Mechanical Behavior And Deformation Mechanism Of Al_0.1CoCrFeNi Gradient High-Entropy Alloy

High-entropy alloys(HEAs),also referred as multicomponent alloys,since its discovery in 2004,have attracted widespread attention due to their excellent mechanical properties.It is known that how to evade the dilemma of strength–ductility trade-off has always been a long-term problem in the alloy strengthening history.Recently,it is reported that the gradient structured(GS)material can achieve the high strength with extraordinary plasticity due to the unique deformation mechanism.The GS HEA realizes the strengthening and toughness of materials through the cooperation of coarse and fine grain layers,which avoids the decrease of material properties caused by uneven interlayer deformation.In this thesis,Al_0.1CoCrFeNi HEAs are subjected to different torsion angles using a torsion testing machine.The microstructure,mechanical properties and corresponding strengthening mechanism of the gradient structured material are systematically investigated.The main research contents are as follows:(1)The Al_0.1CoCrFeNi HEA rod is subjected to quasi-static torsion loading,and the torsion angles are set as 180°,540°and 900°,respectively.Different gradient microstructures along the radial direction can be obtained,in which dislocation slip is dominated in the central area,and deformed twins combined with microbands are crucial in the edge area.The X-ray diffraction(XRD),scanning electron microscope(SEM),electron backscatter diffraction(EBSD)and transmission electron microscope(TEM)techniques are employed to obtain qualitative and quantitative characterizations of microstructures including grain size,dislocation density and deformation twins volume fraction from center to edge.(2)Based on the gradient microstructure along the radial direction,nanohardness and creep behavior present an obvious gradient distribution,where the nanohardness value increases with the increase of the torsion angle and the creep resistance is continuously decreased from center to edge.Moreover,based on grain boundary strengthening,dislocation strengthening and twin strengthening,the strengthening mechanism of gradient alloys is quantitatively characterized,and the role of each strengthening mechanism is discussed.(3)Using TEM and quasi-static tensile experiments,the microstructure and mechanical properties at room temperature are further analyzed for the Al_0.1CoCrFeNi HEA after torsion.The experimental results reveal that the GS HEA can increase the strength with moderate decrease in ductility.In spite of this,a strength-ductility synergy is still attained in GS plate due to its gradient microstructure and extraordinary strain hardening capacity,i.e.,an enhancement in tensile yielding strength beyond the effect of role of mixture(Ro M)while retaining good plasticity.TEM analysis suggests that additional work hardening ability and back stress strengthening lead to the excellent strength-ductility combination of GS sheets.(4)Tensile experiments at 77 K exhibit that in comparison with room temperature loading,both strength and ductility of the Al_0.1CoCrFeNi GS HEA are simultaneously improved,which is probably due to the decreased stacking-fault energy at 77 K and high density lattice defects upon pre-torsion,such as dislocations,stack faults,twin boundaries,etc.