Strategies For Strength-Ductility Optimization Of Face-Centered Cubic High-entropy Alloys

Unlike the design of the conventional alloys with a single host element,such as Fe for steels and Al for aluminium alloys,high-entropy alloys(HEAs)are a new type of alloys and composed of multi principal elements.The features of sluggish diffusion and severe lattice-distortion of HEAs introduced by the multiple elements enable them having good structure stability and superior mechanical properties.Hence,the discovery of HEAs provides the opportunities to explore novel properties that are difficult to be obtained in conventional alloys.However,the strength of HEAs with face-centered cubic(FCC)structure is low,which limits their application potential as structural materials.In this thesis,the CoCrFeMnNi HEA was selected as the model material,and the investigation of increasing strength and improving the strength-ductility synergy was carried out.Then,a new type precipitation-strengthening HEA was developed.These studies would provide theoretical support for optimizing the mechanical properties and realizing industrial application of the HEAs with FCC structure.The main results of this thesis are as follows:(1)The fully recrystallized ultrafine-graind(UFG)microstructure was introduced into CoCrFeMnNi HEA by cold rolling and annealing treatment.The recrystallized UFG grains are clean inside and there is nearly no dislocations in the grains,which is different from the UFG microstructure fabricated by severe plastic deformation(SPD)with high-density dislocations.The strength could be enhanced due to the grain refinement and the strain-hardening ability could be improved because the dislocations can be effectively stored in the clean grains during the plastic deformation process.With the increase of grain size in the alloys,deformation twins could be detected in the deformed microstructures,and the strain-hardening ability was gradually improved.The twinning behavior is dependent on the competition between the flow stress and critical twinning stress(2)With the decrease of deformation temperature,the mechanical properties of the HEAs were improved.Eespecially,the UFG HEA exhibited an ultrahigh yield strength of 1.24 GPa,a considerable ultimate tensile strength of and 1.46 GPa and an excellent uniform elongation of 41%at 77 K.The decrease of temperature led to a transition in deformation mechanism from dislocation slip at 293 K to multiple deformation modes of dislocations,stacking faults and deformation twins at 77 K.It is found that the Hall-Petch relationship between the yield strength and grain size was well fitted in the tempearature range from 77 K to 873 K,and the fitting parameters have a negative relation with temperature.In addition,CoCrFeMnNi HEA with partially recrystallized structure was fabricated,which exhibited an ultrahigh yield strength of～1.7 GPa and a considerable uniform elongation of 10.3%at 77 K.(3)In order to improve the strength of the alloys with FCC structure,prestrain at different temperatures was applied in this thesis.The HEA prestrained at 77 K possessed higher yield strength and uniform elongation than the HEA prestrained at 293 K,which indicates that the trade-off relationship between strength and ductility can be broken by modulating the prestrain history.Furthermore,the yield point phenomenon disappeared after prestrained at 77 K.These can be related to the defects as imposed by different prestrain histories.Deformation twins and dislocations were introduced after prestrain at 77 K,while only dislocations were imposed after prestrain at 293 K.It is also found that the cryogenic strengthening magnitude is sensitive to the prestrain history,which is attributed to the twin boundary strengthening effect via the remarkable increase of Hall-Petch slope with decreasing temperature.(4)A new precipitation hardened Fe53Mn15Ni15Cr10Al4Ti2C1 HEA was designed by adding Al,Ti and C element based on the FCC HEAs.The microstructures of the cold-rolling alloy presented rolling bands(including deformation twin)and high-density dislocation.After the heat treatmen at low temperature for long time,the HEA exhibited inhomogeneous microstructures,including rolling bands,high-density dislocation and precipitate,and the HEA possessed a superior balance of strength and ductility.The superior mechanical properties were resulted from the significant precipitation-strengthening effect of L12 precipitate coherent with matrix in the microstructure,and the improved strain-hardening ability due to the recovery of dislocations.It could be an effective method to develop ultrahigh-strength HEA with excellent strength-ductility relationship by designing precipitated HEAs with inhomogeneous structure through the heat treatment at low temperature for long time.