Phase Stability And Strengthening Of Ni-Co-Cr-Fe Based High Entropy Alloys

Differentiating from traditional one element-based design strategy,high entropy alloys(HEAs)are brand-new metallic strucutal materials with excellent mechanical and functional properties based on multi-component-element design strategy.Phase stability and strengthening mechanisms are basic scientific problems for researchers to develop strong and ductile HEAs.Current investigations mainly focous on the phase selection of single-phase solid-solutions while the phase stability of HEAs is not fully understood.Researchers tend to utlize the traditional strengthening methods to improve the mechanical properties of HEAs,however,understanding on the strengthening mechanisms of HEAs is limited.In the current work,using Ni-Co-Cr-Fe based HEAs as a model alloy system,we systematically investigated the phase stability,grain boundary,solid-solution and precipitation strengthening mechanisms of face centered cubic(FCC)HEAs by scanning electron microscope,transmission electron microscope,3-D atom probe,X-ray diffraction,and mechanical tests.An ultra-strong and ductile HEAs was developed by utilizing multiple strengthening mechanisms.The main research contents and conclusions can be drawn as following:(1)We investigated the effects of mixing entropy,mixing enthopy,and temperature on the phase stability of FCC HEA respectively using experimental and CALculated Phase Diagram(CALPHD)methods.It is revealed that there is a large single-phase region in the Ni-Co-Cr-Fe system due to its composition freedoms.Temperature plays an important role in the phase stability of HEAs.The Ni Co Cr Fe HEA transforms from stable to metastable states when the temperature decreases from 900 to 750?.Slight variation in mixing enthopy resulted in precipitation of different types of intemetallic compounds.With the current results,we believe that the competition of Gibbs free energy of different phases in HEAs determines their phase stabilities.(2)The grain boundary strengthening mechanism of the Ni₂Co Cr Fe HEA was revealed.The yield strength of the Ni₂Co Cr Fe HEA is inversely propotional to the squre root of grain size and the Hall-Petch coefficient is measured to be K_y=412MPa·?m^1/2.The ductility of Ni₂Co Cr Fe HEA firstly increases and then decreases with decreasing grain size.The variation of ductility is attributed to the different strain hardening behaviors caused by different dislocation gliding modes in HEAs with different grain sizes.(3)The sloid-solution strengthening mechanism of the Ni₂Co Cr Fe HEA was clearified.The solutes addition simutaneously enhances the strength and ductility of the Ni₂Co Cr Fe HEA.Classic solid-solution strengthening model indictates that the solid-solution strengthening efficiency is better than that of traditional dilute solid-sloutions due to the formation of short-range ordering(SRO)in the multi-component-element HEAs.The increase of ductility is because the increased strain hardening capicaity cuased by the increase of lattice friction stress,decrease of stacking fault energy,and the formation of SRO.(4)An overall valence electron concentration(OVEC)strategy was proposed for the precipitates design of HEAs.Experimental results and previous studies indictate that the?"phase tends to form when the OVEC of HEAs is higher than 8.4 and the solute elements are from group VB,while??phase is easy to precipitate when OVEC is between 8 and 8.4 and solute elements are Ti and Al.Based on the OVEC strategy,a??-strengthened Ni₂Co Cr Fe Ti_xAl_yand a?"-strengthened Ni₂Co Cr Fe Nb_0.15 were developed.(5)The precipitation strengthening mechanisms of different precipitates were investigated.Ordering strengthening is the dominant mechanism for??phase.Ti/Al ratio is the main factor that affects the precipitaton behavior and strengthening efficiency of??phase.With the same total mole concentration of Ti and Al,a higher Ti/Al ratio enhances the volume fraction,lattice misfit,and anti-phase boundary energy of??phase,resulting in better strengthening effect and higher strength.Ordering strenghthening and coherency strengthening mechanisms simultaneously contribute to yield strength increase caused by?"phase.Compared with??phase,?"phase shows better strengthening effect because of a higher lattice misfit and an anti-phase boundary energy.(6)A multi-phase hierarchical HEA was developed by controlling relative kinetics of cold rolling,recrystallization,and precipitation.The mechanical property of the new HEA was studied.The multi-phase hierarchical HEA showed excellent mechanical properties because it combines the grain boundary,solid-solution,precipitation,work hardening,and gradient microstructure strengthening mechanisms.The Ni₂Co Cr Fe Ti_0.18Al_0.12 HEA showed a yield strength of 1.2 GPa,ultimate strength 1.5 GPa,and ducilty higher than 15%after 70%cold rolling followed by annealed at 800?for 30–60 min.