Atomic Study Of Microstructures In Cold Deformed Fe₄₂Mn₃₈Co₁₀Cr₁₀ High-entropy Alloy

High-entropy alloy,one of the advanced materials developed in the last twenty years,is the key to explore the broad space of multi-component phase diagram and to achieve performance breakthroughs.A well-known high-entropy alloy system is the metastable Fe_80-xMn_xCo₁₀Cr₁₀（30≤x≤45,at.%）,which exhibits excellent mechanical properties and remarkable strain hardening effect at room temperature owing to the activation of dislocation,stacking fault,twin and martensite.However,the core structure,nucleation mechanism,growth mechanism,and interaction mechanism of deformation microstructures of this high-entropy alloy system have not been established.Therefore,the present research targets one of the metastable high-entropy alloys,Fe₄₂Mn₃₈Co₁₀Cr₁₀,to systematically study the deformation microstructures by using atomic-resolved HAADF-STEM.The purpose of the research is to elucidate the crystallographic feature and evolution process of deformed microstructures,providing a theoretical basis for the design of structure and performance of high-entropy alloys.In this study,it is found that the hexagonal close-packed martensite and face-centered cubic twin induced by deformation are both lath with thickness of several to tens of nanometers.With the increase of deformation,martensite and twin laths persistently form and overlap to construct nano-laminates,followed by bending at high strains to support further deformation.The nucleation and growth of most martensite and twin are governed by self-accommodation mechanism,in which Shockley partial dislocations belonging to three different slip systems in one slip plane are activated to minimize the macroscopic strain.Meanwhile,a small quantity of martensite is produced by homogeneous shear of monotype Shockley partial dislocations.When homogenous shear induced parallel martensite plates collide,two new types of dislocation walls form at the interfaces,respectively consisting of all 30°and all 90°Shockley partial dislocations.In addition,Frank partial dislocation wall is found at the twin boundary for the first time,which may derive from the reaction between perfect dislocation generated inside the twin and Shockley partial dislocation located at the twin front.The above three types of dislocation walls can accommodate plastic deformation by absorbing dislocations as well as produce strain hardening effect by blocking dislocations.Intersecting variants of martensite and twin are induced by plastic deformation,resulting in multiple interaction mechanisms.The interaction between incident twin lath and barrier twin lath（twin-twin interaction）leads to{111}_FCC secondary twinning in the intersection region.Incident martensite lath can not only cause the non-coplanar twin lath（martensite-twin interaction）to twin or to rotate,but can also trigger{10（?）2}_HCP or{10（?）3}_HCP twinning near the intersection region.When nano-laminates of martensite and twin interact with non-coplanar martensite laths（complex interaction）,grain rotation,martensitic transformation and the reverse phase transformation happen in and around the intersection region.According to the crystallographic analyses,the angle law for grain rotation in different intersection cases is as follows:if the incident lath is martensite,the barrier lath,either martensite or twin,will be forced to rotate in the range from 8.5°counterclockwise to 19.5°clockwise;if the incident lath is twin,the rotation angle of barrier martensite or twin ranges from 15.7°counterclockwise to 39°clockwise.The exact rotation angle depends on the shear carried by incident lath,namely the dislocations at the front of incident lath.Abundant stacking faults are introduced in the deformed Fe₄₂Mn₃₈Co₁₀Cr₁₀,including four intrinsic types and two extrinsic types.These stacking faults are bordered by different partial dislocations,including Shockley,Frank and unusual 1/6<411>_FCCpartial dislocations.Additionally,seventeen types of stacking fault configurations are also observed,including the well-known Lomer-Cottrell lock,Hirth lock and faulted dipole,the rarely reported intrinsic-extrinsic fault bend,and thirteen hitherto unreported configurations.Moreover,three configurations contain surprisingly a Frank partial dislocation at one edge,in contrast to previously reported configurations that are all bounded exclusively by Shockley partial dislocations.Based on Burgers vector analyses,the evolution of stacking faults during deformation process is deduced as follows:firstly,perfect dislocations dissociate into intrinsic or extrinsic stacking faults,which only contain Shockley partial dislocations at their ends;later,the stacking fault reacts with a perfect dislocation,forming a Frank or 1/6<411>_FCC partial dislocation at one end;then under external force,two conjugate stacking faults react,constructing a V-shaped stacking fault configuration when meeting at their ends or a X-shaped configuration when they cross each other;afterwards,the stacking fault configuration continues to react with isolated stacking fault or stacking fault configuration,constructing T-,Z-,U-,W-shaped or other complex configurations.