| High entropy alloy(HEA)is a new type of multi-principal component alloy composed of four or more major elements.Since its appearance,it has won wide attention by virtue of its excellent properties such as high strength,high plasticity and corrosion resistance.At first,the design philosophy of HEAs followed the principle of equal molar ratio,and the goal of designing stable and disordered single-phase solid solution was achieved through pursuing the maximum configurational entropy.However,with the rapid development of the field of HEAs,the improvement of single-phase HEAs in terms of strength and toughness has fallen into a bottleneck.At the same time,recent studies have found that in addition to mixing entropy,intrinsic stacking fault energy(SFE)also has significant effects on phase structure,microstructure and mechanical properties of HEAs.By changing the type and concentration of alloying elements,the stacking fault energy and phase stability of alloys can be adjusted,and more microstructure defects can be introduced appropriately to induce deformation twins,martensitic transformation and other deformation mechanisms,obtaining outstanding comprehensive mechanical properties.The idea of improving the mechanical properties of alloys by designing metastable phases is called "metastable engineering".As a typical metastable HEA system,the microstructure of Fe-Mn-Co-Cr system contains matrix face-centered cubic(FCC)phase and HCP-martensite phase,and the stability of FCC phase is mainly related to the content of Mn element.With the decrease of Mn content,the stacking fault energy of the system decreases,and the stability of FCC phase decreases.Under stress,martensitic transformation(FCC?HCP)is activated,and the hardening mechanism such as interface hardening is introduced,which realizes the strength and plasticity enhancement at the same time.Although many scholars have analyzed the relationship between micro structure evolution and mechanical properties of metastable Fe-Mn-Co-Cr system at room temperature from the perspective of experimental characterization,there are few reports on the calculation of thermodynamic parameters of martensitic transformation in HEAs.In addition,the mechanical properties and deformation mechanism of the metastable Fe-Mn-Co-Cr system at 77K also need to be further investigated.Therefore,Fe60Mn20Co10Cr10 HEA was selected as the research object in this paper.Alloy samples with different microstructures were obtained through appropriate heat treatment process to study the micro structure evolution,mechanical properties and deformation mechanism of quasi-static tensile under room temperature and low temperature conditions.The main research results are as following:(1)After homogenization annealing and warm rolling,the as-cast Fe60Mn20Co10Cr10 HEA was annealed at 700,800 and 900? for 30min,respectively.Three kinds of alloy samples with different recrystallization states were obtained.The micro structure of the alloy samples all contained FCC matrix phase and lamellar HCP phase.Quasi-static tensile tests at room temperature and cryogenic temperature show successive strain hardening capability and excellent strength-plasticity bonding characteristics.(2)The Gibbs free energy and stacking fault energy of FCC?HCP transformation of Fe60Mn20Co10Cr10 high entropy alloys at room temperature were calculated by regular solution model as well as the values of ?G???=-79.9J/mol and ?sf=15·3mJ/m2,respectively.The relationship between configuration entropy and SFE in the process of alloy composition evolution is analyzed.It is found that the SFE of the alloy system decreases with the increasing of the number of principal elements,indicating that multi-principal element design concept is beneficial to reduce the SFE of alloy system.(3)At room temperature,dislocation slip was the main deformation mechanism in the FCC phase at the initial stage of deformation,and crystal defects such as stacking faults and high-density dislocation walls were gradually formed,which hindered dislocation movement.As the strain increases,the FCC?HCP martensitic transformation is activated,and the dynamic strain partitioning between the two phases results in the continuous and stable strain hardening ability.In the later deformation stage,the HCP laminates become thicker and the internal dislocation density gradually increases,and dislocations are significantly blocked,which eventually leads to stress concentration and fracture.(4)At cryogenic temperature,the alloy has lower SFE than at room temperature,and the stacking fault density increases,which provides more nucleation sites for HCP phase and accelerates the nucleation rate of HCP phase.However,with the rapid increasing of the volume fraction of HCP phase,the FCC phase is consumed so fast that two phases cannot disperse the stress in time.The HCP-martensite phase bears the main task of plastic deformation,and finally leads to the premature fracture of the alloy. |
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