Study On Strain Hardening Of Ni-Co-Cr-Fe Based High Entropy Alloys

High-entropy alloys have attracted much attention due to their excellent mechanical properties such as high fracture toughness,excellent ductility and unique cryogenic temperature deformation mechanism,and have become potential candidates for engineering application.However,the low yield strength of the face-centered cubic(FCC)alloys has limited the alloys applications in engineering areas.Therefore,how to probe the balance between strength and ductility in FCC high-entropy alloy has become a research hotspot in recent years.Strain hardening is an effective method of strengthening,whereas there is no systematic report on the strain hardening of FCC high-entropy alloys.In the current work,the strain hardening behaviors of Ni-Co-Cr-Fe-based high entropy alloys,i.e.as Ni2CoCrFe,NiCoCrFe,and NiCoCrFeMo0.2 FCC single-phase high-entropy alloys was investigated by using confocal laser scanning microscope,X-ray diffraction,scanning electron microscope and mechanical tests.The strain hardening effects at room and cryogenic temperatures were evaluated.The effects of dislocations,deformation twins,and incomplete recrystallization on the strain hardening behaviors of the alloys were clarified.The main research contents and conclusions of this thesis can be drawn as following:(1)The strain hardening behavior of Ni2CoCrFe high-entropy alloy was studied.The microstructures and mechanical properties of the Ni2CoCrFe high-entropy alloys with different rolling reductions at room temperature were investigated.With the increase of rolling reductions,the uniformly distributed dislocations of Ni2CoCrFe high-entropy alloy gradually evolved to dislocation cells and dislocation walls,and the dislocation density increased significantly.The strength of the alloy gradually increased as the rolling deformation increases.The optimal combination of strength and ductility for the alloy was achieved when the rolling deformation is 30%at room temperature.The yield strength,tensile strength and elongation of this alloy are 835.0MPa,882.0MPa and 12.5%,respectively.The microstructure evolution and strengthening effect of the alloy under cryogenic conditions are similar to those at room temperature.Micro structure analysis shows that the deformation mechanism of Ni2CoCrFe high-entropy alloy is mainly dominated by dislocation slip,so the main strengthening mechanism of is the increase of dislocation density.(2)The effect of twinning on the strain strengthening of NiCoCrFe high-entropy alloy was clarified.At room temperature,deformation twins and dislocations exist in some grains when the rolling deformation was low.With the increase of rolling deformation,the microstructure of the alloy gradually evolved into the presence of deformed twins in most grains and the dislocation density increased significantly.Comparing the tensile properties of Ni2CoCrFe and NiCoCrFe high-entropy alloys with different rolling deformations at room temperature,it can be found that NiCoCrFe high-entropy alloys has higher tensile ductility.The ductility retention of NiCoCrFe high-entropy alloy processed by 20%rolling deformation at room temperature is 45.2%,which is higher than that of the Ni2CoCrFe high-entropy alloy.Microstructure analysis showed that the main deformation mechanism of the NiCoCrFe high-entropy alloy is dominated by dislocation slip and deformation twining,which provided additional work hardening capability and resulted in the higher ductility.(3)The effects of solid solution elements and annealing treatment on the strain hardening behavior of the NiCoCrFeMo0.2 high-entropy alloy was revealed.The NiCoCrFeMo0.2 alloys processed by different rolling reductions at room temperature and cryogenic temperature exhibit deformed twins and dislocation structures,which increased with the increasing rolling reduction.With the increase of rolling reduction,the strength of the alloy increased and the elongation decreased.For the NiCoCrFeMo0.2 and NiCoCrFe high-entropy alloys with same grain size,the addition of Mo element significantly improved the strength of the alloy,illustrating typical solid solution strengthening.The yield strength,tensile strength and elongation of NiCoCrFeMo0.2 high-entropy alloy processed by 35%rolling deformation at cryogenic condition are 937.4 MPa,1058.3 MPa and 10.7%,respectively.The partially recrystallized NiCoCrFeMo0.2 high-entropy alloys was featured by coarse deformed grains and fine recrystallized grains and showed excellent comprehensive mechanical properties with yield strength of 539.3MPa and tensile elongation of 46.8%.