by continually cold-rolling and cold-drawing,and its mechanism was studied.This will construct a theoretical foundation for its industrial applications.The innovative achievements obtained in this study are as follows:By phase diagram calculation and criterion-parameters calculation,a novel Fe35Ni35Cr20Mn10 MPEA with single-phase FCC structure was successfully designed and prepared.This alloy is demonstrated to have an excellent stability in phase structure even at high temperature and after a very large plastic deformation.It has good yield strength,excellent elongation and compressive deformation ability,and high strain hardening ability during tensile and compressive deformation process.It also has a better electrochemical corrosion resistance in 0.5 M H₂SO₄solution than the commercial 304 stainless steel.Cold-rolling+recrystallization annealing treatment or hot-forging+recrystallization annealing treatment could effectively solve the strength-toughness trade-off of the prepared L-Fe35Ni35Cr20Mn10 MPEA.After cold-rolling with a reduction of 50%and annealing at800℃for 1 h,the prepared MPEA has a good combination of yield strength（～336 MPa）,ultimate tensile strength（～525 MPa）,and elongation to fracture（～43.7%）,After hot-forging and annealing at 800℃for 1 h,it also has a good combination of yield strength（～411 MPa）,ultimate tensile strength（～583 MPa）,and elongation to fracture（～35.0%）.It is found that different microstructure features in the prepared MPEA before the tensile test would lead to different softening mechanisms during the tensile deformation,and hence result in different mechanical behaviors.L-（Fe35Ni35Cr20Mn10）_98.5C_1.5MPEA containing a great number of nano coherent precipitation particles of M₂₃C₆carbide was successfully prepared by adding 1.5 at.%of carbon to the base alloy,which has a higher yield strength than the base alloy（from 201 MPa to 480 MPa）.It is found that carbon alloying leads to the significant refinement of grains and coherent precipitation of a great number of nano carbide particles,which are the main reasons to the high strength and good plastic toughness（28.3%of elongation）.Adding 4.7 at.%of titanium to the base alloy,a great amount of thick/thinηflakes are formed at the grain boundaries of the homogenized L-（Fe35Ni35Cr20Mn10）_95.3Ti_4.7MPEA.Theseηflakes are coherent with FCC matrix and regularly intersect-array with specific directions that formsη-locks.Titanium alloying greatly increases the yield strength from 201MPa to 331 MPa and doesn’t decrease the plastic toughness（the elongation is still at a high level of 47%）.It is found that,it is the unique and coherentη-Locks constructed at the grain boundaries by titanium alloying that deform coordinately with matrix and hence maintain the high plastic toughness while greatly improving the strength.As a result,the problem of strength-toughness trade-off is solved.Industrial I-Fe35Ni35Cr20Mn10 MPEA large ingot（about 30 kg）was prepared by industrial vacuum induction melting furnace.After cold-rolling with a reduction of 95%,the yield strength of industrial I-Fe35Ni35Cr20Mn10 MPEA is about 1099 MPa,4.4 times higher than that in the initial state.It is found that,the mechanism that the prepared MPEA has excellent continual cold-rolling plastic deformation ability is as follows:the plane slipping of dislocations that causes the flattened grains to become laminating,then sub-graining and further refining（the sub-grain lamellar structure is increasingly thinned）and the cross slipping that forms complex shear bands support the continuous plastic deformation,while the twin bundles,nano-twins between sub-grain lamellae and crossed sub-grains inside a sub-grain lamellae harmonize the plastic deformation between grains and sub-grain lamellae.A rod of 6.34 mm in diameter,made of industrial I-Fe35Ni35Cr20Mn10 MPEA,was continually cold-drawn to a wire of 0.08 mm without inter-annealing,indicating that the prepared I-Fe35Ni35Cr20Mn10 MPEA has a very excellent continual cold-drawing ability at room temperature.The mechanism for it is that:in the high drawing strain stage,plane slipping and cross slipping of dislocations and dynamical recovering of sub-grain layers make the poly-layer structure maintain a dynamic equilibrium state;it is the thickness of layers remains essentially constant that supports the continuous plastic deformation;nano twins between the layers and inside the layer harmonize the deformation between the layers and within layer.By continual cold-drawing process,a single-phase FCC-MPEA wire with the largest cumulative strain and section compression（8.73 and 99.984%,respectively）,the smallest diameter（0.08 mm）and the highest strength（1868 MPa）was successfully produced.